Computer network

Communication and networking

Computer Network:

A computer network consists of two or more computers that are linked in order to share resources such as printers and CD-ROMs, exchange files, or allow electronic communications. The computers on a computer network may be linked through cables, telephone lines, radio waves, satellites, or infrared light beams.

Purpose and benefits of computer networking:

Linking computers into networks provides benefits in the following areas: information sharing, resource sharing and application sharing. These benefits help to increase productivity.

Sharing information: Networks allow users to share information in several different ways. The most common way of sharing information is to share individual files. For example, two or more people can work together on a single spreadsheet file or word-processing document. In most networks, a large hard drive on a central server computer is set up as a common storage area where users can store files to be shared.

In addition to sharing files, networks allow users to communicate with each other in various ways. For example, messaging applications let network users exchange messages with each other using an e-mail application such as Microsoft Outlook. Users can also hold online meetings over the network. In fact, with inexpensive video cameras and the right software, users can hold videoconferences over the network.

Sharing resources: Certain computer resources, such as printers or hard drives, can be set up so that network users can share them. Sharing these resources can result in significant cost savings. For example, it’s cheaper to buy a single high-speed printer with advanced features that can be shared by an entire workgroup than it is to buy separate printers for each user in the group.

Hard drives can also be shared resources. In fact, providing users with access to a shared hard drive is the most common method of sharing files on a network. A computer whose main purpose in life is to host shared hard drives is called a file server.

In actual practice, entire hard drives aren’t usually shared. Instead, individual folders on a networked hard drive are shared. This way, the network administrator can allow different network users to have access to different shared folders. For example, a company may set up shared folders for its sales department and accounting department. Then, sales personnel can access the sales department’s folder, and accounting personnel can access the accounting department’s folder.

You can share other resources on a network. For example, a network can be used to share an Internet connection. In the early days of the Internet, it was common for each user who required access to the Internet to have his or her own modem connection. Nowadays, it’s more common for the network to provide a shared, high-speed Internet connection that everyone on the network can access.

By Er Hari Prasad Ghimire Page 1


Sharing applications: One of the most common reasons for networking in many businesses is so that several users can work together on a single business application. For example, an accounting department may have accounting software that can be used from several computers at the same time. Or a sales-processing department may have an order-entry application that runs on several computers to handle a large volume of orders.

Analog signal, digital signal and modulation:

Analog signal: Is a signal that is transmitted as a continuous wave. Analog signals change all the time.As said earlier, many telephone systems transit sound as analog signals.

Digital signal: Is such signal that is transmitted between computers in which information is represented by discrete states. For eg: high and low voltage. Digital signal may be represented either in 0 or 1.

Modulation: Is the process of altering one signal(a carrier) according to a pattern provided by another signal.There are there types of modulation.they are:

1. Amplitude modulation(AM): Is a method of encoding information in a transmission, such as radio. Using carrier wave of constant frequency but varying amplitude.

2. Frequency modulation(FM): Is a way of encoding information in an electrical signal by varying its frequency.The FM radiio band uses frequency modulation.

3. Phase modulation(PM): Is a method of imposing information onto a waveform signal by shifting the phase of the wave to represent information, Such as the binary digits 0 and 1.

Modes of Channel Operation

SimplexData in a simplex channel is always one way. Simplex channels are not often used because it is not possible to send back error or control signals to the transmit end.


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It's like a one way street. An example of simplex is Television, or Radio.

Half DuplexA half-duplex channel can send and receive, but not at the same time. It's like a one-lane bridge where two way traffic must give way in order to cross. Only one end transmits at a time, the other end receives. In addition, it is possible to perform error detection and request the sender to retransmit information that arrived corrupted. In some aspects, you can think of Internet surfing as being half-duplex, as a user issues a request for a web document, then that document is downloaded and displayed before the user issues another request.

Another example of half-duplex is talk-back radio, and CB Radio (Citizens Band). You might have seen movies where truckies (drivers of very big trucks) communicate to each other, and when they want the other person to speak they say "over". This is because only one person can talk at a time.

Full DuplexData can travel in both directions simultaneously. There is no need to switch from transmit to receive mode like in half duplex. Its like a two lane bridge on a two-lane highway. Have you ever watched these television talk shows where the host has a number of people on the show, and they all try to talk at once. Well, that's full duplex!

