USES OF COMPUTER NETWORKS - WordPress.com

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UNIT-I:Network Hardware: LAN – WAN – MAN – Wireless – Home Networks. Network

Software: Protocol Hierarchies – Design Issues for the Layers – Connection-oriented and

connectionless services – Service Primitives – The Relationship of services to Protocols.

Reference Models: OSI Reference Model – TCP/IP reference Model – Comparison of OSI and

TCP/IP -Critique of OSI and protocols – Critique of the TCP/IP Reference model.

UNIT-I

INTRODUCTION

A group of two or more computing devices connected via a form of communications

technology. For example, a business might use a computer network connected via cables or the

Internet in order to gain access to a common server or to share programs, files and other

information.

Computer Network means an “interconnected collection of autonomous

Computers”.

Two Computers are said to be interconnected if they can exchange information.

The connection usually will be based on a communication medium like copper wire,

fiber optics etc.,

Master/Slave relationship in which one computer forcibly start, stop, or control

another computer is not a network, the computers should be autonomous.

Difference between a Computer Network and Distributed System.

Computer Network Distributed System

The Existence of autonomous computers are

not transparent (they are visible).

The Existence of autonomous computers are

transparent (they are not visible).

The autonomous computer performs the

operation requested by the user.

The best processor is selected by the operating

system for carrying out the operations requested

by the user.

The user is aware of his working

environment.

The user is not aware of his working

environment, which is multiple processor in

nature but looks like a virtual uniprocessor.

All operations (allocation of jobs to

processors, files to disks, movement of files)

are done explicitly.

All operations (allocation of jobs to processors,

files to disks, movement of files) are done

automatically without the user’s knowledge.

Regulation software is enough for computer

networks.

A software that gives a high degree of

cohesiveness and transparency is needed since

distributed system is built on top of a network

A computer network can be two computers connected:

Fig 1.1 Connecting two computer

Characteristics of a Computer Network

The primary purpose of a computer network is to share resources:

You can play a CD music from one computer while sitting on another computer

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You may have a computer that doesn’t have a DVD or BluRay (BD) player. In this case,

you can place a movie disc (DVD or BD) on the computer that has the player, and then

view the movie on a computer that lacks the player

You may have a computer with a CD/DVD/BD writer or a backup system but the other

computer(s) doesn’t (don't) have it. In this case, you can burn discs or make backups on a

computer that has one of these but using data from a computer that doesn’t have a disc

writer or a backup system

You can connect a printer (or a scanner, or a fax machine) to one computer and let other

computers of the network print (or scan, or fax) to that printer (or scanner, or fax

machine)

The computers can be geographically located anywhere

Fig 1.2 Geographically located network

You can place a disc with pictures on one computer and let other computers access those

pictures

You can create files and store them in one computer,then access those files from the other

computer(s) connected to it

Peer-to-Peer network:

Based on their layout (not the physical but the imagined layout, also referred to as

topology), there are various types of networks. A network is referred to as peer-to-peer if

most computers are similar and run workstation operating systems.

In a peer-to-peer network, each computer holds its files and resources. Other computers

can access these resources but a computer that has a particular resource must be turned on

for other computers to access the resource it has. For example, if a printer is connected to

computer A and computer B wants to printer to that printer, computer A must be turned

On.

History Of Computer Network

A computer network, or simply a network, is a collection of computers and other

hardware components interconnected by communication channels that allow sharing of resources

and information. Today, computer networks are the core of modern communication. All modern

aspects of the public switched telephone network (PSTN) are computer-controlled. Telephony

increasingly runs over the Internet Protocol, although not necessarily the public Internet. The

scope of communication has increased significantly in the past decade.

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This boom in communications would not have been possible without the progressively

advancing computer network. Computer networks, and the technologies that make

communication between networked computers possible, continue to drive computer hardware,

software, and peripherals industries. The expansion of related industries is mirrored by growth in

the numbers and types of people using networks, from the researcher to the home user.

The following is a chronology of significant computer network developments:

In the late 1950s, early networks of communicating computers included the military

radar system Semi-Automatic Ground Environment (SAGE).

In 1960, the commercial airline reservation system semi-automatic business research

environment (SABRE) went online with two connected mainframes.

In 1962, J.C.R. Licklider developed a working group he called the "Intergalactic

Computer Network", a precursor to the ARPANET, at the Advanced Research

Projects Agency (ARPA).

In 1964, researchers at Dartmouth developed the Dartmouth Time Sharing System for

distributed users of large computer systems. The same year, at Massachusetts Institute

of Technology, a research group supported by General Electric and Bell Labs used a

computer to route and manage telephone connections.

Throughout the 1960s, Leonard Kleinrock, Paul Baran, and Donald Davies

independently developed network systems that used packets to transfer information

between computers over a network.

In 1965, Thomas Marill and Lawrence G. Roberts created the first wide area network

(WAN). This was an immediate precursor to the ARPANET, of which Roberts

became program manager.

Also in 1965, the first widely used telephone switch that implemented true computer

control was introduced by Western Electric.

In 1969, the University of California at Los Angeles, the Stanford Research Institute,

the University of California at Santa Barbara, and the University of Utah were

connected as the beginning of the ARPANET network using 50 kbit/s circuits.

In 1972, commercial services using X.25 were deployed, and later used as an

underlying infrastructure for expanding TCP/IP networks.

In 1973, Robert Metcalfe wrote a formal memo at Xerox PARC describing Ethernet,

a networking system that was based on the Aloha network, developed in the 1960s by

Norman Abramson and colleagues at the University of Hawaii. In July 1976, Robert

Metcalfe and David Boggs published their paper "Ethernet: Distributed Packet

Switching for Local Computer Networks" and collaborated on several patents

received in 1977 and 1978. In 1979, Robert Metcalfe pursued making Ethernet an

open standard.

In 1976, John Murphy of Datapoint Corporation created ARCNET, a token-passing

network first used to share storage devices.

In 1995, the transmission speed capacity for Ethernet was increased from 10 Mbit/s to

100 Mbit/s. By 1998, Ethernet supported transmission speeds of a Gigabit. The ability

of Ethernet to scale easily (such as quickly adapting to support new fiber optic cable

speeds) is a contributing factor to its continued use today.

USES OF COMPUTER NETWORKS

The use of Computer Networks can be divided into three ways.

1. Business Applications

2. Home Applications

3. Mobile Users

4. Social Issues

Business Application

Many companies have a substantial number of computers, for examples a company may

have separate computers to monitor production, keep track of inventories, do the payroll. Each

of these computers may have worked in isolation from the others, but at some point,

management may have decided to connect them to extract and correlate information about entire

company.

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Use of Computer Network for business can be classified as

1. Resource Sharing

2. High Reliability

3. Saving Money

4. Scalability

5. Communication Medium

The goal is resource sharing, to make all programs, equipment, and especially data

available to anyone on the network without regard to the physical location of the resource and

the user.

A second goal is to provide high reliability by having alternative sources of supply. All

files could be replicated on two or three machines, so if one of them is unavailable (due to a

hardware failure), the other copies could be used. In addition, the presence of multiple CPUs

means that if one goes down, the others may be able to take over its work, although at reduced

performance. Military, banking, air traffic control, nuclear reactor safety, and many other

applications, the ability to continue operating in the face of hardware problems is of utmost

importance.

Another goal is saving money. Small computers have a much better price/performance

ratio than large ones. Mainframes (room-size computers) are faster than personal computers, cost

more. This imbalance has lead to the idea of connecting personal computers, with data kept on

one or more shared file server machines. In this model, the users are called clients, and the

whole arrangement is called the client server model. In the client-server model, communication

generally takes the form of a request message from the client to the server asking for some work

to be done.

The server then does the work and sends back the reply. Usually, there are many clients

using a small number of servers.Another networking goal is scalability, the ability to increase

system performance gradually as the workload grows just by adding more processors. With the

client-server model, new clients and new servers can be added as needed.

A computer network can provide a powerful Communication medium among widely

separated employees. Using a network, it is easy for two or more people who live far apart to

write a report together. When one worker makes a change to an on-line document, the others can

see the change immediately, instead of waiting several days for a letter. Such a speedup makes

cooperation among far-flung groups of people easy where it previously had been impossible.

Home Applications:

The use of Computer Networks for people can be classified as

1. Access to remote information.

2. Person-to-person communication.

3. Interactive entertainment.

4. Electronic commerce.

Client Machine

Fig 1.3 Client Server

Model

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Access to remote information comes in many forms. Information available includes the

Arts, Business, Cooking, Government, Health, History, Hobbies, Recreation, Science, Sports,

Travel and many others. Newspapers will go on-line and be personalized. It will be possible to

download the areas of interest of a person say, politics, big fires, scandals involving celebrities,

and epidemics. The next step beyond newspapers is the on-line digital library. All of the above

applications involve interactions between a person and a remote database.

The second broad category of network use will be person-to-person communication

Electronic mail or email is already widely used by millions of people and will soon contain

audio and video as well as text. Smell in messages will take a bit longer to perfect. Instant

messaging allows two people to type messages at each other in real time. A multiperson version

of this idea is the chat room, in which a group of people can type messages for all to see.

Real-time email will allow remote users to communicate with no delay, possibly seeing

and hearing each other as well. This technology makes it possible to have virtual meetings,

called videoconference, among far-flung people. Virtual meetings could be used for remote

school, getting medical opinions from distant specialists, and numerous other applications.

Worldwide newsgroups, with discussions on every conceivable topic are common among a

select group of people, and this will grow to include the population at large. Here one person

posts a message and all the other subscribers to the newsgroup can read it and can respond with

an answer.

Our third category is entertainment, which is a huge and growing industry. The killer

application here is video on demand. Live television may also become interactive, with the

audience participating in quiz shows, choosing among contestants, and so on. Game playing is

an important application of computer network for people. Multiperson real-time simulation

games, like hide-and-seek in a virtual dungeon, and flight simulators with the players on one

team trying to shoot down the players on the opposing team, 3-dimensional real-time,

photographic-quality moving images, virtual reality games are few to mention.

Tag Full name Example

B2C Business-to-Consumer Ordering books on-line

B2B Business-to-Business Car manufacturer ordering tires from supplier

G2C Government-to-Consumer Government distributing tax forms

electronically

C2C Consumer-to-Consumer Auctioning second-hand products on line

P2P Peer-to-Peer File sharing

Mobile Users Mobile computers, such as notebook computers and personal digital

assistants (PDAs), are one of the fastest-growing segments of the computer industry. A common

reason is the portable office. People on the road want to use their portable electronic equipment

to send and receive telephone calls, faxes and electronic mail, surf the web, access remote files,

and log on remote machines and they want to do this from anywhere on land, sea or air. Wireless

networks are of great value of fleets of trucks, taxis, delivery vehicles and repair persons for

keeping in contact with home.

For example in many cities, taxi drivers are independent business men rather than being

employee’s of a taxi company. The taxi has a display the driver can see, when a customer calls

up, a central dispatcher types in the pick-up and destination points. This information is displayed

on a driver’s displays and a beep sounds. The first driver to hit a button on the display gets the

call. Wireless network are also important to the military.

Distinction between Fixed wireless and mobile wireless

Wireless Mobile Applications

No No Desktop computers in offices

No Yes A notebook computer used in a hotel room

Yes No Networks in older, unwired buildings

Yes Yes Portable offices; PDA for store inventory

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Social issues The widespread introduction of networking has led to

1. Social

2. Ethical and

3. Political problems.

The trouble arises when newsgroups are set up on topics that people contradicting views.

Views posted to such groups may be deeply offensive to some people. Thus the debate rages.

Users rights are violated and freeness of speech is barred. Computer networks offer the potential

for sending anonymous messages, a way to express views without fear of reprisals. This

newfound freedom brings with it many unsolved social, political, and moral issues.

NETWORK HARDWARE There is no generally accepted taxonomy into which all computer networks fit,but two

dimensions Stand out as important.

Transmission Technology

Scale

Transmission Technology: Broadly speaking, there are two types of transmission

technology:

Broadcast networks.

Point-to-point Networks

Broadcast networks have a single communication channel that is shared by all the machines on

the network.

Short messages, called packets in certain contexts sent by any machine are received by

all the others.

An address field within the packet specifies for whom it is intended.

Upon receiving a packet, a machine checks the address field. If the packet is intended for

itself, it processes the packet; if the packet is intended for some other machine, it is just

ignored.

Although the packet may actually be received by many systems, only the intended one

responds. The others just ignore it.

Broadcast systems also allow the possibility of addressing a packet to all destinations by

using a special code in the address field.

When a packet with this code is transmitted, it is received and processed by every

machine on the network. This mode of operation is called broadcasting.

Some broadcast systems also support transmission to a subset of the machines, something

known as multicasting.

One possible scheme is to reserve one bit to indicate multicasting.

The remaining n - 1 address bits can hold a group number. Each machine can "subscribe"

to any or all of the groups. When a packet is sent to a certain group, it is delivered to all

machines subscribing to that group.

Point-to-point Networks

Point-to-Point networks consist of many connections between individual pairs of

machines.

To go from the source to the destination, a packet on this type of network may have to

first visit one or more intermediate machines.

Often multiple routes, of different lengths are possible, so routing algorithms play an

important role in point-to-point networks.

As a general rule smaller, geographically localized networks tend to use broadcasting,

whereas larger networks usually are point-to-point.

Point-to-Point transmission with one sender and one receiver is called Unicasting.

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Computer networks can be classified according to their size:

Personal area network (PAN)

Local area network (LAN)

Metropolitan area network (MAN)

Wide area network (WAN)

Interprocessor Distances Processors located in same Examples

Classification of interconnected processors by scale.

An alternative criterion for classifying networks is their scale. Multiple processor systems

can be arranged by their physical size. At the top are data flow machines, highly parallel

computers with many functional units all working on the same program.

Next come the multicomputers, systems that communicate by sending messages over

very short, very fast buses. Beyond the multicomputers are the true networks, computers that

communicate by exchanging messages over longer cables. These can be divided into local,

metropolitan, and wide area networks, finally, the connection of two or networks are called an

internetwork. The worldwide Internet is a well-known example of an internetwork.

Personal area network

A personal area network (PAN) is a computer network used for data transmission among

devices such as computers, telephones and personal digital assistants.

PANs can be used for communication among the personal devices themselves

(intrapersonal communication), or A PAN is a network that is used for communicating among

computers and computer devices (including telephones) in close proximity of around a few

meters within a room connecting to a higher level network and the Internet (an uplink).

It can be used for communicating between the devices themselves, or for connecting to a

larger network such as the internet PAN’s can be wired or wireless.

PAN’s can be wired with a computer bus such as a universal serial bus: USB (a

serial bus standard for connecting devices to a computer-many devices can be

connected concurrently)

PAN’s can also be wireless through the use of bluetooth (a radio standard

designed for low power consumption for interconnecting computers and devices

such as telephones, printers or keyboards to the computer) or IrDA (infrared data

association) technologies

A wireless personal area network (WPAN) is a PAN carried over wireless network

technologies such as:

INSTEON

IrDA

Wireless USB

Bluetooth

1 m Square meter

10 m Room

100 m Building

1 km Campus

10 km City

100 km Country

1000 km Continent

10000 km Planet

Wide Area Network

Metropolitan Area Network

Personal Area Network

Local area network

The Internet

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Z-Wave

ZigBee

Body Area Network

The reach of a WPAN varies from a few centimeters to a few meters. A PAN may also be

carried over wired computer buses such as USB and FireWire.

Fig1.4 PAN Connected Devices

Local Area Networks

Generally called LANs, are privately-owned network within a single building or campus

of up to a few kilometers in size. They are widely used to connect personal computers and

workstations in company offices and factories to share resources (e.g., printers) and exchange

information.

LANs are distinguished from other kinds of networks by three characteristics:

(1) size.

(2) transmission technology, and

(3) topology.

Size

LANs are restricted in size, which means that the worst-case transmission time is

bounded and known in advance

It simplifies network management.

Transmission Technology

LANs often use a transmission technology consisting of a cable to which all the

machines are attached.

Traditional LANs run at speeds of 10 to 100 Mbps, have low delay (tens of

microseconds), and make very few errors.

Newer LANs may operate at higher speeds, up to hundreds of megabits/sec.

Topology

Various topologies are possible for broadcast LANs.

The network topology defines the way in which computers, printers, and other devices are

connected. A network topology describes the layout of the wire and devices as well as the paths

used by data transmissions.

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Fig 1.5 Toplogies

Bus Topology

Commonly referred to as a linear bus, all the devices on a bus topology are connected by

one single cable.

A linear bus topology consists of a main run of cable with a terminator at each end. All

servers workstations and peripherals are connected to the linear cable

Application of Bus Topology

Transmission Logic

Listen to the bus for traffic

If no traffic is detected, then transmit

Otherwise, if the bus is busy with traffic, wait for a random period of time before

attempting to transmit again

Repeated attempts will be made until the bus is found free

Fig 1.6 Bus Topology used at LAN Network

Collision of Data

Two workstations may find the bus free at the same time

Both would transmit at the same time Collision of data occurs

Both workstations will now wait for a random period of time before attempting to

transmit again

Advantages

Cabling is simple and easy to install in a local setup

Based on well established standards

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o IEEE 802.3

o Also known as the Ethernet standard

Disadvantages

Sharing of a single data bus

o When the traffic increases the performance deteriorates

Waiting period may reach unacceptable lengths of time under heavy data traffic

Cable fault results in the entire LAN becoming inoperative

Solution

Collision domains are present only when a hub used

Using switches will eliminate the collision domains

Fig 1.7 Bus Topology using Switch

Ring Network

A ring topology consists of a set of stations connected serially by cable. In other words,

it’s a circle or ring of computers. There are no terminated ends to the cable; the signal travels

around the circle in a clockwise direction.

A second type of broadcast system is the ring.

In a ring, each bit propagates around on its own, not waiting for the rest of the packet to

which it belongs.Typically, each bit circumnavigates the entire ring in the time it takes to

transmit a few bits, often before the complete packet has even been transmitted.

Like all other broadcast systems, some rule is needed for arbitrating simultaneous

accesses to the ring. The IBM token ring, is a popular ring-based LAN operating at 4 and

16 Mbps.Broadcast networks can be further divided into static and dynamic, depending

on how the channel is allocated.

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Fig 1.8 Ring Topology

A frame travels around the ring, stopping at each node. If a node wants to transmit data, it

adds the data as well as the destination address to the frame. The frame then continues around

the ring until it finds the destination node, which takes the data out of the frame.

Single ring – All the devices on the network share a single cable.

Dual ring – The dual ring topology allows data to be sent in both directions.

A typical static allocation would be to divide up time into discrete intervals and run a

round robin algorithm, allowing each machine to broadcast only when its time slot comes

up.

Static allocation wastes channel capacity when a machine has nothing to say during

its allocated slot, so most systems attempt to allocate the channel dynamically (i.e., on

demand).

Dynamic allocation methods for a common channel are either centralized or

decentralized.In the centralized channel allocation method, there is a single entity, for

example a bus arbitration unit, which determines who goes next.

It might do this by accepting requests and making a decision according to some internal

algorithm.In the decentralized channel allocation method, there is no central entity; each

machine must decide for itself whether or not to transmit.

A counterrotating ring is a ring topology that consists of two rings transmitting in opposite

directions. The intent is to provide fault tolerance in the form of redundancy in the event of a

cable failure. If one ring goes, the data can flow across to the other path, thereby preserving the

ring.

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Fig 1.9 Ring Topology

Advantages of ring topology:

Growth of system has minimal impact on performance

All stations have equal access

Disadvantages of ring topology:

Most expensive topology

Failure of one computer may impact others

Complex

Star & Tree Topology

In a star topology, all computers are connected through one central hub or switch, as

illustrated in Figure below. This is a very common network scenario.

Fig 1.10 Star Topology

Computer in a star topology are all connected to a central hub.A star topology actually

comes from the days of the mainframe system. The mainframe system had a centralized point

where the terminals connected.

Advantages

One advantage of a start topology is the centralization of cabling. With a hub, if one link

fails, the remaining workstations are not affected like they are with other topologies, which we

will look at in this chapter.

Centralizing network components can make an administrator’s life much easier in the

long run. Centralized management and monitoring of network traffic can be vital to network

success. With this type of configuration, it is also easy to add or change configurations with all

the connections coming to a central point.

Easy to add new stations

Easy to monitor and troubleshoot

Can accommodate different wiring

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Disadvantages

On the flip side to this is the fact that if the hub fails, the entire network, or a good

portion of the network, comes down. This is, of course, an easier fix than trying to find a break in

a cable in a bus topology.

Another disadvantage of a star topology is cost: to connect each workstation to a

centralized hub, you have to use much more cable than you do in a bus topology.

Failure of hub cripples attached stations

More cable required

Fig 1.11 Extended Star Topology

Larger networks use the extended star topology also called tree topology. When used

with network devices that filter frames or packets, like bridges, switches, and routers, this

topology significantly reduces the traffic on the wires by sending packets only to the wires of the

destination host

Mesh Topology

The mesh topology connects all devices (nodes) to each other for redundancy and fault

tolerance. It is used in WANs to interconnect LANs and for mission critical networks like those

used by banks and financial institutions. Implementing the mesh topology is expensive and

difficult.

Fig 1.12 Mesh Topology

A mesh topology is not very common in computer networking, but you will have to know

it for the exam. The mesh topology is more commonly seen with something like the national

phone network. With the mesh topology, every workstation has a connection to every other

component of the network, as illustrated in Figure 1-9

Advantages

The biggest advantage of a mesh topology is fault tolerance. If there is a break in a cable

segment, traffic can be rerouted. This fault tolerance means that the network going down due to a

cable fault is almost impossible. (I stress almost because with a network, no matter how many

connections you have, it can crash.)

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Disadvantages

A mesh topology is very hard to administer and manage because of the numerous

connections. Another disadvantage is cost. With a large network, the amount of cable needed to

connect and the interfaces on the workstations would be very expensive.

Costs considerations for choosing a topology

The following factors should be considered when choosing a topology:

Installation

Maintenance and troubleshooting

Expected growth

Distances

Infrastructure

Existing network

As a general rule, a bus topology is the cheapest to install, but may be more expensive to

maintain because it does not provide for redundancy.

Metropolitan Area Networks

Metropolitan Area Networks or Man covers a city.

The best-known example of MAN it is the cable television network available in many

cities. In early systems a large antenna was placed on top of a near by hill and signal was

piped to the subscribers houses. At first, these were locally-designed, ad hoc systems. The

next step was television and even entire channels designed for cable only, starting when the

internet attracted a mass audience a cable TV network operator begun to realize that with

some changes to the system, they could provide two-way internet service in un used parts of

the spectrum, we see both television signals and internet being fed into the centralized head

end for subsequent distribution to homes.

A metropolitan area network (MAN) is computer network larger than a local area network,

covering an area of a few city blocks to the area of an entire city, possibly also including the

surrounding areas.

A MAN is optimized for a larger geographical area than a LAN, ranging from several

blocks of buildings to entire cities. MANs can also depend on communications channels of

moderate-to-high data rates. A MAN might be owned and operated by a single organization, but

it usually will be used by many individuals and organizations. MANs might also be owned and

operated as public utilities. They will often provide means for inter networking of local

networks.

A metropolitan area network, or MAN, covers a city. The best-known example of a MAN

is the cable television network available in many cities. This system grew from earlier

community antenna systems used in areas with poor over-the-air television reception. In these

early systems, a large antenna was placed on top of a nearby hill and signal was then piped to the

subscribers' houses.

At first, these were locally-designed, ad hoc systems. Then companies began jumping

into the business, getting contracts from city governments to wire up an entire city. The next step

was television programming and even entire channels designed for cable only. Often these

channels were highly specialized, such as all news, all sports, all cooking, all gardening, and so

on. But from their inception until the late 1990s, they were intended for television reception only.

To a first approximation, a MAN might look something like the system shown in Fig. In

this figure both television signals and Internet are fed into the centralized head end for

subsequent distribution to people's homes. Cable television is not the only MAN. Recent

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developments in high-speed wireless Internet access resulted in another MAN, which has been

standardized as IEEE 802.16.

Fig 1.13(a)MAN Network

Metropolitan Area Network Basics

• MANs borrow technologies from LANs and WANs.

• MANs support high-speed disaster recovery systems, real-time transaction backup

systems, interconnections between corporate data centers and Internet service providers,

and government, business, medicine, and education high-speed interconnections.

• Almost exclusively fiber optic systems

• MANs have very high transfer speeds

• MANs can recover from network faults very quickly (failover time)

• MANs are very often a ring topology (not a star-wired ring)

• Some MANs can be provisioned dynamically

Fig 1.13 (b) MAN Network

Implementation

Also known as a Municipal Area Network, networking technologies used in municipal

networks include Asynchronous Transfer Mode (ATM), FDDI, and SMDS. However, these

technologies are increasingly being displaced by Ethernet-based connections (e.g., Metro

Ethernet). MAN links between local area networks have been built with wireless links using

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either microwave, radio, or infra-red laser transmission. Most companies rent or lease circuits

from common carriers because laying long stretches of cable is expensive.

Distributed-queue dual-bus (DQDB) refers to the metropolitan area network standard for

data communication specified in the IEEE 802.6 standard. With DQDB, networks can extend up

to 20 miles (30 km) long and operate at speeds of 34–155 Mbit/s.

MAN ( Metropolitan Area Network ) is a computer network that is large and spacious

which is usually used in areas such as schools , colleges , malls and even the city as well .

Actually there are two types of connections are typically used the first is by using a wireless

connection and the second using fiber optic cable . For instance , a school or college has a MAN

connection consisting of multiple LANs and are at a radius of a few pounds around the place .

Then connect the campus with the MAN also has relationships with other universities to

form a WAN or the Internet . There are also other examples with some technology that uses

MAN connections such as ATM , FDDI and SMDS . Here below I give also an explanation of

the ATM , FDDI and SMDS.

1 . ATM , or Asynchronous Transfer Mode instead Automated Teller yes . ATM is a cell relay

network protocol that clicking - encoded traffic or traffic data to a smaller cell forms such as 53

bytes , 48 bytes and 5 bytes .

2 . FDDI or Fiber Distributed Data Interface is a standard data transmission in a LAN that

includes a sizable range is up to 200 kilometers away . FDDI can also include thousands of users

. Standard medium is used to connect fiber optic , even though it could also use copper wires ,

but with the proviso shall be in accordance with the FDDI technology if not then the

transmission will be interrupted .

3 . Or SMDS Switched Multi -megabit Data Services is a service connection to the LAN , MAN

and WAN to the data exchange standard based on the IEEE 802.6 DQDB . For the connection

between MAN and LAN can be done by using radio signals , microwaves and infrared .

Metropolitan Area Network (MAN). A MAN can connect multiple LANs. Some large

network of large universities may be classified as MAN. MAN is owned by single organization

but can be used by many individuals and organizatios.

The MAN network usually provides connectivity to local Internet Service Providers

(ISPs), cable TV or large organizations. IT is larger than LAN and smaller than WAN. A MAN

can extend up to a city or a larger geographical area. A MAN can be heterogeneous system

interconnecting different communication media and different types of protocols.

The connecting media of MAN can be copper cables or high speed optical cables. Along

with wired media a MAN can also have wireless media. A MAN often provides efficient

connection to a wide area network (WAN) or the Internet. A MAN is less secure than LAN

because of its accessibility to large number of users. There is a high chance of data leakage if

proper security measures are not taken i MAN.

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Fig 1.14 MAN Network

When the computer network increases beyond local area network then it is called

A metropolitan area network (MAN) is a computer network that usually spans a city or

a large campus. A MAN usually interconnects a number of local area networks (LANs) using a

high-capacity backbone technology, such as fiber-optical links, and provides up-link services to

wide area networks (or WAN) and the Internet.

A MAN is optimized for a larger geographical area than a LAN, ranging from several

blocks of buildings to entire cities. MANs can also depend on communications channels of

moderate-to-high data rates. A MAN might be owned and operated by a single organization, but

it usually will be used by many individuals and organizations. MANs might also be owned and

operated as public utilities. They will often provide means for internetworking of local networks

A Metropolitan Area Network (MAN) is a large computer network that spans a

metropolitan area or campus. Its geographic scope falls between a WAN and LAN.

MANs provide Internet connectivity for LANs in a metropolitan region, and connect

them to wider area networks like the Internet. ” It can also be used in cable television

Fig 1.15 MAN Network

Wide Area Networks

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Wide Area Networks or WAN spans a large geographical area often a country or continent.

It contains collections of machines for running user programs called Hosts.

The hosts are connected by a communication subnet.

The hosts are owned by the customers whereas the communication subnet owned and

operated by Telephone Company or ISP.

The job of the subnet is to carry messages from host to host. The subnet consists of two

distinct components: transmission lines and switching elements.Transmission lines

move bits between machines that are made of copper wire, optical fiber, or even radio

links.

Switching elements are specialized computers that connect 3 or more transmission lines.

When data arrive on an incoming line the switching element must choose an outgoing line to

forward them.

The switching elements are also called router. The collection communication lines and

routers(but not the hosts) form the subnet.

A short subnet is a collection of communication lines that moved packets from the source

host to the destination host.In most WAN the network contains numerous transmission

lines, each one connecting a pair of routers.

If two routers that do not share a transmission line wish to communicate, they must do

this indirectly, via other routers.

When a packet is sent from one router to another via one or more intermediate routers the

packet is received at each intermediate routers and stored there until the required line free

and then forwarded.

A subnet organized according to this principle is called store and forward or packet

switched subnet. When the packets are small and all the same size they are often called

as cells.

The principle of packet switched WAN, when a process on some host as a message to be

sent to a process on some other host the sending host first cuts the message into packets,

each one bearing its number in the sequence.

Fig 1.17 Transferring of information

A

B

A

D

C E

Sending Process

Sending Host

Receiving Process Packet

Receiving Host

Subnet Routers

Fig 1.16 Relation between hosts and the subnet.

19

The packets are then transported individual over the network and deposited at the

receiving hosts where they are reassembled into the original message and delivered to

the receiving process. A second possibility for a WAN is a satellite or ground radio

system.

Each router has an antenna through which it can send and receive.

Sometimes the routers are connected to a substantial point-to-point subnet, with only

some of them having a satellite antenna.

Satellite networks are inherently broadcast and are most useful when the broadcast

property is important.

Wireless networks: Wireless network can be divided into three main categories:

System interconnection

Wireless LANs

Wireless WANs

System interconnection:

System interconnection is all about interconnecting the components of a computer using

short range radio. Every computer has monitor, keyboard, mouse and printer connected to main

unit by cables. New users have a hard time plugging all the cables into right little holes.

Some companies got together to design short range wireless network called Bluetooth to

connect these components without wires. Bluetooth also allows digital cameras, headsets,

scanners and other devices to connect the computer about within range. No cables, no driver

installation just put down them on and they work.

Wireless LANs:

These are systems in which every computer has a radio modem and antenna which it can

communicate with other systems. If systems are close enough they can communicate directly

with one another with peer-to-peer configuration.Wireless LANs are becoming increasingly

common in small offices and homes. There is a standard for wireless LANs called IEEE 802.11.

Wireless WANs:

The radio network used for cellular telephones is an example of low bandwidth wireless

systems. System has already gone through 3 generation. The first generation was analog and for

voice only. The second generation was digital and for voice only. The third generation is digital

and it is for both voice and data. Wireless LANs can operate rate up to 15 Mbps over distance of

ten of meters. Cellular system operate below 10 Mbps but the distance between way station and

the computer or telephone is measured in kilometers rather than meters.

A standard for a called IEEE 802.16. For example an airplane with number peoples using

modem and seat-back telephones to call the office.

Each call is independent of other ones. Next case a flying LAN each seats comes

equipped with an Ethernet connection into which passengers can plug their computers. A single

router on the aircraft maintain radio link with some router on the ground, changing router as its

flies along.

Several options are available for WAN connectivity:

Leased line: Point-to-Point connection between two computers or Local Area Networks (LANs)

Circuit switching: A dedicated circuit path is created between end points. Best example is

dialup connections

Packet switching (Connection oriented): Devices transport packets via a shared single point-

to-point or point-to-multipoint link across a carrier internetwork. Before information can be

exchanged between two endpoints, they first establish a Virtual Circuit. Variable length packets

are transmitted over Permanent Virtual Circuits (PVC) or Switched Virtual Circuits (SVC)

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Packet switching (Connectionless): Devices transport packets via a shared single point-to-point

or point-to-multipoint link across a carrier internetwork. Variable length packets are transmitted.

Between endpoints no connection is build; endpoints can just offer packets to the network,

addressed to any other endpoint and the network will try to deliver the packet. As an example:

the Internet works this way.

Cell relay:Similar to packet switching, but uses fixed length cells instead of variable length

packets. Data is divided into fixed-length cells and then transported across virtual circuits

Home network:

Home network is on the horizon. The fundamental idea is that in the future most homes

will be setup for networking. Every device in home will be capable of communicating with every

other device, and all of them will be accessible over the internet. Many devices are capable of

being a network some of more obvious categories are as follows:

Computers ( desktop PC, notebook PC, PDA, shared peripherals)

Entertainment ( TV, DVD, VCR, Camcorder, camera, stereo, MP3)

Telecommunication ( telephone, mobile telephone, intercom, fax)

Appliances ( microwave, refrigerator, clock, furnace, airco, lights)

Telemetry ( utility meter, smoke/burglar alarm, thermostat, babycam)

A home network or home area network (HAN) is a type of local area network that

develops from the need to facilitate communication and interoperability among digital devices

present inside or within the close vicinity of a home.

Devices capable of participating in this network–smart devices such as network printers

and handheld mobile computers–often gain enhanced emergent capabilities through their ability

to interact. These additional capabilities can then be used to increase the quality of life inside the

home in a variety of ways, such as automation of repetitious tasks, increased personal

productivity, enhanced home security, and easier access to entertainment.

Infrastructure

A home network usually relies on one of the following equipment to establish physical

layer, data link layer, and network layer connectivity both internally amongst devices and

externally with outside networks:

A modem is usually provided by an ISP to expose an Ethernet interface to the WAN via

their telecommunications infrastructure. In homes these usually come in the form of a

DSL modem or cable modem.

A router manages network layer connectivity between a WAN and the HAN. Most home

networks feature a particular class of small, passively-cooled, table-top device with an

integrated wireless access point and 4 port Ethernet switch. These devices aim to make

the installation, configuration, and management of a home network as automated, user

friendly, and "plug-and-play" as possible.

A network switch is used to allow devices on the home network to talk to one another via

Ethernet. While the needs of most home networks are satisfied with Wi-Fi or the built-in

switching capacity of their router, certain situations require the introduction of a distinct

switch. For example:

o When the router's switching capacity is exceeded. Most home routers expose only

4 to 6 Ethernet ports.

o When Power over Ethernet is required by devices such as IP cameras and IP

phones

o When distant rooms have a large amount of wired devices in close proximity

A wireless access point is required for connecting wireless devices to a network. Most

home networks rely on one "Wireless Router" combination device to fill this role.

A network bridge connecting two network interfaces to each other, often in order to grant

a wired-only device, e.g. Xbox, access to a wireless network medium.

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A home network is a group of devices – such as computers, game systems, printers, and mobile

devices – that connect to the Internet and each other. Home networks connect in two ways:

1.A wired network, which connects devices like printers and scanners with cables.

2.A wireless network, which connects devices like tablets and e-readers without cables

Set Up a Home Network

There are many reasons to establish a home network. Here are just a few of the things

home networking allows you to do:

Connect to the Internet from multiple computers, game systems, mobile devices, and

more.

Access files and folders on all devices connected to the network.

Print from multiple computers on a single printer.

Manage security settings for all networked devices in one place.

