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Data Communication & Networking Introduction:

• In simple terms network means an interconnected set of some objects. • For decades we are familiar with the Radio, Television, railway, Highway, Bank and other

types of networks. • In recent years, the network that is making significant impact in our day-to-day life is the

Computer network. • By computer network we mean an interconnected set of autonomous computers. • The term autonomous implies that the computers can function independent of others. • However, these computers can exchange information with each other through the

communication network system. History of Networking:

• In a short period of time computer networks have become an indispensable part of business, industry, entertainment as well as a common-man's life.

• Initially, computer network was developed for defense purpose, to have a secure communication network that can even withstand a nuclear attack.

• After a decade or so, companies, in various fields, started using computer networks for keeping track of inventories, monitor productivity, communication between their different branches offices located at different locations.

• For example, Railways started using computer networks by connecting their nationwide reservation counters to provide the facility of reservation and enquiry from anywhere across the country.

• And now after almost two decades, computer networks have entered a new dimension; they are now an integral part of the society and people.

• In 1990s, computer network started delivering services to private individuals at home. • Some of the services are access to remote information, person-person communication,

and interactive entertainment. Network Goals:

1. Resource Sharing. 2. Providing High Reliability. 3. Distribution of Work Load. 4. Saving Money. 5. Expandability. 6. Powerful Communication Medium. 7. Protecting Information. 8. Preserving Information.

Applications of Networking:

1. Marketing and sales 2. Financial services 3. Manufacturing 4. Directory services 5. Information services 6. Electronic data interchange 7. Electronic mail 8. Teleconferencing 9. Voice over IP 10. Video on demand

Source: Source is where the data is originated. Typically it is a computer, but it can be any other electronic equipment such as telephone handset, video camera, etc, which can generate data for transmission to some destination. Transmitter: As data cannot be sent in its native form, it is necessary to convert it into signal. This is performed with the help of a transmitter such as modem. Important Terms: Data:

• Data refers to information that conveys some meaning based on some mutually agreed up rules or conventions between a sender and a receiver.

• It comes in a variety of forms such as text, graphics, audio, video and animation. • Data can be of two types; analog and digital. • Analog data take on continuous values on some interval. Typical examples of analog data are

voice and video. The data that are collected from the real world with the help of transducers (are continuous-valued or analog in nature.

• Digital data take on discrete values. Text or character strings can be considered as examples of digital data. Characters are represented by suitable codes, e.g. ASCII code, where each character is represented by a 7-bit code.

Signal:

• It is an electrical or electronic representation of data, which can be sent over a communication medium.

• Stated in mathematical terms, a signal is merely a function of the data. • For example, a microphone converts voice data into voice signal, which can be sent over a

pair of wire. • Analog signals are continuous-valued; digital signals are discrete-valued.

Network Technologies:

• There is no generally accepted taxonomy into which all computer networks fit, but two dimensions stand out as important: Transmission Technology and Scale.

Classification Based on Transmission Technology:

• Computer networks can be broadly categorized into two types based on transmission technologies:

– Broadcast networks – Point-to-point networks

• As a general rule smaller, geographically localized networks tend to use broadcasting, whereas larger networks normally use are point-to-point communication.

1. Broadcast Networks:

• Broadcast network have a single communication channel that is shared by all the machines on the network.

• All the machines on the network receive short messages, called packets sent by any machine. • An address field within the packet specifies the intended recipient. • Upon receiving a packet, machine checks the address field. • If packet is intended for itself, it processes the packet; if packet is not intended for itself it is

simply ignored.

• This system also allows for addressing the packet to all destinations (all nodes on the network).

• Such a packet is transmitted and is received by all the machines on the network. • This mode of operation is known as Broadcast Mode. • The end devices that wish to communicate are called stations. • The switching devices are called nodes. • Some Nodes connect to other nodes and some to attached stations. • It uses FDM or TDM for node-to-node communication. • There may exist multiple paths between a source and a destination for better network

reliability. • The switching nodes are not concerned with the contents of data. • Their purpose is to provide a switching facility that will move data from node to node until

they reach the destination. Classification based on Scale: Alternative criteria for classifying networks are their scale. They are divided into :

• Local Area Network (LAN), • Metropolitan Area Network (MAN), • Wide Area Networks (WAN).

Local Area Network:

• LAN is usually privately owned. • It links the devices in a single office, building or campus of up to few kilometers in size. • These are used to share resources (may be hardware or software resources) and to exchange

information. • LANs are distinguished from other kinds of networks by three categories: their size,

transmission technology and topology. • LANs are restricted in size, which means that their worst-case transmission time is bounded

and known in advance. • LAN typically used transmission technology consisting of single cable to which all machines

are connected. • The most common LAN topologies are bus, ring and star.

Metropolitan Area Networks (MAN): • MAN is designed to extend over the entire city. • It may be a single network as a cable TV network or it may be means of connecting a number

of LANs into a larger network so that resources may be shared. • For example, a company can use a MAN to connect the LANs in all its offices in a city. • MAN is wholly owned and operated by a private company or may be a service provided by a

public company. • The main reason for distinguishing MANs as a special category is that a standard has been

adopted for them. It is DQDB (Distributed Queue Dual Bus) or IEEE 802.6.