Of course, in the world of data communications, full duplex allows both way communication simultaneously. An example can be a consumer which uses a cable connection to not only receive TV channels, but also the same cable to support their phone and Internet surfing. All these activities can occur simultaneously.

Types of Network:

Network based on services:

Based on how the computers in a network are configured and how they access information, networks are classified into two types: peer to peer and client-server.

Peer-to-peer network





In a peer-to-peer network a group of computers is connected together so that users can share resources and information. There is no central location for authenticating users, storing files, or accessing resources. This means that users must remember which computers in the workgroup have the shared resource or information that they want to access. It also means that users must log on to each computer to access the shared resources on that computer.

In most peer-to-peer networks, it is difficult for users to track where information is located because data is generally stored on multiple computers. This makes it difficult to back up critical business information, and it often results in small businesses not completing backups. Often, there are multiple versions of the same file on different computers in the workgroup.

In some peer-to-peer networks, the small business uses one computer that is running a client operating system, such as Microsoft Windows 98 or Windows XP Professional, as the designated "server" for the network. Although this helps with saving data in a central location, it does not provide a robust solution for many of the needs of a small business, such as collaborating on documents.

Client-Server network

In a server-based network, the server is the central location where users share and access network resources . This dedicated computer controls the level of access that users have to shared resources. Shared data is in one location, making it easy to back up critical business information. Each computer that connects to the network is called a client computer. In a server-based network, users have one user account and password to log on to the server and to access shared resources. Server operating systems are designed to handle the load when multiple client computers access server-based resources.

Windows SBS 2008 is installed and configured as the central server on a server-based network. Windows SBS 2008 provides the central point for authenticating users, accessing resources, and storing information.



Network based on geographical area:

Computer networks can be classified based on various parameters . One such classification parameter is geographical area the networks are spread out. Based on the span and scope the network types can be :

LAN – Local Area Network WLAN – Wireless Local Area Network WAN – Wide Area Network MAN – Metropolitan Area Network SAN – Storage Area Network CAN – Campus Area network PAN – Personal Area Network DAN – Desktop Area Network.

Of all these, LAN, WLAN, WAN and MAN is mostly used.

LAN (Local Area Network)

1. Local Area Network is a network type which is used to connect computer devices which are located within small geographical area like home , offices, schools and small group of buildings.

2. The typical range of LAN is few hundred meters but not more than a mile.3. Data transfer rates in LAN are very high typically in the range of 0Mbps to 10Gbps4. Cost of setting up and maintaining LAN network is low as compared to other types of

network.5. LANs can be either wired or wireless. For wired connection, twisted pair or coaxial cable

or fiber optic can be used.6. IEEE defines standards for LAN in IEEE 802. Ethernet or IEEE 802.3 is one of those.7. Nodes in LAN can be organized as Bus, Ring and Star.

Fig: Bus Topology

MAN (Metropolitan Area Network)

MAN is a computer network type which usually spans a city or large campus. MAN networks lie in between LAN and WAN

MAN interconnects a number of LANs using high capacity backbone technology, for example fiber optic. MAN also provides up-link services to WAN and Internet


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WAN (Wide Area Network)

WAN is a network type which is used to describe network which spans regional, national or global area. For example, offices located at NewDelhi , Mumbai and New York are connected by WAN.

WANs are connected using Leased Line, Dialup connection, Packet Switching and Cell Relay

WANs are also used to connect LANs located at different locations. Transmission rated in WANs usually range from 1200 bits/sec to 24 Mbps. With

connections like ATM and Leased Lines, the transmission rate can be reached greater than 156 Mbps

Network Topology:

The physical topology of a network refers to the configuration of cables, computers, and other peripherals. Physical topology should not be confused with logical topology which is the method used to pass information between workstations. Logical topology was discussed in the Protocol chapter.

Main Types of Network Topologies In networking, the term "topology" refers to the layout of connected devices on a network. This article introduces the standard topologies of computer networking.

One can think of a topology as a network's virtual shape or structure. This shape does not necessarily correspond to the actual physical layout of the devices on the network. For example, the computers on a home LAN may be arranged in a circle in a family room, but it would be highly unlikely to find an actual ring topology there.