Need to Set Up a Home Network

To set up home networking, need the following:

XFINITY Internet Service subscription (or subscription to another Internet provider)

A modem, which connects to the Internet, and a router, which connects your devices to

each other and to the Internet through your modem (or a gateway, which functions as

both a modem and a router)

A computer or other device to connect to the network

The Wireless Gateway 1 (model numbers TG852G, TG862G, SMCD3GNV, TC8305C)

and Wireless Gateway 2 (model number DPC3939) function as an all-in-one modem, router, and

phone device. They automatically provide users with the best security settings available for a

home network. Find out more about wireless gateways.

Wireless Home Network

A wireless network, often called Wi-Fi, connects devices to each other and to the Internet

without using cables. Read our rundown of wireless networking and its benefits.

Wired Home Network

A wired home network connects devices to each other and to the Internet using Ethernet

cables.

Fig1.18 Wired Home Network

There are several benefits to having a wired home network:

Faster and more reliable connection to the Internet

Increased security, as no outside users can access your network

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Easier set-up and troubleshooting than wireless connections

Mixed Home Network

Many people find that a mix of wireless and wired networking meets their needs best. For

instance, devices that stream movies benefit from the quicker and more stable wired connection.

Devices like laptops or tablets, however, benefit from the mobility available with a wireless

connection.

Both the Wireless Gateway 1 and Wireless Gateway 2 come with wireless capability and

four Ethernet ports, allowing you to connect devices with and without cables at the same time.

Internetworks:

A collection of interconnected networks called internetwork or internet. A common form

of internet is a collection of LANs connected by WANs. Subnet makes the most sense in the

context of wide area network, Where it refers to collection of routers to the communication lines

owned by network operator. Telephone system consist of telephone switching offices connected

to one another by high speed lines and houses and businesses by low speed lines.

Lines and equipment owned by telephone companies form the subnet of telephone

system. The combination of a subnet and its host forms a network. An internetwork is formed

when distinct network are interconnected.

A Brief History

A network is a group of connected communicating devices such as computers and

printers. An internet (note the lowercase letter i) is two or more networks that can communicate

with each other. The most notable internet is called the Internet (uppercase letter I), a

collaboration of more than hundreds of thousands of interconnected networks. Private

individuals as well as various organizations such as government agencies, schools, research

facilities, corporations, and libraries in more than 100 countries use the Internet. Millions of

people are users.

Yet this extraordinary communication system only came into being in 1969. In the mid-

1960s, mainframe computers in research organizations were standalone devices. Computers from

different manufacturers were unable to communicate with one another. The Advanced Research

Projects Agency (ARPA) in the Department of Defense (DoD) was interested in finding a way to

connect computers so that the researchers they funded could share their findings, thereby

reducing costs and eliminating duplication of effort.

In 1967, at an Association for Computing Machinery (ACM) meeting, ARPA presented

its ideas for ARPANET, a small network of connected computers. The idea was that each host

computer (not necessarily from the same manufacturer) would be attached to a specialized

computer, called an inteiface message processor (IMP). The IMPs, in tum, would be connected

to one another. Each IMP had to be able to communicate with other IMPs as well as with its own

attached host. By 1969, ARPANET was a reality. Four nodes, at the University of California at

Los Angeles (UCLA), the University of California at Santa Barbara (UCSB), Stanford Research

Institute (SRI), and the University of Utah, were connected via the IMPs to form a network.

Software called the Network Control Protocol (NCP) provided communication between the

hosts.

In 1972, Vint Cerf and Bob Kahn, both of whom were part of the core ARPANET group,

collaborated on what they called the Internetting Project1.

Cerf and Kahn's landmark 1973 paper outlined the protocols to achieve end-to-end

delivery of packets. This paper on Transmission Control Protocol (TCP) included concepts such

as encapsulation, the datagram, and the functions of a gateway. Shortly thereafter, authorities

made a decision to split TCP into two protocols: Transmission Control Protocol (TCP) and

Internetworking Protocol (lP). IP would handle datagram routing while TCP would be

responsible for higher-level functions such as segmentation, reassembly, and error detection. The

internetworking protocol became known as TCPIIP.

The Internet Today

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The Internet has come a long way since the 1960s. The Internet today is not a simple

hierarchical structure. It is made up of many wide- and local-area networks joined by connecting

devices and switching stations. It is difficult to give an accurate representation of the Internet

because it is continually changing-new networks are being added, existing networks are adding

addresses, and networks of defunct companies are being removed. Today most end users who

want Internet connection use the services of Internet service providers (lSPs). There are

international service providers, national service providers, regional service providers, and local

service providers. The Internet today is run by private companies, not the government.

International Internet Service Providers:

At the top of the hierarchy are the international service providers that connect nations

together.

National Internet Service Providers:

The national Internet service providers are backbone networks created and maintained by

specialized companies. There are many national ISPs operating in North America; some of the

most well known are SprintLink, PSINet, UUNet Technology, AGIS, and internet Mel. To

provide connectivity between the end users, these backbone networks are connected by complex

switching stations (normally run by a third party) called network access points (NAPs). Some

national ISP networks are also connected to one another by private switching stations called

peering points. These normally operate at a high data rate (up to 600 Mbps).

Regional Internet Service Providers:

Regional internet service providers or regional ISPs are smaller ISPs that are connected

to one or more national ISPs. They are at the third level of the hierarchy with a smaller data rate.

Local Internet Service Providers:

Local Internet service providers provide direct service to the end users. The local ISPs

can be connected to regional ISPs or directly to national ISPs. Most end users are connected to

the local ISPs. Note that in this sense, a local ISP can be a company that just provides Internet

services, a corporation with a network that supplies services to its own employees, or a nonprofit

organization, such as a college or a university, that runs its own network. Each of these local

ISPs can be connected to a regional or national service provider.

Protocols:

In computer networks, communication occurs between entities in different systems. An

entity is anything capable of sending or receiving information. However, two entities cannot

simply send bit streams to each other and expect to be understood. For communication to occur,

the entities must agree on a protocol. A protocol is a set of rules that govern data

communications. A protocol defines what is communicated, how it is communicated, and when

it is communicated.

The key elements of a protocol are syntax, semantics, and timing.

Syntax. The term syntax refers to the structure or format of the data, meaning the order in

which they are presented. For example, a simple protocol might expect the first 8 bits of

data to be the address of the sender, the second 8 bits to be the address of the receiver,

and the rest of the stream to be the message itself.

Semantics. The word semantics refers to the meaning of each section of bits. How is a

particular pattern to be interpreted, and what action is to be taken based on that

interpretation? For example, does an address identify the route to be taken or the final

destination of the message?

Timing. The term timing refers to two characteristics: when data should be sent and how

fast they can be sent. For example, if a sender produces data at 100 Mbps but the receiver

can process data at only 1 Mbps, the transmission will overload the receiver and some

data will be lost.

Standards Standards are essential in creating and maintaining an open and competitive market for

equipment manufacturers and in guaranteeing national and international interoperability of data

and telecommunications technology and processes. Standards provide guidelines to

24

manufacturers, vendors, government agencies, and other service providers to ensure the kind of

interconnectivity necessary in today's marketplace and in international communications.

Data communication standards fall into two categories: de facto (meaning "by fact" or

"by convention") and de jure (meaning "by law" or "by regulation").

De facto. Standards that have not been approved by an organized body but have been

adopted as standards through widespread use are de facto standards. De facto standards

are often established originally by manufacturers who seek to define the functionality of a

new product or technology.

De jure. Those standards that have been legislated by an officially recognized body are de

jure standards.

Layered Tasks:

We use the concept of layers in our daily life. As an example, let us consider two friends

who communicate through postal maiL The process of sending a letter to a friend would be

complex if there were no services available from the post office. Below Figure shows the steps in

this task.

Sender, Receiver, and Carrier

In Figure we have a sender, a receiver, and a carrier that transports the letter. There is a

hierarchy of tasks.

At the Sender Site

Let us first describe, in order, the activities that take place at the sender site.

Higher layer. The sender writes the letter, inserts the letter in an envelope, writes the

sender and receiver addresses, and drops the letter in a mailbox.

Middle layer. The letter is picked up by a letter carrier and delivered to the post office.

Lower layer. The letter is sorted at the post office; a carrier transports the letter.

0n the Way: The letter is then on its way to the recipient. On the way to the recipient's local post

office, the letter may actually go through a central office. In addition, it may be transported by

truck, train, airplane, boat, or a combination of these.

Fig 1.19 (a) Layere working Concept

25

At the Receiver Site

Lower layer. The carrier transports the letter to the post office.

Middle layer. The letter is sorted and delivered to the recipient's mailbox.

Higher layer. The receiver picks up the letter, opens the envelope, and reads it.

NETWORK SOFTWARE

The first computer networks were designed with the hardware as the main concern and the

software as an afterthought. Network software is now highly structured. Protocol Hierarchies To

reduce their design complexity, most networks are organized as a series of layers or levels, each

one built upon the one below it. The number of layers, the name of each layer, the contents of

each layer, and the function of each 1ayer differ from network to network.

However, in all networks, the purpose of each layer is to offer certain services to the higher

layers, shielding those layers from the details of how the offered services are actually

implemented. Layer n on one machine carries on a conversation with layer n on another

machine.

The rules and conventions used in this conversation are collectively known as the layer n

protocol.

Basically, a protocol is an agreement between the communicating parties on how

communication is to proceed. Violating the protocol will make communication more difficult if

not impossible. The entities comprising the corresponding layers on different machines are called

peers. The peers communicate using protocol.

In reality, no data are directly transferred from layer n on one machine to layer n on

another machine. Instead, each layer passes data and control information to the layer

immediately below it, until the lowest layer is reached. Below layer 1 is the physical medium

through which actual communication occurs Between each pair of adjacent layers there is an

interface. The interface defines which primitive operations and services the lower layer offers to

the upper one. One of the most important considerations is defining clean interfaces between the

layers.

Doing so, in turn, requires that each layer perform a specific collection of well-understood

functions. In addition to minimizing the amount of information that must be passed between

layers, clean-cut interfaces also make it simpler to replace the implementation of one layer with a

completely different implementation because all that is required of the new implementation is

that it offers exactly the same set of services to its upstairs neighbor as the old implementation

did.

Doing so, in turn, requires that each layer perform a specific collection of well-understood

functions. In addition to minimizing the amount of information that must be passed between

layers, clean-cut interfaces also make it simpler to replace the implementation of one layer with a

completely different implementation because all that is required of the new implementation is

that it offers exactly the same set of services to its upstairs neighbor as the old implementation

did.

A set of layers and protocols is called network architecture. The specification of

architecture contains enough information to build the hardware/software for each layer so that it

correctly obeys the appropriate protocol.

A list of protocols used by a certain system, one protocol per layer, is called a protocol

stack. A message M, produced by the application process puts a header in front of the message to

identify the message and passes the result to the next layer. The header includes control

information, such as a sequence numbers to allow the next layer in the destination machine to

deliver messages in the right order. Headers may also contain sizes, times and other control

fields. The layers break up the incoming messages into smaller units, packets.

For example, message M is split into two parts, m1 and m2. A Layer decides which of the

outgoing lines to use and passes the packets to next layer. This Layer adds not only a header to

each piece, but also a trailer, and give the resulting unit to layer below it for physical

26

transmission. At the receiving machine the message moves upward, from layer to layer, with

headers being stripped off as it progresses.

Fig 1.19 Layers Hierarchies

None of the headers for layers below n are passed up to layer n. The peer process

abstraction is crucial to all network design. Using it, the unmanageable task of designing the

complete network can be broken into several smaller, manageable, design problems, namely the

design of the individual layers.

Design Issues for the Layers

Some of the key design issues that occur in computer networking are present in several

Layers. Every layer needs a mechanism for identifying senders and receivers. Since a network

normally has many computers, some of which have multiple processes, a means is needed for a

process on one machine to specify with whom it want to talk.

Layers, Protocols and Interfaces

Host 2

Layer 1/2 Interface

Layer 2/3 Interface

Layer 3/4 Interface

Layer 4/5 Interface

Layer 4 Protocol

Layer 3 Protocol

Layer 2 Protocol

Layer 1 Protocol

Layer 5 Protocol Host 1

Layer 5

Layer 2

Layer 1 Layer 1

Layer 2

Layer 3

Layer 4

Layer 5

Layer 3

Layer 4

Physical Medium

Layer 5 Protocol M M

H4 M H4 M

H3 H4 M1 H3 H4 M1 H3 M2 H3 M2

H2 H3 M2 T2 H2 H3 H4 M1 T2 H2 H3 M2 T2

Source Machine Destination Machine

Layer 4 Protocol

Layer 3 Protocol

Layer 2

Protocol H2 H3 H4 M1 T2

27

As a consequence of having multiple destinations, some form of addressing is needed in

order to specify a specific destination.

Another set of design decisions concerns the rules for data transfer. In some systems,

data only travel in one direction (simplex communication). In others they can travel in either

direction, but not simultaneously (half-duplex communication). In still others they travel in

both directions at once (full-duplex communication). The protocol must also determine how

many logical channels the connection corresponds to, and what their priorities are. Many

networks provide at least two logical channels per connection, one for normal data and one for

urgent data.

Error control is an important issue because physical communication circuits are not

perfect. Many error-detecting and error-correcting codes are known, but both ends of the

connection must agree on which one is being used. In addition the receiver must have some way

of telling the sender which messages have been correctly received and which has not. Not all

communication channels preserve the order of messages sent on them to deal with a possible loss

of sequencing; the protocol must make explicit provision for the receiver to allow the pieces to

be put back together properly.

An issue that occurs at every level is how to keep a fast sender from swamping a slow

receiver with data. Some of them involve some kind of feedback from the receiver to the

sender, either directly or indirectly, about the receiver's current situation. This subject is called

flow control.

Another problem that must be solved at several levels is the inability of all processes to

accept arbitrarily long messages. This property leads to mechanisms for disassembling,

transmitting, and then reassembling messages. A related issue is what to do when processes insist

upon transmitting data in units that are so small that sending each one separately is inefficient.

Here the solution is to gather together several small messages heading toward a common

destination into a single large message and dismember the large message at the other side. When

it is inconvenient or expensive to set up a separate connection for each pair of communicating

processes, the underlying layer may decide to use the same connection for multiple, unrelated

conversations.

As long as this multiplexing and de-multiplexing is done transparently, it can be used

by any layer. Multiplexing is needed in the physical layer, for example, where all the traffic for

all connections has to be sent over at most a few physical circuits. When there are multiple paths

between source and destination, a. route must be chosen. Sometimes this decision must be split

over two or more layers.

Connection-Oriented and Connectionless Services

Connection-Oriented and Connectionless Services Layers can offer two different types

of service to the layers above them:

1.Connection-oriented and

2.Connectionless.

Connection-oriented service is modeled after the telephone system. To talk to someone,

you pick up the phone, dial the number, talk, and then hang up. Similarly, to use a connection-

oriented network service, the service user first establishes a connection, uses the connection, and

then releases the connection. The essential aspect of a connection is that it acts like a tube: the

sender pushes objects (bits) in at one end, and the receiver takes them out in the same order at the

other end.

Connectionless service is modeled after the postal system. Each message carries the full

destination address, and each one is routed through the system independent of all the others.

Normally, when two messages are sent to the same destination, the first one sent will be the first

one to arrive. However, it is possible that the first one sent can be delayed so that the second one

arrives first. Each service can be characterized by a quality of service.

28

Some services are reliable in the sense that they never lose data. Usually, a reliable

service is implemented by having the receiver acknowledge the receipt of each message, so the

sender is sure that it arrived. The acknowledgement process introduces overhead and delays,

which are often worth it but are sometimes undesirable.

A typical situation in which a reliable connection-oriented service is appropriate is file

transfer. The owner of the file wants to be sure that all the bits arrive correctly and in the same

order they were sent. Reliable connection-oriented service has two minor variations: message

sequences and byte streams. In the former, the message boundaries are preserved when two 1-

KB messages are sent, they arrive as two distinct 1-KB messages never as one 2-KB message. In

the latter, the connection is simply a stream of bytes, with no message boundaries. Not all

applications require connections.

Unreliable (not acknowledged) connectionless service is often called datagram service,

which does not provide an acknowledgement back to the sender. In other situations, the

convenience of not having to establish a connection to send one short message is desired, but

reliability is essential.

The acknowledged datagram service can be provided for these applications. Still

another service is the request-reply service.

In this service, the sender transmits a single datagram containing a request, the reply

contains the answer. Request-reply is commonly used to implement communication in the client-

server model: the client issues a request and the server responds to it.

Service Primitives

A service is formally specified by a set of primitives (operations) available to a user or

other entity to access the service.

These primitives tell the service to perform some action or report on an action taken by a

peer entity.

One way to classify the service primitives is to divide them into four classes:

Primitive Meaning

LISTEN Block waiting for a incoming connection.

CONNECT Establish a connection with a waiting peer.

RECEIVE Block waiting for an incoming message.

SEND Send the message to the peer

DISCONNECT Terminate a connection

Five classes of service primitives.

First the server executes LISTEN to indicate that is prepared to accept the incoming

connection. A common way to implement LISTEN is make it a blocking system call. After

Connection Oriented

Connectionless

Six different types of Service

29

executing primitive a server process a block until a request for connection appears. A client

process executed CONNECT to establish a connection with the server.

The CONNECT call needs to specify who to connect to, parameter gives the servers

address.

The operating system sends the packet to the peer asking it to connect as shown by (1) fig

(refer class notes). The client process is connected until there is a response. When a packet

arrives at the server it is processed by the operating system. When a system sees the packet is

requesting a connection, it checks to see there is a listener. Unblock the listener and sends back

the acknowledgement (2).

The arrival of acknowledgement releases the client. At this point the client and server are

both are running and they have a connection established. Next step for the server to execute

RECEIVE to prepare to accept the first request.

The server does this immediately upon being released from the LISTEN, before the

acknowledgement can get back to the client. The RECEIVE call blocks the srever. The client

executes send to transmit the request (3) followed by the execution to receive to get the reply.

The arrival of the request packet at the server machine unblocks the server process so it can

process the request. After it has done the work it uses SEND to return the answer to the client

(4).

If it is done it use DISCONNECT to terminate the connection. An initial is a blocking

call, suspending the client and sending a packet to the server saying that connection is no longer

needed (5). When the server gets the packet it also issues a DISCONNECT of its own,

acknowledging the client and releasing the connection when the servers packet (6) gets back to

the client machine, the client process is released and connection is broken.

The Relationship of Services to Protocols

The service defines what operations the layer is prepared to perform or behalf of its users,

but it says nothing at all about how these operations are implemented. A service relates to an

interface between two layers, with the lower layer being the service provider and the upper layer

being the service user.

A service is a set of primitives (operations) that a layer provides to the layer above it. A

protocol, in contrast, is a set of rules governing the format and meaning of the frames, packets,

or messages that are exchanged by the peer entities within a layer.

Entities use protocols in order to implement their service definitions. They are free to

change their protocols at will, provided they do not change the service visible to their users. In

this way, the service and the protocol are completely decoupled.

A service is like an abstract data type or an object in an object-oriented language. It

defines operations that can be performed on an object but does not specify how these operations

are implemented. A protocol relates to the implementation of the service and as such is not

visible to the user of the service.

The OSI Reference Model

This model is based on a proposal developed by the International Standards Organization

(ISO) as a first step toward international standardization of the protocols used in the various

layers The model is called the ISO OSI (Open Systems Interconnection) Reference Model

because it deals with connecting open systems—that is, systems that are open for

communication with .

30

The OSI model has seven layers. The principles that were applied to arrive at the seven

layers are as follows:

1. A layer should be created where a different level of abstraction is needed.

2. Each layer should perform a well-defined function.

3. The function of each layer should be chosen with an eye toward defining internationally

Standardized protocols.

4. The layer boundaries should be chosen to minimize the information flow across the

interfaces.

5. The number of layers should be large enough that distinct functions need not be thrown

Together in the same layer out of necessity, and small enough that the architecture does not

become unwieldy.

The Physical Layer

The physical layer is concerned with transmitting raw bits over a communication

channel. The design issues have to do with making sure that when one side sends a 1 bit, it is

received by the other side as a 1 bit, not as a 0 bit.

Typical questions here are how many volts should be used to represent a 1 and how many

for a 0, how many microseconds a bit lasts, whether transmission may proceed simultaneously in

both directions, how the initial connection is established and how it is torn down when both sides

are finished, etc., the design issues here largely deal with mechanical, electrical, and procedural

interfaces, and the physical transmission medium, which lies below the physical layer.

The Data Link Layer

The main task of the data link layer is to take a raw transmission facility and transform

it into a line that appears free of undetected transmission errors to the network layer. It

accomplishes this task by having the sender break the input data up into data frames transmit

the frames sequentially, and process the acknowledgement frames sent back by the receiver.

Fig 1.20 OSI Reference Model

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Since the physical layer merely accepts and transmits a stream of bits without any regard to

meaning or structure, it is up to the data link layer to create and recognize frame boundaries.

This can be accomplished by attaching special bit patterns to the beginning and end of the

frame. If these bit patterns can accidentally occur in the data, special care must be taken to make

sure these patterns are not incorrectly interpreted as frame delimiters. A noise burst on the line

can destroy a frame completely. In this case, the data link layer software on the source machine

can retransmit the frame.

However, multiple transmissions of the same frame introduce the possibility of duplicate

frames. A duplicate frame could be sent if the acknowledgement frame from the receiver back to

the sender were lost. It is up to this layer to solve the problems caused by damaged, lost, and

duplicate frames. The data link layer may offer several different service classes to the network

layer, each of a different quality and with a different price.

Another issue that arises in the data link layer is how to keep a fast transmitter from

drowning a slow receiver in data. Some traffic regulation mechanism must be employed to let

the transmitter know how much buffer space the receiver has at the moment. Frequently, this

flow regulation and the error handling are integrated. If the line can be used to transmit data in

both directions, this introduces a new complication that the data link layer software must deal

with. The problem is that the acknowledgement frames for A to B traffic compete for the use of

the line with data frames for the B to A traffic.

Broadcast networks have an additional issue in the data link layer: how, to control access

to the shared channel. A special sub layer of the data link layer, the medium access sublayer,

deals with this problem.

The Network Layer

The network layer is concerned with controlling the operation of the subnet. A key

design issue is determining how packets are routed from source to destination. Routes can be

based on static tables that are "wired into" the network and rarely changed. They can also be

determined at the start of each conversation, for example a terminal session. Finally, they can be

highly dynamic, being determined a new for each packet, to reflect the current network load.

If too many packets are present in the subnet at the same time, they will get in each

other's way forming bottlenecks. The control of such congestion also belongs to the network

layer. There should be software that must count how many packets or characters or bits are sent

by each customer. When a packet crosses between layers, with different rates on each side, the

accounting can become complicated.

When a packet has to travel from one network to another to get to its destination, many

problems can arise. The addressing used by the second network may be different from the first

one. The second one may not accept the packet because it is too large, the protocols may differ,

and so on. It is up to the network layer to overcome all these problems to allow heterogeneous

networks get interconnected. In broadcast networks, the routing problem is simple, so the

network layer often is thin or even nonexistent.

The Transport Layer

The basic function of the transport layer is to accept data from the session layer, split it

up into smaller units if needed, pass these to the network layer, and ensure that the pieces all

arrive correctly at the other end. Under normal conditions, the transport layer creates a distinct

network connection for each transport connection required by the session layer. If the transport

connection requires a high throughput, however, the transport layer might create multiple

network connections, dividing the data among the network connections to improve throughput.

On the other hand, if creating or maintaining network connection is expensive, the

transport layer might multiplex several transport connections onto the same network connection

to reduce the cost. In cases, the transport layer is required to make the multiplexing transparent

to the session layer. The transport layer also determines what type of service to provide the

session layer and ultimately, the users of the network.

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The most popular type of transport connection is an error-free point-to-point channel that

delivers messages or bytes in the order in which they were sent. However, other possible kinds

of transport service are transport of isolated messages with no guarantee about the order of

delivery, and broadcasting of messages to multiple destinations. The type of service is

determined when the connection is established. The transport layer is a true end-to-end layer,

from source to destination. In other words, a program on the source machine carries on a

conversation with a similar program on the destination machine, using the message headers and

control messages. In the lower layers, the protocols are between each machine and its immediate

neighbors, and not by the ultimate source and destination machines, which may be separated by

many routers.

Many hosts are multiprogrammed, which implies that multiple connections will be

entering and leaving each host. There needs to be some way to tell which message belongs to

which connection. In addition to multiplexing several message streams onto one channel, the

transport layer must take care of establishing and deleting connections across the network. This

requires some kind of naming mechanism, so that a process on one machine has a way of

describing with whom it wishes to converse. There must also be a mechanism to regulate the

flow of information, so that a fast host cannot overrun a slow one. Such a mechanism is called

flow control and plays a key role in the transport layer.

The Session Layer

The session layer allows users on different machines to establish sessions between them.

A session allows ordinary data transport, as does the transport layer, but it also provides

enhanced services useful in some applications. A session might be used to allow a user to log

into a remote timesharing system or to transfer a File between two machines. One of the services

of the session layer is to manage dialogue control. Sessions can allow traffic to go in both

directions at the same time, or in only one direction at a time.

A related session service is token management. For some protocols, it is essential that

both sides do not attempt the same operation at the same time. To manage these activities, the

session layer provides tokens that can be exchanged. Only the side holding the token may

perform the critical operation.

Another session service is synchronization, the session layer provides a way to insert

checkpoints into the data stream, so that after a crash, only the data transferred after the last

checkpoint have to be repeated.

The Presentation Layer

The presentation layer performs certain functions that are requested sufficiently often to

warrant finding a general solution for them, rather than letting each user solve the problems. The

presentation layer is concerned with the syntax and semantics of the information transmitted. A

typical example of a presentation service is encoding data in a standard way. Most user programs

do not exchange random binary bit strings. They exchange things such as people's names, dates,

amounts of money, and invoices.

These items are represented as character strings, integers, floating-point numbers, and

data structures composed of several simpler items. Different computers have different codes for

representing character strings (e.g., ASCII and Unicode), integers (e.g., one's complement and

two's complement), and these different representations should be made to communicate. The

presentation layer manages these abstract data structures and converts from the representation

used inside the computer to the network standard representation and back.

The Application Layer

The application layer contains a variety of protocols that are commonly needed. For

example, there are hundreds of incompatible terminal types in the world. Consider the plight of a

full screen editor that is supposed to work over a network with many different terminal types,

each with different screen layouts, escape sequences for inserting and deleting text, moving the

cursor, etc. One way to solve this problem is to define an abstract network virtual terminal that

editors and other programs can be written to deal with.

33

To handle each terminal type, a piece of software must be written to map the functions of

the network virtual terminal onto the real terminal. For example, when the editor moves the

virtual terminal's cursor to the upper left-hand corner of the screen, this software must issue the

proper command sequence to the real terminal to get its cursor there too. All the virtual terminal

software is in the application layer.

Another application layer function is file transfer. Different file systems have different

file naming conventions, different ways of representing text lines, and so on. Transferring a file

between two different systems requires handling these and other incompatibilities. This work,

too, belongs to the application layer, as do electronic mail, remote job entry, directory lookup,

and various other general purpose and special-purpose facilities.

The TCP/IP Reference model

ARPANET was a research network sponsored by the DOD (U.S Department of

Defense), connecting hundreds of universities and government installations, using leased

telephone lines.

When satellite and radio networks were added later, the existing protocols had trouble

interworking with them, so new reference architecture was needed. Thus, the ability to connect

multiple networks in a seamless way was one of the major design goals from the very beginning.

This architecture later became known as the TCP/IP Reference model. DOD wanted connections

to remain intact as long as the source and destination machines were functioning.

The Internet Layer

All these requirements led to the choice of a packet-switching network based on a

connectionless internetwork layer. This layer, called the internet layer, is the linchpin that holds

the whole architecture together. Its job is to permit hosts to inject packets into any network and

have them travel independently to the destination. They arrive in a different order than they were

sent, the job of higher layers to rearrange them.

The analogy here is with the (snail) mail system. A person can drop a sequence of

international letters into a mail box in one country and with a little luck, most of them will be

delivered to the correct address in the destination country, letters will travel through one or more

international mail gateways.

Each country (i.e., each network) has its own stamps, preferred envelope sizes and

delivery rules is hidden from the users. The internet layer defines an official packet format and

protocol called IP (INTERNET PROTOCOL). The job of the internet layer is to deliver IP

packets where they are supposed to go.

The Transport Layer

The layer above the internet layer in the TCP/IP model is the transport layer. It is

designed to allow peer entities on the source and destination hosts to carry on a conversation, just

as OSI transport layer. Two end-to-end transport protocols are defined, first one TCP

(Transmission Control Protocol), is a reliable connection-oriented protocol that allows a byte

stream originating on one machine to be delivered without error on any other machine in the

internet. It fragments the incoming byte stream into discrete messages and passes each one to the

internet layer. At the destination, the receiving TCP process reassembles the received messages

into the output stream. TCP also handles flow control to make fast sender cannot swamp slow

receiver with more messages than it can handle.

The second protocol is UDP (USER DATAGRAM PROTOCOL) is an un-reliable,

connectionless protocol for applications protocol for applications that do not want TCP’s

sequencing or flow control and wish to provide their own. It is also widely used for one-shot,

client-server-type request-reply queries and applications in which prompt delivery is more

important than accurate delivery.

The Application layer:

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On the top of the transport layer is application layer. It contains all the higher level

protocols. The early ones included virtual terminal (TELNET), file transfer (FTP), and electronic

mail (SMTP). The virtual terminal allows a user on one machine to log on to a distant machine

and work there. The file transfer protocol provides the way to move data efficiently from one

machine to another. Electronic mail was originally just a kind of file transfer but later a

specialized protocol (SMTP) was developed for it. Some other protocol are Domain Name

System (DNS) for mapping host names on to their network address, NNTP, the protocol for

moving USENET news article around, HTTP, the protocol for fetching pages on the World Wide

Web.

The Host-to-Network layer

It tells only to point out that the host has to connect to the network using some protocol

so it can send IP packets to it.

Fig 1.21 Comparison

A Comparison of the OSI and TCP/IP Reference Models:-

The OSI and TCP/IP reference model have much in common. Both are based on the

concept of stack of independent protocols. Also the functionality of the layers is roughly similar.

For example in the both models the layer up thru & including the transport layer are there to

provide an end-to-end, network independent transport services to processes to wishing to

communicate. The layers form the transport provider. Again in both models, the layers above

transport are application-oriented users of the transport service.

Three concepts are central to the OSI Model:

1. Services

2. Interfaces

3. Protocols

The services definition tells what the layer does, not how entities above it access it or

how the layer works. It defines the layer’s semantics. A layer’s interface tells the processes

above it how to access it. It specifies what the parameter are and what results to expect. Finally,

the peer protocols used in a layer are the layer’s own business. It can use any protocol it wants

to, as long as it gets the job done. It can also change them at will without affecting software in

higher layers.

The TCP/IP model did not distinguish between service, interface, and protocol.

For example the only real services offered by the internet layer are SEND IP PACKET and

RECEIVE IP PACKET.

The OSI reference model was devised before the corresponding protocols were

invented.With TCP/IP the reverse was true:

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The protocol came first, and the model was really just a description of the existing

protocols.

OSI model has seven layers and the TCP/IP has four layers. Both have (inter)network,

transport, and application layers, but the other layers are different.

OSI model supports both connection less and connection oriented communication in the

network layer, but only connection oriented communication in the transport layer.

The TCP/IP model has only one mode in the network layer but supports both modes in

the transport layer giving the user a choice. This choice is especially important for

simple request response protocols.

A Critique of the OSI Model and protocols:-

These lessons can be summarized as:

1. Bad timing.

2. Bad technology

3. Bad implementations

4. Bad politics

Bad timing

The time at which a standard is established is absolutely critical to its success. When the

subject is first discovered, there is a burst of research activity in the form of discussions, papers

and meetings. After a while this activity subsides, corporation discover the subject, and the

billion-dollar wave of investment hits. It is essential that the standard be return in the trough in

between the two elephants. If the standards are written too early, before the research is finished,

the subject may still be poorly understood; the result is bad standards. If they are written too late

so many companies may have already made major investments in different ways of doing things

that the standards are effectively ignored.

Bad technology

The choice of seven layers was more political than technical, and two of the layers

(session and presentation) are nearly empty, whereas two other ones (data link and network) are

overfull. The OSI model, along with the associated service definition and protocol, is

extraordinarily complex. Another problem with OSI that some functions, such as addressing,

flow control and error control, reappear again and again in each layer.

Bad Implementation

Complexity of the model and the protocols, it will come as no surprise the initial

implementation were huge, unwieldy and slow.

Bad Politics

OSI was widely thought to be the creature of the European telecommunication ministries,

the European community, and later the U.S. Government. Some people viewed this development

in the same light as IBM announcing in the 1960’s that PL/I was the language of the future or

DoD correcting this later by announcing it was actually Ada.

A Critique of the TCP/IP Reference Model The TCP/IP model and protocols have their problems too.

The model does not clearly distinguish the concepts of service, interface and protocol.

The model is not all general and is poorly suited to describing any protocol stack other

than TCP/IP.

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Fig 1.21 Layers

The Host-to-network layer is not really a layer at all in the normal sense of a term as used

in the context of layered protocol. It is an interface between the network and the data link

layer.

TCP/IP model does not distinguish the physical and data link layer and they are

completely different. The physical layer has to do with the transmission characteristic of

copper wire, Fiber optics, wireless communication. The data link layer job’s is to delimit

the start and end of frames and get them from one side to the other with the desired

degree of reliability.

Application Layer

Transport Layer

Internet Layer

Host-to-Network Layer

Fig 1.21 TCP/IP Reference model

Network Glossaries:

Network: Two or more computers connected together that can share resources and pass data

Sneakernet Sharing data on computers by running around (in sneakers) with removable media

(such as floppies) to and from each standalone computer.

Transmission Media: The connection between computers in a network. Examples include

various types of (copper) cables, fiber-optic cables, radio signals (wireless), and infra-red signals.

NIC Network Interface Card: (Also Network Board.)

Local Computer: The computer in front of you, the one you are physically interacting with now.

Remote Computer: The computer you are working on via a network. Host Another term for

computer. Node Any networked device (usually a host).

Peer-to-peer:Network A network of computers (peers) that each can communicate with each

other, and make and respond to requests for data and access to shard devices. (Also referred to

as p2p networks.)

Client-Server: Also referred to as server based networking, the more common type of network

today.

Client: Any node that makes requests of servers. The term may also refer to any user or

software that makes requests of a server, it doesn't have to be a separate piece of hardware.

Server: A node that responds to requests made by clients. (A server may not be a separate piece

of hardware.)

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NOS Network Operating System: It refers to an operating system (OS) that supports

networking

Network Model: Also the Network Architecture. LAN Local Area Network WAN Wide Area

network MAN Metropolitan Area Network internet A collection of related, connected networks

Enterprise network: A network that spans an entire organization regardless of size, but often

multiple sites.

Internet: A global internet that grew from the ARPANET Address A number given to a NIC (not

a host, although since most hosts have a single NIC it often comes to the same thing) to uniquely

identify it on a network.

Addressing: A scheme used to assign addresses to nodes on a network.

Topology: The shape of a network: star, ring, bus (hub), mesh, partial mess, and cell(ular) are

common topologies.

Protocol: The rules used between nodes to communicate.

Protocol Stack (Also Protocol Suite.): A hierarchical group of protocols designed to work

together. Examples include Ethernet, Netware, and TCP/IP.

Gateway: A combination of hardware and software that allows different kinds of networks to

exchange data.