Network topologies are categorized into the following basic types:

Star Topology Ring Topology Bus Topology Tree Topology Mesh Topology Hybrid Topology

More complex networks can be built as hybrids of two or more of the above basic topologies.

Star Topology Many home networks use the star topology. A star network features a central connection point called a "hub" that may be a hub, switch or router. Devices typically connect to the hub with Unshielded Twisted Pair (UTP) Ethernet.

Compared to the bus topology, a star network generally requires more cable, but a failure in any star network cable will only take down one computer's network access and not the entire LAN. (If the hub fails, however, the entire network also fails.)



See the illustration of Star Network Topology.

Advantages of a Star Topology

Easy to install and wire. No disruptions to the network then connecting or removing devices. Easy to detect faults and to remove parts.

Disadvantages of a Star Topology

Requires more cable length than a linear topology. If the hub or concentrator fails, nodes attached are disabled. More expensive than linear bus topologies because of the cost of the concentrators.

The protocols used with star configurations are usually Ethernet or LocalTalk. Token Ring uses a similar topology, called the star-wired ring.

Star-Wired Ring

A star-wired ring topology may appear (externally) to be the same as a star topology. Internally, the MAU of a star-wired ring contains wiring that allows information to pass from one device to another in a circle or ring (See fig. 3). The Token Ring protocol uses a star-wired ring topology.

Ring Topology In a ring network, every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network.

To implement a ring network, one typically uses FDDI, SONET, or Token Ring technology. Ring topologies are found in some office buildings or school campuses.

Advantage:



Because every computer is given equal access to the token.No one computer can monopolize the network.

Disadvantage:

Failure of one computer on the ring can affect the entire network. It is difficult to troubleshoot a ring network. Adding or removing computers disrupts the network. More cabling is needed than in a bus network.

See the illustration of Ring Topology.

Bus Topology Bus networks (not to be confused with the system bus of a computer) use a common backbone to connect all devices. A single cable, the backbone functions as a shared communication medium that devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message.

Ethernet bus topologies are relatively easy to install and don't require much cabling compared to the alternatives. 10Base-2 ("ThinNet") and 10Base-5 ("ThickNet") both were popular Ethernet cabling options many years ago for bus topologies. However, bus networks work best with a limited number of devices. If more than a few dozen computers are added to a network bus, performance problems will likely result. In addition, if the backbone cable fails, the entire network effectively becomes unusable.

See the illustration of Bus Network Topology.



Advantages of a Linear Bus Topology

Easy to connect a computer or peripheral to a linear bus. Requires less cable length than a star topology.

Disadvantages of a Linear Bus Topology

Entire network shuts down if there is a break in the main cable. Terminators are required at both ends of the backbone cable. Difficult to identify the problem if the entire network shuts down. Not meant to be used as a stand-alone solution in a large building.

Tree Topology Tree topologies integrate multiple star topologies together onto a bus. In its simplest form, only hub devices connect directly to the tree bus, and each hub functions as the "root" of a tree of devices. This bus/star hybrid approach supports future expandability of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub connection points) alone.

See the illustration of Tree Network Topology.



Advantages of a Tree Topology

Point-to-point wiring for individual segments. Supported by several hardware and software venders.

Disadvantages of a Tree Topology

Overall length of each segment is limited by the type of cabling used. If the backbone line breaks, the entire segment goes down. More difficult to configure and wire than other topologies.

Mesh Topology Mesh topologies involve the concept of routes. Unlike each of the previous topologies, messages sent on a mesh network can take any of several possible paths from source to destination. (Recall that even in a ring, although two cable paths exist, messages can only travel in one direction.) Some WANs, most notably the Internet, employ mesh routing.

A mesh network in which every device connects to every other is called a full mesh. As shown in the illustration below, partial mesh networks also exist in which some devices connect only indirectly to others.

See the illustration of Mesh Network Topology.

Hybrid Topology

A combination of any two or more network topologies. Note 1: Instances can occur where two basic network topologies, when connected together, can still retain the basic network character, and therefore not be a hybrid network. For example, a tree network connected to a tree network is still a tree network. Therefore, a hybrid network accrues only when two basic networks are connected and the resulting network topology fails to meet one of the basic topology definitions. For example, two star networks connected together exhibit hybrid network topologies. Note 2: A hybrid topology always accrues when two different basic network topologies are connected.

TRANSMISSION MEDIA



The means through which data is transformed from one place to another is called transmission or communication media. There are two categories of transmission media used in computer communications.

BOUNDED/GUIDED MEDIA UNBOUNDED/UNGUIDED MEDIA

1. BOUNDED MEDIA:

Bounded media are the physical links through which signals are confined to narrow path. These are also called guide media. Bounded media are made up o a external conductor (Usually Copper) bounded by jacket material. Bounded media are great for LABS because they offer high speed, good security and low cast. However, some time they cannot be used due distance communication. Three common types of bounded media are used of the data transmission. These are

Coaxial Cable Twisted Pairs Cable Fiber Optics Cable

COAXIAL CABLE:

Coaxial cable is very common & widely used commutation media. For example TV wire is usually coaxial.

Coaxial cable gets its name because it contains two conductors that are parallel to each other. The center conductor in the cable is usually copper. The copper can be either a solid wire or stranded martial.

Outside this central Conductor is a non-conductive material. It is usually white, plastic material used to separate the inner Conductor form the outer Conductor. The other Conductor is a fine mesh made from Copper. It is used to help shield the cable form EMI.

Outside the copper mesh is the final protective cover. (as shown in Fig)

The actual data travels through the center conductor in the cable. EMI interference is caught by outer copper mesh. There are different types of coaxial cable vary by gauge & impedance.

Gauge is the measure of the cable thickness. It is measured by the Radio grade measurement, or RG number. The high the RG number, the thinner the central conductor core, the lower the number the thicker the core.

Here the most common coaxial standards.

50-Ohm RG-7 or RG-11 : used with thick Ethernet. 50-Ohm RG-58 : used with thin Ethernet



75-Ohm RG-59 : used with cable television 93-Ohm RG-62 : used with ARCNET.

CHARACTERISTICS OF COAXIAL CABLE

Low cost Easy to install Up to 10Mbps capacity Medium immunity form EMI Medium of attenuation

ADVANTAGES COAXIAL CABLE

Inexpensive Easy to wire Easy to expand Moderate level of EMI immunity

DISADVANTAGE COAXIAL CABLE

Single cable failure can take down an entire network

STP

UTP

Twisted Pair Cable

The most popular network cabling is Twisted pair. It is light weight, easy to install, inexpensive and support many different types of network. It also supports the speed of 100 mps. Twisted pair cabling is made of pairs of solid or stranded copper twisted along each other. The twists are done to reduce vulnerably to EMI and cross talk. The number of pairs in the cable depends on the



type. The copper core is usually 22-AWG or 24-AWG, as measured on the American wire gauge standard. There are two types of twisted pairs cabling

1. Unshielded twisted pair (UTP)

2. Shielded twisted pair (STP)

1. Unshielded twisted pair (UTP)

UTP is more common. It can be either voice grade or data grade depending on the condition. UTP cable normally has an impedance of 100 ohm. UTP cost less than STP and easily available due to its many use. There are five levels of data cabling

Category 1

These are used in telephone lines and low speed data cable.

Category 2

These cables can support up to 4 mps implementation.

Category 3

These cable supports up to 16 mps and are mostly used in 10 mps.

Category 4

These are used for large distance and high speed. It can support 20mps.

Category 5

This is the highest rating for UTP cable and can support up to 100mps.

UTP cables consist of 2 or 4 pairs of twisted cable. Cable with 2 pair use RJ-11 connector and 4 pair cable use RJ-45 connector.

Characteristics of UTP

low cost easy to install High speed capacity High attenuation Effective to EMI 100 meter limit

Advantages of UTP



Easy installation Capable of high speed for LAN Low cost

Disadvantages of UTP

Short distance due to attenuation

2. Shielded twisted pair (STP)

It is similar to UTP but has a mesh shielding that’s protects it from EMI which allows for higher transmission rate.

IBM has defined category for STP cable.