PacketA packet is a unit of data sent between devices. When you load a web page, your

computer sends packets to the server requesting the web page and the server responds with many

different packets of its own, which your computer stitches together to form the web page. The

packet is the basic unit of data that computers on a network exchange.

Segment: (1) A section of a network, typically a single cable or hub. (2) A packet of data.

38

Bridge: A Layer to device that connects 2 (or more) segments into a single network. A bridge

looks at the layer 2 packet header to determine which port (NIC) or ports to send a packet out.

Repeater: A layer 1 device that is used to extend a LAN. A repeater simply sends all received

packets out all ports.

Hub: The center of a star network topology, a layer 1 device that have replaced the cable

segment in a bus topology. A hub may be thought of as a multi-port repeater.

Network Services: The common services provided by a network are: file/application, database,

print, remote access (a.k.a. communication, RAS, NAS), e-mail, Internet (www, FTP, email, ...),

security, and management services (traffic monitoring, load balancing, fault alerting, assess

management, license tracking, security, patches, configuration, address management, and backup

and restore).

Load Balancing: Splitting the workload over several servers.

Traffic: A term that refers to the data flowing through a network. Workstation A host connected

to a network primarily used for a single user at a time.

MAC Address

Each network interface has a media access control address, or MAC address — also

known as a physical address. This is a unique identifier designed to identify different computers

on a network. MAC addresses are usually assigned when a manufacturer creates a network

device.

For example, when you visit an airport and use 30 minutes of free Wi-Fi before being

kicked off and refused access to further Wi-FI without paying, the airport’s Wi-Fi network has

likely noted your device’s MAC address and is using it to track your PC and prevent you from

taking advantage of more free time. MAC addresses could also be used to assign static IP

addresses to specific devices, so they’d always get the same IP address when they connected to a

router with DHCP.

MAC addresses are actually more fluid in practice, as you can change your network

interface’s MAC address. (Yes, this means you can often gain access to more free airport Wi-Fi

by changing your device’s MAC address.)

Network Interface / Network Adapter

Your computer’s wired Ethernet connection and Wi-Fi connection are basically both

network interfaces. If your laptop was connected to both a wired connection and a Wi-Fi

network, each network interface would have its own IP address. Each is a different connection.

39

Network interfaces can also be implemented entirely in software, so they don’t always

directly correspond to hardware devices.

Localhost:

The hostname “localhost” always corresponds to the device you’re using. This uses the

loopback network interface — a network interface implemented in software — to connect

directly to your own PC.

localhost actually points to the IPv4 address 127.0.0.1 or the IPv6 address ::1 . Each

always corresponds to the current device.

IP Address

An Internet Protocol address, or IP address, is a numerical address that corresponds to

your computer on a network. When a computer wants to connect to another computer, it

connects to that computer’s IP address.

IPv4 and IPv6

There are two types of IP address in common use. Older IPv4 (IP version 4) addresses are

the most common, followed by newer IPv6 (IP version 6) addresses. IPv6 is necessary because

we just don’t have enough IPv4 addresses for all the people and devices in the world.

NAT

Network Address Translation, or NAT, is used by routers to share a single IP address

among many devices. For example, you probably have a wireless router at home that creates a

40

Wi-Fi network your laptops, smartphones, tablets, and other devices connect to. Your ISP

provides you with a single IP address that’s reachable from anywhere on the Internet, sometimes

called a public IP address.

Your router creates a LAN and assigns local IP addresses to your devices. The router then

functions as a gateway. To devices outside your LAN, it appears as if you have one device (the

router) using a single IP address.

DHCP

The dynamic host configuration protocol allows computers to automatically request and

be assigned IP addresses and other network settings. For example, when you connect your laptop

or smartphone to your Wi-Fi network, your device asks the router for an IP address using DHCP

and the router assigns an IP address. This simplifies things — you don’t have to set up static IP

addresses manually.

Data Communication

When we communicate, we are sharing information. This sharing can be local or remote.

Between individuals, local communication usually occurs face to face, while remote

communication takes place over distance.

Components

A data communications system has five components.

1. Message. The message is the information (data) to be communicated. Popular forms of

information include text, numbers, pictures, audio, and video.

2. Sender. The sender is the device that sends the data message. It can be a computer,

workstation, telephone handset, video camera, and so on.

3. Receiver. The receiver is the device that receives the message. It can be a computer,

workstation, telephone handset, television, and so on.

4. Transmission medium. The transmission medium is the physical path by which a message

travels from sender to receiver. Some examples of transmission media include twisted-pair wire,

coaxial cable, fiber-optic cable, and radio waves

5. Protocol. A protocol is a set of rules that govern data communications. It represents an

agreement between the communicating devices. Without a protocol, two devices may be

connected but not communicating, just as a person speaking French cannot be understood by a

person who speaks only Japanese.

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Data Representation:

Information today comes in different forms such as text, numbers, images, audio, and

video.

Text:

In data communications, text is represented as a bit pattern, a sequence of bits (Os or Is).

Different sets of bit patterns have been designed to represent text symbols. Each set is called a

code, and the process of representing symbols is called coding. Today, the prevalent coding

system is called Unicode, which uses 32 bits to represent a symbol or character used in any

language in the world. The American Standard Code for Information Interchange (ASCII),

developed some decades ago in the United States, now constitutes the first 127 characters in

Unicode and is also referred to as Basic Latin.

Numbers:

Numbers are also represented by bit patterns. However, a code such as ASCII is not used

to represent numbers; the number is directly converted to a binary number to simplify

mathematical operations. Appendix B discusses several different numbering systems.

Images:

Images are also represented by bit patterns. In its simplest form, an image is composed

of a matrix of pixels (picture elements), where each pixel is a small dot. The size of the pixel

depends on the resolution. For example, an image can be divided into 1000 pixels or 10,000

pixels. In the second case, there is a better representation of the image (better resolution), but

more memory is needed to store the image. After an image is divided into pixels, each pixel is

assigned a bit pattern. The size and the value of the pattern depend on the image. For an image

made of only blackand- white dots (e.g., a chessboard), a I-bit pattern is enough to represent a

pixel.

If an image is not made of pure white and pure black pixels, you can increase the size of

the bit pattern to include gray scale. For example, to show four levels of gray scale, you can use

2-bit patterns.

A black pixel can be represented by 00, a dark gray pixel by 01, a light gray pixel by 10,

and a white pixel by 11. There are several methods to represent color images. One method is

called RGB, so called because each color is made of a combination of three primary colors: red,

green, and blue. The intensity of each color is measured, and a bit pattern is assigned to it.

Another method is called YCM, in which a color is made of a combination of three other primary

colors: yellow, cyan, and magenta.

Audio:

Audio refers to the recording or broadcasting of sound or music. Audio is by nature different

from text, numbers, or images. It is continuous, not discrete. Even when we use a microphone to

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change voice or music to an electric signal, we create a continuous signal. In Chapters 4 and 5, we

learn how to change sound or music to a digital or an analog signal.

Video:

Video refers to the recording or broadcasting of a picture or movie. Video can either be

produced as a continuous entity (e.g., by a TV camera), or it can be a combination of images, each a

discrete entity, arranged to convey the idea of motion. Again we can change video to a digital or an

analog signal.

Data Flow

Communication between two devices can be simplex, half-duplex, or full-duplex as shown in

Figure

Simplex:

In simplex mode, the communication is unidirectional, as on a one-way street. Only one of

the two devices on a link can transmit; the other can only receive (see Figure a). Keyboards and

traditional monitors are examples of simplex devices. The keyboard can only introduce input; the

monitor can only accept output. The simplex mode can use the entire capacity of the channel to send

data in one direction.

Half-Duplex:

In half-duplex mode, each station can both transmit and receive, but not at the same time.

When one device is sending, the other can only receive, and vice versa The half-duplex mode is like

a one-lane road with traffic allowed in both directions.

When cars are traveling in one direction, cars going the other way must wait. In a half-duplex

transmission, the entire capacity of a channel is taken over by whichever of the two devices is

transmitting at the time. Walkie-talkies and CB (citizens band) radios are both half-duplex systems.

The half-The half-duplex mode is used in cases where there is no need for communication in both

directions at the same time; the entire capacity of the channel can be utilized for each direction.

Full-Duplex

In full-duplex both stations can transmit and receive simultaneously (see Figure c). The full-

duplex mode is like a tW<D-way street with traffic flowing in both directions at the same time. In

full-duplex mode, si~nals going in one direction share the capacity of the link: with signals going in

the other din~c~on. This sharing can occur in two ways:

Either the link must contain two physically separate t:nmsmissiIDn paths, one for sending

and the other for receiving; or the capacity of the ch:arillilel is divided between signals traveling in

both directions. One common example of full-duplex communication is the telephone network.

When two people are communicating by a telephone line, both can talk and listen at the same time.

43

The full-duplex mode is used when communication in both directions is required all the time. The

capacity of the channel, however, must be divided between the two directions.

NETWORKS

A network is a set of devices (often referred to as nodes) connected by communication links.

A node can be a computer, printer, or any other device capable of sending and/or receiving data

generated by other nodes on the network.

Distributed Processing

Most networks use distributed processing, in which a task is divided among multiple

computers. Instead of one single large machine being responsible for all aspects of a process,

separate computers (usually a personal computer or workstation) handle a subset.

Network Criteria

A network must be able to meet a certain number of criteria. The most important of these are

performance, reliability, and security.

Performance:

Performance can be measured in many ways, including transit time and response time.Transit

time is the amount of time required for a message to travel from one device to another. Response

time is the elapsed time between an inquiry and a response. The performance of a network depends

on a number of factors, including the number of users, the type of transmission medium, the

capabilities of the connected hardware, and the efficiency of the software. Performance is often

evaluated by two networking metrics: throughput and delay. We often need more throughput and less

delay. However, these two criteria are often contradictory. If we try to send more data to the network,

we may increase throughput but we increase the delay because of traffic congestion in the network.

Reliability:

In addition to accuracy of delivery, network reliability is measured by the frequency of

failure, the time it takes a link to recover from a failure, and the network's robustness in a

catastrophe.

Security:

Network security issues include protecting data from unauthorized access, protecting data

from damage and development, and implementing policies and procedures for recovery from

breaches and data losses.

Physical Structures:

Type of Connection

A network is two or more devices connected through links. A link is a communications

pathway that transfers data from one device to another. For visualization purposes, it is simplest to

imagine any link as a line drawn between two points. For communication to occur, two devices must

be connected in some way to the same link at the same time. There are two possible types of

connections: point-to-point and multipoint.

Point-to-Point :

A point-to-point connection provides a dedicated link between two devices. The entire

capacity of the link is reserved for transmission between those two devices. Most point-to-point

connections use an actual length of wire or cable to connect the two ends, but other options, such as

microwave or satellite links, are also possible. When you change television channels by infrared

remote control, you are establishing a point-to-point connection between the remote control and the

television's control system.

Multipoint

A multipoint (also called multidrop) connection is one in which more than two specific

devices share a single link. In a multipoint environment, the capacity of the channel is shared, either

44

spatially or temporally. If several devices can use the link simultaneously, it is a spatially shared

connection. If users must take turns, it is a timeshared connection.

Unit -1 Questions

Section-A 1. Point to point transmission with one sender and one receiver is sometimes called

_________.

2. __________is an agreement between the communicating parties on how communication

is to proceed.

3. A set of layers and protocols is called a _________

4. The primitive ________can establish a connection with a waiting peer.

5. The ___________is to transform a raw transmission facility into a line.

6. The _______ Layer controls the operation of the subnet.

7. _______ are specialized computers that connect 3 or more transmission lines.

8. A set of layers and protocols is called _______

9. ________have a single communication channel that is shared by all the machines on the

network.

10. In OSI network architecture the dialogue control and token management are

responsibilities of__________

Section-B 1. Explain LAN of Network hardware?

2. Explain the uses of Computer Network?

3. What are the five Services Primitives for implementing a simple connection oriented

Service?

4. Explain the relationship of services to protocols?

5. Explain the different types of services provided by the network software?

6. Define Broadcast network and point to point network?

45

7. Define UniCasting and Multi casting

8. Write about the transmission technology?

9. What is datagram service?

10. Explain the three main categories of the wireless networks?

Section-C

1. Explain the topologies of the network?

2. Explain about the TCP/IP Reference model?

3. Differentiate between OSI and TCP/IP Reference Models?

4. Briefly Explain Network Hardware?

5. Explain about the uses of computer network?

6. What is the Critique of the OSI model and TCP/IP Reference Model?

7. Differentiate between broadcast network and Point to Point Network

8. Compare connection oriented and connection less service

9. Explain ISO/OSI reference model?

10. Briefly Explain Network Software?

UNIT-II:PHYSICAL LAYER - Guided Transmission Media: Magnetic Media – Twisted

Pair – Coaxial Cable – Fiber Optics. Wireless Transmission: Electromagnetic Spectrum–Radio

Transmission – Microwave Transmission – Infrared and Millimeter Waves – Light Waves.

Communication Satellites: Geostationary, Medium-Earth Orbit, Low Earth-orbitSatellites –

Satellites versus Fiber.

UNIT-II PHYSICAL LAYER Classes Of Transmission Media:

Conducted or guided media:use a conductor such as a wire or a fiber optic cable to move

the signal from sender to receiver.

Wireless or unguided media:use radio waves of different frequencies and do not need a

wire or cable conductor to transmit signals.

Design Factors for Transmission Media:

Bandwidth: All other factors remaining constant, the greater the band-width of a signal,

the higher the data rate that can be achieved.

Transmission impairments. Limit the distance a signal can travel.

Interference: Competing signals in overlapping frequency bands can distort or wipe out a

signal.

Number of receivers: Each attachment introduces some attenuation and distortion,

limiting distance and/or data rate.

Guided Transmission Media

46

The purpose of physical layer is to transport a raw bit stream from one machine to another.

Various physical media can be used for the actual transmission. Each one has its own niche in terms of

bandwidth, delay, cost, and ease of installation and maintenance. Media are grouped in to guided

media such as copper wire and fiber optics, and unguided media, such as radio and lasers through air.

Transmission capacity depends on the distance and on whether the medium is point-to-point or

multipoint.Examples:Twisted pair wires,Coaxial cables,optical Fiber

Magnetic Media:

One of the most common ways to transport data from one computer to another is to write them

onto magnetic tape or removable media (e.g., recordable DVDs) physically transport the tape or disks

to he destination machine, and read them back in again. Although this method is not as sophisticated

as using a geosynchronous communication satellite, it is more cost effective, and useful when high

bandwidth or cost per bit transported is the key factor. Though, the bandwidth characteristics of

magnetic tape are excellent, the delay characteristics are poor. Transmission time is measured in

minutes, hours or even days.

Twisted Pair:

Although the band width characteristics of magnetic tape are excellent, the delay

characteristics are poor. Many applications need an on-line connection. The most common medium for

transmission is twisted pair. A twisted pair consists of two insulated copper wires about 1mm thick.

The wires are twisted together in a helical form, the structure of a DNA molecule to avoid electrical

interference to similar pairs of wires close by. Twisting is done because two parallel wires constitute a

fine antenna. When the wires are twisted, the waves from different twists cancel out, so the wires

radiates less effectively.

The most common application of twisted pair is the telephone system. Nearly all telephones are

connected to the telephone company (telco) office by a twisted pair. Wires used can run for several

kilometers without amplification, but for longer distances repeaters are used. When many twisted pairs

run in parallel for a substantial distance, such as all the wires coming from an apartment building to

the telephone company office, there are bundled together and encased in a protective sheath. The pairs

in the bundles would interfere with each other if they are not twisted.

Twisted pairs can be used for either analog or digital transmission. The bandwidth depends on

the thickness of the wires and the distance traveled.

Due to their adequate performance and low cost, twisted pairs are widely used. Twisted pair

cabling comes in several varieties, two of which are important for computer networks. Category 3

twisted pairs consist of two insulated wires gently twisted together.

Four such pairs are grouped in a plastic sheath to protect the wires and keep them together,

category 3 cable running from a central wiring closet on each floor into each office. This scheme

allowed up to four regular telephones or two multiline telephones in each office to connect to the

telephone company equipment in the wiring closet.

The most advanced category 5 twisted pairs were introduced. They are similar to category 3

pairs, but with more twists per centimeter, which results in less crosstalk and a better-quality signal

over longer distances, making them more suitable for high-speed computer communication. Up-and-

coming categories are 6 and 7, which are capable of handling signals with bandwidths of 250 MHz

and 600 MHz, respectively. All of these wiring types are often referred as UTP (Unshielded Twisted

Pair), to contrast them with the bulky, expensive, shielded twisted pair cables IBM introduced in the

early 1980s.

Fig 2.1 Twisted PairsUnshielded twisted pair (UTP)

UTP cables are found in many Ethernet networks and telephone systems. For indoor telephone

applications, UTP is often grouped into sets of 25 pairs according to a standard 25-pair color code originally developed by AT&T Corporation. A typical subset of these colors (white/blue, blue/white, white/orange,

47

orange/white) shows up in most UTP cables. The cables are typically made with copper wires measured at 22 or 24 American Wire Gauge (AWG), with the colored insulation typically made from an insulator such as

polyethylene or FEP and the total package covered in a polyethylene jacket.

For urban outdoor telephone cables containing hundreds or thousands of pairs, the cable is

divided into smaller but identical bundles. Each bundle consists of twisted pairs that have different twist rates. The bundles are in turn twisted together to make up the cable. Pairs having the same twist rate

within the cable can still experience some degree of crosstalk. Wire pairs are selected carefully to

minimize crosstalk within a large cable.Unshielded twisted pair cable with different twist rates.

UTP cable is also the most common cable used in computer networking. Modern Ethernet, the

most common data networking standard, can use UTP cables. Twisted pair cabling is often used in data networks for short and medium length connections because of its relatively lower costs compared to

optical fiber and coaxial cable.

UTP is also finding increasing use in video applications, primarily in security cameras. Many cameras include a UTP output with screw terminals; UTP cable bandwidth has improved to match the

baseband of television signals. As UTP is a balanced transmission line, a balun is needed to connect to

unbalanced equipment, for example any using BNC connectors and designed for coaxial cable.

Cable shielding

Twisted pair cables are often shielded in an attempt to prevent electromagnetic

interference. Shielding provides an electric conductive barrier to attenuate electromagnetic

waves external to the shield and provides conduction path by which induced currents can be

circulated and returned to the source, via ground reference connection.

This shielding can be applied to individual pairs or quads, or to the collection of pairs.

Individual pairs are foiled, while overall cable may use braided screen, foil, or braiding with foil.

ISO/IEC 11801:2002 (Annex E) attempts to internationally standardise the various

designations for shielded cables by using combinations of three letters - U for unshielded, S for

braided shielding, and F for foiled shielding - to explicitly indicate the type of screen for overall

cable protection and for individual pairs or quads, using a two-part abbreviation in the form of

xx/xTP.

When shielding is applied to the collection of pairs, this is usually referred to as

screening, however different vendors and authors use different terminology, employing

"screening" and "shielding" interchangeably; for example, STP (shielded twisted pair) or ScTP

(screened twisted pair) has been used to denote U/FTP, S/UTP, F/UTP, SF/UTP and S/FTP

construction.

Because the shielding is made of metal, it may also serve as a ground. Usually a shielded

or a screened twisted pair cable has a special grounding wire added called a drain wire which is

electrically connected to the shield or screen. The drain wire simplifies connection to ground at

the connectors.

An early example of shielded twisted-pair is IBM STP-A, which was a two-pair 150 ohm

S/FTP cable defined in 1985 by the IBM Cabling System specifications, and used with token

ring or FDDI networks.

Common shielded cable types used by Cat. 6a, Cat.7 and Cat.8 cables include:

Shielded twisted pair (U/FTP)

Also pair in metal foil. Individual shielding with foil for each twisted pair or quad. This

type of shielding protects cable from external EMI entering or exiting the cable and also protects

neighboring pairs from crosstalk.

Screened twisted pair (F/UTP, S/UTP and SF/UTP)

Also foiled twisted pair for F/UTP. Overall foil, braided shield or braiding with foil

across all of the pairs within the 100 Ohm twisted pair cable. This type of shielding protects EMI

from entering or exiting the cable.

48

Screened shielded twisted pair (F/FTP and S/FTP)

Also fully shielded twisted pair, shielded screened twisted pair, screened foiled twisted

pair, shielded foiled twisted pair. Individual shielding using foil between the twisted pair sets,

and also an outer metal and/or foil shielding within the 100 Ohm twisted pair cable.[6] This type

of shielding protects EMI from entering or exiting the cable and also protects neighboring pairs

from crosstalk.

A series of new standards are in the works to include power over some of the unused wire

pairs in Twisted Pairs:

CAT 1-6. This would eliminate the need for separate power cables to some network

devices, and will likely be very popular. CAT 1 Unshielded, un-twisted cable with 2 pairs, usually used for voice but can support

up to 128 Kbps

CAT 2 UTP or STP with 4 pairs; can support up to 4 Mbps

CAT 3 UTP or STP with 4 pairs; can support up to 10 Mbps and was common in older

installations that only supported 4 Mbps token ring or 10 Mbps Ethernet. Has 3 to 4

twists per foot.

CAT 4 UTP with 4 pairs; can support up to 20 Mbps and was common to support

16 Mbps token ring. Has about 10 twists per foot. CAT 5UTP or STP with 4 pairs; can

support 100 Mbps. Has 36 to 48 twists per foot. (Very common today.)

CAT 5eUTP or STP with 4 pairs; can support 250 MHz. Has 48+ twists per foot. When

adding connectors, you must not untwist more than 1/2 inch of cable at each end, and

strip no more than 1 inch of insulation. (Note that even one inch of untwisted wires can

reduce throughput to less than 30 Mbps!) This is the current standard in new

construction and will support Gigabit Ethernet.

CAT 6 STP with 4 pairs; can support 100 Mbps. This is rarely used.

CAT 7 STP with 4 pairs; will support 750 MHz. This category of cable is not currently

standardized (3/2003). This cable will use different connectors than CAT 3-6 (i.e., not

RJ-45 connectors).

Fig 2.1 Twisted Pairs Connection

Networking media can be defined simply as the means by which signals (data) are sent

from one computer to another (either by cable or wireless means).

Advantages

It is a thin, flexible cable that is easy to string between walls

More lines can be run through the same wiring ducts

Electrical noise going into or coming from the cable can be prevented.

Cross-talk is minimized.

Disadvantages

Twisted pair's susceptibility to electromagnetic interference greatly depends on the pair

twisting schemes (usually patented by the manufacturers) staying intact during the installation.

As a result, twisted pair cables usually have stringent requirements for maximum pulling tension

as well as minimum bend radius. This relative fragility of twisted pair cables makes the

installation practices an important part of ensuring the cable's performance.

In video applications that send information across multiple parallel signal wires, twisted

pair cabling can introduce signaling delays known as skew which cause subtle color defects and

ghosting due to the image components not aligning correctly when recombined in the display

49

device. The skew occurs because twisted pairs within the same cable often use a different

number of twists per meter in order to prevent crosstalk between pairs with identical numbers of

twists. The skew can be compensated by varying the length of pairs in the termination box, in

order to introduce delay lines that take up the slack between shorter and longer pairs, though the

precise lengths required are difficult to calculate and vary depending on the overall cable length.

Coaxial Cable

Another common transmission medium is the Coaxial cable; it has better shielding than

twisted pairs, so it can span longer distances at higher speeds. Two kinds of coaxial cable are widely

used. One kind, 50-ohm cable, is commonly used when it is intended for digital transmission from the

start. The other kind, 75-ohm cable, is commonly used for analog transmission and cable television

but is becoming more important with the advent of Internet over cable.

A coaxial cable consists of a stiff copper wire as the core surrounded by an insulating material.

The insulator is encased by a cylindrical conductor, often as a close woven braided mesh.

The outer conductor is covered in a protective plastic sheath. The construction of the coaxial

cable gives it a good combination of high bandwidth and excellent noise immunity.

The ossible bandwidth depends on the cable quality, length and signal-to-noise ratio of the data

signal. Higher data rates are possible for shorter cables. Longer cables offer lower data rates. The

coaxial cables are used within the telephone system for long-distance lines but have now largely been

replaced by fiber optics on long-haul routes. Coax is still widely used for cable television and

metropolitan area networks

A Coaxial cable

History Of Coaxial cable

1880 Coaxial cable patented in England by Oliver Heaviside, patent no. 1,407.[39]

1884 — Siemens & Halske patent coaxial cable in Germany (Patent No. 28,978, 27

March 1884).[40]

1894 — Oliver Lodge demonstrates waveguide transmission at the Royal Institution.

1894 — Nikola Tesla Patent of an electrical conductor. An early example of the coaxial

cable [41]

1929 — First modern coaxial cable patented by Lloyd Espenschied and Herman Affel of

AT&T's Bell Telephone Laboratories.[42]

Baseband(digital) Broadband(analog)

Baseband is simple and inexpensive

to Install.

Broadband requires expensive radio

Frequency engineers to plan the cable and

Amplifier layout to install the system.

Maintenance cost is less. Skilled personnel is required to maintain the

System and periodically tune the amplifiers

During its use.

It requires inexpensive interfaces. The broadband interfaces are very expensive.

It offers a digital channel with a data

Rate of about 10 Mbps over a

distance of 1 km Using off-the-shelf

coaxial cable.

Broadband offers multiple channels and can

Transmit data, voice, so on in the same cable

Copper Insulating Braided Outer Protective Plastic

Core Material Conductor Covering

50

1936 — First closed circuit transmission of TV pictures on coaxial cable, from the 1936

Summer Olympics in Berlin to Leipzig.[43]

1936 — World's first underwater coaxial cable installed between Apollo Bay, near

Melbourne, Australia, and Stanley, Tasmania. The 300 km cable can carry one 8.5-kHz

broadcast channel and seven telephone channels.[44]

1936 — AT&T installs experimental coaxial telephone and television cable between New

York and Philadelphia, with automatic booster stations every ten miles. Completed in

December, it can transmit 240 telephone calls simultaneously.[45][46]

1936 — Coaxial cable laid by the General Post Office (now BT) between London and

Birmingham, providing 40 telephone channels.[47][48]

1941 — First commercial use in USA by AT&T, between Minneapolis, Minnesota and

Stevens Point, Wisconsin. L1 system with capacity of one TV channel or 480 telephone

circuits.

1956 — First transatlantic coaxial cable laid, TAT-1

Coaxial cables are a type of cable that is used by cable TV and that is common for data

communications.

Taking a a round cross-section of the cable, one would find a single center solid wire

symmetrically surrounded by a braided or foil conductor. Between the center wire and foil is a

insulating dialectric.

This dielectric has a large affect on the fundamental characteristics of the cable. In this

lab, we show the how the permittivity and permeability of the dialectric contributes to the cable's

inductance and capacitance. Also, these values affect how quickly electrical data is travels

through the wire. Data is transmitted through the center wire, while the outer braided layer serves

as a line to ground. Both of these conductors are parallel and share the same axis. This is why the

wire is called coaxial!

Just like all electrical components, coaxial cables have a characteristic impedance. This

impedance depends on the dialectric material and the radii of each conducting material As

shown in this lab, the impedance affects how the cable interacts with other electrical

components.

In this lab we used a RG-580/U coaxial cable. This is just one of many types of cables

that are used today to transmit data. The dialectric of the RG-580/U was made of polyethylene.

The radius of our cable's inner copper wire was .42mm and there was 2.208mm of polyethylene

between the inner wire and outer mesh conductors.

Fiber Optics:

It has been made possible to transfer data by pulses of light. A Light signal is used to

transmit a 1, the absence of light to transmit 0 bit. Visible light frequency is 108 MHz, so the

bandwidth of an optical transmission is enormous.

There are three components in Optical transmission.

1. The transmission medium

2. The Light source

3. The detector

The transmission medium is an ultra-thin fiber of glass or fused silica. The detector generates

an electrical pulse when light falls on it. By attaching a light source to one end of an optical fiber and

a detector to the other, we have a unidirectional data transmission system that accepts an electrical

signal, converts and transmits it by light pulses, and then reconverts the output to an electrical signal

at the receiving end. The transmission leaks light.

When a light ray passes from one medium to another, for example, from fused silica to air, the

ray is refracted (bent). The amount of refraction depends on the two medias. For angle of incidence

above the certain critical value, the light is refracted back into the silica; none escapes into the air.

Thus a light ray incident at or above the critical angle is trapped inside the fiber, and can propagate for

many kilo meters with virtually no loss. Can be trapped inside without any loss.

51

Since any light ray incident on the boundary above the critical angle may be refracted

internally, many different rays will be bouncing around at different angles.

β1 β2 β3

Each ray is said to have a different mode so a fiber having this property is called Multimode

fiber.If the fiber’s diameter is reduced to a few wavelength of light, the fiber acts like a wave guide,

and the light will propagate in a straight line, without bouncing, yielding a single mode fiber. Single

mode fibers are more expensive but are widely used for longer distances. Currently available single-

mode fibers can transmit data at 50 Gbps for 100 km without amplification. Even higher data rates

have been achieved in the laboratory for shorter distances.

Transmission of Light through Fiber

Optical fibers are of made of glass, which, in turn is made from sand, an inexpensive raw

material available in unlimited amounts. Glassmaking was known to the ancient Egyptians, but their

glass had to be no more than 1 mm thick or the light could not shine through. Glass transparent to be

useful for windows was developed during the Renaissance. The glass used for modern optical fibers is

so transparent that if the oceans were full of it instead of water, the seabed would be as visible from

the surface as the ground is from an airplane on a clear day. The attenuation of light through glass

depends on the wavelength of the light. For the kind of glass used in fibers, the attenuation in decibels

per linear kilometer of fiber. The attenuation in decibels is given by the formula

Attenuation in decibels= 10 log 10 Transmitted power

Received power

For example, a factor of two loss gives an attenuation of 10 log 10 2=3 dB.

The figure shows the near infrared part of the spectrum, which is what is used in practice. Visible

light has slightly shorter wavelengths, from 0.4 to 0.7 microns. The true metric purist would refer to

these wavelengths as 400 nm to 700 nm, but we will stick with traditional usage. Three wavelength

bands are used for optical communication. They are centered at 0.85, 1.3 and 1.55 microns

respectively. The last two have good attenuation properties, the 085 micron band has higher

attenuation, but at that wavelength the lasers and electronics can be made from the same material

(gallium arsenide). All three bands are 25,000 to 30,000 GHz wide.

Light pulses sent down a fiber spread out in length as they propagate. This spreading is called

chromatic dispersion. The amount of it is wavelength dependent. One way to keep these spread-out

pulses from overlapping is to increase the distance them, but this can be done only by reducing the

signaling rate. It has been discovered that by making the pulses in special shape related to the

reciprocal of the hyperbolic cosine, nearly all the dispersion effects cancel out, and it is possible to

send pulses for thousands of kilometers without appreciable shape distortion. These pulses are called

solitons.

Fiber Cables

Fiber optic cables are similar to coax, except without the braid. At the center is the glass core

through which the light propagates. In multimode fibers, the core is 50 microns in diameter, about the

thickness of a human hair. In single-mode fibers, the core is 8 to 10 microns. The core is surrounded

Air/Silica Boundary

α1 α2 α3

Light Source

Light ray inside silica bouncing at

different angles Light trapped by total internal reflection

Total internal

reflection

Silica

52

by a glass cladding with a lower index of refraction than the core, to keep all the light in the core. Next

comes a thin plastic jacket to protect the cladding. Fibers are grouped in bundles, protected by an outer

sheath.

An optical fiber (or optical fibre) is a flexible, transparent fiber made of extruded glass

(silica) or plastic, slightly thicker than a human hair. It can function as a waveguide, or “light pipe”, to

transmit light between the two ends of the fiber.The field of applied science and engineering

concerned with the design and application of optical fibers is known as fiber optics.

Terrestrial fiber sheaths are normally laid in the ground within a meter of the surface, where

they are occasionally subject to attacks by backhoes or gophers. Near the shore, transoceanic fiber

sheaths are buried in trenches by a kind of seaplow. In deep water, they just lie on the bottom, where

they can be snagged by fishing trawlers or attacked by giant squid.

Fibers can be connected in three different ways.

1. First, they can terminate in connectors and be plugged into fiber sockets. Connectors lose about

10 to 20 percent of the light, but they make is easy to reconfigure systems.

2. Second, they can be spliced mechanically. Mechanical splices just lay the two carefully-cut

ends next to each other in s special sleeve and clamp them in place. Alignment can be

improved by passing light through the junction and then making small adjustments to

maximize the signal. Mechanical splices take trained personnel about 5 minutes and result in a

10 percent light loss.

3. Third, two pieces of fiber can be fused (melted) to form a solid connection. A fusion splice is

as good as a single drawn fiber, but a small amount of attenuation occurs. For all three kinds of

splices, reflections can occur at the point of the splice, and the reflected energy can interfere

with the signal.

Two kinds of light sources are used to do the signaling, LEDs (Light Emitting Diodes) and

semiconductor lasers. They have different properties shown below.

Item LED Semiconductor laser

Data rate Low High

Fiber type Multimode Multimode or single mode

Distance Short Long

Lifetime Long life Short life

Temperature sensitivity Minor Substantial

Cost Low cost Expensive

They can be tuned in wavelength by inserting Fabry-perot or Mach-Zehnder interferometers

between the source and the fiber. Fabry-perot interferometers are simple resonant cavities consisting

of two parallel mirrors. The light is incident perpendicular to the mirrors. The length of the cavity

selects out those wavelengths that fit inside an integral number of times. Mach- Zehnder

interferometers separate the light in to two beams. The two beams travel slightly different distances.

They are recombined at the end and are in phase for only certain wavelengths. The receiving

end of an optical fiber consists of a photodiode, which gives off an electrical pulse when struck by

53

mitter

light. The response time of a photodiode is 1 nsec, which limits data rates to about 1 Gbps. Thermal

noise is also an issue, so a pulse of light must carry energy to be detected. By making the pulse

powerful, the error rate can be made arbitrarily small.

Fiber Optic Networks

Fiber optics can be used for LAN’s but the technology becomes complex. The basic problem

is that while vampire taps can be made on fiber LAN’s by fusing the incoming fiber from the

computer with the LAN fiber, the process of making a tap is very tricky and substantial light is lost.

Another way is to treat as a ring network. The interface at each computer passes the light pulse stream

through to the next link and also serves as a T junction to allow the computer to send and accept

messages.

Two types of interfaces are used. A passive interface consists of two taps fused onto the main

fiber. One tap has a LED or laser diode at the end of it for transmitting and the other has a photodiode

for receiving.

The tap itself is completely passive and thus extremely reliable because, a broken LED or

photodiode does not break the ring, it just takes one computer off-line.

The other interface type is the active repeaters. The incoming light is converted into an

electrical signal, regenerated to full strength and retransmitted as light. The interface with the

computer is an ordinary copper wire that comes with the signal regenerator. Purely optical repeaters

are being used, these devices do not require the optical to electrical to optical conversions, which

means they can operate at extremely high bandwidths. If an active repeater fails, the ring is broken

down and the network goes down. On the other hand, the signal is regenerated at each interface, the

individual computer-to-computer links can be kilometers long, with virtually no limit on the total size

of the ring. The Passive interfaces lose light at each junction, so the number of computers and total

ring length are greatly restricted.

A ring topology is not the only way to build a LAN using fiber optics. It is also possible to

have hardware broadcasting using the passive star construction. In this design, each interface

has a fiber running from its transmitter to a silica cylinder, with the incoming fibers fused to one

end of the cylinder. Similarly, fibers fused to the other end of the cylinder are run to each of the

receivers. Whenever an interface emits a light pulse, it is diffused inside the passive star to

illuminate all the receivers, thus achieving broadcast. In effect, the passive star performs a

Boolean OR of all the incoming signals and transmits the result out on all lines. Since the

incoming energy is divided among all the outgoing lines, the number of nodes in the network is

limited by the sensitivity of the photodiodes.