Type 1

STP features two pairs of 22-AWG

Type 2

This type include type 1 with 4 telephone pairs

Type 6

This type feature two pairs of standard shielded 26-AWG

Type 7

This type of STP consist of 1 pair of standard shielded 26-AWG

Type 9

This type consist of shielded 26-AWG wire

Characteristics of STP

Medium cost Easy to install Higher capacity than UTP Higher attenuation, but same as UTP Medium immunity from EMI 100 meter limit

Advantages of STP:



Shielded Faster than UTP and coaxial

Disadvantages of STP:

More expensive than UTP and coaxial More difficult installation High attenuation rate

Fiber Optics

Fiber optic cable uses electrical signals to transmit data. It uses light. In fiber optic cable light only moves in one direction for two way communication to take place a second connection must be made between the two devices. It is actually two stands of cable. Each stand is responsible for one direction of communication. A laser at one device sends pulse of light through this cable to other device. These pulses translated into “1’s” and “0’s” at the other end.

In the center of fiber cable is a glass stand or core. The light from the laser moves through this glass to the other device around the internal core is a reflective material known as CLADDING. No light escapes the glass core because of this reflective cladding.

Fiber optic cable has bandwidth more than 2 gbps (Gigabytes per Second)

Characteristics Of Fiber Optic Cable:

Expensive Very hard to install Capable of extremely high speed Extremely low attenuation No EMI interference

Advantages Of Fiber Optic Cable:

Fast Low attenuation No EMI interference

Disadvantages Fiber Optics:

Very costly Hard to install

Unguided Media: Transmission media then looking at analysis of using them unguided transmission media is data signals that flow through the air. They are not guided or bound to a channel to follow. Following are unguided media used for data communication:



Radio Transmission Microwave Satellite Communication

. RF Propagation: There are three types of RF (radio frequency) propagation:

Ground Wave Ionospheric Line of Sight (LOS)

Ground wave propagation follows the curvature of the Earth. Ground waves have carrier frequencies up to 2 MHz. AM radio is an example of ground wave propagation. Ionospheric propagation bounces off of the Earth’s ionospheric layer in the upper atmosphere.

It is sometimes called double hop propagation. It operates in the frequency range of 30 - 85 MHz. Because it depends on the Earth’s ionosphere, it changes with the weather and time of day. The signal bounces off of the ionosphere and back to earth. Ham radios operate in this range.

Line of sight propagation transmits exactly in the line of sight. The receive station must be in the view of the transmit station. It is sometimes called space waves or tropospheric propagation. It is limited by the curvature of the Earth for ground-based stations (100 km, from horizon to horizon). Reflected waves can cause problems. Examples of line of sight propagation are: FM radio, microwave and satellite.

Radio Frequencies : The frequency spectrum operates from 0 Hz (DC) to gamma rays (1019 Hz). Radio frequencies are in the range of 300 kHz to 10 GHz. We are seeing an emerging technology called wireless LANs. Some use radio frequencies to connect the workstations together, some use infrared technology.

Microwave : Microwave transmission is line of sight transmission. The transmit station must be in visible contact with the receive station. This sets a limit on the distance between stations depending on the local geography. Typically the line of sight due to the Earth’s curvature is only 50 km to the horizon! Repeater stations must be placed so the data signal can hop, skip and jump across the country.

Microwaves operate at high operating frequencies of 3 to 10 GHz. This allows them to carry large quantities of data due to their large bandwidth.

Advantages:



(a) They require no right of way acquisition between towers.

(b) They can carry high quantities of information due to their high operating frequencies.

(c) Low cost land purchase: each tower occupies only a small area.

(d) High frequency/short wavelength signals require small antennae.

Disadvantages:

(a) Attenuation by solid objects: birds, rain, snow and fog.

(b) Reflected from flat surfaces like water and metal.

(c) Diffracted (split) around solid objects.

(d) Reflected by atmosphere, thus causing beam to be projected away from receiver.

Satellite: Satellites are transponders (units that receive on one frequency and retransmit on another) that are set in geostationary orbits directly over the equator. These geostationary orbits are 36,000 km from the Earth’s surface. At this point, the gravitational pull of the Earth and the centrifugal force of Earth’s rotation are balanced and cancel each other out. Centrifugal force is the rotational force placed on the satellite that wants to fling it out into space.

The uplink is the transmitter of data to the satellite. The downlink is the receiver of data. Uplinks and downlinks are also called Earth stations because they are located on the Earth. The footprint is the “shadow” that the satellite can transmit to, the shadow being the area that can receive the satellite’s transmitted signal.

Network Connecting Devices

HUB

Networks using a Star topology require a central point for the devices to connect. Originally this device was called a concentrator since it consolidated the cable runs from all network devices. The basic form of concentrator is the hub.

As shown in Figure; the hub is a hardware device that contains multiple, independent ports that match the cable type of the network. Most common hubs interconnect Category 3 or 5 twisted-pair cable with RJ-45 ends, although Coax BNC and Fiber Optic BNC hubs also exist. The hub



is considered the least common denominator in device concentrators. Hubs offer an inexpensive option for transporting data between devices, but hubs don't offer any form of intelligence. Hubs can be active or passive.

An active hub strengthens and regenerates the incoming signals before sending the data on to its destination.

Passive hubs do nothing with the signal.

Switches

Switches are a special type of hub that offers an additional layer of intelligence to basic, physical-layer repeater hubs. A switch must be able to read the MAC address of each frame it receives. This information allows switches to repeat incoming data frames only to the computer or computers to which a frame is addressed. This speeds up the network and reduces congestion.

Switches operate at both the physical layer and the data link layer of the OSI Model.

Bridges

A bridge is used to join two network segments together, it allows computers on either segment to access resources on the other. They can also be used to divide large networks into smaller segments. Bridges have all the features of repeaters, but can have more nodes, and since the network is divided, there is fewer computers competing for resources on each segment thus improving network performance.



Bridges can also connect networks that run at different speeds, different topologies, or different protocols. But they cannot, join an Ethernet segment with a Token Ring segment, because these use different networking standards. Bridges operate at both the Physical Layer and the MAC sublayer of the Data Link layer. Bridges read the MAC header of each frame to determine on which side of the bridge the destination device is located, the bridge then repeats the transmission to the segment where the device is located.

Routers

Routers Are networking devices used to extend or segment networks by forwarding packets from one logical network to another. Routers are most often used in large internetworks that use the TCP/IP protocol suite and for connecting TCP/IP hosts and local area networks (LANs) to the Internet using dedicated leased lines.

Routers work at the network layer (layer 3) of the Open Systems Interconnection (OSI) reference model for networking to move packets between networks using their logical addresses (which, in the case of TCP/IP, are the IP addresses of destination hosts on the network). Because routers operate at a higher OSI level than bridges do, they have better packet-routing and filtering capabilities and greater processing power, which results in routers costing more than bridges.

Gateways

A gateway is a device used to connect networks using different protocols. Gateways operate at the network layer of the OSI model. In order to communicate with a host on another network, an IP host must be configured with a route to the destination network. If a configuration route is not



found, the host uses the gateway (default IP router) to transmit the traffic to the destination host. The default t gateway is where the IP sends packets that are destined for remote networks. If no default gateway is specified, communication is limited to the local network. Gateways receive data from a network using one type of protocol stack, removes that protocol stack and repackages it with the protocol stack that the other network can use.

Examples

E-mail gateways-for example, a gateway that receives Simple Mail Transfer Protocol (SMTP) e-mail, translates it into a standard X.400 format, and forwards it to its destination

Gateway Service for NetWare (GSNW), which enables a machine running Microsoft Windows NT Server or Windows Server to be a gateway for Windows clients so that they can access file and print resources on a NetWare server

Gateways between a Systems Network Architecture (SNA) host and computers on a TCP/IP network, such as the one provided by Microsoft SNA Server

A packet assembler/disassembler (PAD) that provides connectivity between a local area network (LAN) and an X.25 packet-switching network

NICs (Network Interface Card)

Network Interface Card, or NIC is a hardware card installed in a computer so it can communicate on a network. The network adapter provides one or more ports for the network cable to connect to, and it transmits and receives data onto the network cable.

Wireless Lan card

Every networked computer must also have a network adapter driver, which controls the network adapter. Each network adapter driver is configured to run with a certain type of network adapter.



Network card

Network Interface Adapter Functions Network interface adapters perform a variety of functions that are crucial to getting data to and from the computer over the network.