Comparison of Fiber Optics and Copper Wire

FIBER COPPER

It is thin and light weight, two fibers have more

capacity and weight only 100 kg

One thousands twisted pairs 1 km long weight

8000 kg.

It can handle much higher bandwidth than

copper

It can handle less bandwidth than fiber

Direction of light

propagation

54

Due to the low attenuation, repeaters are needed

only about every 50 km on long lines.

Due to the low attenuation, repeaters are needed

only about every 5 km for copper a substantial

cost saving.

It is affected by corrosive chemicals in the air. Copper is affected by power surges,

electromagnetic interference or power failures.

It greatly reduces the need for expensive

mechanical support systems, fiber wins hands

down due to its lower installation cost. Fiber

interfaces cost more than electrical interfaces.

It is inexpensive.

Fibers do not leak light and are quite difficult to

tap. Fibers can be damaged easily by being bent

too much.

The copper has excellent resale value to copper

refiners who see it as very high grade ore.

Fiber excellent security against wiretappers,

since optical transmission is inherently

unidirectional, two-way communication

requires either two fiber or two frequency bands

on one fiber

Copper wire is used for two-way

communications.

Optical Fibre

Three components: light source, transmission system, and a detector

The detector generates an electric pulse when hit by light

1-a pulse of light; 0-missing pulse of light.

optical rays travel in glass or plastic core

When light move from one medium to another it bend at the boundary. The amount of

bending depends on the properties of the media.

Light at shallow angles propagate along the fibre, and those that are less than critical

angle are absorbed in the jacket

The cladding is a glass or plastic with properties that differ from those of the core.

Used in long distance communication, in locations having small amount of space, and

with reduction in price is starting to get also to LANs.

Not affected by external electromagnetic fields, and do not radiate energy. Hence,

providing high degree of security from eavesdropping.

Provide for multimode of propagation at different angles of reflections. Cause signal

elements to spread out in time, which limits the rate in which data can be accurately

received

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Reduction of the radius of the core implies less reflected angles. Single mode is achieved

with sufficient small radius.

A multimode graded index transmission is obtained by varying the index of reflection

of the core to improve on the multi mode option without resolving to the cost of single

mode. (index of reflection=speed in vacuum / speed in medium.)

1 Gbps is the current limitation, with the bottle neck in the conversion from electrical to

optical signals. Large improvements are expected.

Optical fibers are widely used in fiber-optic communications, where they permit

transmission over longer distances and at higher bandwidths (data rates) than wire cables. Fibers

are used instead of metal wires because signals travel along them with less loss and are also

immune to electromagnetic interference. Fibers are also used for illumination, and are wrapped

in bundles so that they may be used to carry images, thus allowing viewing in confined spaces.

Specially designed fibers are used for a variety of other applications, including sensors and fiber

lasers.

Optical fibers typically include a transparent core surrounded by a transparent cladding

material with a lower index of refraction. Light is kept in the core by total internal reflection.

This causes the fiber to act as a waveguide. Fibers that support many propagation paths or

transverse modes are called multi-mode fibers (MMF), while those that only support a single

mode are called single-mode fibers (SMF). Multi-mode fibers generally have a wider core

diameter, and are used for short-distance communication links and for applications where high

power must be transmitted. Single-mode fibers are used for most communication links longer

than 1,000 meters (3,300 ft).

Joining lengths of optical fiber is more complex than joining electrical wire or cable. The

ends of the fibers must be carefully cleaved, and then carefully spliced together with the cores

perfectly aligned. A mechanical splice holds the ends of the fibers together mechanically, while

fusion splicing uses heat to fuse the ends of the fibers together. Special optical fiber connectors

for temporary or semi-permanent connections are also available.

Advantages over copper wiring

The advantages of optical fiber communication with respect to copper wire systems are:

Broad bandwidth

A single optical fiber can carry 3,000,000 full-duplex voice calls or 90,000 TV channels.

Immunity to electromagnetic interference

Light transmission through optical fibers is unaffected by other electromagnetic radiation

nearby.

The optical fiber is electrically non-conductive, so it does not act as an antenna to pick up

electromagnetic signals.

Information traveling inside the optical fiber is immune to electromagnetic interference,

even electromagnetic pulses generated by nuclear devices.

Low attenuation loss over long distances

Attenuation loss can be as low as 0.2 dB/km in optical fiber cables, allowing transmission

over long distances without the need for repeaters.

Electrical insulator

Optical fibers do not conduct electricity, preventing problems with ground loops and

conduction of lightning.

Optical fibers can be strung on poles alongside high voltage power cables.

Material cost and theft prevention

Conventional cable systems use large amounts of copper. In some places, this copper is a

target for theft due to its value on the scrap market.

Media Comparison:

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Due to continually changing technology, the costs and other data in the chart below may

be out of date. Factors in the cost include cost of: installation, modification, maintenance and

support, having a lower throughput (affects productivity), obsolescence, and the type of

connectors required. Factors in the capacity include the maximum size and scalability, the

maximum nodes per segment, the maximum number of segments, maximum segment length, etc.

HOW FIBER OPTICS WORK

You hear about fiber-optic cables whenever people talk about the telephone system, the

cable TV system or the Internet. Fiber-optic lines are strands of optically pure glass as thin as a

human hair that carry digital information over long distances. They are also used in medical

imaging and mechanical engineering inspection.

What are Fiber Optics?

Fiber optics (optical fibers) are long, thin strands of very pure glass about the diameter

of a human hair. They are arranged in bundles called optical cables and used to transmit light

signals over long distances.

Parts of a single optical fiber

If you look closely at a single optical fiber, you will see that it has the following parts:

Core - Thin glass center of the fiber where the light travels

Cladding - Outer optical material surrounding the core that reflects the light back into the

core

Buffer coating - Plastic coating that protects the fiber from damage and moisture

Hundreds or thousands of these optical fibers are arranged in bundles in optical cables. The

bundles are protected by the cable's outer covering, called a jacket.

Optical fibers come in two types:

Single-mode fibers

Multi-mode fibers

Single-mode fibers have small cores (about 3.5 x 10-4 inches or 9 microns in diameter) and

transmit infrared laser light (wavelength = 1,300 to 1,550 nanometers).

Multi-mode fibers have larger cores (about 2.5 x 10-3 inches or 62.5 microns in diameter)

and transmit infrared light (wavelength = 850 to 1,300 nm) from light-emitting diodes (LEDs).

Some optical fibers can be made from plastic. These fibers have a large core (0.04 inches or 1

mm diameter) and transmit visible red light (wavelength = 650 nm) from LEDs.

How Does an Optical Fiber Transmit Light?

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Suppose you want to shine a flashlight beam down a long, straight hallway. Just point the

beam straight down the hallway -- light travels in straight lines, so it is no problem. What if the

hallway has a bend in it? You could place a mirror at the bend to reflect the light beam around

the corner. What if the hallway is very winding with multiple bends? You might line the walls

with mirrors and angle the beam so that it bounces from side-to-side all along the hallway. This

is exactly what happens in an optical fiber.

The light in a fiber-optic cable travels through the core (hallway) by constantly bouncing

from the cladding (mirror-lined walls), a principle called total internal reflection.

Because the cladding does not absorb any light from the core, the light wave can travel

great distances. However, some of the light signal degrades within the fiber, mostly due to

impurities in the glass. The extent that the signal degrades depends on the purity of the glass and

the wavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm =

50 to 60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium optical fibers

show much less signal degradation -- less than 10 percent/km at 1,550 nm.

A fiber-optic relay system

To understand how optical fibers are used in communications systems, let's look at an

example from a World War II movie or documentary where two naval ships in a fleet need to

communicate with each other while maintaining radio silence or on stormy seas. One ship pulls

up alongside the other. The captain of one ship sends a message to a sailor on deck. The sailor

translates the message into Morse code (dots and dashes) and uses a signal light (floodlight with

a venetian blind type shutter on it) to send the message to the other ship. A sailor on the deck of

the other ship sees the Morse code message, decodes it into English and sends the message up to

the captain.

Imagine doing this when the ships are on either side of the ocean separated by thousands of

miles and you have a fiber-optic communication system in place between the two ships. Fiber-

optic relay systems consist of the following:

Transmitter - Produces and encodes the light signals

Optical fiber - Conducts the light signals over a distance

Optical regenerator - May be necessary to boost the light signal (for long distances)

Optical receiver - Receives and decodes the light signals

Transmitter

The transmitter is like the sailor on the deck of the sending ship. It receives and directs

the optical device to turn the light "on" and "off" in the correct sequence, thereby generating a

light signal.

The transmitter is physically close to the optical fiber and may even have a lens to focus

the light into the fiber. Lasers have more power than LEDs, but vary more with changes in

temperature and are more expensive. The most common wavelengths of light signals are 850 nm,

1,300 nm, and 1,550 nm (infrared, non-visible portions of the spectrum).

Optical Regenerator

As mentioned above, some signal loss occurs when the light is transmitted through the

fiber, especially over long distances (more than a half mile, or about 1 km) such as with undersea

cables.

Therefore, one or more optical regenerators is spliced along the cable to boost the

degraded light signals.

An optical regenerator consists of optical fibers with a special coating (doping). The

doped portion is "pumped" with a laser. When the degraded signal comes into the doped coating,

the energy from the laser allows the doped molecules to become lasers themselves. The doped

molecules then emit a new, stronger light signal with the same characteristics as the incoming

weak light signal. Basically, the regenerator is a laser amplifier for the incoming signal.

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Optical Receiver

The optical receiver is like the sailor on the deck of the receiving ship. It takes the

incoming digital light signals, decodes them and sends the electrical signal to the other user's

computer, TV or telephone (receiving ship's captain). The receiver uses a photocell or

photodiode to detect the light.

Advantages of Fiber Optics

Why are fiber-optic systems revolutionizing telecommunications? Compared to conventional

metal wire (copper wire), optical fibers are:

Less expensive

Thinner

Higher carrying capacity

Non-flammable

Lightweight

Flexible

Medical imaging

Mechanical imaging

Plumbing

How Are Optical Fibers Made?

Optical fibers are made of extremely pure optical glass. We think of a glass window as

transparent, but the thicker the glass gets, the less transparent it becomes due to impurities in the

glass. However, the glass in an optical fiber has far fewer impurities than window-pane glass.

One company's description of the quality of glass is as follows: If you were on top of an ocean

that is miles of solid core optical fiber glass, you could see the bottom clearly.

Testing the Finished Optical Fiber

The finished optical fiber is tested for the following:

Tensile strength - Must withstand 100,000 lb/in2 or more

Refractive index profile - Determine numerical aperture as well as screen for optical

defects

Fiber geometry - Core diameter, cladding dimensions and coating diameter are uniform

Attenuation - Determine the extent that light signals of various wavelengths degrade over

distance

Information carrying capacity (bandwidth) - Number of signals that can be carried at one

time (multi-mode fibers)

Chromatic dispersion - Spread of various wavelengths of light through the core

(important for bandwidth)

Operating temperature/humidity range

Temperature dependence of attenuation

Ability to conduct light underwater - Important for undersea cables

Once the fibers have passed the quality control, they are sold to telephone companies, cable

companies and network providers. Many companies are currently replacing their old copper-

wire-based systems with new fiber-optic-based systems to improve speed, capacity and clarity.

Wireless Transmission

Our age has given rise to information junkies: people who need to be online all the time.

For these mobile users, twisted pair, coax, and fiber optics are of no use. They need to get their

‘‘hits’’ of data for their laptop, notebook, shirt pocket,palmtop, or wristwatch computers without

being tethered to the terrestrial communication infrastructure. For these users, wireless

communication is the answer.

In the following sections, we will look at wireless communication in general.It has many

other important applications besides providing connectivity to users who want to surf the Web

from the beach.Wireless has advantages for even fixed devices in some circumstances. For

example, if running a fiber to a building is difficult due to the terrain (mountains, jungles,

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swamps, etc.), wireless may be better. It is noteworthy that modern wireless digital

communication began in the Hawaiian Islands, where large chunks of Pacific Ocean

separated the users from their computer center and the telephone system was inadequate.

Wireless transmission is a form of unguided media. Wireless communication involves no

physical link established between two or more devices, communicating wirelessly. Wireless

signals are spread over in the air and are received and interpret by appropriate antennas.

When an antenna is attached to electrical circuit of a computer or wireless device, it

converts the digital data into wireless signals and spread all over within its frequency range. The

receptor on the other end receives these signals and converts them back to digital data.

A little part of electromagnetic spectrum can be used for wireless transmission.

Electro magnetic Spectrum

When electrons move, they create electromagnetic waves that can propagate through

space (even in a vacuum). These waves were predicted by the British physicist James Clerk

Maxwell in 1865 and first observed by the Germanphysicist Heinrich Hertz in 1887. The number

of oscillations per second of a wave is called its frequency ,f, and is measured in Hz (in honor of

Heinrich Hertz). The distance between two consecutive maxima (or minima) is called the

wavelength , which is universally designated by the Greek letter λ (lambda).

When an antenna of the appropriate size is attached to an electrical circuit, the

electromagnetic waves can be broadcast efficiently and received by a receiver some distance

away. All wireless communication is based on this principle. In a vacuum, all electromagnetic

waves travel at the same speed, no matter what their frequency. This speed, usually called the

speed of light,c, is approxi-mately 3×108m/sec, or about 1 foot (30 cm) per nanosecond. (Acase

could be made for redefining the foot as the distance light travels in a vacuum in 1 nsec rather

than basing it on the shoe size of some long-dead king.) In copper or fiber the speed slows to

about 2/3 of this value and becomes slightly frequency dependent. The speed of light is the

ultimate speed limit. No object or signal can ever move faster than it.

The fundamental relation between f , λ , and c (in a vacuum) is λ f = c (2-4) Since c is a

constant, if we know f , we can find λ , and vice versa. As a rule of thumb, when λ is in meters

and f is in MHz, λ f ∼ ∼ 300. For example, 100-MHz waves are about 3 meters long, 1000-MHz

waves are 0.3 meters long, and 0.1-meter waves have a frequency of 3000 MHz. The

electromagnetic spectrum is shown in Fig.

The radio, microwave,infrared, and visible light portions of the spectrum can all be used

for transmitting information by modulating the amplitude, frequency, or phase of the waves.

Ultraviolet light, X-rays, and gamma rays would be even better, due to their higher frequencies,

but they are hard to produce and modulate, do not propagate well through buildings, and are

dangerous to living things. The bands listed at the bottom of Fig. 2-10 are the official ITU

(International Telecommunication Union) names and are based on the wavelengths, so the LF

band goes from 1 km to 10 km (approximately 30 kHz to 300 kHz). The terms LF, MF, and HF

refer to Low, Medium, and High Frequency, respectively. Clearly, when the names were

assigned nobody expected to go above 10 MHz, so the higher bands were later named the Very,

Ultra, Super, Extremely, and Tremendously High Frequency bands. Beyond that there are no

names, but Incredibly, Astonishingly, and Prodigiously High Frequency (IHF, AHF, and PHF)

would sound nice.

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Fig 2.10 The electromagnetic spectrum and its uses for communication

The electromagnetic spectrum is the range of all possible frequencies of

electromagnetic radiation. The "electromagnetic spectrum" of an object has a different meaning,

and is instead the characteristic distribution of electromagnetic radiation emitted or absorbed by

that particular object.

The electromagnetic spectrum extends from below the low frequencies used for modern

radio communication to gamma radiation at the short-wavelength (high-frequency) end, thereby

covering wavelengths from thousands of kilometers down to a fraction of the size of an atom.

The limit for long wavelengths is the size of the universe itself, while it is thought that the short

wavelength limit is in the vicinity of the Planck length. Until the middle of last century it was

believed by most physicists that this spectrum was infinite and continuous.

Most parts of the electromagnetic spectrum are used in science for spectroscopic and

other probing interactions, as ways to study and characterize matter. In addition, radiation from

various parts of the spectrum has found many other uses for communications and manufacturing.

History of electromagnetic spectrum discovery

For most of history, visible light was the only known part of the electromagnetic

spectrum. The ancient Greeks recognized that light traveled in straight lines and studied some of

its properties, including reflection and refraction. Over the years the study of light continued and

during the 16th and 17th centuries there were conflicting theories which regarded light as either a

wave or a particle.

The first discovery of electromagnetic radiation other than visible light came in 1800,

when William Herschel discovered infrared radiation. He was studying the temperature of

different colors by moving a thermometer through light split by a prism. He noticed that the

highest temperature was beyond red. He theorized that this temperature change was due to

"calorific rays" which would be in fact a type of light ray that could not be seen. The next year,

Johann Ritter worked at the other end of the spectrum and noticed what he called "chemical

rays" (invisible light rays that induced certain chemical reactions) that behaved similar to visible

violet light rays, but were beyond them in the spectrum. They were later renamed ultraviolet

radiation.

Electromagnetic radiation had been first linked to electromagnetism in 1845, when

Michael Faraday noticed that the polarization of light traveling through a transparent material

responded to a magnetic field (see Faraday effect). During the 1860s James Maxwell developed

four partial differential equations for the electromagnetic field. Two of these equations predicted

the possibility of, and behavior of, waves in the field. Analyzing the speed of these theoretical

waves, Maxwell realized that they must travel at a speed that was about the known speed of

light. This startling coincidence in value led Maxwell to make the inference that light itself is a

type of electromagnetic wave.

Maxwell's equations predicted an infinite number of frequencies of electromagnetic

waves, all traveling at the speed of light. This was the first indication of the existence of the

entire electromagnetic spectrum.

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Maxwell's predicted waves included waves at very low frequencies compared to infrared,

which in theory might be created by oscillating charges in an ordinary electrical circuit of a

certain type.

Attempting to prove Maxwell's equations and detect such low frequency electromagnetic

radiation, in 1886 the physicist Heinrich Hertz built an apparatus to generate and detect what is

now called radio waves.

Hertz found the waves and was able to infer (by measuring their wavelength and

multiplying it by their frequency) that they traveled at the speed of light. Hertz also demonstrated

that the new radiation could be both reflected and refracted by various dielectric media, in the

same manner as light. For example, Hertz was able to focus the waves using a lens made of tree

resin. In a later experiment, Hertz similarly produced and measured the properties of

microwaves. These new types of waves paved the way for inventions such as the wireless

telegraph and the radio.

In 1895 Wilhelm Röntgen noticed a new type of radiation emitted during an experiment

with an evacuated tube subjected to a high voltage. He called these radiations x-rays and found

that they were able to travel through parts of the human body but were reflected or stopped by

denser matter such as bones. Before long, many uses were found for them in the field of

medicine.

The last portion of the electromagnetic spectrum was filled in with the discovery of

gamma rays. In 1900 Paul Villard was studying the radioactive emissions of radium when he

identified a new type of radiation that he first thought consisted of particles similar to known

alpha and beta particles, but with the power of being far more penetrating than either. However,

in 1910, British physicist William Henry Bragg demonstrated that gamma rays are

electromagnetic radiation, not particles, and in 1914, Ernest Rutherford (who had named them

gamma rays in 1903 when he realized that they were fundamentally different from charged alpha

and beta rays) and Edward Andrade measured their wavelengths, and found that gamma rays

were similar to X-rays, but with shorter wavelengths and higher frequencies.

Radio Transmission

Radio waves are very easy to generate andcan penetrate buildings easily, can travel long

distances because of which they can be used for communication indoor and outdoor both. Radio

waves travel in all the direction from the source because of which physical alignment of

transmitter and receiver is not important.

The property of radio waves varies as the frequency varies.Radio waves pass through

obstacles well at low frequencies,but the power falls roughly as 1/r2with distance from the

source, in air. Radio waves tend to travel in straight lines at high frequenciesand bounce off

obstacles but are also absorbed by rain. At allfrequencies, radio waves are subject to interference

from motors and other electrical equipment.

In the VLF, LF, and MF bands, radio waves follow the ground, as shown in figure. These

waves can bedetected for perhaps 1000 km at the lower frequencies, less at the higher ones. AM

radio broadcasting uses theMF band, which is why the ground wavescannot be heard easily in

New York from Boston AM radio stations.Radio waves in these bands pass through buildings

easily, that’s why portable radios work indoors. Low bandwidth is the mainproblem for using

these bands for data communication.

In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. (B) In the

HFband, they bounce off the ionosphere.

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In the VHF and HF bands, the ground waves tend to be absorbed by the earth. The waves

that reach the ionosphere, a layer of charged particles circling the earth at a height of 100 to 500

km, are as shown in the figure. Under certain atmospheric conditions, the signals can bounce

several times. Amateur radio operators (hams) use these bands to talk long distance. The military

also communicate in the HF and VHF bands.

Using Radio Signals as a Data Transmission Medium

Now that you've become a little more familiar with what radio signals are and how they

can be used in a public band of frequencies without creating general chaos, let's take a look at

how those radio signals are used to create a network transmission medium.

Computer networks use variations in electrical current to transmit data from one

computer to another. While each type of cable (coaxial, thin coaxial, and unshielded twisted pair)

has its own electrical properties, there is a commonality in how the electrical signals are

transmitted from one network card to another using these media. Using a telephone "line,"

whether analog or digital, adds a little complexity to the process, but not a lot. However, when

using fiber optical cable, which uses light waves as a medium, and radio signals, which use radio

waves as a medium, the process is a bit more complex.

Microwave Radio Transceivers

To create a computer network connection over radio waves, two puzzle pieces are

needed. First, a network device such as a bridge or a router is needed. The network bridge/router

handles the data traffic. It routes the appropriate data signals bound from the computer network

in one building to the network at the other end of the radio connection. Second, a radio

transmitter and receiver, commonly called a transceiver, is required. The radio transceiver

handles the radio signal communications between locations. The interesting part of this marriage

of technologies is that radios have always dealt with electrical signals. The radio transmitter

modulates, or changes, an electrical signal so that its frequency is raised to one appropriate to

radio communications. Then the signal is passed on to a radio antenna. We'll discuss the work of

antennas more in the section "How the Antennas Work."

At the other end of the transmission, the receiving portion of the radio transceiver takes

the radio signal and de-modulates it back to its normal frequency. Then the resulting electrical

signal is passed to the bridge/router side for processing by the network. While the actual process

of modulation/demodulation is technical, the concept of radio transmission is very simple.

Likewise, when a response is sent back to the originating site, the radio transceiver "flips" from

reception mode to transmission mode. The radio transceivers at each end have this characteristic.

Transmit-receive, transmit-receive. They change modes as many as thousands of times per

second. This characteristic leads to a delay in communications called latency. It is idiosyncratic

to radio communications and negatively affects data throughput. See the section "Throughput vs.

Data Rates" below for more information.

Although there is no clear-cut demarcation between radio waves and microwaves,

electromagnetic waves ranging in frequencies between 3 kHz and 1 GHz are normally called

radio waves; waves ranging in frequencies between 1 and 300 GHz are called microwaves.

However, the behavior of the waves, rather than the frequencies, is a better criterion for

classification.

Radio waves, for the most part, are omnidirectional. When an antenna transmits radio

waves, they are propagated in all directions.

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This means that the sending and receiving antennas do not have to be aligned. A sending

antenna sends waves that can be received by any receiving antenna. The omnidirectional

property has a disadvantage, too. The radio waves transmitted by one antenna are susceptible to

interference by another antenna that may send signals using the same frequency or band. Radio

waves,in the sky mode,can travel long distances.

Microwave Transmission

Above 100 MHz, the waves travel in nearly straight lines and can therefore be narrowly

focused. Concentrating all the energy into a small beam by means of a parabolic antenna (like

the familiar satellite TV dish) gives a much higher signal-to-noise ratio, but the transmitting and

receiving antennas must be accurately aligned with each other.

In addition, this directionality allows multiple transmitters lined up in arow to

communicate with multiple receivers in a row without interference, provided some minimum

spacing rules are observed. Before fiber optics, for decades these microwaves formed the heart of

the long-distance telephone transmission system. In fact, MCI, one of AT&T’s first competitors

after it was deregulated, built its entire system with microwave communications passing between

towers tens of kilometers apart. Even the company’s name reflected this (MCI stood for

Microwave Communications, Inc.). MCI has since gone over to fiber and through a long series

of corporate mergers and bankruptcies in the telecommunications shuffle has become part of

Verizon

Microwaves travel in a straight line, so if the towers are too far apart, the earth will get in

the way (think about a Seattle-to-Amsterdam link). Thus, repeaters are needed periodically. The

higher the towers are, the farther apart they can be. The distance between repeaters goes up very

roughly with the square root of the tower height. For 100-meter-high towers, repeaters can be 80

km apart.Unlike radio waves at lower frequencies, microwaves do not pass through buildings

well. In addition, even though the beam may be well focused at the transmitter, there is still some

divergence in space. Some waves may be refracted off low-lying atmospheric layers and may

take slightly longer to arrive than the direct waves.

The delayed waves may arrive out of phase with the direct wave and thus cancel the

signal. This effect is called multipath fading and is often a serious problem. It is weather and

frequency dependent. Some operators keep 10% of their channels idle as spares to switch on

when multipath fading temporarily wipes out some frequency band.

The demand for more and more spectrum drives operators to yet higher frequencies. Bands up to

10 GHz are now in routine use, but at about 4 GHz a new problem sets in: absorption by water.

These waves are only a few centimeters long and are absorbed by rain. This effect would be fine

if one were planning to build a huge outdoor microwave oven for roasting passing birds, but for

communication it is a severe problem. As with multipath fading, the only solution is to shut off

links that are being rained on and route around them.

In summary, microwave communication is so widely used for long-distance telephone

communication, mobile phones, television distribution, and other purposes that a severe shortage

of spectrum has developed. It has several key advantages over fiber. The main one is that no

right of way is needed to lay down cables. By buying a small plot of ground every 50 km and

putting a microwave tower on it, one can bypass the telephone system entirely.

This is how MCI managed to get started as a new long-distance telephone company so

quickly. (Sprint,another early competitor to the deregulated AT&T, went a completely different

route: it was formed by the Southern Pacific Railroad, which already owned a large amount of

right of way and just buried fiber next to the tracks.) Microwave is also relatively inexpensive.

Putting up two simple towers (which can be just big poles with four guy wires) and putting

antennas on each one may be cheaper than burying 50 km of fiber through a congested urban

area or up over a mountain, and it may also be cheaper than leasing the telephone company’s

fiber, especially if the telephone company has not yet even fully paid for the copper it ripped out

when it put in the fiber.

Infrared Transmission

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Unguided infrared waves are widely used for short-range communication.The remote

controls used for televisions, VCRs, and stereos all use infrared communication. They are

relatively directional, cheap, and easy to build but have a major drawback: they do not pass

through solid objects. (Try standing between your remote control and your television and see if it

still works.) In general, as we go from long-wave radio toward visible light, the waves behave

more and more like light and less and less like radio.

On the other hand, the fact that infrared waves do not pass through solid walls well is also

a plus. It means that an infrared system in one room of a building will not interfere with a similar

system in adjacent rooms or buildings: you cannot control your neighbor’s television with your

remote control. Furthermore, security of infrared systems against eavesdropping is better than

that of radio systems precisely for this reason. Therefore, no government license is needed to

operate an infrared system, in contrast to radio systems, which must be licensed outside the ISM

bands.

Infrared communication has a limited use on the desktop, for example, to connect

notebook computers and printers with the IrDA(Infrared Data Association ) standard, but it is not

a major player in the communication game.

Infrared reflectography (fr/it/es), as called by art historians, are taken of paintings to

reveal underlying layers, in particular the underdrawing or outline drawn by the artist as a guide.

This often uses carbon black, which shows up well in reflectograms, as long as it has not also

been used in the ground underlying the whole painting. Art historians are looking to see whether

the visible layers of paint differ from the under-drawing or layers in between – such alterations

are called pentimenti when made by the original artist. This is very useful information in

deciding whether a painting is the prime version by the original artist or a copy, and whether it

has been altered by over-enthusiastic restoration work. In general, the more pentimenti the more

likely a painting is to be the prime version. It also gives useful insights into working practices.

Among many other changes in the Arnolfini Portrait of 1434 (left), the man's face was

originally higher by about the height of his eye; the woman's was higher, and her eyes looked

more to the front. Each of his feet was underdrawn in one position, painted in another, and then

overpainted in a third. These alterations are seen in infra-red reflectograms.

Similar uses of infrared are made by historians on various types of objects, especially

very old written documents such as the Dead Sea Scrolls, the Roman works in the Villa of the

Papyri, and the Silk Road texts found in the Dunhuang Caves. Carbon black used in ink can

show up extremely well.

Biological systems

Thermographic image of a snake eating a mouse Thermographic image of a fruit bat.The

pit viper has a pair of infrared sensory pits on its head. There is uncertainty regarding the exact

thermal sensitivity of this biological infrared detection system.Other organisms that have

thermoreceptive organs are pythons (family Pythonidae), some boas (family Boidae), the

Common Vampire Bat (Desmodus rotundus), a variety of jewel beetles (Melanophila acuminata)

darkly pigmented butterflies (Pachliopta aristolochiae and Troides rhadamantus plateni), and

possibly blood-sucking bugs (Triatoma infestans).

Although near-infrared vision (780–1000 nm) has long been deemed impossible due to

noise in visual pigments, sensation of near-infrared light was reported in the common carp and in

three cichlid species. Fish use NIR to capture prey and for phototactic swimming orientation.

NIR sensation in fish may be relevant under poor lighting conditions during twilight and in turbid

surface waters.

Photobiomodulation

Near-infrared light, or photobiomodulation, is used for treatment of chemotherapy-

induced oral ulceration as well as wound healing. There is some work relating to anti-herpes

virus treatment. Research projects include work on central nervous system healing effects via

cytochrome c oxidase upregulation and other possible mechanisms.

65

Health hazard

Strong infrared radiation in certain industry high-heat settings may be hazard to the eyes,

resulting in damage or blindness to the user. Since the radiation is invisible, special IR-proof

goggles must be worn in such places.

History of infrared science

The discovery of infrared radiation is ascribed to William Herschel, the astronomer, in

the early 19th century. Herschel published his results in 1800 before the Royal Society of

London. Herschel used a prism to refract light from the sun and detected the infrared, beyond the

red part of the spectrum, through an increase in the temperature recorded on a thermometer. He

was surprised at the result and called them "Calorific Rays". The term 'Infrared' did not appear

until late in the 19th century.

Infrared radiation was discovered in 1800 by William Herschel.

1737: Émilie du Châtelet predicted what is today known as infrared radiation in

Dissertation sur la nature et la propagation du feu.

1835: Macedonio Melloni made the first thermopile IR detector.

1860: Gustav Kirchhoff formulated the blackbody theorem .

1873: Willoughby Smith discovered the photoconductivity of selenium.

1879: Stefan-Boltzmann law formulated empirically that the power radiated by a

blackbody is proportional to T4.

1880s & 1890s: Lord Rayleigh and Wilhelm Wien solved part of the blackbody equation,

but both solutions diverged in parts of the electromagnetic spectrum. This problem was

called the "Ultraviolet catastrophe and Infrared Catastrophe".

1901: Max Planck published the blackbody equation and theorem. He solved the problem

by quantizing the allowable energy transitions.

1905: Albert Einstein developed the theory of the photoelectric effect.

1917: Theodore Case developed the thallous sulfide detector; British scientist built the

first infra-red search and track (IRST) device able to detect aircraft at a range of one mile

(1.6 km).

1935: Lead salts – early missile guidance in World War II.

1938: Teau Ta – predicted that the pyroelectric effect could be used to detect infrared

radiation.

1945: The Zielgerät 1229 "Vampir" infrared weapon system was introduced as the first

portable infrared device for military applications.

1952: H. Welker grew synthetic InSb crystals.

1950s: Paul Kruse (at Honeywell) and Texas Instruments recorded infrared images.

1950s and 1960s: Nomenclature and radiometric units defined by Fred Nicodemenus,

G.J. Zissis and R. Clark; Robert Clark Jones defined D*.

1958: W.D. Lawson (Royal Radar Establishment in Malvern) discovered IR detection

properties of HgCdTe.

1958: Falcon and Sidewinder missiles were developed using infrared technology.

1961: J. Cooper demonstrated pyroelectric detection.

1964: W.G. Evans discovered infrared thermoreceptors in a pyrophile beetle.[32]

1965: First IR Handbook; first commercial imagers (Barnes, Agema {now part of FLIR

Systems Inc.}; Richard Hudson's landmark text; F4 TRAM FLIR by Hughes;

phenomenology pioneered by Fred Simmons and A.T. Stair; U.S. Army's night vision lab

formed (now Night Vision and Electronic Sensors Directorate (NVESD), and Rachets

develops detection, recognition and identification modeling there.

1970: Willard Boyle and George E. Smith proposed CCD at Bell Labs for picture phone.

1972: Common module program started by NVESD.

1978: Infrared imaging astronomy came of age, observatories planned, IRTF on Mauna

Kea opened; 32 by 32 and 64 by 64 arrays produced using InSb, HgCdTe and other

materials.

2013: On February 14 researchers developed a neural implant that gives rats the ability to

sense infrared light which for the first time provides living creatures with new abilities,

instead of simply replacing or augmenting existing abilities

66

Lightwave Transmission

Unguided optical signaling or free-space optics has been in use for centuries. Paul Revere

used binary optical signaling from the Old North Church just prior to his famous ride. A more

modern application is to connect the LANs in two buildings via lasers mounted on their rooftops.

Optical signaling using lasers is inherently unidirectional, so each end needs its own laser and its

own photodetector. This scheme offers very high bandwidth at very low cost and is relatively

secure because it is difficult to tap a narrow laser beam. It is also relatively easy to install and,

unlike microwave transmission, does not require an FCC license.

The laser’s strength, a very narrow beam, is also its weakness here.

Aiming a laser beam 1 mm wide at a target the size of a pin head 500 meters away

requires the marksmanship of a latter-day Annie Oakley.

Usually, lenses are put into the system to defocus the beam slightly. To add to the

difficulty, wind and temperature changes can distort the beam and laser beams also cannot

penetrate rain or thick fog, although they normally work well on sunny days. However, many of

these factors are not an issue when the use is to connect two spacecraft.

One of the authors (AST) once attended a conference at a modern hotel in Europe at

which the conference organizers thoughtfully provided a room full of terminals to allow the

attendees to read their email during boring presentations. Since the local PTT was unwilling to

install a large number of telephone lines for just 3 days, the organizers put a laser on the roof and

aimed it at their university’s computer science building a few kilometers away. They tested it the

night before the conference and it worked perfectly. At 9A.M.on a bright, sunny day, the link

failed completely and stayed down all day. The pattern repeated itself the next two days. It was

not until after the conference that the organizers discovered the problem: heat from the sun

during the daytime caused convection currents to rise up from the roof of the building, as shown

in Fig.

This turbulent air diverted the beam and made it dance around the detector, much like a

shimmering road on a hot day. The lesson here is that to work well in difficult conditions as well

as good conditions, unguided optical links need to be engineered with a sufficient margin of error

Unguided optical communication may seem like an exotic networking technology today, but it

might soon become much more prevalent. We are surrounded by cameras (that sense light) and

displays (that emit light using LEDs and other technology). Data communication can be layered

on top of these displays by encoding information in the pattern at which LEDs turn on and off

that is below the threshold of human perception. Communicating with visible light in this way is

inherently safe and creates a low-speed network in the immediate vicinity of the display. This

could enable all sorts of fanciful ubiquitous computing scenarios.

The flashing lights on emergency vehicles might alert nearby traffic lights and vehicles to help

clear a path. Informational signs might broadcast maps. Even festive lights might broadcast

songs that are synchronized with their display.

Communication Satellites

67

In the 1950s and early 1960s, people tried to set up communication systems by bouncing

signals off metallized weather balloons. The received signals were too weak so the U.S navy

noticed a kind of permanent weather balloon in the sky-the moon-and built an operation system for

ship-to-shore communication by bouncing signals off it. The celestial communication field had to

wait until the first communication satellite was launched. The key difference between an artificial

satellite and a real one is that the artificial one can amplify the signals before sending them back,

turning a strange curiosity in to powerful communication system.

Communication satellites have some interesting properties that make them attractive for

certain applications. A communication satellite can be thought of as a big microwave repeater in

the sky.

It contains several transponders, each of which listens to some portion of the spectrum,

amplifies the incoming signal, and then rebroadcasts it at another frequency, to avoid interference

with the incoming signal.

The downward beams can be broad, covering a substantial fraction of the earth's surface,

or narrow, covering an area only hundreds of kilometers in diameter. This mode of operation is

known as a bent pipe.

According to Kepler’s law, the orbital period of a satellite varies as the radius of the orbit

to the 3/2 power. The higher the satellite, the longer the period. Near the surface of the earth, the

period is about 90 minutes. Low-orbit satellites pass out of view fairly quickly, so many of them

are needed to provide continuous coverage. A satellite’s period is important, but it is not only issue

in determining where to place it. Another issue is the presence of the Van Allen belts, layers of

highly charged particles trapped by the earth’s magnetic field. Any satellite flying with in them

would be destroyed fairly quickly by the highly-energetic charged particles trapped there by the

earth’s magnetic field. These factors lead to three regions in which satellite can be placed safely.

They are: -

1) Geostationary satellites

2) Medium-Earth Orbit satellites

3) Low-Earth Orbit Satellites

Fig: -Communication satellites and some of their properties, includes altitude above the earth,

round-trip delay time, and number of satellites needed for global coverage.

Geostationary Satellites

In 1045, the science fiction writer Authur C.Clarke calculated that a satellite at an altitude

of 35,800 km in a circular equatorial orbit would appear to remain motionless in the sky. So it can

be tracked. Complete communication system that used these (manned) Geostationary Satellites

including the orbits, solar panels, radio frequencies, and launch procedures. Unfortunately he

35,000

30,000

25,000

20,000

15,000

10,000

5,000

0

Attitude (km) Type Latency (ms) Sats needed

Upper Van Allen belt

Lower Van Allen belt

GEO

MEO

LEO

270 3

35-85 10

1-7 50

68

concluded that satellites is impractical that due to the impossibility of putting power-hungry,

fragile, vacuum tube amplifiers into orbit.

The invention of the transistor changed all that and the first artificial communication

satellite, Telstar, was launched in July 1962. Communication satellites have become a multibillion

dollar business and the only aspect of outer space that has become highly profitable. These high-

flying satellites are called GEO (Geostationary Earth Orbit) satellites. With the current technology

Geostationary satellites spaced much closer than 2 degrees in the 360-degree equatorial plane, to

avoid interference. With a spacing of 2 degrees, there can only be 360/2=180 of these satellites in

the sky at once. Each transponder can use multiple frequencies and polarizations to increase the

available bandwidth. To prevent total chaos in the sky, orbit slot allocation is done by ITU. This

process is highly political, with countries barely out of the stone age demanding their orbits slots.

Maintain that national property rights do not extend up to the moon and that no country has a legal

right to the orbit slots above its territory. Modern satellites can be quite large, weighting up to 4000

kg and several kilowatts of electric power produced by the solar panels. The effects of solar, lunar,

and planetary gravity tend to move them away from their assigned slots and orientations, an effect

countered by on-board rocket motors. This fine-tuning activity is called station keeping. When the

fuel for the motors has been exhausted in about 10 years, the satellite drifts and tumbles helplessly,

so it has to be turned off. The orbit decays and the satellite reenters the atmosphere and burns up or

occasionally crashes to earth.

Orbit slots are not only bone of contention. Frequencies are too, because the downlink

transmissions interfere with existing microwave users. ITU has allocated certain frequency bands

to satellite users. The C band was the first to be designated for commercial satellite traffic. Two

frequency ranges are assigned in it, the lower one for Downlink traffic (from the satellite) and the

upper one for Uplink traffic (to the satellite).

To allow traffic to go both ways at the same time, two channels are required, one going each way.

These bands are already overcrowded because they are also by the common carriers for terrestrial

microwave links. The L and S bands were added by international agreement in 2000. They are

narrow and crowded. The next highest available to commercial telecommunication carriers is the

Ku (K under) band.

This band is not congested, and at these frequencies, satellites can be spaced as close as 1 degree.

Another problem exists: Rain. Water is an excellent absorber of these short microwaves. Heavy

storms are usually localized, so using several widely separated ground stations but at the price of

extra antennas, extra cables, and extra electronics to enable rapid switching between stations.

Bandwidth has been allocated in the Ka (K above) band for commercial satellite traffic, but the

equipment used is expensive. A modern satellite has around 40 transponders, each with an 80-

MHz bandwidth. Each transponder operates as a bent pipe, but recent satellites have some on-

board processing capacity, allowing more sophisticated operation. In the earliest satellites, the

division of the transponders into channels was static: the bandwidth was split up into fixed

frequency bands. Each transponder beam is divided into time slots, with various users taking turns.

Band Downlink Uplink Bandwidth Problems

L 1.5 GHz 1.6 GHz 15 MHz Low bandwidth; crowded

Software 1.9 GHz 2.2 GHz 70 MHz Low bandwidth; crowded

C 4.0 GHz 6.0 GHz 500 MHz Terrestrial interference

Ku 11 GHz 14 GHz 500 MHz Rain

Ka 20 GHz 30 GHz 3500 MHz Rain, equipment cost

The first geostationary satellites had a single spatial beam that illuminated about 1/3 of the earth’s

surface called its footprint.

With the enormous decline in the price, size, and power requirements of microelectronics, a much

more sophisticated broadcasting strategy has become possible. Each satellite is equipped with

multiple antennas and multiple transponders. Each downward beam can be focused on a small

geographical area, so multiple upward and downward transmissions can take place simultaneously.

These so-called spot beams are elliptically shaped, and can be as small as a few hundred km in

69

diameter. A communication satellite for the United States has one wide beam for the contiguous 48

states, plus spot beams for Alaska and Hawaii.

A new development in the communication satellite world is the development of low-cost

microstations called VSATs (Very Small Aperture Terminals). These tiny terminals have 1-

meter or smaller antennas versus 10 m for a standard GEO antenna and can put out about 1 watt of

power. The uplink is good for 19.2 kbps, but the downlink is more 512 kbps or more. Direct

broadcast satellite television uses this technology for one-way transmission. In many VSAT

systems, the microstations do not have enough power to communicate directly with one another.

A special ground station, the hub, with a large high-gain antenna is needed to relay traffic

between VSATs.

In this mode of operation, either the sender or the receiver has a large antenna and a

powerful amplifier. The trade-off is a longer delay in return for having cheaper end-user stations.

VSATs have great potential in rural areas. Another important property of satellite is broadcast

media. It does not cost more to send a message to thousands of stations within a transponder’s

footprint than it does to send to one. On the other hand, from a security and privacy point of view,

satellites are a complete disaster; everybody can hear everything. Encryption is essential when

security is required. Satellites also have the property that the cost of transmitting a message is

independent of the distance traversed. A call across the ocean costs no more to service than a call

across the street. A major consideration for military communication.

Medium-Earth Orbit Satellites

At much lower altitudes, between the two Van Allen belts, the MEO Satellites. As viewed

from the earth, these drift slowly in longitude, taking something like 6 hours to circle the earth.

They must be tracked as they move through the sky. Because they are lowest than the

GEOs they have a smaller footprint on the ground and require less powerful transmitters to reach

them.

The 24 GPS (Global Positioning System) satellites orbiting at about 18,000 km are

examples of MEO satellites.

Low-Earth Orbit Satellites

Moving down in altitude, we come to the LEO ( Low-Earth Orbit ) satellites. Due to their

rapid motion, large numbers of them are needed for a complete system. On the other

hand,because the satellites are so close to the earth, the ground stations do not need much power,

and the round-trip delay is only a few milliseconds. The launch cost is substantially cheaper too.

In this section we will examine two examples of satellite constellations for voice service, Iridium

and Globalstar.

For the first 30 years of the satellite era,low-orbit satellites were rarely used because they

zip into and out of view so quickly. In 1990, Motorola broke new ground by filing an application

with the FCC asking for permission to launch 77 low-orbit satellites for the Iridium project

70

(element 77 is iridium). The plan was later revised to use only 66 satellites, so the project should

have been renamed Dysprosium (element 66), but that probably sounded too much like a disease.

The idea was that as soon as one satellite went out of view, another would replace it.

This proposal set off a feeding frenzy among other communication companies. All of a

sudden, everyone wanted to launch a chain of low-orbit satellites.

After seven years of cobbling together partners and financing, communication service

began in November 1998. Unfortunately, the commercial demand for large, heavy satellite

telephones was negligible because the mobile phone network had grown in a spectacular way

since 1990. As a consequence, Iridium was not profitable and was forced into bankruptcy in

August 1999 in one of the most spectacular corporate fiascos in history.

The satellites and other assets (worth $5 billion) were later purchased by an investor for

$25 million at a kind of extraterrestrial garage sale. Other satellite business ventures promptly

followed suit. The Iridium service restarted in March 2001 and has been growing ever since. It

provides voice, data, paging, fax, and navigation service everywhere on land,air, and sea, via

hand-held devices that communicate directly with the Iridium satellites.

Customers include the maritime,aviation, and oil exploration industries, as well as people

traveling in parts of the world lacking a telecom infrastructure (e.g., deserts, mountains, the

South Pole, and some Third World countries).The Iridium satellites are positioned at an altitude

of 750 km, in circular polar orbits. They are arranged in north-south necklaces, with one satellite

every 32 degrees of latitude, as shown in Fig. Each satellite has a maximum of 48 cells (spot

beams) and a capacity of 3840 channels, some of which are used for paging and navigation,

while others are used for data and voice.

With six satellite necklaces the entire earth is covered, as suggested by Fig.An interesting

property of Iridium is that communication between distant customers takes place in space, as

shown in Fig. Here we see a caller at the North Pole contacting a satellite directly overhead.

Each satellite has four neighbors with which it can communicate, two in the same necklace

(shown) and two in adjacent necklace s (not shown). The satellites relay the call across this grid

until it is finally sent down to the callee at the South Pole.

An alternative design to Iridium is Globalstar . It is based on 48 LEO satellites but uses a

different switching scheme than that of Iridium. Whereas Iridium relays calls from satellite to

satellite, which requires sophisticated switching equipment in the satellites, Globalstar uses a

traditional bent-pipe design.

71

The call originating at the North Pole in Fig. is sent back to earth and picked up by the

large ground station at Santa’s Workshop. The call is then routed via a terrestrial network to the

ground station nearest the callee and delivered by a bent-pipe connection as shown.

The advantage of this scheme is that it puts much of the complexity on the ground, where

it is easier to manage.

Also, the use of large ground station antennas that can put out a powerful signal and

receive a weak one means that lower-powered telephones can be used. After all, the telephone

puts out only a few milliwatts of power, so the signal that gets back to the ground station is fairly

weak, even after having been amplified by the satellite.Satellites continue to be launched at a rate

of around 20 per year, including ever-larger satellites that now weigh over 5000 kilograms. But

there are also very small satellites for the more budget- conscious organization.

To make space research more accessible,academics from CalPoly and Stanford got

together in 1999 to define a standard for miniature satellites and an associated launcher that

would greatly lower launch costs (Nugent et al., 2008).CubeSats are satellites in units of 10 cm × 10 cm × 10 cm cubes, each weighing no more than 1 kilogram, that can be launched for as little

as $40,000 each. The launcher flies as a secondary payload on commercial space missions. It is

basically a tube that takes up to three units of cubesats and uses springs to release them into

orbit. Roughly 20 cubesats have launched so far, with many more in the works. Most of them

communicate with ground stations on the UHF and VHF bands.

Satellite versus Fiber

A comparison between satellite communication and terrestrial communication is

instructive. As recently as 25 years ago, a case could be made that the future of communication

lay with communication satellites. had changed little in the previous 100 years and showed no

signs of changing in the next 100 years. This glacial movement was caused in no small part by

the regulatory environment in which the telephone companies were expected to provide good

voice service at reasonable prices (which they did), and in return got a guaranteed profit on their

investment. For people with data to transmit, 1200-bps modems were available. That was pretty

much all there was.The introduction of competition in 1984 in the United States and somewhat

later in Europe changed all that radically.

Telephone companies began replacing their long-haul networks with fiber and introduced

high-bandwidth services like ADSL (Asymmetric Digital Subscriber Line). They also stopped

their long-time practice of charging artificially high prices to long-distance users to subsidize

local service. All of a sudden, terrestrial fiber connections looked like the winner. Nevertheless,

communication satellites have some major niche markets that fiber does not (and, sometimes,

cannot) address. First, when rapid deployment is critical, satellites win easily. A quick response

is useful for military communication systems in times of war and disaster response in times of

peace.

Following the massive December 2004 Sumatra earthquake and subsequent tsunami, for

example, communications satellites were able to restore communications to first responders

within 24 hours. This rapid response was possible because there is a developed satellite service

72

provider market in which large players, such as Intelsat with over 50 satellites, can rent out

capacity pretty much anywhere it is needed.

For customers served by existing satellite networks, a VSAT can be set up easily and

quickly to provide a megabit/sec link to elsewhere in the world.

A second niche is for communication in places where the terrestrial infra structure is

poorly developed. Many people nowadays want to communicate everywhere they go. Mobile

phone networks cover those locations with good population density, but do not do an adequate

job in other places (e.g., at sea or in the desert).

Conversely, Iridium provides voice service everywhere on Earth, even at the South Pole.

Terrestrial infrastructure can also be expensive to install,depending on the terrain and necessary

rights of way. Indonesia, for example, has its own satellite for domestic telephone traffic.

Launching one satellite was cheaper than stringing thousands of undersea cables among the

13,677 islands in the archipelago. A third niche is when broadcasting is essential.

A message sent by satellite can be received by thousands of ground stations at once.

Satellites are used to distribute much network TV programming to local stations for this reason.

There is now a large market for satellite broadcasts of digital TV and radio directly to end users

with satellite receivers in their homes and cars. All sorts of other content can be broadcast too.

For example, an organization transmitting a stream of stock, bond, or commodity prices to

thousands of dealers might find a satellite system to be much cheaper than simulating

broadcasting on the ground. In short, it looks like the mainstream communication of the future

will be terrestrial fiber optics combined with cellular radio, but for some specialized uses,

satellites are better.

However, there is one caveat that applies to all of this: economics. Although fiber offers

more bandwidth, it is conceivable that terrestrial and satellite communication could compete

aggressively on price. If advances in technology radically cut the cost of deploying a satellite

(e.g.,if some future space vehicle can toss out dozens of satellites on one launch) or low-orbit

satellites catch on in a big way, it is not certain that fiber will win all markets.

HUB

An Ethernet hub, active hub, network hub, repeater hub, multiport repeater or hub

is a device for connecting multiple Ethernet devices together and making them act as a single

network segment. It has multiple input/output (I/O) ports, in which a signal introduced at the

input of any port appears at the output of every port except the original incoming. A hub works

at the physical layer (layer 1) of the OSI model.[2] Repeater hubs also participate in collision

detection, forwarding a jam signal to all ports if it detects a collision. In addition to standard

8P8C ("RJ45") ports, some hubs may also come with a BNC and/or Attachment Unit Interface

(AUI) connector to allow connection to legacy 10BASE2 or 10BASE5 network segments.

Hubs are now largely obsolete, having been replaced by network switches except in very

old installations or specialized applications.

Technical information

Physical layer function

A network hub is an unsophisticated device in comparison with a switch. As a multiport

repeater it works by repeating bits (symbols) received from one of its ports to all other ports. It is

aware of physical layer packets, that is it can detect their start (preamble), an idle line

(interpacket gap) and sense a collision which it also propagates by sending a jam signal. A hub

cannot further examine or manage any of the traffic that comes through it: any packet entering

any port is rebroadcast on all other ports.[3] A hub/repeater has no memory to store any data in –

a packet must be transmitted while it is received or is lost when a collision occurs (the sender

should detect this and retry the transmission). Due to this, hubs can only run in half duplex mode.

Consequently, due to a larger collision domain, packet collisions are more frequent in networks

connected using hubs than in networks connected using more sophisticated devices.[2]

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Connecting multiple hubs

The need for hosts to be able to detect collisions limits the number of hubs and the total

size of a network built using hubs (a network built using switches does not have these

limitations). For 10 Mbit/s networks built using repeater hubs, the 5-4-3 rule must be followed:

up to 5 segments (4 hubs) are allowed between any two end stations.[3] For 10BASE-T networks,

up to five segments and four repeaters are allowed between any two hosts.[4] For 100 Mbit/s

networks, the limit is reduced to 3 segments (2 hubs) between any two end stations, and even

that is only allowed if the hubs are of Class II. Some hubs have manufacturer specific stack ports

allowing them to be combined in a way that allows more hubs than simple chaining through

Ethernet cables, but even so, a large Fast Ethernet network is likely to require switches to avoid

the chaining limits of hubs.[2]

Additional functions

Most hubs detect typical problems, such as excessive collisions and jabbering on

individual ports, and partition the port, disconnecting it from the shared medium. Thus, hub-

based twisted-pair Ethernet is generally more robust than coaxial cable-based Ethernet (e.g.

10BASE2), where a misbehaving device can adversely affect the entire collision domain.[3] Even

if not partitioned automatically, a hub simplifies troubleshooting because hubs remove the need

to troubleshoot faults on a long cable with multiple taps; status lights on the hub can indicate the

possible problem source or, as a last resort, devices can be disconnected from a hub one at a time

much more easily than from a coaxial cable.

To pass data through the repeater in a usable fashion from one segment to the next, the

framing and data rate must be the same on each segment. This means that a repeater cannot

connect an 802.3 segment (Ethernet) and an 802.5 segment (Token Ring) or a 10 Mbit/s segment

to 100 Mbit/s Ethernet.

Fast Ethernet classes

100 Mbit/s hubs and repeaters come in two different speed grades: Class I delay the

signal for a maximum of 140 bit times (enabling translation/recoding between 100Base-TX,

100Base-FX and 100Base-T4) and Class II hubs delay the signal for a maximum of 92 bit times

(enabling installation of two hubs in a single collision domain).[5]

Dual-speed hub

In the early days of Fast Ethernet, Ethernet switches were relatively expensive devices.

Hubs suffered from the problem that if there were any 10BASE-T devices connected then the

whole network needed to run at 10 Mbit/s.

Therefore a compromise between a hub and a switch was developed, known as a dual-

speed hub. These devices make use of an internal two-port switch, bridging the 10 Mbit/s and

100 Mbit/s segments.

The device would typically consist of more than two physical ports. When a network

device becomes active on any of the physical ports, the device attaches it to either the 10 Mbit/s

segment or the 100 Mbit/s segment, as appropriate. This obviated the need for an all-or-nothing

migration to Fast Ethernet networks. These devices are considered hubs because the traffic

between devices connected at the same speed is not switched.

Gigabit Ethernet hubs

Repeater hubs have been defined for Gigabit Ethernet but commercial products have

failed to appear due to the industry's transition to switching.

74

Uses

Historically, the main reason for purchasing hubs rather than switches was their price.

This motivator has largely been eliminated by reductions in the price of switches, but hubs can

still be useful in special circumstances:

For inserting a protocol analyzer into a network connection, a hub is an alternative to a

network tap or port mirroring.

When a switch is accessible for end users to make connections, for example, in a

conference room, an inexperienced or careless user (or saboteur) can bring down the

network by connecting two ports together, causing a switching loop. This can be

prevented by using a hub, where a loop will break other users on the hub, but not the rest

of the network (more precisely, it will break the current collision domain up to the next

switch/bridge port). This hazard can also be avoided by using switches that can detect

and deal with loops, for example by implementing the spanning tree protocol.A hub with

a 10BASE2 port can be used to connect devices that only support 10BASE2 to a modern

network.

A hub with an AUI port can be used to connect to a 10BASE5 network.

Network switch

A network switch is a computer networking device that connects devices together on a

computer network, by using a form of packet switching to forward data to the destination device.

A network switch is considered more advanced than a (repeater) hub because a switch

will only forward a message to one or multiple devices that need to receive it, rather than

broadcasting the same message out of each of its ports.

A network switch (also called switching hub, bridging hub, officially MAC bridge) is

a multi-port network bridge that processes and forwards data at the data link layer (layer 2) of the

OSI model. Switches can also incorporate routing in addition to bridging; these switches are

commonly known as layer-3 or multilayer switches. Switches exist for various types of networks

including Fibre Channel, Asynchronous Transfer Mode, InfiniBand, Ethernet and others. The

first Ethernet switch was introduced by Kalpana in 1990.

Overview

Cisco small business SG300-28 28-port Gigabit Ethernet rackmount switch and its

internals

A switch is a device used on a computer network to physically connect devices together.

Multiple cables can be connected to a switch to enable networked devices to communicate with

each other. Switches manage the flow of data across a network by only transmitting a received

message to the device for which the message was intended. Each networked device connected to

a switch can be identified using a MAC address, allowing the switch to regulate the flow of

traffic. This maximises security and efficiency of the network.

Because of these features, a switch is often considered more "intelligent" than a network

hub. Hubs neither provide security, or identification of connected devices. This means that

messages have to be transmitted out of every port of the hub, greatly degrading the efficiency of

the network.

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Network design

An Ethernet switch operates at the data link layer of the OSI model to create a separate

collision domain for each switch port. Each computer connected to a switch port can transfer

data to any of the other ones at a time, and the transmissions will not interfere with the limitation

that, in half duplex mode, each line can only either receive from or transmit to its connected

computer at a certain time. In full duplex mode, each line can simultaneously transmit and

receive, regardless of the partner.

In the case of using a repeater hub, only a single transmission could take place at a time

for all ports combined, so they would all share the bandwidth and run in half duplex. Necessary

arbitration would also result in collisions requiring retransmissions.

Applications

The network switch plays an integral part in most modern Ethernet local area networks

(LANs). Mid-to-large sized LANs contain a number of linked managed switches. Small

office/home office (SOHO) applications typically use a single switch, or an all-purpose

converged device such as a residential gateway to access small office/home broadband services

such as DSL or cable Internet. In most of these cases, the end-user device contains a router and

components that interface to the particular physical broadband technology. User devices may

also include a telephone interface for Voice over IP (VoIP) protocol.

Microsegmentation

Segmentation is the use of a bridge or a switch (or a router) to split a larger collision

domain into smaller ones in order to reduce collision probability and improve overall throughput.

In the extreme, i. e. microsegmentation, each device is located on a dedicated switch port. In

contrast to an Ethernet hub, there is a separate collision domain on each of the switch ports. This

allows computers to have dedicated bandwidth on point-to-point connections to the network and

also to run in full-duplex without collisions. Full-duplex mode has only one transmitter and one

receiver per 'collision domain', making collisions impossible.

Role of switches in a network

Switches may operate at one or more layers of the OSI model, including the data link and

network layers. A device that operates simultaneously at more than one of these layers is known

as a multilayer switch.

In switches intended for commercial use, built-in or modular interfaces make it possible

to connect different types of networks, including Ethernet, Fibre Channel, ATM, ITU-T G.hn

and 802.11. This connectivity can be at any of the layers mentioned. While layer-2 functionality

is adequate for bandwidth-shifting within one technology, interconnecting technologies such as

Ethernet and token ring is easier at layer 3.

Devices that interconnect at layer 3 are traditionally called routers, so layer-3 switches

can also be regarded as (relatively primitive) routers.

Where there is a need for a great deal of analysis of network performance and security,

switches may be connected between WAN routers as places for analytic modules. Some vendors

provide firewall, network intrusion detection, and performance analysis modules that can plug

into switch ports. Some of these functions may be on combined modules.

In other cases, the switch is used to create a mirror image of data that can go to an external

device. Since most switch port mirroring provides only one mirrored stream, network hubs can

be useful for fanning out data to several read-only analyzers, such as intrusion detection systems

and packet sniffers.

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Layer-specific functionality

A modular network switch with three network modules (a total of 24 Ethernet and 14

Fast Ethernet ports) and one power supply.

While switches may learn about topologies at many layers, and forward at one or more

layers, they do tend to have common features. Other than for high-performance applications,

modern commercial switches use primarily Ethernet interfaces.

At any layer, a modern switch may implement power over Ethernet (PoE), which avoids

the need for attached devices, such as a VoIP phone or wireless access point, to have a separate

power supply. Since switches can have redundant power circuits connected to uninterruptible

power supplies, the connected device can continue operating even when regular office power

fails.

Layer 1 (Hubs versus higher-layer switches)

A network hub, or repeater, is a simple network device. Repeater hubs do not manage any

of the traffic that comes through them. Any packet entering a port is flooded out or "repeated" on

every other port, except for the port of entry. Since every packet is repeated on every other port,

packet collisions affect the entire network, limiting its capacity.

A switch creates the – originally mandatory – Layer 1 end-to-end connection only

virtually. Its bridge function selects which packets are forwarded to which port(s) on the basis of

information taken from layer 2 (or higher), removing the requirement that every node be

presented with all data. The connection lines are not "switched" literally, it only appears like this

on the packet level. "Bridging hub", "switching hub", or "multiport bridge" would be more

appropriate terms.

There are specialized applications where a hub can be useful, such as copying traffic to

multiple network sensors. High end switches have a feature which does the same thing called

port mirroring.

By the early 2000s, there was little price difference between a hub and a low-end switch.

Layer 2

A network bridge, operating at the data link layer, may interconnect a small number of

devices in a home or the office. This is a trivial case of bridging, in which the bridge learns the

MAC address of each connected device.

Single bridges also can provide extremely high performance in specialized applications

such as storage area networks.

Classic bridges may also interconnect using a spanning tree protocol that disables links so

that the resulting local area network is a tree without loops. In contrast to routers, spanning tree

bridges must have topologies with only one active path between two points. The older IEEE

802.1D spanning tree protocol could be quite slow, with forwarding stopping for 30 seconds

while the spanning tree reconverged. A Rapid Spanning Tree Protocol was introduced as IEEE

802.1w. The newest standard Shortest path bridging (IEEE 802.1aq) is the next logical

progression and incorporates all the older Spanning Tree Protocols (IEEE 802.1D STP, IEEE

802.1w RSTP, IEEE 802.1s MSTP) that blocked traffic on all but one alternative path. IEEE

802.1aq (Shortest Path Bridging SPB) allows all paths to be active with multiple equal cost

paths, provides much larger layer 2 topologies (up to 16 million compared to the 4096 VLANs

limit), faster convergence, and improves the use of the mesh topologies through increase

bandwidth and redundancy between all devices by allowing traffic to load share across all paths

of a mesh network.[11][12][13][14]

While layer 2 switch remains more of a marketing term than a technical term,[citation needed]

the products that were introduced as "switches" tended to use microsegmentation and Full duplex

to prevent collisions among devices connected to Ethernet. By using an internal forwarding plane

77

much faster than any interface, they give the impression of simultaneous paths among multiple

devices. 'Non-blocking' devices use a forwarding plane or equivalent method fast enough to

allow full duplex traffic for each port simultaneously.

Once a bridge learns the addresses of its connected nodes, it forwards data link layer

frames using a layer 2 forwarding method. There are four forwarding methods a bridge can use,

of which the second through fourth method were performance-increasing methods when used on

"switch" products with the same input and output port bandwidths:

1. Store and forward: The switch buffers and verifies each frame before forwarding it.

2. Cut through: The switch reads only up to the frame's hardware address before starting to

forward it. Cut-through switches have to fall back to store and forward if the outgoing

port is busy at the time the packet arrives. There is no error checking with this method.

3. Fragment free: A method that attempts to retain the benefits of both store and forward

and cut through. Fragment free checks the first 64 bytes of the frame, where addressing

information is stored. According to Ethernet specifications, collisions should be detected

during the first 64 bytes of the frame, so frames that are in error because of a collision

will not be forwarded. This way the frame will always reach its intended destination.

Error checking of the actual data in the packet is left for the end device.

4. Adaptive switching: A method of automatically selecting between the other three modes.

While there are specialized applications, such as storage area networks, where the input

and output interfaces are the same bandwidth, this is not always the case in general LAN

applications. In LANs, a switch used for end user access typically concentrates lower bandwidth

and uplinks into a higher bandwidth.

Layer 3

Within the confines of the Ethernet physical layer, a layer-3 switch can perform some or

all of the functions normally performed by a router. The most common layer-3 capability is

awareness of IP multicast through IGMP snooping. With this awareness, a layer-3 switch can

increase efficiency by delivering the traffic of a multicast group only to ports where the attached

device has signaled that it wants to listen to that group.

Layer 4

While the exact meaning of the term layer-4 switch is vendor-dependent, it almost always

starts with a capability for network address translation, but then adds some type of load

distribution based on TCP sessions.[15]

The device may include a stateful firewall, a VPN concentrator, or be an IPSec security gateway.

Layer 7

Layer-7 switches may distribute loads based on Uniform Resource Locator URL or by

some installation-specific technique to recognize application-level transactions. A layer-7 switch

may include a web cache and participate in a content delivery network.[16]

Types of switches

Form factor

Desktop, not mounted in an enclosure, typically intended to be used in a home or office

environment outside of a wiring closet.

Rack-mounted, a switch that mounts in an equipment rack.

Chassis, with swappable module cards.

DIN rail–mounted, normally seen in industrial environments.

Configuration options

Unmanaged switches – these switches have no configuration interface or options. They

are plug and play. They are typically the least expensive switches, and therefore often

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used in a small office/home office environment. Unmanaged switches can be desktop or

rack mounted.

Managed switches – these switches have one or more methods to modify the operation

of the switch. Common management methods include: a command-line interface (CLI)

accessed via serial console, telnet or Secure Shell, an embedded Simple Network

Management Protocol (SNMP) agent allowing management from a remote console or

management station, or a web interface for management from a web browser. Examples

of configuration changes that one can do from a managed switch include: enabling

features such as Spanning Tree Protocol or port mirroring, setting port bandwidth,

creating or modifying Virtual LANs (VLANs), etc. Two sub-classes of managed switches

are marketed today:

o Smart (or intelligent) switches – these are managed switches with a limited set

of management features. Likewise "web-managed" switches are switches which

fall into a market niche between unmanaged and managed. For a price much

lower than a fully managed switch they provide a web interface (and usually no

CLI access) and allow configuration of basic settings, such as VLANs, port-

bandwidth and duplex.[17]

o Enterprise Managed (or fully managed) switches – these have a full set of

management features, including CLI, SNMP agent, and web interface. They may

have additional features to manipulate configurations, such as the ability to

display, modify, backup and restore configurations. Compared with smart

switches, enterprise switches have more features that can be customized or

optimized, and are generally more expensive than smart switches. Enterprise

switches are typically found in networks with larger number of switches and

connections, where centralized management is a significant savings in

administrative time and effort. A stackable switch is a version of enterprise-

managed switch.

HP Procurve rack-mounted switches mounted in a standard Telco Rack 19-inch rack with

network cables

Turn particular port range on or off

Link bandwidth and duplex settings

Priority settings for ports

IP Management by IP Clustering

MAC filtering and other types of "port security" features which prevent MAC flooding

Use of Spanning Tree Protocol

SNMP monitoring of device and link health

Port mirroring (also known as: port monitoring, spanning port, SPAN port, roving

analysis port or link mode port)

Link aggregation (also known as bonding, trunking or teaming) allows the use of multiple

ports for the same connection achieving higher data transfer rates

VLAN settings. Creating VLANs can serve security and performance goals by reducing

the size of the broadcast domain

802.1X network access control

IGMP snooping

Traffic monitoring on a switched network

Unless port mirroring or other methods such as RMON, SMON or Flow are implemented

in a switch, it is difficult to monitor traffic that is bridged using a switch because only the

sending and receiving ports can see the traffic. These monitoring features are rarely present on

consumer-grade switches.

Two popular methods that are specifically designed to allow a network analyst to monitor traffic

are:

Port mirroring – the switch sends a copy of network packets to a monitoring network

connection.

SMON – "Switch Monitoring" is described by RFC 2613 and is a protocol for controlling

facilities such as port mirroring.

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Another method to monitor may be to connect a layer-1 hub between the monitored device and

its switch port. This will induce minor delay, but will provide multiple interfaces that can be used

to monitor the individual switch port.

Router (computing)

A router is a networking device, commonly specialized hardware, that forwards data

packets between computer networks. This creates an overlay internetwork, as a router is

connected to two or more data lines from different networks. When a data packet comes in one

of the lines, the router reads the address information in the packet to determine its ultimate

destination. Then, using information in its routing table or routing policy, it directs the packet to

the next network on its journey. Routers perform the "traffic directing" functions on the Internet.

A data packet is typically forwarded from one router to another through the networks that

constitute the internetwork until it reaches its destination node.

The most familiar type of routers are home and small office routers that simply pass data,

such as web pages, email, IM, and videos between the home computers and the Internet. An

example of a router would be the owner's cable or DSL router, which connects to the Internet

through an ISP. More sophisticated routers, such as enterprise routers, connect large business or

ISP networks up to the powerful core routers that forward data at high speed along the optical

fiber lines of the Internet backbone. Though routers are typically dedicated hardware devices,

use of software-based routers has grown increasingly common.

Applications

When multiple routers are used in interconnected networks, the routers exchange

information about destination addresses using a dynamic routing protocol. Each router builds up

a table listing the preferred routes between any two systems on the interconnected networks. A

router has interfaces for different physical types of network connections, such as copper cables,

fiber optic, or wireless transmission. It also contains firmware for different networking

communications protocol standards. Each network interface uses this specialized computer

software to enable data packets to be forwarded from one protocol transmission system to

another.

Routers may also be used to connect two or more logical groups of computer devices

known as subnets, each with a different sub-network address.

The subnet addresses recorded in the router do not necessarily map directly to the

physical interface connections.

A router has two stages of operation called planes:

Control plane: A router maintains a routing table that lists which route should be used to

forward a data packet, and through which physical interface connection. It does this using

internal pre-configured directives, called static routes, or by learning routes using a

dynamic routing protocol. Static and dynamic routes are stored in the Routing

Information Base (RIB). The control-plane logic then strips the RIB from non essential

directives and builds a Forwarding Information Base (FIB) to be used by the forwarding-

plane.

Forwarding plane: The router forwards data packets between incoming and outgoing

interface connections. It routes them to the correct network type using information that

the packet header contains. It uses data recorded in the routing table control plane.

Routers may provide connectivity within enterprises, between enterprises and the

Internet, or between internet service providers' (ISPs) networks. The largest routers (such as the

Cisco CRS-1 or Juniper T1600) interconnect the various ISPs, or may be used in large enterprise

networks. Smaller routers usually provide connectivity for typical home and office networks.

Other networking solutions may be provided by a backbone Wireless Distribution System

(WDS), which avoids the costs of introducing networking cables into buildings.

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All sizes of routers may be found inside enterprises. The most powerful routers are

usually found in ISPs, academic and research facilities. Large businesses may also need more

powerful routers to cope with ever increasing demands of intranet data traffic. A three-layer

model is in common use, not all of which need be present in smaller networks.

Access

A screenshot of the LuCI web interface used by OpenWrt. This page configures Dynamic

DNS.

Access routers, including 'small office/home office' (SOHO) models, are located at

customer sites such as branch offices that do not need hierarchical routing of their own.