These functions are as follows:

Data encapsulationThe network interface adapter and its driver are responsible for building the frame around the data generated by the network layer protocol, in preparation for transmission. The network interface adapter also reads the contents of incoming frames and passes the data to the appropriate network layer protocol.

Signal encoding and decodingThe network interface adapter implements the physical layer encoding scheme that converts the binary data generated by the network layer-now encapsulated in the frame-into electrical voltages, light pulses, or whatever other signal type the network medium uses, and converts received signals to binary data for use by the network layer.

transmission and receptionThe primary function of the network interface adapter is to generate and transmit signals of the appropriate type over the network and to receive incoming signals. The nature of the signals depends on the network medium and the data-link layer protocol. On a typical LAN, every computer receives all of the packets transmitted over the network, and the network interface adapter examines the destination address in each packet, to see if it is intended for that computer. If so, the network interface adapter passes the packet to the computer for processing by the next layer in the protocol stack; if not, the network interface adapter discards the packet.

Data buffering Network interface adapters transmit and receive data one frame at a time, so they have built-in buffers that enable them to store data arriving either from the computer or from the network until a frame is complete and ready for processing.

Serial/parallel conversionThe communication between the computer and the network interface adapter runs in parallel, that is, either 16 or 32 bits at a time, depending on the bus the adapter uses. Network



communications, however, are serial (running one bit at a time), so the network interface adapter is responsible for performing the conversion between the two types of transmissions.

Media access controlThe network interface adapter also implements the MAC mechanism that the data-link layer protocol uses to regulate access to the network medium. The nature of the MAC mechanism depends on the protocol used.

Network protocols

A networked computer must also have one or more protocol drivers (sometimes called a transport protocol or just a protocol). The protocol driver works between the upper-level network software and the network adapter to package data to be sent on the network.

In most cases, for two computers to communicate on a network, they must use identical protocols. Sometimes, a computer is configured to use multiple protocols. In this case, two computers need only one protocol in common to communicate. For example, a computer running File and Printer Sharing for Microsoft Networks that uses both NetBEUI and TCP/IP can communicate with computers using only NetBEUI or TCP/IP.

ISDN (Integrated Services Digital Network) adapters

Integrated Services Digital Network adapters can be used to send voice, data, audio, or video over standard telephone cabling. ISDN adapters must be connected directly to a digital telephone network. ISDN adapters are not actually modems, since they neither modulate nor demodulate the digital ISDN signal.

Like standard modems, ISDN adapters are available both as internal devices that connect directly to a computer's expansion bus and as external devices that connect to one of a computer's serial or parallel ports. ISDN can provide data throughput rates from 56 Kbps to 1.544 Mbps (using a T1 carrier service).



ISDN hardware requires a NT (network termination) device, which converts network data signals into the signaling protocols used by ISDN. Some times, the NT interface is included, or integrated, with ISDN adapters and ISDN-compatible routers. In other cases, an NT device separate from the adapter or router must be implemented. ISDN works at the physical, data link, network, and transport layers of the OSI Model.

WAPs (Wireless Access Point)

A wireless network adapter card with a transceiver sometimes called an access point, broadcasts and receives signals to and from the surrounding computers and passes back and forth between the wireless computers and the cabled network.

Access points act as wireless hubs to link multiple wireless NICs into a single subnet. Access points also have at least one fixed Ethernet port to allow the wireless network to be bridged to a traditional wired Ethernet network.

Modems

A modem is a device that makes it possible for computers to communicate over telephone lines. The word modem comes from Modulate and Demodulate. Because standard telephone lines use analog signals, and computers digital signals, a sending modem must modulate its digital signals into analog signals. The computers modem on the receiving end must then demodulate the analog signals into digital signals.



Modems can be external, connected to the computers serial port by an RS-232 cable or internal in one of the computers expansion slots. Modems connect to the phone line using standard telephone RJ-11 connectors.

The OSI Reference Model

The OSI reference model specifies standards for describing "Open Systems Interconnection" with the term 'open' chosen to emphasise the fact that by using these international standards, a system may be defined which is open to all other systems obeying the same standards throughout the world. The definition of a common technical language has been a major catalyst to the standardisation of communications protocols and the functions of a protocol layer.