Typically, they are optimized for low cost. Some SOHO routers are capable of running

alternative free Linux-based firmwares like Tomato, OpenWrt or DD-WRT.

Distribution

Distribution routers aggregate traffic from multiple access routers, either at the same site,

or to collect the data streams from multiple sites to a major enterprise location. Distribution

routers are often responsible for enforcing quality of service across a WAN, so they may have

considerable memory installed, multiple WAN interface connections, and substantial onboard

data processing routines. They may also provide connectivity to groups of file servers or other

external networks.

Security

External networks must be carefully considered as part of the overall security strategy. A

router may include a firewall, VPN handling, and other security functions, or these may be

handled by separate devices. Many companies produced security-oriented routers, including

Cisco Systems' PIX and ASA5500 series, Juniper's Netscreen, Watchguard's Firebox,

Barracuda's variety of mail-oriented devices, and many others. Routers also commonly perform

network address translation, which allows multiple devices on a network to share a single public

IP address.

Core

In enterprises, a core router may provide a "collapsed backbone" interconnecting the

distribution tier routers from multiple buildings of a campus, or large enterprise locations. They

tend to be optimized for high bandwidth, but lack some of the features of Edge Routers.

Internet connectivity and internal use

Routers intended for ISP and major enterprise connectivity usually exchange routing

information using the Border Gateway Protocol (BGP). RFC 4098 standard defines the types of

BGP routers according to their functions:

Edge router: Also called a Provider Edge router, is placed at the edge of an ISP network.

The router uses External BGP to EBGP routers in other ISPs, or a large enterprise

Autonomous System.

Subscriber edge router: Also called a Customer Edge router, is located at the edge of the

subscriber's network, it also uses EBGP to its provider's Autonomous System. It is

typically used in an (enterprise) organization.

Inter-provider border router: Interconnecting ISPs, is a BGP router that maintains BGP

sessions with other BGP routers in ISP Autonomous Systems.

Core router: A core router resides within an Autonomous System as a back bone to carry

traffic between edge routers.

Within an ISP: In the ISP's Autonomous System, a router uses internal BGP to

communicate with other ISP edge routers, other intranet core routers, or the ISP's intranet

provider border routers.

"Internet backbone:" The Internet no longer has a clearly identifiable backbone, unlike its

predecessor networks. See default-free zone (DFZ). The major ISPs' system routers make

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up what could be considered to be the current Internet backbone core.ISPs operate all

four types of the BGP routers described here. An ISP "core" router is used to interconnect

its edge and border routers. Core routers may also have specialized functions in virtual

private networks based on a combination of BGP and Multi-Protocol Label Switching

protocols.

Port forwarding: Routers are also used for port forwarding between private Internet

connected servers.

Voice/Data/Fax/Video Processing Routers: Commonly referred to as access servers or

gateways, these devices are used to route and process voice, data, video and fax traffic on

the Internet. Since 2005, most long-distance phone calls have been processed as IP traffic

(VOIP) through a voice gateway. Voice traffic that the traditional cable networks once

carried. Use of access server type routers expanded with the advent of the Internet, first

with dial-up access and another resurgence with voice phone service.

Historical and technical information

The very first device that had fundamentally the same functionality as a router does

today, was the Interface Message Processor (IMP); IMPs were the devices that made up the

ARPANET, the first packet network. The idea for a router (called "gateways" at the time)

initially came about through an international group of computer networking researchers called

the International Network Working Group (INWG). Set up in 1972 as an informal group to

consider the technical issues involved in connecting different networks, later that year it became

a subcommittee of the International Federation for Information Processing.[16]

These devices were different from most previous packet networks in two ways. First,

they connected dissimilar kinds of networks, such as serial lines and local area networks.

Second, they were connectionless devices, which had no role in assuring that traffic was

delivered reliably, leaving that entirely to the hosts (this particular idea had been previously

pioneered in the CYCLADES network).

The idea was explored in more detail, with the intention to produce a prototype system,

as part of two contemporaneous programs. One was the initial DARPA-initiated program, which

created the TCP/IP architecture in use today.[17] The other was a program at Xerox PARC to

explore new networking technologies, which produced the PARC Universal Packet system; due

to corporate intellectual property concerns it received little attention outside Xerox for years.[18]

Some time after early 1974 the first Xerox routers became operational. The first true IP

router was developed by Virginia Strazisar at BBN, as part of that DARPA-initiated effort,

during 1975-1976. By the end of 1976, three PDP-11-based routers were in service in the

experimental prototype Internet.[19]

The first multiprotocol routers were independently created by staff researchers at MIT

and Stanford in 1981; the Stanford router was done by William Yeager, and the MIT one by

Noel Chiappa; both were also based on PDP-11s.[20][21][22][23]

Virtually all networking now uses TCP/IP, but multiprotocol routers are still

manufactured. They were important in the early stages of the growth of computer networking,

when protocols other than TCP/IP were in use. Modern Internet routers that handle both IPv4

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and IPv6 are multiprotocol, but are simpler devices than routers processing AppleTalk, DECnet,

IP and Xerox protocols.

From the mid-1970s and in the 1980s, general-purpose mini-computers served as routers.

Modern high-speed routers are highly specialized computers with extra hardware added to speed

both common routing functions, such as packet forwarding, and specialised functions such as

IPsec encryption.

There is substantial use of Linux and Unix software based machines, running open source

routing code, for research and other applications. Cisco's operating system was independently

designed. Major router operating systems, such as those from Juniper Networks and Extreme

Networks, are extensively modified versions of Unix software.

Forwarding

For pure Internet Protocol (IP) forwarding function, a router is designed to minimize the

state information associated with individual packets.

The main purpose of a router is to connect multiple networks and forward packets

destined either for its own networks or other networks.

A router is considered a Layer 3 device because its primary forwarding decision is based

on the information in the Layer 3 IP packet, specifically the destination IP address. This process

is known as routing. When each router receives a packet, it searches its routing table to find the

best match between the destination IP address of the packet and one of the network addresses in

the routing table. Once a match is found, the packet is encapsulated in the Layer 2 data link

frame for that outgoing interface. A router does not look into the actual data contents that the

packet carries.but only at the layer 3 addresses to make a forwarding decision, plus optionally

other information in the header for hints on, for example, quality of service (QoS). Once a packet

is forwarded, the router does not retain any historical information about the packet, but the

forwarding action can be collected into the statistical data, if so configured.

The routing table itself can contain information derived from a variety of sources, such as

a default or static route that is configured manually, or dynamic routing protocols where the

router learns routes from other routers. A default route is one that is used to route all traffic

whose destination does not otherwise appear in the routing table; this is common – even

necessary – in small networks, such as a home or small business where the default route simply

sends all non-local traffic to the Internet service provider. The default route can be manually

configured (as a static route), or learned by dynamic routing protocols, or be obtained by DHCP.

(A router can serve as a DHCP client or as a DHCP server.) A router can run more than one

routing protocol at a time, particularly if it serves as an autonomous system border router

between parts of a network that run different routing protocols; if it does so, then redistribution

may be used (usually selectively) to share information between the different protocols running

on the same router.

Forwarding decisions can involve decisions at layers other than layer 3. A function that

forwards based on layer 2 information is properly called a bridge.

This function is referred to as layer 2 bridging, as the addresses it uses to forward the

traffic are layer 2 addresses (e.g. MAC addresses on Ethernet).

Yet another function a router performs is called policy-based routing where special rules

are constructed to override the rules derived from the routing table when a packet forwarding

decision is made.

Besides making decision as to which interface a packet is forwarded to, which is

handledprimarily via the routing table, a router also has to manage congestion, when packets

arrive at arate higher than the router can process.

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Three policies commonly used in the Internet are tail drop, random early detection

(RED), and weighted random early detection (WRED). Tail drop is the simplest and most easily

implemented; the router simply drops packets once the length of the queue exceeds the size of

the buffers in the router. RED probabilistically drops datagrams early when the queue exceeds a

pre-configured portion of the buffer, until a pre-determined max, when it becomes tail drop.

WRED requires a weight on the average queue size to act upon when the traffic is about to

exceed the pre-configured size, so that short bursts will not trigger random drops.

Another function a router performs is to decide which packet should be processed first

when multiple queues exist. This is managed through QoS, which is critical when Voice over IP

is deployed, so that delays between packets do not exceed 150ms to maintain the quality of voice

conversations.

These functions may be performed through the same internal paths that the packets travel

inside the router. Some of the functions may be performed through an application-specific

integrated circuit (ASIC) to avoid overhead caused by multiple CPU cycles, and others may have

to be performed through the CPU as these packets need special attention that cannot be handled

by an ASIC.

Wireless access point

In computer networking, a wireless Access Point (AP) is a device that allows wireless

devices to connect to a wired network using Wi-Fi, or related standards. The AP usually

connects to a router (via a wired network) as a standalone device, but it can also be an integral

component of the router itself.

Introduction

Access Points connecting university campus; APs are controlled by a single, common

WLAN Controller

Embedded RouterBoard 112 with U.FL-RSMA pigtail and R52 mini PCI Wi-Fi card

widely used by wireless Internet service providers (WISPs) across the world

Prior to wireless networks, setting up a computer network in a business, home or school

often required running many cables through walls and ceilings in order to deliver network access

to all of the network-enabled devices in the building.

With the creation of the wireless Access Point (AP), network users are now able to add

devices that access the network with few or no cables. An AP normally connects directly to a

wired Ethernet connection and the AP then provides wireless connections using radio frequency

links for other devices to utilize that wired connection. Most APs support the connection of

multiple wireless devices to one wired connection. Modern APs are built to support a standard

for sending and receiving data using, these radio frequencies. Those standards, and the

frequencies they use are defined by the IEEE. Most APs use IEEE 802.11 standards.

Common AP applications

Typical corporate use involves attaching several APs to a wired network and then

providing wireless access to the office LAN. The wireless access points are managed by a

WLAN Controller which handles automatic adjustments to RF power, channels, authentication,

and security. Furthermore, controllers can be combined to form a wireless mobility group to

allow inter-controller roaming. The controllers can be part of a mobility domain to allow clients

access throughout large or regional office locations. This saves the clients time and

administrators overhead because it can automatically re-associate or re-authenticate.

A hotspot is a common public application of APs, where wireless clients can connect to

the Internet without regard for the particular networks to which they have attached for the

moment.

84

The concept has become common in large cities, where a combination of coffeehouses,

libraries, as well as privately owned open access points, allow clients to stay more or less

continuously connected to the Internet, while moving around. A collection of connected hotspots

can be referred to as a lily pad network.

APs are commonly used in home wireless networks. Home networks generally have only

one AP to connect all the computers in a home. Most are wireless routers, meaning converged

devices that include the AP, a router, and, often, an Ethernet switch. Many also include a

broadband modem. In places where most homes have their own AP within range of the

neighbours' AP, it's possible for technically savvy people to turn off their encryption and set up a

wireless community network, creating an intra-city communication network although this does

not negate the requirement for a wired network.

An AP may also act as the network's arbitrator, negotiating when each nearby client

device can transmit. However, the vast majority of currently installed IEEE 802.11 networks do

not implement this, using a distributed pseudo-random algorithm called CSMA/CA instead.

Wireless access point vs. ad hoc network

Some people confuse wireless access points with wireless ad hoc networks. An ad hoc

network uses a connection between two or more devices without using a wireless access point:

the devices communicate directly when in range. An ad hoc network is used in situations such as

a quick data exchange or a multiplayer LAN game because setup is easy and does not require an

access point. Due to its peer-to-peer layout, ad hoc connections are similar to Bluetooth ones and

are generally not recommended for a permanent installation.[citation needed]

Internet access via ad hoc networks, using features like Windows' Internet Connection

Sharing, may work well with a small number of devices that are close to each other, but ad hoc

networks don't scale well. Internet traffic will converge to the nodes with direct internet

connection, potentially congesting these nodes. For internet-enabled nodes, access points have a

clear advantage, with the possibility of having multiple access points connected by a wired LAN.

Limitations

One IEEE 802.11 AP can typically communicate with 30 client systems located within a

radius of 103 m.However, the actual range of communication can vary significantly, depending

on such variables as indoor or outdoor placement, height above ground, nearby obstructions,

other electronic devices that might actively interfere with the signal by broadcasting on the same

frequency, type of antenna, the current weather, operating radio frequency, and the power output

of devices. Network designers can extend the range of APs through the use of repeaters and

reflectors, which can bounce or amplify radio signals that ordinarily would go un-received. In

experimental conditions, wireless networking has operated over distances of several hundred

kilometers.

Most jurisdictions have only a limited number of frequencies legally available for use by

wireless networks. Usually, adjacent WAPs will use different frequencies (Channels) to

communicate with their clients in order to avoid interference between the two nearby systems.

Wireless devices can "listen" for data traffic on other frequencies, and can rapidly switch from

one frequency to another to achieve better reception. However, the limited number of

frequencies becomes problematic in crowded downtown areas with tall buildings using multiple

WAPs. In such an environment, signal overlap becomes an issue causing interference, which

results in signal droppage and data errors.

Wireless networking lags wired networking in terms of increasing bandwidth and

throughput. While (as of 2013) high-density 256-QAM (TurboQAM) modulation, 3-antenna

wireless devices for the consumer market can reach sustained real-world speeds of some 240

Mbit/s at 13 m behind two standing walls (NLOS) depending on their nature &c or 360 Mbit/s at

10 m line of sight or 380 Mbit/s at 2 m line of sight (IEEE 802.11ac) or 20 to 25 Mbit/s at 2 m

line of sight (IEEE 802.11g), wired hardware of similar cost reaches somewhat less than 1000

Mbit/s up to specified distance of 100 m with twisted-pair cabling (Cat-5, Cat-5e, Cat-6, or

Cat-7) (Gigabit Ethernet). One impediment to increasing the speed of wireless communications

comes from Wi-Fi's use of a shared communications medium: Thus, two stations in

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infrastructure mode that are communicating with each other even over the same AP must have

each and every frame transmitted twice: from the sender to the AP, then from the AP to the

receiver. This approximately halves the effective bandwidth, so an AP is only able to use

somewhat less than half the actual over-the-air rate for data throughput. Thus a typical 54 Mbit/s

wireless connection actually carries TCP/IP data at 20 to 25 Mbit/s. Users of legacy wired

networks expect faster speeds, and people using wireless connections keenly want to see the

wireless networks catch up.

By 2012, 802.11n based access points and client devices have already taken a fair share

of the marketplace and with the finalization of the 802.11n standard in 2009 inherent problems

integrating products from different vendors are less prevalent.

Security

Wireless access has special security considerations. Many wired networks base the

security on physical access control, trusting all the users on the local network, but if wireless

access points are connected to the network, anybody within range of the AP (which typically

extends farther than the intended area) can attach to the network.

The most common solution is wireless traffic encryption. Modern access points come

with built-in encryption.

The first generation encryption scheme WEP proved easy to crack; the second and third

generation schemes, WPA and WPA2, are considered secure if a strong enough password or

passphrase is used.

Some WAPs support hotspot style authentication using RADIUS and other authentication

servers.

Opinions about wireless network security vary widely. For example, in a 2008 article for

Wired magazine, Bruce Schneier asserted the net benefits of open wifi without passwords

outweigh the risks, a position supported in 2014 by Peter Eckersley at eff.org. Peter Eckersley

The opposite position was taken by Nick Mediati in an article for PCWorld, in which he

takes the position that every wireless access point should be locked down with a password.

Industrial Wireless Access Point

Industrial grade APs are rugged, with a metal cover and a DIN rail mount. During

operations they can tolerate a wide temperature range, high humidity and exposure to water,

dust, and oil. Wireless security includes: WPA-PSK, WPA2, IEEE 802.1x/RADIUS, WDS,

WEP, TKIP, and CCMP (AES) encryption. Unlike some home consumer models, industrial

wireless access points can also act as a bridge, router, or a client.

Modem

A modem (modulator-demodulator) is a device that modulates signals to encode digital

information and demodulates signals to decode the transmitted information. The goal is to

produce a signal that can be transmitted easily and decoded to reproduce the original digital data.

Modems can be used with any means of transmitting analog signals, from light emitting diodes

to radio.

A common type of modem is one that turns the digital data of a computer into modulated

electrical signal for transmission over telephone lines and demodulated by another modem at the

receiver side to recover the digital data.

Modems are generally classified by the amount of data they can send in a given unit of

time, usually expressed in bits per second (symbol bit/s, sometimes abbreviated "bps"), or bytes

per second (symbol B/s). Modems can also be classified by their symbol rate, measured in baud.

The baud unit denotes symbols per second, or the number of times per second the modem sends

a new signal. For example, the ITU V.21 standard used audio frequency shift keying with two

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possible frequencies, corresponding to two distinct symbols (or one bit per symbol), to carry 300

bits per second using 300 baud. By contrast, the original ITU V.22 standard, which could

transmit and receive four distinct symbols (two bits per symbol), transmitted 1,200 bits by

sending 600 symbols per second (600 baud) using phase shift keying.

Dialup modem

News wire services in the 1920s used multiplex devices that satisfied the definition of a

modem. However the modem function was incidental to the multiplexing function, so they are

not commonly included in the history of modems. Modems grew out of the need to connect

teleprinters over ordinary phone lines instead of the more expensive leased lines which had

previously been used for current loop–based teleprinters and automated telegraphs. In 1942, IBM

adapted this technology to their unit record equipment and were able to transmit punched cards

at 25 bits/second.

Mass-produced modems in the United States began as part of the SAGE air-defense

system in 1958 (the year the word modem was first used[1]), connecting terminals at various

airbases, radar sites, and command-and-control centers to the SAGE director centers scattered

around the U.S. and Canada. SAGE modems were described by AT&T's Bell Labs as

conforming to their newly published Bell 101 dataset standard. While they ran on dedicated

telephone lines, the devices at each end were no different from commercial acoustically coupled

Bell 101, 110 baud modems.

In summer 1960, the name Data-Phone was introduced to replace the earlier term digital

subset. The 202 Data-Phone was a half-duplex asynchronous service that was marketed

extensively in late 1960. In 1962, the 201A and 201B Data-Phones were introduced. They were

synchronous modems using two-bit-per-baud phase-shift keying (PSK). The 201A operated half-

duplex at 2,000 bit/s over normal phone lines, while the 201B provided full duplex 2,400 bit/s

service on four-wire leased lines, the send and receive channels each running on their own set of

two wires.

The famous Bell 103A dataset standard was also introduced by AT&T in 1962. It

provided full-duplex service at 300 bit/s over normal phone lines. Frequency-shift keying was

used, with the call originator transmitting at 1,070 or 1,270 Hz and the answering modem

transmitting at 2,025 or 2,225 Hz. The readily available 103A2 gave an important boost to the

use of remote low-speed terminals such as the Teletype Model 33 ASR and KSR, and the IBM

2741. AT&T reduced modem costs by introducing the originate-only 113D and the answer-only

113B/C modems.

For many years, the Bell System (AT&T) maintained a monopoly on the use of its phone

lines, and what devices could be connected to its lines. However, the seminal Hush-a-Phone v.

FCC case of 1956 concluded that it was within the FCC's jurisdiction to regulate the operation of

the System. Subsequently, the FCC examiner found that as long as the device was not

electrically attached it would not threaten to degenerate the system. This led to a number of

devices that mechanically connected to the phone, through a standard handset. Since most

handsets were supplied from Western Electric, it was relatively easy to build such an acoustic

coupler, and this style of connection was used for many devices like answering machines.

Acoustically coupled Bell 103A-compatible 300 bit/s modems became common during

the 1970s, with well-known models including the Novation CAT and the Anderson-Jacobson,

the latter spun off from an in-house project at Stanford Research Institute (now SRI

International). An even lower-cost option was the Pennywhistle modem, designed to be built

using parts found at electronics scrap and surplus stores.

In December 1972, Vadic introduced the VA3400, which was notable because it provided

full duplex operation at 1,200 bit/s over the phone network. Like the 103A, it used different

frequency bands for transmit and receive. In November 1976, AT&T introduced the 212A

modem to compete with Vadic. It was similar in design to Vadic's model, but used the lower

frequency set for transmission. One could also use the 212A with a 103A modem at 300 bit/s.

According to Vadic, the change in frequency assignments made the 212 intentionally

incompatible with acoustic coupling, thereby locking out many potential modem manufacturers.

In 1977, Vadic responded with the VA3467 triple modem, an answer-only modem sold to

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computer center operators that supported Vadic's 1,200-bit/s mode, AT&T's 212A mode, and

103A operation.

Carterfone and direct connection

The Hush-a-Phone decision applied only to mechanical collections, but the Carterfone

decision of 1968 led to the FCC introducing a rule setting stringent AT&T-designed tests for

electronically coupling a device to the phone lines. AT&T's tests were complex, making

electronically coupled modems expensive,[citation needed] so acoustically coupled modems remained

common into the early 1980s.

However, the rapidly falling prices of electronics in the late 1970s led to an increasing

number of direct-connect models around 1980. In spite of being directly connected, these

modems were generally operated like their earlier acoustic versions—dialling and other phone-

control operations were completed by hand, using an attached handset. A small number of

modems added the ability to automatically answer incoming calls, or automatically place an

outgoing call to a single number, but even these limited features were relatively rare or limited to

special models in a lineup. When more flexible solutions were needed, 3rd party "diallers" were

used to automate calling, normally using a separate serial port.

The Smartmodem and the rise of BBSs

The next major advance in modems was the Hayes Smartmodem, introduced in 1981. The

Smartmodem was an otherwise standard 103A 300-bit/s modem, but it was attached to a small

microcontroller that let the computer send it commands. The command set included instructions

for picking up and hanging up the phone, dialing numbers, and answering calls. This eliminated

the need for any manual operation, a handset, or a dialler. Terminal programs that maintained

lists of phone numbers and sent the dialing commands became common. The basic Hayes

command set remains the basis for computer control of most modern modems.

The Smartmodem and its clones also aided the spread of bulletin board systems (BBSs)

because it was the first low-cost modem that could answer calls. Modems had previously been

typically either the call-only, acoustically coupled models used on the client side, or the much

more expensive, answer-only models used on the server side. These were fine for large computer

installations, but useless for the hobbyist who wanted to run a BBS using the same telephone line

to call other systems. The first hobby BBS system, CBBS, started as an experiment in ways to

better use the Smartmodem.

Almost all modern modems can inter-operate with fax machines. Digital faxes,

introduced in the 1980s, are simply an image format sent over a high-speed (commonly

14.4 kbit/s) modem. Software running on the host computer can convert any image into fax

format, which can then be sent using the modem. Such software was at one time an add-on, but

has since become largely universal.

1200 and 2400 bit/s

The 300 bit/s modems used audio frequency-shift keying to send data. In this system the

stream of 1s and 0s in computer data is translated into sounds which can be easily sent on the

phone lines. In the Bell 103 system, the originating modem sends 0s by playing a 1,070 Hz tone,

and 1s at 1,270 Hz, with the answering modem transmitting its 0s on 2,025 Hz and 1s on

2,225 Hz. These frequencies were chosen carefully; they are in the range that suffers minimum

distortion on the phone system and are not harmonics of each other.

In the 1,200 bit/s and faster systems, phase-shift keying was used. In this system the two

tones for any one side of the connection are sent at similar frequencies as in the 300 bit/s

systems, but slightly out of phase. Voiceband modems generally remained at 300 and 1,200 bit/s

(V.21 and V.22) into the mid-1980s. A V.22bis 2,400-bit/s system similar in concept to the

1,200-bit/s Bell 212 signaling was introduced in the U.S., and a slightly different one in Europe.

The limited available frequency range meant the symbol rate of 1,200 bit/s modems was still

only 600 baud (symbols per second). The bit rate increases were achieved by defining 4 or 8

distinct symbols, which allowed the encoding of 2 or 3 bits per symbol instead of only 1. The use

of smaller shifts had the drawback of making each symbol more vulnerable to interference, but

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improvements in phone line quality at the same time helped compensate for this. By the late

1980s, most modems could support all of these standards and 2,400-bit/s operation was

becoming common.

Proprietary standards

Many other standards were also introduced for special purposes, commonly using a high-

speed channel for receiving, and a lower-speed channel for sending. One typical example was

used in the French Minitel system, in which the user's terminals spent the majority of their time

receiving information. The modem in the Minitel terminal thus operated at 1,200 bit/s for

reception, and 75 bit/s for sending commands back to the servers.

Three U.S. companies became famous for high-speed versions of the same concept.

Telebit introduced its Trailblazer modem in 1984, which used a large number of 36 bit/s

channels to send data one-way at rates up to 18,432 bit/s. A single additional channel in the

reverse direction allowed the two modems to communicate how much data was waiting at either

end of the link, and the modems could change direction on the fly. The Trailblazer modems also

supported a feature that allowed them to spoof the UUCP g protocol, commonly used on Unix

systems to send e-mail, and thereby speed UUCP up by a tremendous amount. Trailblazers thus

became extremely common on Unix systems, and maintained their dominance in this market

well into the 1990s.

USRobotics (USR) introduced a similar system, known as HST, although this supplied

only 9,600 bit/s (in early versions at least) and provided for a larger backchannel. Rather than

offer spoofing, USR instead created a large market among Fidonet users by offering its modems

to BBS sysops at a much lower price, resulting in sales to end users who wanted faster file

transfers. Hayes was forced to compete, and introduced its own 9,600-bit/s standard, Express 96

(also known as Ping-Pong), which was generally similar to Telebit's PEP. Hayes, however,

offered neither protocol spoofing nor sysop discounts, and its high-speed modems remained rare.

Echo cancellation, 9600 and 14,400

Echo cancellation was the next major advance in modem design.

Local telephone lines use the same wires to send and receive, which results in a small

amount of the outgoing signal being reflected back. This is useful for people talking on the

phone, as it provides a signal to the speaker that their voice is making it through the system.

However, this reflected signal causes problems for the modem, which is unable to distinguish

between a signal from the remote modem and the echo of its own signal. This was why earlier

modems split the signal frequencies into "answer" and "originate"; the modem could then ignore

any signals in the frequency range it was using for transmission. Even with improvements to the

phone system allowing higher speeds, this splitting of available phone signal bandwidth still

imposed a half-speed limit on modems.

Echo cancellation eliminated this problem. Measuring the echo delays and magnitudes

allowed the modem to tell if the received signal was from itself or the remote modem, and create

an equal and opposite signal to cancel its own. Modems were then able to send over the whole

frequency spectrum in both directions at the same time, leading to the development of 4,800 and

9,600 bit/s modems.

Increases in speed have used increasingly complicated communications theory. 1,200 and

2,400 bit/s modems used the phase shift key (PSK) concept. This could transmit two or three bits

per symbol.

The next major advance encoded four bits into a combination of amplitude and phase,

known as Quadrature Amplitude Modulation (QAM).

The new V.27ter and V.32 standards were able to transmit 4 bits per symbol, at a rate of

1,200 or 2,400 baud, giving an effective bit rate of 4,800 or 9,600 bit/s. The carrier frequency

was 1,650 Hz. For many years, most engineers considered this rate to be the limit of data

communications over telephone networks.

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Error correction and compression

Operations at these speeds pushed the limits of the phone lines, resulting in high error

rates. This led to the introduction of error-correction systems built into the modems, made most

famous with Microcom's MNP systems. A string of MNP standards came out in the 1980s, each

increasing the effective data rate by minimizing overhead, from about 75% theoretical maximum

in MNP 1, to 95% in MNP 4. The new method called MNP 5 added data compression to the

system, thereby increasing overall throughput above the modem's rating. Generally the user

could expect an MNP5 modem to transfer at about 130% the normal data rate of the modem.

Details of MNP were later released and became popular on a series of 2,400-bit/s modems, and

ultimately led to the development of V.42 and V.42bis ITU standards. V.42 and V.42bis were

non-compatible with MNP but were similar in concept because they featured error correction and

compression.

Another common feature of these high-speed modems was the concept of fallback, or

speed hunting, allowing them to communicate with less-capable modems. During the call

initiation, the modem would transmit a series of signals and wait for the remote modem to

respond. They would start at high speeds and get progressively slower until there was a response.

Thus, two USR modems would be able to connect at 9,600 bit/s, but, when a user with a 2,400-

bit/s modem called in, the USR would fall back to the common 2,400-bit/s speed. This would

also happen if a V.32 modem and a HST modem were connected. Because they used a different

standard at 9,600 bit/s, they would fall back to their highest commonly supported standard at

2,400 bit/s. The same applies to V.32bis and 14,400 bit/s HST modem, which would still be able

to communicate with each other at 2,400 bit/s.

Breaking the 9.6k barrier

In 1980, Gottfried Ungerboeck from IBM Zurich Research Laboratory applied channel

coding techniques to search for new ways to increase the speed of modems. His results were

astonishing but only conveyed to a few colleagues.[2] In 1982, he agreed to publish what is now a

landmark paper in the theory of information coding.[citation needed] By applying parity check coding

to the bits in each symbol, and mapping the encoded bits into a two-dimensional diamond

pattern, Ungerboeck showed that it was possible to increase the speed by a factor of two with the

same error rate. The new technique was called mapping by set partitions, now known as trellis

modulation.

Error correcting codes, which encode code words (sets of bits) in such a way that they are

far from each other, so that in case of error they are still closest to the original word (and not

confused with another) can be thought of as analogous to sphere packing or packing pennies on a

surface: the further two bit sequences are from one another, the easier it is to correct minor

errors.

V.32bis was so successful that the older high-speed standards had little to recommend

them. USR fought back with a 16,800 bit/s version of HST, while AT&T introduced a one-off

19,200 bit/s method they referred to as V.32ter, but neither non-standard modem sold well.

Any interest in these systems was destroyed during the lengthy introduction of the

28,800 bit/s V.34 standard. While waiting, several companies decided to release hardware and

introduced modems they referred to as V.FAST. In order to guarantee compatibility with V.34

modems once the standard was ratified (1994), the manufacturers were forced to use more

flexible parts, generally a DSP and microcontroller, as opposed to purpose-designed ASIC

modem chips.

Today, the ITU standard V.34 represents the culmination of the joint efforts. It employs

the most powerful coding techniques including channel encoding and shape encoding. From the

mere 4 bits per symbol (9.6 kbit/s), the new standards used the functional equivalent of 6 to 10

bits per symbol, plus increasing baud rates from 2,400 to 3,429, to create 14.4, 28.8, and

33.6 kbit/s modems. This rate is near the theoretical Shannon limit. When calculated, the

Shannon capacity of a narrowband line is

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with the (linear) signal-to-noise ratio. Narrowband phone lines have a bandwidth

of 3000 Hz so using (SNR = 30 dB), the capacity is approximately 30 kbit/s.

Without the discovery and eventual application of trellis modulation, maximum

telephone rates using voice-bandwidth channels would have been limited to 3,429 baud ×

4 bit/symbol = approximately 14 kbit/s using traditional QAM.

V.61/V.70 Analog/Digital Simultaneous Voice and Data

The V.61 Standard introduced Analog Simultaneous Voice and Data (ASVD). This

technology allowed users of v.61 modems to engage in point-to-point voice conversations with

each other while their respective modems communicated.

In 1995, the first DSVD (Digital Simultaneous Voice and Data) modems became

available to consumers, and the standard was ratified as v.70 by the International

Telecommunication Union (ITU) in 1996.

Two DSVD modems can establish a completely digital link between each other over

standard phone lines. Sometimes referred to as "the poor man's ISDN", and employing a similar

technology, v.70 compatible modems allow for a maximum speed of 33.6 kbit/s between peers.

By using a majority of the bandwidth for data and reserving part for voice transmission, DSVD

modems allow users to pick up a telephone handset interfaced with the modem, and initiate a call

to the other peer.

One practical use for this technology was realized by early two-player video gamers, who

could hold voice communication with each other over the phone while playing.

Using digital lines and PCM (V.90/92)

In the late 1990s Rockwell/Lucent and USRobotics introduced new competing

technologies based upon the digital transmission used in modern telephony networks. The

standard digital transmission in modern networks is 64 kbit/s but some networks use a part of the

bandwidth for remote office signaling (e.g. to hang up the phone), limiting the effective rate to

56 kbit/s DS0. This new technology was adopted into ITU standards V.90 and is common in

modern computers. The 56 kbit/s rate is only possible from the central office to the user site

(downlink). In the United States, government regulation limits the maximum power output,

resulting in a maximum data rate of 53.3 kbit/s. The uplink (from the user to the central office)

still uses V.34 technology at 33.6 kbit/s.

Later in V.92, the digital PCM technique was applied to increase the upload speed to a

maximum of 48 kbit/s, but at the expense of download rates. A 48 kbit/s upstream rate would

reduce the downstream as low as 40 kbit/s due to echo on the telephone line. To avoid this

problem, V.92 modems offer the option to turn off the digital upstream and instead use a

33.6 kbit/s analog connection, in order to maintain a high digital downstream of 50 kbit/s or

higher.[4] V.92 also adds two other features. The first is the ability for users who have call

waiting to put their dial-up Internet connection on hold for extended periods[vague] of time while

they answer a call. The second feature is the ability to quickly connect to one's ISP. This is

achieved by remembering the analog and digital characteristics of the telephone line, and using

this saved information when reconnecting.

Using compression to exceed 56k

Today's V.42, V.42bis and V.44 standards allow the modem to transmit data faster than

its basic rate would imply. For instance, a 53.3 kbit/s connection with V.44 can transmit up to

53.3*6 == 320 kbit/s using pure text. However, the compression ratio tends to vary due to noise

on the line, or due to the transfer of already-compressed files (ZIP files, JPEG images, MP3

audio, MPEG video). At some points the modem will be sending compressed files at

approximately 50 kbit/s, uncompressed files at 160 kbit/s, and pure text at 320 kbit/s, or any

value in between.

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In such situations a small amount of memory in the modem, a buffer, is used to hold the

data while it is being compressed and sent across the phone line, but in order to prevent overflow

of the buffer, it sometimes becomes necessary to tell the computer to pause the datastream.

This is accomplished through hardware flow control using extra lines on the modem–

computer connection. The computer is then set to supply the modem at some higher rate, such as

320 kbit/s, and the modem will tell the computer when to start or stop sending data.

Compression by the ISP

As telephone-based 56k modems began losing popularity, some Internet service

providers such as Netzero/Juno, Netscape, and others started using pre-compression to increase

the throughput and maintain their customer base. The server-side compression operates much

more efficiently than the on-the-fly compression done by modems due to the fact these

compression techniques are application-specific (JPEG, text, EXE, etc.). The website text,

images, and Flash executables are compacted to approximately 4%, 12%, and 30%, respectively.

The drawback of this approach is a loss in quality, which causes image content to become

pixelated and smeared. ISPs employing this approach often advertise it as "accelerated dial-up."

These accelerated downloads are now integrated into the Opera and Amazon Silk web

browsers, using their own server-side text and image compression.

Softmodem

A PCI Winmodem/softmodem (on the left) next to a traditional ISA modem (on the

right).

A Winmodem or softmodem is a stripped-down modem that replaces tasks traditionally

handled in hardware with software. In this case the modem is a simple interface designed to act

as a digital-to-analog and an analog-to-digital converter. Softmodems are cheaper than traditional

modems because they have fewer hardware components. However, the software generating and

interpreting the modem tones to be sent to the softmodem uses many system resources. For

online gaming, this can be a real concern. Another problem is the lack of cross-platform

compatibility, meaning that non-Windows operating systems (such as Linux) often do not have

an equivalent driver to operate the modem.

List of dialup speeds

These values are maximum values, and actual values may be slower under certain

conditions (for example, noisy phone lines).[7] For a complete list see the companion article list

of device bandwidths. A baud is one symbol per second; each symbol may encode one or more

data bits.