The seven layers of the OSI reference model showing a connection between two end systems communicating using one intermediate system.

The structure of the OSI architecture is given in the figure above, which indicates the protocols used to exchange data between two users A and B. The figure shows bidirectional (duplex)



information flow; information in either direction passes through all seven layers at the end points. When the communication is via a network of intermediate systems, only the lower three layers of the OSI protocols are used in the intermediate systems.

Services provided by each Protocol Layer

The OSI layers may be summarised by:

1. Physical layer: Provides electrical, functional, and procedural characteristics to activate, maintain, and deactivate physical links that transparently send the bit stream; only recognises individual bits, not characters or multicharacter frames.

2. Data link layer: Provides functional and procedural means to transfer data between network entities and (possibly) correct transmission errors; provides for activation, maintenance, and deactivation of data link connections, grouping of bits into characters and message frames, character and frame synchronisation, error control, media access control, and flow control (examples include HDLC and Ethernet)

3. Network layer: Provides independence from data transfer technology and relaying and routing considerations; masks peculiarities of data transfer medium from higher layers and provides switching and routing functions to establish, maintain, and terminate network layer connections and transfer data between users.

4. Transport layer: Provides transparent transfer of data between systems, relieving upper layers from concern with providing reliable and cost effective data transfer; provides end-to-end control and information interchange with quality of service needed by the application program; first true end-to-end layer.

5. Session layer: Provides mechanisms for organising and structuring dialogues between application processes; mechanisms allow for two-way simultaneous or two-way alternate operation, establishment of major and minor synchronisation points, and techniques for structuring data exchanges.

6. Presentation layer: Provides independence to application processes from differences in data representation, that is, in syntax; syntax selection and conversion provided by allowing the user to select a "presentation context" with conversion between alternative contexts.

7. Application layer: Concerned with the requirements of application. All application processes use the service elements provided by the application layer. The elements include library routines which perform interprocess communication, provide common procedures for constructing application protocols and for accessing the services provided by servers which reside on the network.

The communications engineer is concerned mainly with the protocols operating at the bottom four layers (physical, data link, network, and transport) in the OSI reference model. These layers provide the basic communications service. The layers above are primarily the concern of computer scientists who wish to build distributed applications programs using the services provided by the network.

Switching: Large internet works may have multiple paths linking sender and receiver devices. In much the same way that trains are switched on railored tracks. Information may be switched as it



travels through various communication channels. The different types of data switching techniques are follows.

Circuit switching Message switching Packet switching

Circuit switching network establishes a fixed bandwidth circuit (channel) between nodes before the users may communicate, as if the nodes were physically connected with an electrical circuit. The bit delay is constant during the connection, as opposed to packet switching, where packet queues may cause varying delay.Each circuit cannot be used by other callers until the circuit is released and a new connection is set up. Even if no communication is taking place in a dedicated circuit then, that channel still remains unavailable to other users. Channels that are available for new calls to be set up are said to be idle. Telephone network is example of circuit switching system.Virtual circuit switching is a packet switching technology that may emulate circuit switching, in the sense that the connection is established before any packets are transferred, and that packets are delivered in order.

Message switching was the precursor of packet switching, where messages were routed in their entirety, one hop at a time. It was first introduced in 1961. Nowadays, message switching systems are mostly implemented over packet-switched or circuit-switched data networks. E-mail is example of a message switching system.

Packet switching is a communications paradigm in which packets (discrete blocks of data) are routed between nodes over data links shared with other traffic. The term "packets" refers to the fact that the data stream from your computer is broken up into packets of about 200 bytes (on average), which are then sent out onto the network. Each packet contains a "header" with information necessary for routing the packet from source to destination. Each packet in a data stream is independent.The main advantage of packet-switching is that it permits "statistical multiplexing" on the communications lines. The packets from many different sources can share a line, allowing for very efficient use of the fixed capacity. With current technology, packets are generally accepted onto the network on a first-come, first-served basis. If the network becomes overloaded, packets are delayed or discarded ("dropped").


Computer network

Engineering

information sharing

network administrator

networkingcomputer network

shared resources

digital signal

application sharing

way of encoding information

shared hard drives