Popularity

A CEA study in 2006 found that dial-up Internet access is declining in the U.S. In 2000,

dial-up Internet connections accounted for 74% of all U.S. residential Internet connections. The

US demographic pattern for dial-up modem users per capita has been more or less mirrored in

Canada and Australia for the past 20 years.

Dial-up modem use in the US had dropped to 60% by 2003, and in 2006 stood at 36.

Voiceband modems were once the most popular means of Internet access in the U.S., but with

the advent of new ways of accessing the Internet, the traditional 56K modem is losing popularity.

The dial up modem is still widely used by customers in rural areas, where DSL, Cable or Fiber

Optic Service is not available, or they are unwilling to pay what these companies charge.[13] AOL

in its 2012 annual report showed it still collects around $700 million in fees from dial-up users;

about 3 million people.

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Radio Routers

Direct broadcast satellite, WiFi, and mobile phones all use modems to communicate, as

do most other wireless services today. Modern telecommunications and data networks also make

extensive use of radio modems where long distance data links are required. Such systems are an

important part of the PSTN, and are also in common use for high-speed computer network links

to outlying areas where fibre is not economical.

Even where a cable is installed, it is often possible to get better performance or make

other parts of the system simpler by using radio frequencies and modulation techniques through a

cable.

Coaxial cable has a very large bandwidth, however signal attenuation becomes a major

problem at high data rates if a baseband digital signal is used.

By using a modem, a much larger amount of digital data can be transmitted through a

single wire. Digital cable television and cable Internet services use radio frequency modems to

provide the increasing bandwidth needs of modern households. Using a modem also allows for

frequency-division multiple access to be used, making full-duplex digital communication with

many users possible using a single wire.

Wireless modems come in a variety of types, bandwidths, and speeds. Wireless modems

are often referred to as transparent or smart. They transmit information that is modulated onto a

carrier frequency to allow many simultaneous wireless communication links to work

simultaneously on different frequencies.

Transparent modems operate in a manner similar to their phone line modem cousins.

Typically, they were half duplex, meaning that they could not send and receive data at the same

time. Typically transparent modems are polled in a round robin manner to collect small amounts

of data from scattered locations that do not have easy access to wired infrastructure. Transparent

modems are most commonly used by utility companies for data collection.

Smart modems come with media access controllers inside, which prevents random data

from colliding and resends data that is not correctly received. Smart modems typically require

more bandwidth than transparent modems, and typically achieve higher data rates. The IEEE

802.11 standard defines a short range modulation scheme that is used on a large scale throughout

the world.

WiFi and WiMax

The WiFi and WiMax standards use wireless mobile broadband modems operating at

microwave frequencies.

Mobile broadband modems

Modems which use a mobile telephone system (GPRS, UMTS, HSPA, EVDO, WiMax,

etc.), are known as mobile broadband modems (sometimes also called wireless modems).

Wireless modems can be embedded inside a laptop or appliance, or be external to it. External

wireless modems are connect cards, USB modems for mobile broadband and cellular routers.

A connect card is a PC Card or ExpressCard which slides into a PCMCIA/PC

card/ExpressCard slot on a computer. USB wireless modems use a USB port on the laptop

instead of a PC card or ExpressCard slot.

A USB modem used for mobile broadband Internet is also sometimes referred to as a

dongle. A cellular router may have an external datacard (AirCard) that slides into it. Most

cellular routers do allow such datacards or USB modems. Cellular routers may not be modems

by definition, but they contain modems or allow modems to be slid into them. The difference

between a cellular router and a wireless modem is that a cellular router normally allows multiple

people to connect to it (since it can route data or support multipoint to multipoint connections),

while a modem is designed for one connection.

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Most of GSM wireless modems come with an integrated SIM cardholder (i.e., Huawei

E220, Sierra 881, etc.) and some models are also provided with a microSD memory slot and/or

jack for additional external antenna such as Huawei E1762 and Sierra Wireless Compass 885.

The CDMA (EVDO) versions do not use R-UIM cards, but use Electronic Serial Number (ESN)

instead.

The cost of using a wireless modem varies from country to country. Some carriers

implement flat rate plans for unlimited data transfers. Some have caps (or maximum limits) on

the amount of data that can be transferred per month. Other countries have plans that charge a

fixed rate per data transferred—per megabyte or even kilobyte of data downloaded; this tends to

add up quickly in today's content-filled world, which is why many people[who?] are pushing for

flat data rates.

The faster data rates of the newest wireless modem technologies (UMTS, HSPA, EVDO,

WiMax) are also considered to be broadband wireless modems and compete with other

broadband modems below.

Until the end of April 2011, worldwide shipments of USB modems surpassed embedded

3G and 4G modules by 3:1 because USB modems can be easily discarded, but embedded

modems could start to gain popularity as tablet sales grow and as the incremental cost of the

modems shrinks, so by 2016 the ratio may change to 1:1.[17]

Like mobile phones, mobile broadband modems can be SIM locked to a particular

network provider. Unlocking a modem is achieved the same way as unlocking a phone, by using

an 'unlock code'.

Broadband

ADSL (asymmetric digital subscriber line) modems, a more recent development, are not

limited to the telephone's voiceband audio frequencies. Early proprietary ADSL modems used

carrierless amplitude phase (CAP) modulation. All standardized asymmetric DSL variants,

including ANSI T1.413 Issue 2, G.dmt, ADSL2, ADSL2+, VDSL2, and G.fast, use discrete

multi-tone (DMT) modulation, also called (coded) orthogonal frequency-division multiplexing

(OFDM or COFDM).

Standard twisted-pair telephone cable can, for short distances, carry signals with much

higher frequencies than the cable's maximum frequency rating. ADSL broadband takes

advantage of this capability.

However, ADSL's performance gradually declines as the telephone cable's length

increases. This limits ADSL broadband service to subscribers within a relatively short distance

from the telephone exchange.

Cable modems use a range of radio frequencies originally intended to carry television

signals. A single cable can carry radio and television signals at the same time as broadband

internet service without interference. Multiple cable modems attached to a single cable can use

the same frequency band by employing a low-level media access protocol to avoid conflicts. In

the prevalent DOCSIS system, frequency-division duplexing (FDD) separates uplink and

downlink signals. For a single-cable distribution system, the return signals from customers

require bidirectional amplifiers or reverse path amplifiers that send specific customer frequency

bands upstream to the cable plant amongst the downstream frequency bands.

Newer types of broadband modems are available, including satellite and power line

modems.

Most consumers did not know about networking and routers when broadband became

available. However, many people knew that a modem connected a computer to the Internet over

a telephone line. To take advantage of consumers' familiarity with modems, companies called

these devices broadband modems rather use less familiar terms such as adapter, interface,

transceiver, or bridge. In fact, broadband modems fit the definition of modem because they use

complex waveforms to carry digital data. They use more advanced technology than dial-up

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modems: typically they can modulate and demodulate hundreds of channels simultaneously or

use much wider channels than dial-up modems.

Residential Gateways

Some devices referred to as "broadband modems" are residential gateways, integrating

the functions of a modem, network address translation (NAT) router, Ethernet switch, WiFi

access point, DHCP server, firewall, among others. Some residential gateway offer a so-called

"bridged mode", which disables the built-in routing function and makes the device function

similarly to a plain modem. This bridged mode is separate from RFC 1483 bridging.

Home networking

Although the name modem is seldom used in this case, modems are also used for high-

speed home networking applications, especially those using existing home wiring. One example

is the G.hn standard, developed by ITU-T, which provides a high-speed (up to 1 Gbit/s) Local

area network using existing home wiring (power lines, phone lines and coaxial cables). G.hn

devices use orthogonal frequency-division multiplexing (OFDM) to modulate a digital signal for

transmission over the wire.

The phrase "null modem" was used to describe attaching a specially wired cable between

the serial ports of two personal computers. Basically, the transmit output of one computer was

wired to the receive input of the other; this was true for both computers. The same software used

with modems (such as Procomm or Minicom) could be used with the null modem connection.

Deep-space communications

Many modern modems have their origin in deep space telecommunications systems from

the 1960s.

Differences between deep space telecom modems and landline modems include:

Digital modulation formats that have high Doppler immunity are typically used.

Waveform complexity tends to be low—typically binary phase shift keying.

Error correction varies mission to mission, but is typically much stronger than most

landline modems.

Voice modem

Voice modems are regular modems that are capable of recording or playing audio over

the telephone line. They are used for telephony applications. See Voice modem command set for

more details on voice modems. This type of modem can be used as an FXO card for Private

branch exchange systems (compare V.92).

Questions: Section-A

1. Protocols in which stations listen for carrier and act accordingly or called

________________

2. Systems in which multiple users share a common channel in a way that can lead to

conflicts or widely known as _________systems

3. The protocols used to determine who goes next on a multi-access channel belong to a sub

layer of the data link layer is called _____________

4. The data link layer takes the packets it gets from the network layer and encapsulates them

into _______for transmission

5. The function of data link layer is to provide services to the ___________

6. The use of error correcting codes is often referred to as _________

7. The number of bit positions in which to code words differ is called _________

8. CRC means ___________

9. Protocol in which sender sends one frame and then waits for an acknowledgement before

proceeding is called _________

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10. The bits in each address positions from different stations or Boolean OR ed together called

________

Section-B

1. Write about the twisted pair?

2. Write about the coaxial cable?

3. Explain about the electronic magnetic spectrum?

4. Explain about the Radio Transmission?

5. Explain about the microwave transmission?

6. Write about the infrared and Millimeter waves?

7. Explain about the light wave transmission?

8. What is an Geostationary satellites?

9. Write short notes on VSATs?

10. Explain about medium earth orbit satellite?

Section-C

1. Write about Fiber Optics in details?

2. Compare Fiber Optics Vs Copper Wire?

3. Explain about Electromagnetic Spectrum?

4. Write about Geostationary Satellites?

5. Write about Low-Earth Orbit Satellites?

6. What are the types of wireless network? Explain with example?

7. Explain about the communication satellites with its type?

8. Compare Satellites Vs Fiber?

9. Explain any two low-earth orbit satellites.

10. Discuss about Guided Transmission Media?

UNIT-III: DATA-LINK LAYER: Error Detection and correction – Elementary Data-link

Protocols – Sliding Window Protocols. MEDIUM-ACCESS CONTROL SUB LAYER: Multiple

Access Protocols – Ethernet – Wireless LANs - Broadband Wireless – Bluetooth.

DATA-LINK LAYER:

The Data-Link layer is the protocol layer in a program that handles the moving of data in

and out across a physical link in a network.

The Data-Link layer is layer 2 in the Open Systems Interconnect (OSI) model for a set of

telecommunication protocols.

This layer is one of the most complicated layers and has complex functionalities and

liabilities. Data link layers hides the details of underlying hardware and represents itself to upper

layer as the medium to communicate

This layer is one of the most complicated layers and has complex functionalities and

liabilities. Data link layers hides the details of underlying hardware and represents itself to upper

layer as the medium to communicate

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Data link layer works between two hosts which are directly connected in some sense.

This direct connection could be point to point or broadcast. Systems on broadcast network are

said to be on same link. The work of data link layer tends to get more complex when it is dealing

with multiple hosts on single collision domain.

Data link layer is responsible for converting data stream to signals bit by bit and to send

that over the underlying hardware. At the receiving end, Data link layer picks up data from

hardware which are in the form of electrical signals assembles them in a recognizable frame

format, and hands over to upper layer.

Data link layer has two sub-layers:

Logical Link Control: Deals with protocols, flow-control and error control

Media Access Control: Deals with actual control of media

ERROR DETECTION AND CORRECTION

Basic approach used for error detection is the use of redundancy, where additional bits

are added to facilitate detection and correction of errors.

Popular techniques are:

1. Simple Parity check

2. Cyclic Redundancy Check

3. Check Sum

Interferences can change the timing and shape of the signal. If the signal is carrying

binary encoded data, such changes can alter the meaning of the data. These errors can be divided

into two types: Single-bit error and Burst error.

Single-bit Error The term single-bit error means that only one bit of given data unit (such as a byte, character, or

data unit) is changed from 1 to 0 or from 0 to 1. For a single bit error to occur noise must have

duration of only 0.1 μs (micro-second), which is very rare. However, a single-bit error can

happen if we are having a parallel data transmission. For example, if 16 wires are used to send all

16 bits of a word at the same time and one of the wires is noisy, one bit is corrupted in each

word.

Burst Error

The term burst error means that two or more bits in the data unit have changed from 0 to 1 or

vice-versa. Note that burst error doesn’t necessary means that error occurs in consecutive bits.

The length of the burst error is measured from the first corrupted bit to the last corrupted bit.

Some bits in between may not be corrupted.

Redundancy

The central concept in detecting or correcting errors is redundancy. To be able to detect

or correct errors, we need to send some extra bits with our data. These redundant bits are added

by the sender and removed by the receiver. Their presence allows the receiver to detect or correct

corrupted bits. The concept of including extra information in the transmission for error detection

is a good one. But instead of repeating the entire data stream, a shorter group of bits may be

appended to the end of each unit. This technique is called redundancy because the extra bits are

redundant to the information: they are discarded as soon as the accuracy of the transmission has

been determined.

Error Detecting Codes Basic approach used for error detection is the use of redundancy, where additional bits

are added to facilitate detection and correction of errors.

Popular techniques are:

4. Simple Parity check

5. Cyclic Redundancy Check

6. Check Sum

7.

1). Simple Parity Check

The most common and least expensive mechanism for error- detection is the simple

parity check. In this technique, a redundant bit called parity bit, is appended to every data unit

so that the number of 1s in the unit (including the parity becomes even).

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Blocks of data from the source are subjected to a check bit or Parity bit generator form,

where a parity of 1 is added to the block if it contains an odd number of 1’s (ON bits) and 0 is

added if it contains an even number of 1’s. At the receiving end the parity bit is computed from

the received data bits and compared with the received parity bit, as shown in Figure. This scheme

makes the total number of 1’s even, that is why it is called even parity checking.

2). Two Dimensional Parity Check

Performance can be improved by using two-dimensional parity check, which organizes

the block of bits in the form of a table. Parity check bits are calculated for each row, which is

equivalent to a simple parity check bit. Parity check bits are also calculated for all columns then

both are sent along with the data. At the receiving end these are compared with the parity bits

calculated on the received data.

Two- Dimension Parity Checking increases the likelihood of detecting burst errors. A

burst error of more than n bits is also detected by 2-D Parity check with a high-probability. There

is, however, one pattern of error that remains elusive.

If two bits in one data unit are damaged and two bits in exactly same position in another

data unit are also damaged, the 2-D Parity check checker will not detect an error.

For example, if two data units: 11001100 and 10101100. If first and second from last bits in

each of them is changed, making the data units as 01001110 and 00101110, the error cannot be

detected by 2-D Parity check.

3). Checksum

In checksum error detection scheme, the data is divided into k segments each of m bits. In

the sender’s end the segments are added using 1’s complement arithmetic to get the sum. The

sum is complemented to get the checksum.

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At the receiver’s end, all received segments are added using 1’s complement arithmetic to

get the sum. The sum is complemented. If the result is zero, the received data is accepted;

otherwise discarded.

4). Cyclic Redundancy Check

This Cyclic Redundancy Check is the most powerful and easy to implement technique.

Unlike checksum scheme, which is based on addition, CRC is based on binary division. In CRC,

a sequence of redundant bits, called cyclic redundancy check bits, are appended to the end of

data unit so that the resulting data unit becomes exactly divisible by a second, predetermined

binary number.

At the destination, the incoming data unit is divided by the same number. If at this step

there is no remainder, the data unit is assumed to be correct and is therefore accepted.

A remainder indicates that the data unit has been damaged in transit and therefore must be

rejected. The generalized technique can be explained as follows.

If a k bit message is to be transmitted, the transmitter generates an r-bit sequence, known

as Frame Check Sequence (FCS) so that the (k+r) bits are actually being transmitted. Now this r-

bit FCS is generated by dividing the original number, appended by r zeros, by a predetermined

number. This number, which is (r+1) bit in length, can also be considered as the coefficients of a

polynomial, called Generator Polynomial.

The remainder of this division process generates the r-bit FCS. On receiving the packet,

the receiver divides the (k+r) bit frame by the same predetermined number and if it produces no

remainder, it can be assumed that no error has occurred during the transmission. Operations at

both the sender and receiver end are shown in Figure.

The transmitter can generate the CRC by using a feedback shift register circuit. The same

circuit can also be used at the receiving end to check whether any error has occurred. All the

values can be expressed as polynomials of a dummy variable X. For example, for P = 11001 the

corresponding polynomial is X4+X3+1. A polynomial is selected to have at least the following

properties:

It should not be divisible by X.

It should not be divisible by (X+1).

The first condition guarantees that all burst errors of a length equal to the degree of

polynomial are detected. The second condition guarantees that all burst errors affecting an odd

number of bits are detected.

CRC process can be expressed as XnM(X)/P(X) = Q(X) + R(X) / P(X)

Commonly used divisor polynomials are:

CRC-16 = X16 + X15 + X2 + 1

CRC-CCITT = X16 + X12 + X5 + 1

CRC-32 = X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10 + X8 + X7 + X5 + X4 +

X2 + 1

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Performance CRC is a very effective error detection technique. If the divisor is chosen according to the

previously mentioned rules, its performance can be summarized as follows:

CRC can detect all single-bit errors

CRC can detect all double-bit errors (three 1’s)

CRC can detect any odd number of errors (X+1)

CRC can detect all burst errors of less than the degree of the polynomial.

CRC detects most of the larger burst errors with a high probability.

For example CRC-12 detects 99.97% of errors with a length 12 or more.

ERROR CORRECTION TECHNIQUES

The techniques that we have discussed so far can detect errors, but do not correct them.

Error correction can be handled in two ways.

One is when an error is discovered; the receiver can have the sender retransmit the entire

data unit. This is known as backward error correction.

In the other, receiver can use an error-correcting code, which automatically corrects

certain errors. This is known as forward error correction.

1). Automatic repeat request (ARQ)

Automatic Repeat request (ARQ) is an error control method for data transmission that

makes use of error-detection codes, acknowledgment and/or negative acknowledgment

messages, and timeouts to achieve reliable data transmission. An acknowledgment is a message

sent by the receiver to indicate that it has correctly received a data frame.

Usually, when the transmitter does not receive the acknowledgment before the timeout

occurs (i.e., within a reasonable amount of time after sending the data frame), it retransmits the

frame until it is either correctly received or the error persists beyond a predetermined number of

retransmissions.

ARQ is appropriate if the communication channel has varying or unknown capacity, such

as is the case on the Internet. However, ARQ requires the availability of a back channel, results

in possibly increased latency due to retransmissions, and requires the maintenance of buffers and

timers for retransmissions, which in the case of network congestion can put a strain on the server

and overall network capacity.

ARQ is used on shortwave radio data links in the form of ARQ-E or combined with

multiplexing as ARQ-M.

2). Error-correcting code

An error-correcting code (ECC) or forward error correction (FEC) code is a system of

adding redundant data, or parity data, to a message, such that it can be recovered by a receiver

even when a number of errors (up to the capability of the code being used) were introduced,

either during the process of transmission, or on storage.

Since the receiver does not have to ask the sender for retransmission of the data, a back-

channel is not required in forward error correction, and it is therefore suitable for simplex

communication such as broadcasting. Error-correcting codes are frequently used in lower-layer

communication, as well as for reliable storage in media such as CDs, DVDs, hard disks, and

RAM.

Error-correcting codes are usually distinguished between convolutional codes and block codes:

Convolution codes are processed on a bit-by-bit basis. They are particularly suitable for

implementation in hardware, and the Viterbi decoder allows optimal decoding.

Block codes are processed on a block-by-block basis. Early examples of block codes are

repetition codes, Hamming codes and multidimensional parity-check codes. They were

followed by a number of efficient codes, Reed–Solomon codes being the most notable

due to their current widespread use. Turbo codes and low-density parity-check codes

(LDPC) are relatively new constructions that can provide almost optimal efficiency.

Shannon's theorem is an important theorem in forward error correction, and describes the

maximum information rate at which reliable communication is possible over a channel that has a

certain error probability or signal-to-noise ratio (SNR). This strict upper limit is expressed in

terms of the channel capacity. More specifically, the theorem says that there exist codes such that

with increasing encoding length the probability of error on a discrete memory less channel can

be made arbitrarily small, provided that the code rate is smaller than the channel capacity. The

code rate is defined as the fraction k/n of k source symbols and n encoded symbols.

ELEMENTARY DATA-LINK PROTOCOLS An unrestricted simplex protocol is a protocol for communication between computers in

which the data flows in only one direction. Because of that fact, the sender has no way of

knowing whether the receiver received a particular piece of information. Processing time is not a

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constraint and buffer space is always available (hence there is no need for flow control.) In

addition to its value for teaching, such a protocol could be the right choice for communications

networks where, unlike the Internet, packets are guaranteed to not get lost or reordered. You can

implement an unrestricted simplex protocol in your software applications.

Another key assumption is that machine A wants to send a long stream of data to

machine B, using a reliable, connection-oriented service. Later, we will consider the case where

B also wants to send data to A simultaneously. A is assumed to have an infinite supply of data

ready to send and never has to wait for data to be produced. Instead, when A’s data link layer

asks for data, the network layer is always able to comply immediately. (This restriction, too, will

be dropped later.)

1). A Utopian Simplex Protocol

As an initial example we will consider a protocol that is as simple as it can be because it

does not worry about the possibility of anything going wrong.

Data are transmitted in one direction only. Both the transmitting and receiving network

layers are always ready.

Processing time can be ignored. Infinite buffer space is available. And best of all, the

communication channel between the data link layers never damages or loses frames. This

thoroughly unrealistic protocol, which we will nickname ‘‘Utopia,’’ is simply to show the basic

structure on which we will build.

The protocol consists of two distinct procedures, a sender and a receiver. The sender runs

in the data link layer of the source machine, and the receiver runs in the data link layer of the

destination machine. No sequence numbers or acknowledgements are used here, so MAX SEQ is

not needed. The only event type possible is frame arrival (i.e., the arrival of an undamaged

frame).

The sender is in an infinite while loop just pumping data out onto the line as fast as it can.

The body of the loop consists of three actions: go fetch a packet from the (always obliging)

network layer, construct an outbound frame using the variable s, and send the frame on its way.

Only the info field of the frame is used by this protocol, because the other fields have to do with

error and flow control and there are no errors or flow control restrictions here.

The receiver is equally simple. Initially, it waits for something to happen, the only

possibility being the arrival of an undamaged frame. Eventually, the frame arrives and the

procedure wait for event returns, with event set to frame arrival (which is ignored anyway). The

call to from physical layer removes the newly arrived frame from the hardware buffer and puts it

in the variable r, where the receivern code can get at it. Finally, the data portion is passed on to

the network layer, and the data link layer settles back to wait for the next frame, effectively

suspending itself until the frame arrives.

The utopia protocol is unrealistic because it does not handle either flow control or error

correction. Its processing is close to that of an unacknowledged connectionless service that relies

on higher layers to solve these problems, though even an unacknowledged connectionless service

would do some error detection.

2). Simplex Stop-and-Wait Protocol for an Error-Free Channel

Now we will tackle the problem of preventing the sender from flooding the receiver with

frames faster than the latter is able to process them. This situation can easily happen in practice

so being able to prevent it is of great importance.

The communication channel is still assumed to be error free, however, and the data traffic

is still simplex.

One solution is to build the receiver to be powerful enough to process a continuous

stream of back-to-back frames (or, equivalently, define the link layer to be slow enough that the

receiver can keep up). It must have sufficient buffering and processing abilities to run at the line

rate and must be able to pass the frames that are received to the network layer quickly enough.

However, this is a worst-case solution. It requires dedicated hardware and can be wasteful of

resources if the utilization of the link is mostly low. Moreover, it just shifts the problem of

dealing with a sender that is too fast elsewhere; in this case to the network layer.

Protocols in which the sender sends one frame and then waits for an acknowledgement

before proceeding are called stop-and-wait.

Although data traffic in this example is simplex, going only from the sender to the

receiver, frames do travel in both directions. Consequently, the communication channel between

the two data link layers needs to be capable of bidirectional information transfer. However, this

protocol entails a strict alternation of flow: first the sender sends a frame, then the receiver sends

a frame, then the sender sends another frame, then the receiver sends another one, and so on. A

halfduplex physical channel would suffice here.

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As in protocol 1, the sender starts out by fetching a packet from the network layer, using

it to construct a frame, and sending it on its way. But now, unlike in protocol 1, the sender must

wait until an acknowledgement frame arrives before looping back and fetching the next packet

from the network layer. The sending data link layer need not even inspect the incoming frame as

there is only one possibility.

The incoming frame is always an acknowledgement.

The only difference between receiver1 and receiver2 is that after delivering a packet to the

network layer, receiver2 sends an acknowledgement frame back to the sender before entering the

wait loop again. Because only the arrival of the frame back at the sender is important, not its

contents, the receiver need not put any particular information in it.

/* Protocol 2 (Stop-and-wait) also provides for a one-directional flow of data from sender to

receiver. The communication channel is once again assumed to be error free, as in protocol 1.

However, this time the receiver has only a finite buffer capacity and a finite processing speed, so

the protocol must explicitly prevent the sender from flooding the receiver with data faster than it

can be handled. */

typedef enum {frame arrival} event type;

#include "protocol.h"

void sender2(void)

{

frame s; /* buffer for an outbound frame */

packet buffer; /* buffer for an outbound packet */

event type event; /* frame arrival is the only possibility */

while (true) {

from network layer(&buffer); /* go get something to send */

s.info = buffer; /* copy it into s for transmission */

to physical layer(&s); /* bye-bye little frame */

wait for event(&event); /* do not proceed until given the go ahead */

}

}

void receiver2(void)

{

frame r, s; /* buffers for frames */

event type event; /* frame arrival is the only possibility */

while (true) {

wait for event(&event); /* only possibility is frame arrival */

from physical layer(&r); /* go get the inbound frame */

to network layer(&r.info); /* pass the data to the network layer */

to physical layer(&s); /* send a dummy frame to awaken sender */

}

}

3). A Simplex Stop-and-Wait Protocol for a Noisy Channel

Now let us consider the normal situation of a communication channel that makes errors.

Frames may be either damaged or lost completely. However, we assume that if a frame is

damaged in transit, the receiver hardware will detect this when it computes the checksum. If the

frame is damaged in such a way that the checksum is nevertheless correct—an unlikely

occurrence—this protocol (and all other protocols) can fail (i.e., deliver an incorrect packet to the

network layer).

At first glance it might seem that a variation of protocol 2 would work: adding a timer.

The sender could send a frame, but the receiver would only send an acknowledgement frame if

the data were correctly received. If a damaged frame arrived at the receiver, it would be

discarded. After a while the sender would time out and sends the frame again. This process

would be repeated until the frame finally arrived intact.

One suggestion is that the sender would send a frame, the receiver would send an ACK

frame only if the frame is received correctly. If the frame is in error the receiver simply ignores

it; the transmitter would time out and would retransmit it.

One fatal flaw with the above scheme is that if the ACK frame is lost or damaged,

duplicate frames are accepted at the receiver without the receiver knowing it.

To overcome this problem it is required that the receiver be able to distinguish a frame

that it is seeing for the first time from a retransmission. One way to achieve this is to have the

sender put a sequence number in the header of each frame it sends. The receiver then can check

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the sequence number of each arriving frame to see if it is a new frame or a duplicate to be

discarded.

SLIDING WINDOW PROTOCOLS When a data frame arrives, instead of immediately sending a separate control frame, the

receiver restrains itself and waits until the network layer passes it the next packet. The

acknowledgement is attached to the outgoing data frame (using the ack field in the frame

header). In effect, the acknowledgement gets a free ride on the next outgoing data frame. The

technique of temporarily delaying outgoing acknowledgements so that they can be hooked onto

the next outgoing data frame is known as piggybacking.

The principal advantage of using piggybacking over having distinct acknowledgement

frames is a better use of the available channel bandwidth. The ack field in the frame header costs

only a few bits, whereas a separate frame would need a header, the acknowledgement, and a

checksum. In addition, fewer frames sent means fewer ‘‘frame arrival’’ interrupts, and perhaps

fewer buffers in the receiver, depending on how the receiver’s software is organized. In the next

protocol to be examined, the piggyback field costs only 1 bit in the frame header. It rarely costs

more than a few bits.

If a new packet arrives quickly, the acknowledgement is piggybacked onto it; otherwise,

if no new packet has arrived by the end of this time period, the data link layer just sends a

separate acknowledgement frame. The next three protocols are bidirectional protocols that

belong to a class called sliding window protocols. There are 3 types are used as following:

1. One-bit sliding window

2. Go-back-N Protocol

3. Selective repeat protocol

1). One-bit sliding window Before tackling the general case, let us first examine a sliding window protocol with a

maximum window size of 1. Such a protocol uses stop-and-wait since the sender transmits a

frame and waits for its acknowledgement before sending the next one.

Like the others, it starts out by defining some variables. Next3frame3to3send tells which

frame the sender is trying to send. Similarly, frame3expected tells which frame the receiver is

expecting. In both cases, 0 and 1 are the only possibilities.

Under normal circumstances, one of the two data link layers goes first and transmits the

first frame. In other words, only one of the data link layer programs should contain the

to3physical3layer and start3timer procedure calls outside the main loop. In the event that both

data link layers start off simultaneously.

The starting machine fetches the first packet from its network layer, builds a frame from

it, and sends it. When this (or any) frame arrives, the receiving data link layer checks to see if it

is a duplicate, just as in protocol 3. If the frame is the one expected, it is passed to the network

layer and the receiver’s window is slid up. The acknowledgement field contains the number of

the last frame received without error. If this number agrees with the sequence number of the

frame the sender is trying to send, the sender knows it is done with the frame stored in buffer and

can fetch the next packet from its network layer. If the sequence

Number disagrees; it must continue trying to send the same frame. Whenever a frame is

received, a frame is also sent back. Now let us examine protocol 4 to see how resilient it is to

pathological scenarios. Assume that computer A is trying to send its frame 0 to computer Band

that B is trying to send its frame 0 to A. Suppose that A sends a frame to B, but A’s timeout

interval is a little too short. Consequently, A may time out repeatedly, sending a series of

identical frames, all with seq 0 and ack 1.

When the first valid frame arrives at computer B, it will be accepted and frame3expected

will be set to 1.

All the subsequent frames will be rejected because B is now expecting frames with

sequence number 1, not 0. Furthermore, since all the duplicates have ack 1 and B is still

waiting for an acknowledgement of 0, B will not fetch a new packet from its network layer.

After every rejected duplicate comes in, B sends A a frame containing seq 0 and ack

0. Eventually, one of these arrives correctly at A, causing A to begin sending the next packet.

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No combination of lost frames or premature timeouts can cause the protocol to deliver duplicate

packets to either network layer, to skip a packet, or to deadlock.

2). Go-back-N Protocol As a consequence of the rule requiring a sender to wait for an acknowledgement before

sending another frame. If we relax that restriction, much better efficiency can be achieved.

Basically, the solution lies in allowing the sender to transmit up to w frames before blocking,

instead of just 1. With an appropriate choice of w the sender will be able to continuously transmit

frames for a time equal to the round-trip transit time without filling up the window. In the

example above, w should be at least 26.

The sender begins sending frame 0 as before. By the time it has finished sending 26

frames, at t = 520, the acknowledgement for frame 0 will have just arrived. Thereafter,

acknowledgements arrive every 20 msec, so the sender always gets permission to continue just

when it needs it. At all times, 25 or 26 unacknowledged frames are outstanding. Put in other

terms, the sender’s maximum window size is 26.

If the bandwidth is high, even for a moderate delay, the sender will exhaust its window

quickly unless it has a large window.

If the delay is high (e.g., on a geostationary satellite channel), the sender will exhaust its

window even for a moderate bandwidth. The product of these two factors basically tells what the

capacity of the pipe is, and the sender needs the ability to fill it without stopping in order to

operate at peak efficiency. This technique is known as pipelining.

If the channel capacity is b bits/sec, the frame size l bits, and the round-trip propagation

time R sec, the time required to transmit a single frame is l /b sec. After the last bit of a data

frame has been sent, there is a delay of R /2 before that bit arrives at the receiver and another

delay of at least R /2 for the acknowledgement to come back, for a total delay of R. In stop-and-

wait the line is busy for l /b and idle for R, giving line utilization = l /(l + bR)

Two basic approaches are available for dealing with errors in the presence of pipelining.

One way, called go back n, is for the receiver simply to discard all subsequent frames, sending

no acknowledgements for the discarded frames.

This strategy corresponds to a receive window of size 1. In other words, the data link

layer refuses to accept any frame except the next one it must give to the network layer. If the

sender’s window fills up before the timer runs out, the pipeline will begin to empty. Eventually,

the sender will time out and retransmit all unacknowledged frames in order, starting with the

damaged or lost one. This approach can waste a lot of bandwidth if the error rate is high.

In the figure, we see go back n for the case in which the receiver’s window is large.

Frames 0 and 1 are correctly received and acknowledged. Frame 2, however, is damaged or lost.

The sender, unaware of this problem, continues to send frames until the timer for frame 2

expires. Then it backs up to frame 2 and starts all over with it, sending 2, 3, 4, etc. all over again.

3). A Protocol Using Selective Repeat

The other general strategy for handling errors when frames are pipelined is called

selective repeat. When it is used, a bad frame that is received is discarded, but good frames

received after it are buffered. When the sender times out, only the oldest unacknowledged frame

is retransmitted. If that frame arrives correctly, the receiver can deliver to the network layer, in

sequence, all the frames it has buffered. Selective repeat is often combined with having the

receiver send a negative acknowledgement (NAK) when it detects an error, for example, when it

receives a checksum error or a frame out of sequence. NAKs stimulate retransmission before the

corresponding timer expires and thus improve performance.

In this protocol, both sender and receiver maintain a window of acceptable sequence

numbers. The sender’s window size starts out at 0 and grows to some predefined maximum,

MAX3SEQ.

The receiver’s window, in contrast, is always fixed in size and equal to MAX3SEQ. The

receiver has a buffer reserved for each sequence number within its fixed window.

Associated with each buffer is a bit (arrived ) telling whether the buffer is full or empty.

Whenever a frame arrives, its sequence number is checked by the function between to see if it

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falls within the window. If so and if it has not already been received, it is accepted and stored.

This action is taken without regard to whether or not it contains the next packet expected by the

network layer. Of course, it must be kept within the data link layer and not passed to the network

layer until all the lower-numbered frames have already been delivered to the network layer in the

correct order.

/* Protocol 5 (go back n) allows multiple outstanding frames. The sender may transmit up to

MAX3SEQ frames without waiting for an ack. In addition, unlike in the previous protocols, the

network layer is not assumed to have a new packet all the time. Instead, the network layer causes

a network3layer3ready event when there is a packet to send. */

#define MAX3SEQ 7 /* should be 2ˆn − 1 */

typedef enum {frame3arrival, cksum3err, timeout, network3layer3ready} event3type;

#include "protocol.h"

static boolean between(seq3nr a, seq3nr b, seq3nr c)

{

/* Return true if a <=b < c circularly; false

otherwise. */

if (((a <= b) && (b < c)) || ((c < a) && (a <= b)) || ((b < c) && (c < a)))

return(true);

else

return(false);

}

static void send3data(seq3nr frame3nr, seq3nr frame3expected, packet buffer[ ])

{

/* Construct and send a data frame.

*/

frame s; /* scratch variable */

s.info = buffer[frame3nr]; /* insert packet into frame */

s.seq = frame3nr; /* insert sequence number into

frame */

s.ack = (frame3expected + MAX3SEQ) % (MAX3SEQ + 1); /* piggyback ack */

to3physical3layer(&s); /* transmit the frame */

start3timer(frame3nr); /* start the timer

running */

}

void protocol5(void)

{

seq3nr next3frame3to3send; /* MAX3SEQ > 1; used for outbound

stream */

seq3nr ack3expected; /* oldest frame as yet unacknowledged */

seq3nr frame3expected; /* next frame expected on inbound stream

*/

frame r; /* scratch variable */

packet buffer[MAX3SEQ + 1]; /* buffers for the outbound stream */

seq3nr nbuffered; /* # output buffers currently in use */

seq3nr i; /* used to index into the buffer array */

event3type event;

enable3network3layer(); /* allow network3layer3ready events */

ack3expected = 0; /* next ack expected inbound */

next3frame3to3send = 0; /* next frame going out */

frame3expected = 0; /* number of frame expected inbound */

nbuffered = 0; /* initially no packets are buffered */

while (true) {

wait3for3event(&event); /* four possibilities: see event3type above */

switch(event) {

case network3layer3ready: /* the network layer has a packet to send */

/* Accept, save, and transmit a new frame. */

from3network3layer(&buffer[next3frame3to3send]); /* fetch new packet */

nbuffered = nbuffered + 1; /* expand the sender’s window */

send3data(next3frame3to3send, frame3expected, buffer);/* transmit the frame */

inc(next3frame3to3send); /* advance sender’s upper window edge */

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break;

case frame3arrival: /* a data or control frame has arrived */

from3physical3layer(&r); /* get incoming frame from physical layer */

if (r.seq == frame3expected) {

/* Frames are accepted only in order. */

to3network3layer(&r.info); /* pass packet to network layer */

inc(frame3expected); /* advance lower edge of receiver’s window */

}

/* Ack n implies n − 1, n − 2, etc. Check for this. */

while (between(ack3expected, r.ack, next3frame3to3send)) {

/* Handle piggybacked ack. */

nbuffered = nbuffered − 1; /* one frame fewer buffered */

stop3timer(ack3expected); /* frame arrived intact; stop timer */

inc(ack3expected); /* contract sender’s window */

}

break;

case cksum3err: break; /* just ignore bad frames */

case timeout: /* trouble; retransmit all outstanding frames */

next3frame3to3send = ack3expected; /* start retransmitting here */

for (i = 1; i <= nbuffered; i++) {

send3data(next3frame3to3send, frame3expected, buffer); /* resend 1 frame */

inc(next3frame3to3send); /* prepare to send the next one */

}

}

if (nbuffered < MAX3SEQ)

enable3network3layer();

else

disable3network3layer();

}

}

THE MEDIUM ACCESS CONTROL SUB LAYER

The Media Access Control (MAC) data communication Networks protocol sub-layer,

also known as the Medium Access Control, is a sub-layer of the data link layer specified

in the seven-layer OSI model.

The medium access layer was made necessary by systems that share a common

communications medium. Typically these are local area networks.

In LAN nodes uses the same communication channel for transmission. The MAC sub-

layer has two primary responsibilities:

Data encapsulation, including frame assembly before transmission, and frame

parsing/error detection during and after reception.

Media access control, including initiation of frame transmission and recovery from transmission

failure.

Following Protocols are used by Medium Access Layer :

1. ALOHA

2. Carrier Sense Multiple Access

3. CSMA/Collision Detection

4. Wireless LAN Protocols

1). ALOHA

It is a system for coordinating and arbitrating access to a shared communication

Networks channel. It was developed in the 1970s by Norman Abramson and his colleagues at the

University of Hawaii. The original system used for ground based radio broadcasting, but the

system has been implemented in satellite communication systems.

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A shared communication system like ALOHA requires a method of handling collisions that

occur when two or more systems attempt to transmit on the channel at the same time.

In the ALOHA system, a node transmits whenever data is available to send. If another

node transmits at the same time, a collision occurs, and the frames that were transmitted are lost.

However, a node can listen to broadcasts on the medium, even its own, and determine whether

the frames were transmitted.

Aloha means "Hello". Aloha is a multiple access protocol at the datalink layer and proposes

how multiple terminals access the medium without interference or collision. In 1972 Roberts

developed a protocol that would increase the capacity of aloha two fold. The Slotted Aloha

protocol involves dividing the time interval into discrete slots and each slot interval corresponds

to the time period of one frame. This method requires synchronization between the sending

nodes to prevent collisions.

There are two different types of ALOHA:

(i) Pure ALOHA

(ii) slotted ALOHA

(i) Pure ALOHA

In pure ALOHA, the stations transmit frames whenever they have data to send.

When two or more stations transmit simultaneously, there is collision and the frames are

destroyed.

In pure ALOHA, whenever any station transmits a frame, it expects the

acknowledgement from the receiver.

If acknowledgement is not received within specified time, the station assumes that the

frame (or acknowledgement) has been destroyed.

If the frame is destroyed because of collision the station waits for a random amount of

time and sends it again. This waiting time must be random otherwise same frames will

collide again and again.

Therefore pure ALOHA dictates that when time-out period passes, each station must wait

for a random amount of time before resending its frame. This randomness will help avoid

more collisions.

Figure shows an example of frame collisions in pure ALOHA.

(ii) Slotted ALOHA

Slotted ALOHA was invented to improve the efficiency of pure ALOHA as chances of

collision in pure ALOHA are very high.

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In slotted ALOHA, the time of the shared channel is divided into discrete intervals called

slots.

The stations can send a frame only at the beginning of the slot and only one frame is sent

in each slot.

In slotted ALOHA, if any station is not able to place the frame onto the channel at the

beginning of the slot i.e. it misses the time slot then the station has to wait until the

beginning of the next time slot.

In slotted ALOHA, there is still a possibility of collision if two stations try to send at the

beginning of the same time slot as shown in fig.

Slotted ALOHA still has an edge over pure ALOHA as chances of collision are reduced

to one-half.

2). CSMA Protocols

Carrier sense multiple access (CSMA) is a probabilistic media access control (MAC)

protocol in which a node verifies the absence of other traffic before transmitting on a shared

transmission medium, such as an electrical bus, or a band of the electromagnetic spectrum.

Carrier sense means that a transmitter uses feedback from a receiver to determine

whether another transmission is in progress before initiating a transmission. That is, it tries to

detect the presence of a carrier wave from another station before attempting to transmit. If a

carrier is sensed, the station waits for the transmission in progress to finish before initiating its

own transmission. In other words, CSMA is based on the principle "sense before transmit" or

"listen before talk".

Multiple access means that multiple stations send and receive on the medium.

Transmissions by one node are generally received by all other stations connected to the medium.

Virtual time CSMA

VTCSMA is designed to avoid collision generated by nodes transmitting signals

simultaneously, used mostly in hard real-time systems. The VTCSMA uses two clocks at every

node, a virtual clock (vc) and a real clock (rc) which tells "real time". When the channel is

sensed to be busy, the vc freezes, when channel is free, it is reset. Hence, calculating vc runs

faster than rc when channel is free, and vc is not initiated when channel is busy.

CSMA access modes

1-persistent

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1-persistent CSMA is an aggressive transmission algorithm. When the sender (station) is

ready to transmit data, it senses the transmission medium for idle or busy. If idle, then it

transmits immediately. If busy, then it senses the transmission medium continuously until it

becomes idle, then transmits the message (a frame) unconditionally (i.e. with probability=1). In

case of a collision, the sender waits for a random period of time and attempts to transmit again

unconditionally (i.e. with probability=1). 1-persistent CSMA is used in CSMA/CD systems

including Ethernet.

Non-persistent

Non persistent CSMA is a non aggressive transmission algorithm. When the sender

(station) is ready to transmit data, it senses the transmission medium for idle or busy. If idle, then

it transmits immediately. If busy, then it waits for a random period of time (during which it does

not sense the transmission medium) before repeating the whole logic cycle (which started with

sensing the transmission medium for idle or busy) again. This approach reduces collision, results

in overall higher medium throughput but with a penalty of longer initial delay compared to 1–

persistent.

P-persistent

This is an approach between 1-persistent and non-persistent CSMA access modes. When

the sender (station) is ready to transmit data, it senses the transmission medium for idle or busy.

If idle, then it transmits immediately. If busy, then it senses the transmission medium

continuously until it becomes idle, then transmits a frame with probability p. If the sender

chooses not to transmit (the probability of this event is 1-p), the sender waits until the next

available time slot. If the transmission medium is still not busy, it transmits again with the same

probability p. This probabilistic hold-off repeats until the frame is finally transmitted or when the

medium is found to become busy again (i.e. some other sender has already started transmitting

their data). In the latter case the sender repeats the whole logic cycle (which started with sensing

the transmission medium for idle or busy) again. p-persistent CSMA is used in CSMA/CA

systems including Wi-Fi and other packet radio systems.

O-persistent

Each station is assigned a transmission order by a supervisor station. When medium goes

idle, stations wait for their time slot in accordance with their assigned transmission order. The

station assigned to transmit first transmits immediately. The station assigned to transmit second

waits one time slot (but by that time the first station has already started transmitting). Stations

monitor the medium for transmissions from other stations and update their assigned order with

each detected transmission (i.e. they move one position closer to the front of the queue). O-

persistent CSMA is used by CobraNet, LonWorks and the controller area network.

CSMA with collision detection

CSMA/CD is used to improve CSMA performance by terminating transmission as soon

as a collision is detected, thus shortening the time required before a retry can be attempted. is a

media access control method used most notably in local area networking using early Ethernet

technology. It uses a carrier sensing scheme in which a transmitting data station detects other

signals while transmitting a frame, and stops transmitting that frame, transmits a jam signal, and

then waits for a random time interval before trying to resend the frame.

CSMA/CD is modifications of pure carrier sense multiple accesses (CSMA). CSMA/CD

is used to improve CSMA performance by terminating transmission as soon as a collision is

detected, thus shortening the time required before a retry can be attempted.

Main procedure

1. Is my frame ready for transmission? If yes, it goes on to the next point.

2. Is medium idle? If not, wait until it becomes ready

3. Start transmitting.

4. Did a collision occur? If so, go to collision detected procedure.

5. Reset retransmission counters and end frame transmission.

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Collision detected procedure

6. Continue transmission (with a jam signal instead of frame header/data/CRC) until

minimum packet time is reached to ensure that all receivers detect the collision.

7. Increment retransmission counter.

8. Was the maximum number of transmission attempts reached? If so, abort transmission.

9. Calculate and wait random backoff period based on number of collisions.

10. Re-enter main procedure at stage 1.

CSMA with collision avoidance

In CSMA/CA collision avoidance is used to improve the performance of CSMA by

attempting to be less "greedy" on the channel. If the channel is sensed busy before transmission

then the transmission is deferred for a "random" interval. This reduces the probability of

collisions on the channel.

It is particularly important for wireless networks, where the collision detection of the

alternative CSMA/CD is unreliable due to the hidden node problem. CSMA/CA is a protocol

that operates in the Data Link Layer (Layer 2) of the OSI model.

Collision avoidance is used to improve the performance of the CSMA method by attempting to

divide the channel somewhat equally among all transmitting nodes within the collision domain.

1. Carrier Sense: prior to transmitting, a node first listens to the shared medium (such as

listening for wireless signals in a wireless network) to determine whether another node is

transmitting or not. Note that the hidden node problem means another node may be

transmitting which goes undetected at this stage.

2. Collision Avoidance: if another node was heard, we wait for a period of time for the

node to stop transmitting before listening again for a free communications channel.

Request to Send/Clear to Send (RTS/CTS) may optionally be used at this point

to mediate access to the shared medium. This goes some way to alleviating the

problem of hidden nodes because, for instance, in a wireless network, the Access

Point only issues a Clear to Send to one node at a time. However, wireless 802.11

implementations do not typically implement RTS/CTS for all transmissions; they

may turn it off completely, or at least not use it for small packets (the overhead of

RTS, CTS and transmission is too great for small data transfers).

Transmission: if the medium was identified as being clear or the node received a

CTS to explicitly indicate it can send, it sends the frame in its entirety. Unlike

CSMA/CD, it is very challenging for a wireless node to listen at the same time as

it transmits (its transmission will dwarf any attempt to listen). Continuing the

wireless example, the node awaits receipt of an acknowledgement packet from the

Access Point to indicate the packet was received and check summed correctly. If

such acknowledgement does not arrive after a timely manner, it assumes the

packet collided with some other transmission, causing the node to enter a period

of binary exponential backoff prior to attempting to re-transmit.

ETHERNET

Digital Equipment Corporation and Xerox (DIX) worked together to develop the first

Ethernet standards.

Ethernet is a passive, contention-based broadcast technology that uses baseband

signaling. Ethernet topology, which is based on bus and bus-star physical configurations, is

currently the most frequently configured LAN network architecture.

A bus is a common pathway (usually copper wire or fiber cable) between multiple

devices such as computers. A bus is often used as a backbone between devices. It is a technology

that has been evolving for more than 25 years and is still evolving to meet the ever increasing

and changing needs of the internetworking community.

Digital Equipment Corporation and Xerox (DIX) worked together to develop the first

Ethernet standards. These standards are the DIX Ethernet standards and are still in use today. As

110

Ethernet topology became more popular, industry-wide standards became necessary. In 1985, the

IEEE adopted the current Ethernet standards. These standards are called the IEEE 802.2 and

802.3 standards. They differ slightly from the DIX standards, but both define the protocols for

the physical and data link layers of the OSI Model. These standards include cabling

specifications, frame format, and network access conventions.

Ethernet is a passive, contention-based broadcast technology that uses baseband

signaling. Baseband signaling uses the entire bandwidth of a cable for a single transmission.

Only one signal can be transmitted at a time and every device on the shared network hears

broadcast transmissions. Passive technology means that there is no one device controlling the

network. Contention-based means that every device must compete with every o ther device for

access to the shared network. In other words, devices take turns. They can transmit only when no

other device is transmitting.

Ethernet popularity is a result of several factors. Ethernet technology is:

· Inexpensive

· Easy to install, maintain, troubleshoot and expand

· A widely accepted industry standard, which means compatibility and equipment access

are less of an issue

· Structured to allow compatibility with network operating systems

(NOS)

· Very reliable

Ethernet is a broadcast topology that may be structured as a physical bus or physical star with a

logical bus.

Ethernet Cables

Generally, some people use the term "Ethernet" or ether refers to cable. Ethernet was the

original product designed by Xerox PARC based on Bob Metcalfe's idea. It was later upgraded

to 10 Mbps by Xerox, Intel and DEC.

This formed the basis for the IEEE 802.3 standard, which then became an ISO standard.

Actually, the Ethernet was the implementation of standard 802.3 between the computers of

Hawaiian Islands.

Cabling for 802.3: The term Ethernet refers to a network of cables. Various types of

cables are being used in different 802.3 implementations.

The following four cable types are the most common among them:

1. 10Base5 cable. 10Base5 cable or "Thick Ethernet" or Thicknet is the cable which is

the oldest in the category. it is called as thicknet because of the use of thick coaxial cable. The

cable is marked after each 2.5 meters. The thicknet uses bus topology. These marks are provided

for attaching tap points. The connections to this cable are made by "Vampire Taps". In this type

of connection, a pin is carefully forced half a way into the coaxial cable core. The cable operates

at 10Mbps and it can support a maximum segment length of 500 meters. It supports 100 nodes

per segment. 10Base5 cable allows, at maximum, five segments (each of max of 500 meter) to be

connected. These segment are connected with the help of repeaters. Therefore four repeaters are

allowed to be used providing the effective length of2.5 km.

It make use of transceiver (transmitter/receiver) connected via a vampire tap. The

transceiver is responsible for transmitting, receiving and detecting collisions. This transceiver is

connected to the station via a transceiver cable that provides separate path for sending and

receiving.

2. 10Base2 Cable. 10Base2 cable also called "Thin Ethernet" or Thinnet or cheapnet or

cheapernet, was designed after the thick Ethernet cable. This type of cable is usually thin,

flexible and bends easily. It also make use of bus topology. It is also a coaxial cable that is

having a smaller diameter than the 10Base5 cable. The connections to this cable are made by

BNC connectors or UTP connectors. The connections are created by forming T junctions. These

are easier to use and more reliable than vampire connections. The Ethernet based on this type of

cables is cheaper and easy to install but it can run for 200 meters and it supports 30 nodes per

segment. In both of these network cables, detecting cable breaks, bad taps or loose connections

111

can be a major problem. Here, a special technique, called "Time Domain Reflectometry" is

used to detect such kind of errors.

3. 10BaseT Cable. l0BaseT cable or "Twisted Pair" Cable is cheapest and easiest to

maintain. This type of cabling is most popular among local area networks. It make use of

unshielded twisted pair and provides maximum segment length of 100 m. It make use of start

topology. In this type of network, every station is having a wired link to a central device, called

"Hub". Telephone company twisted pair cable of category 5 is used in this type of network. This

is an older and known technology of connections. It can support 1024 nodes per cable segment.

The maximum length of a segment from hub to station can be 150 meters. This type of networks

involves the extra cost of hubs.

4. l0BaseF Cable. 10BaseF cable or "Fiber Optics Cable" is the most efficient and fastest

cable in the category of cables for 802 LANs. The fiber optic cable is very expensive as

compared to above discussed cables but it offers a very high data transmission speed and noise

immunity. This type of cabling is preferred for running networks between buildings or widely

separated hubs. It has the highest length per cable segment i.e. 2000 meters and it can support

1024 nodes per cable segment.

Gigabit Ethernet

Gigabit Ethernet provides the data rate of 1 Gbps or 1000 Mbps. IEEE created Gigabit

Ethernet under the name 802.3z.

• It is compatible with Standard or Fast Ethernet.

• It also uses similarly 48 bit hexadecimal addressing scheme.

• The frame format is also similar to standard Ethernet.

• It operates in both half-duplex and full duplex mode.

• In half duplex mode, CSMNCD access method IS used whereas in full duplex mode CSMNCD

is not required.

MAC Sublayer Functions

MAC sub layer remains almost same in Gigabit Ethernet. There are two distinct approaches for

medium access: half-duplex and full duplex.

• In full duplex mode

(a) There is no collision and CSMNCD is not used.

(b) Each computer is connected to central switch or other switches as shown in fig.

(c) In this mode, each switch has buffers for each input port in which data are stored until they

are transmitted.

(d) The maximum length of the cable is determined by the signal attenuation in the cable.

112

• In Half duplex mode

(a) Switch is not used rather a hub is used.

(b) All the collisions occur in this hub.

(c) To control this, CSMA/CD approach is used.

(d) The maximum length of the network in this approach is totally dependent on the minimum

frame size.

Physical layer function

Topology

• In Gigabit Ethernet, two or more stations can be connected.

• Only two stations can be connected in point to point mode as shown in fig.

• Multiple stations (two or more) can be connected in a star topology with a switch or hub.

• The various possible implementations are shown in fig.

Physical layer implementation

• Two different implementation of Gigabit Ethernet are two wires and a four Wire.

• Two wire implementation uses fiber optic cable. The various two Wire implementations are

1000Base-SX, 1000Base-LX, 1000Base-CX

• Four wire implementation uses category 5 twisted pair cable. It includes 100 Base-T.

113

1000 Base-SX

(a) It uses multimode optical fiber with 2 wires.

(b) It uses short wave laser.

(c) The maximum length of segment supported by 1000Base-SX is 550 meters.

(d) It uses 8B/10B block encoding and NRZ line encoding as shown in Fig.

1000 Base-LX

• It uses multimode or single mode optical fiber (2 wires).

• It makes use of long wave laser.

• The maximum length of segment supported is 550 meters (in multimode) and 5000 meters (in

single mode).

• It also uses 8B/IOB block encoding with NRZ line encoding.

1000 Base-CX

• It uses shielded twisted pair cable (2 wires) that carry electrical signal.

• The maximum length of segment supported by it is~5 meters.

• It also uses 8B/6B blocking encoding and NRZ line encoding.

1000 Base-T

• It uses category 5 UTP (4 wires).

• The maximum segment length supported is 100 meters.

• It makes use of 4D-P AM5 encoding to reduce the bandwidth.

• In 4D-PAM 5 encoding, are four wires are evolved input and output.

• Each wire carries 250 Mbps which is in the range for cat 5 UTP cable.

Fast Ethernet

Fast Ethernet is a version of Ethernet with a 100 Mbps data rate.

• Fast Ethernet was designed to compete with LAN protocols such as FDDI or Fiber Channel.

• IEEE created Fast Ethernet under the name 802.3u.

• The frame format of fast Ethernet is same as that of the traditional Ethernet.

114

• Addressing scheme of Fast Ethernet is also same as that of traditional Ethernet.

• It also makes use of 48 bit hexadecimal address.

IEEE has designed two categories of Fast Ethernet: 100Base-X and 100BaseT4.

100Base-X uses two cables between the station and the hub and 100Base-T4 uses four cables

between the station and hub.

100 Base-TX

(a) 1OOBase-TX uses two pairs of category 5 unshielded twisted pair (UTP) or two pairs of

shielded twisted pair (STP) cables to connect a station to the hub.

(b) One pair is used to carry frames from the station to the hub and the other to carry frames from

the hub to the station.

(c) The distance between hub and station should be less than 100 meters.

(d) For this implementation, the MLT-3 scheme is used. However as MLT-3 is not a self

synchronous line coding scheme, 4B/5B block coding is used to provide bit synchronization.

(e) This creates a data rate of 125 Mbps, which is fed into MLT-3 for encoding.

(f) The encoding and decoding is implemented in two steps as shown in Fig.

100 Base-FX

(a) It users two pairs fiber-optic cables.

(b) One pair carries frame from the station to the hub and the other from hub to the station.

(c) The distance between the station and the hub (or switch) should be less than 2000 meters.

115

(d) It makes use of NRZ-I encoding scheme.

(e) As NRZ-I has a bit synchronization problem for long sequences, 100Base-FX uses 4B/5B

block encoding that increases the bit rate from 100 to 125 mbps.

(f) The encoding scheme for 100Base-FX is shown in Fig.

100 Base-T4

(a) It uses four pairs of category 3 UTP.

(b) Two of the four pairs are bi-directional, the other two are unidirectional.

(c) In each direction, three pairs are used at the same time to carry data as shown in fig.

(d) Encoding/decoding in 100Base-T4 is more complicated.

(e) As this implementation uses category 3 UTP, each twisted pair cannot easily handle more

than 25 Mbaud.

(f) As one pair switches between sending and receiving, three pairs of UTP category 3 can

handle only 75 Mbaud (25 Mbaud each).

(g) Thus it requires an encoding scheme that converts 100 Mbps to a 75 Mbaud signal. This is

done by using 8B/6T (eight binary/six ternary) encoding scheme.

WIRELESS LAN

Although Ethernet is widely used, it is about to get some competition. Wire- less LANs

are increasingly popular, and more and more office buildings, airports, and other public places

are being outfitted with them.

The 802.11 Protocol Stack

The protocols used by all the 802 variants, including Ethernet, have a certain

commonality of structure.

The physical layer corresponds to the OSI physical layer fairly well, but the data link

layer in all the 802 protocols is split into two or more sublayers. In 802.11, the MAC (Medium

Access Control) sublayer determines how the chan- nel is allocated, that is, who gets to

transmit next. Above it is the LLC (Logical Link Control) sublayer, whose job it is to hide the

differences between the dif- ferent 802 variants and make them indistinguishable as far as the

network layer is concerned. We studied the LLC when examining Ethernet earlier in this

chapter and will not repeat that material here.

The 1997 802.11 standard specifies three transmission techniques allowed in the physical

layer. The infrared method uses much the same technology as televi- sion remote controls do. The other

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two use short-range radio, using techniques called FHSS and DSSS. Both of these use a part of the spectrum that does not require licensing (the 2.4-GHz ISM band). Radio-controlled garage door

openers also use this piece of the spectrum, so your notebook computer may find itself in competition

with your garage door. Cordless telephones and microwave ovens al- so use this band. All of these

techniques operate at 1 or 2 Mbps and at low enough power that they do not conflict too much. In 1999, two new techniques were introduced to achieve higher bandwidth. These are called OFDM and

HR- DSSS. They operate at up to 54 Mbps and 11 Mbps, respectively. In 2001, a second OFDM

modulation was introduced, but in a different frequency band from the first one.

Logical link control

802.11

Infrared

802.11

FHSS

802.11

DSSS

802.11a

OFDM

802.11b

HR-DSSS

802.11g

OFDM

The 802.11 Physical Layer

Each of the five permitted transmission techniques makes it possible to send a MAC

frame from one station to another. They differ, however, in the technology used and speeds

achievable. A detailed discussion of these technologies is far beyond the scope of this book,

but a few words on each one, along with some of the key words, may provide interested

readers with terms to search for on the Internet or elsewhere for more information.

The infrared option uses diffused (i.e., not line of sight) transmission at 0.85 or 0.95

microns. Two speeds are permitted: 1 Mbps and 2 Mbps. At 1 Mbps, an encoding scheme is

used in which a group of 4 bits is encoded as a 16-bit code- word containing fifteen 0s and a

single 1, using what is called Gray code. This code has the property that a small error in time

synchronization leads to only a single bit error in the output. At 2 Mbps, the encoding takes 2

bits and produces a 4-bit codeword, also with only a single 1, that is one of 0001, 0010, 0100, or

1000. Infrared signals cannot penetrate walls, so cells in different rooms are well iso- lated

from each other. Nevertheless, due to the low bandwidth (and the fact that sunlight swamps

infrared signals), this is not a popular option.

FHSS (Frequency Hopping Spread Spectrum) uses 79 channels, each 1- number

generator is used to produce the sequence of frequencies hopped to.

As long as all stations use the same seed to the pseudorandom number generator and stay

synchronized in time, they will hop to the same frequencies simultaneously. The amount of

time spent at each frequency, the dwell time, is an adjustable parameter, but must be less than

400 msec. FHSS’ randomization provides a fair way to allocate spectrum in the unregulated

ISM band. It also provides a modicum of security since an intruder who does not know the

hopping sequence or dwell time cannot eavesdrop on transmissions. Over longer distances,

multipath fading can be an issue, and FHSS offers good resistance to it. It is also relatively

insensi- tive to radio interference, which makes it popular for building-to-building links. Its

main disadvantage is its low bandwidth.

The third modulation method, DSSS (Direct Sequence Spread Spectrum ), is also

restricted to 1 or 2 Mbps. The scheme used has some similarities to the CDMA system we

examined in Sec. 2.6.2, but differs in other ways. Each bit is transmitted as 11 chips, using

what is called a Barker sequence . It uses phase shift modulation at 1 Mbaud, transmitting

1 bit per baud when operating at 1Mbps and 2 bits per baud when operating at 2 Mbps. For

years, the FCC required all wireless communications equipment operating in the ISM bands in

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the U.S. to use spread spectrum, but in May 2002, that rule was dropped as new technologies

emerged.

The first of the high-speed wireless LANs, 802.11a, uses OFDM (Orthogo- nal

Frequency Division Multiplexing) to deliver up to 54 Mbps in the wider 5- GHz ISM band.

As the term FDM suggests, different frequencies are used—52 of them, 48 for data and 4 for

synchronization—not unlike ADSL. Since transmis- sions are present on multiple frequencies

at the same time, this technique is con- sidered a form of spread spectrum, but different from

both CDMA and FHSS. Splitting the signal into many narrow bands has some key advantages

over using a single wide band, including better immunity to narrowband interference and the

possibility of using noncontiguous bands. A complex encoding system is used, based on phase-

shift modulation for speeds up to 18 Mbps and on QAM above that. At 54 Mbps, 216 data

bits are encoded into 288-bit symbols. Part of the motivation for OFDM is compatibility with

the European HiperLAN/2 system (Doufexi et al., 2002). The technique has a good spectrum

efficiency in terms of bits/Hz and good immunity to multipath fading.

Next, we come to HR-DSSS (High Rate Direct Sequence Spread Spec- trum),

another spread spectrum technique, which uses 11 million chips/sec to achieve 11 Mbps in the

2.4-GHz band.

It is called 802.11b but is not a follow-up to 802.11a. In fact, its standard was approved first

and it got to market first. Data rates supported by 802.11b are 1, 2, 5.5, and 11 Mbps. The two

slow rates run at

1 Mbaud, with 1 and 2 bits per baud, respectively, using phase shift modulation (for

compatibility with DSSS). The two faster rates run at 1.375 Mbaud, with 4 and 8 bits per baud,

respectively, using Walsh/Hadamard codes.

The 802.11 MAC Sublayer Protocol

The 802.11 MAC sublayer protocol is quite different from that of Ethernet due to the

inherent complexity of the wireless environment compared to that of a wired system. With

Ethernet, a station just waits until the ether goes silent and starts transmitting. If it does not

receive a noise burst back within the first 64 bytes, the frame has almost assuredly been

delivered correctly. With wireless, this situation does not hold.

In this protocol, both physical channel sensing and virtual channel sensing are used.

Two methods of operation are supported by CSMA/CA. In the first method, when a station

wants to transmit, it senses the channel. If it is idle, it just starts transmitting. It does not sense

the channel while transmitting but emits its entire frame, which may well be destroyed at the

re- ceiver due to interference there. If the channel is busy, the sender defers until it goes idle

and then starts transmitting. If a collision occurs, the colliding stations wait a random time,

using the Ethernet binary exponential backoff algorithm, and then try again later.

To deal with the problem of noisy channels, 802.11 allows frames to be frag- mented

into smaller pieces, each with its own checksum. The fragments are indi- vidually numbered

and acknowledged using a stop-and-wait protocol (i.e., the sender may not transmit fragment k

+ 1 until it has received the acknowledgment for fragment k). Once the channel has been

acquired using RTS and CTS, multiple fragments can be sent in a row.. Sequence of fragments

is called a fragment burst.

Fragmentation increases the throughput by restricting retransmissions to the bad

fragments rather than the entire frame. The fragment size is not fixed by the standard but is a

parameter of each cell and can be adjusted by the base station. The NAV mechanism keeps

other stations quiet only until the next acknowledgement, but another mechanism (described

below) is used to allow a whole fragment burst to be sent without interference.

All of the above discussion applies to the 802.11 DCF mode. In this mode, there is

no central control, and stations compete for air time, just as they do with Ethernet. The other

allowed mode is PCF, in which the base station polls the other stations, asking them if they

have any frames to send. Since transmission order is completely controlled by the base

station in PCF mode, no collisions ever occur. The standard prescribes the mechanism for

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polling, but not the polling frequency, polling order, or even whether all stations need to get

equal service.

The basic mechanism is for the base station to broadcast a beacon frame per- iodically

(10 to 100 times per second). The beacon frame contains system parameters, such as hopping

sequences and dwell times (for FHSS), clock synchronization, etc. It also invites new stations

to sign up for polling service. Once a station has signed up for polling service at a certain

rate, it is effectively guaranteed a certain fraction of the bandwidth, thus making it

possible to give quality-of- service guarantees.

Battery life is always an issue with mobile wireless devices, so 802.11 pays attention

to the issue of power management. In particular, the base station can direct a mobile station

to go into sleep state until explicitly awakened by the base station or the user. Having told a

station to go to sleep, however, means that the base station has the responsibility for buffering

any frames directed at it while the mobile station is asleep. These can be collected later.

PCF and DCF can coexist within one cell. At first it might seem impossible to have

central control and distributed control operating at the same time, but 802.11 provides a way

to achieve this goal. It works by carefully defining the interframe time interval. After a frame has

been sent, a certain amount of dead time is required before any station may send a frame. Four

different intervals are defined, each for a specific purpose. The four intervals are depicted in Fig. 4-29.

The shortest interval is SIFS (Short InterFrame Spacing). It is used to allow the

parties in a single dialog the chance to go first. This includes letting the receiver send a CTS to

respond to an RTS, letting the receiver send an ACK for a fragment or full data frame, and

letting the sender of a fragment burst transmit the next fragment without having to send an RTS

again. There is always exactly one station that is entitled to respond after a SIF Spacing)

elapses, the base station may send a beacon frame or poll frame. This mechanism allows a

station sending a data frame or fragment sequence to finish its frame without anyone else

getting in the way, but gives the base station a chance to grab the channel when the

previous sender is done without having to compete with eager users.

If the base station has nothing to say and a time DIFS (DCF InterFrame Spacing)

elapses, any station may attempt to acquire the channel to send a new frame. The usual

contention rules apply, and binary exponential backoff may be needed if a collision occurs.

The last time interval, EIFS (Extended InterFrame Spacing), is used only by a station that

has just received a bad or unknown frame to report the bad frame. The idea of giving this event the

lowest priority is that since the receiver may have no idea of what is going on, it should wait a

substantial time to avoid interfering with an ongoing dialog between two stations.

Frame Format of 802.11

The MAC layer frame consists of nine fields.

1. Frame Control (FC). This is 2 byte field and defines the type of frame and some control

information. This field contains several different subfields.

These are listed in the table below:

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2. D. It stands for duration and is of 2 bytes. This field defines the duration for which the frame

and its acknowledgement will occupy the channel. It is also used to set the value of NA V for

other stations.

3. Addresses. There are 4 address fields of 6 bytes length. These four addresses represent source,

destination, source base station and destination base station.

4. Sequence Control (SC). This 2 byte field defines the sequence number of frame to be used in

flow control.

5. Frame body. This field can be between 0 and 2312 bytes. It contains the information.

6. FCS. This field is 4 bytes long and contains 'cRC-32 error detection sequence.

IEEE 802.11 Frame types

There are three different types of frames:

1. Management frame

2. Control frame

3. Data frame

1. Management frame. These are used for initial communication between stations and access

points.

2. Control frame. These are used for accessing the channel and acknowledging frames. The

control frames are RTS and CTS.

3. Data frame. These are used for carrying data and control information.

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802.11 Addressing

• There are four different addressing cases depending upon the value of To DS And from DS

subfields of FC field.

• Each flag can be 0 or 1, resulting in 4 different situations.

1. If To DS = 0 and From DS = 0, it indicates that frame is not going to distribution system and

is not coming from a distribution system. The frame is going from one station in a BSS to

another.

2. If To DS = 0 and From DS = 1, it indicates that the frame is coming from a distribution

system. The frame is coming from an AP and is going to a station. The address 3 contains

original sender of the frame (in another BSS).

3. If To DS = 1 and From DS = 0, it indicates that the frame is going to a distribution system.

The frame is going from a station to an AP. The address 3 field contains the final destination of

the frame.

4. If To DS = 1 and From DS = 1,it indicates that frame is going from one AP to another AP in a

wireless distributed system.

The table below specifies the addresses of all four cases.

WIRELESS BROADBAND

Originally the word "broadband" had a technical meaning, but became a marketing term

for any kind of relatively high-speed computer network or Internet access technology.

According to the 802.16-2004 standard, broadband means "having instantaneous

bandwidths greater than 1 MHz and supporting data rates greater than about 1.5 Mbit/s."

Wireless networks can feature data rates roughly equivalent to some wired networks,

such as that of asymmetric digital subscriber line (ADSL) or a cable modem.

Wireless networks can also be symmetrical, meaning the same rate in both directions

(downstream and upstream), which is most commonly associated with fixed wireless networks.

A fixed wireless network link is a stationary terrestrial wireless connection, which can support

higher data rates for the same power as mobile or satellite systems.

Few wireless Internet service providers (WISPs) provide download speeds of over 100

Mbit/s; most broadband wireless access (BWA) services are estimated to have a range of 50 km

(31 mi) from a tower.

Technologies used include LMDS and MMDS, as well as heavy use of the ISM bands

and one particular access technology was standardized by IEEE 802.16, with products known as

WiMAX.

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802.16e-2005 Technology

The 802.16 standard essentially standardizes two aspects of the air interface – the physical layer