Wide Area Network (WAN):

• WAN provides long-distance transmission of data, voice, image and information over large geographical areas that may comprise a country, continent or even the whole world.

• In contrast to LANs, WANs may utilize public, leased or private communication devices, usually in combinations, and can therefore span an unlimited number of miles.

• A WAN that is wholly owned and used by a single company is often referred to as enterprise network.

The Internet: • Internet is a collection of networks or network of networks. • Various networks such as LAN and WAN connected through suitable hardware and software

to work in a seamless manner. • It allows various applications such as e-mail; file transfer, remote log-in, World Wide Web,

Multimedia, etc, run across the internet. • The basic difference between WAN and Internet is that WAN is owned by a single

organization while internet is not so. • But with the time the line between WAN and Internet is shrinking, and these terms are

sometimes used interchangeably. Modes of Communication: There are three possible modes in communication: simplex, full duplex and half duplex. 1. Simplex:

• In simplex mode, the communication is unidirectional, such as from a computer to a printer. 2. Half Duplex:

• In half-duplex mode of communication, each station can both send and receive data. But, when one is sending, the other one can only receive and vice versa.

3. Full Duplex: • In full-duplex mode both the sides can communicate simultaneously.

Network Topologies: Introduction:

• Topology refers to the way in which the network of computers is connected. • Topology is defined as the geometrically interconnection pattern by which the stations

(nodes/computers) are connected using suitable transmission media (which can be point-to-point and broadcast).

• The choice of topology is dependent upon type and number of equipment being used, planned applications and rate of data transfer required, response time, and cost.

Mesh Topology: • In this topology each node or station is connected to every other station.

Features:

• Two nodes are connected by dedicated point-point links between them. • Media used for the connection (links) can be twisted pair, co-axial cable or optical fiber. • With this topology there is no need to provide any additional information that is from where

the packet is coming, along with the packet because two nodes have a point-point dedicated link between them.

• Mesh Topology is not flexible and has a poor expandability as to add a new node n links have to be laid because that new node has to be connected to each of the existing nodes via dedicated link.

• For the same reason the cost of cabling will be very high for a larger area. Key Characteristics:

• Fully connected • Robust – Highly reliable • Not flexible • Poor expandability

Bus Topology • In Bus Topology, all stations attach through appropriate hardware interfacing known as a tap,

directly to a linear transmission medium, or bus. • Full-duplex operation between the station and the tap allows data to be transmitted onto the

bus and received from the bus. • A transmission from any station propagates the length of the medium in both directions and

can be received by all other stations. • At each end of the bus there is a terminator, which absorbs any signal, preventing reflection

of signal from the endpoints. • If the terminator is not present, the endpoint acts like a mirror and reflects the signal back

causing interference and other problems.

Features: • A shared link is used between different stations. Hence it is very cost effective. • One can easily add any new node or delete any node without affecting other nodes; this

makes this topology easily expandable. • Because of the shared medium, it is necessary to provide some extra information about the

desired destination, i.e. to explicitly specify the destination in the packet, as compared to mesh topology.

• This is because the same medium is shared among many nodes. As each station has a unique address in the network, a station copies a packet only when the destination address of the packet matches with the self-address.

• This is how data communications take place among the stations on the bus. • As there are dedicated links in the mesh topology, there is a possibility of transferring data in

parallel. But in bus topology, only one station is allowed to send data at a time and all other stations listen to it, as it works in a broadcast mode.

• Hence, only one station can transfer the data at any given time. Key Characteristics of this topology are:

• Flexible • Expandable • Moderate Reliability • Moderate performance

STAR Topology

• In the star topology, each station is directly connected to a common central node. • Typically, each station is attached to a central node, referred to as the star coupler, via two

point-to-point links, one for transmission and one for reception. • In general, there are two alternatives for the operation of the central node.

• One approach is for the central node to operate in a broadcast fashion. • A transmission of a frame from one station to the node is retransmitted on all of the

outgoing links. • In this case, although the arrangement is physically a star, it is logically a bus. • A transmission from any station is received by all other stations, and only one station

at a time may successfully transmit. • In this case the central node acts as a repeater. • Another approach is for the central node to act as a frame-switching device. • An incoming frame is buffered in the node and then retransmitted on an outgoing link

to the destination station. • In this approach, the central node acts as a switch and performs the switching or

routing function.

• This mode of operation can be compared with the working of a telephone exchange, where the caller party is connected to a single called party and each pair of subscriber who needs to talk have a different connection.

• Very High speeds of data transfer can be achieved by using star topology, particularly when the star coupler is used in the switch mode.

• This topology is the easiest to maintain, among the other topologies. • As the number of links is proportional to n, this topology is very flexible and is the most

preferred topology. Key features:

• High Speed • Very Flexible • High Reliability • High Maintainability

Ring Topology:

• In the ring topology, the network consists of a set of repeaters joined by point-to-point links in a closed loop.

• The repeater is a comparatively simple device, capable of receiving data on one link and transmitting them, bit by bit, on the other link as fast as they are received, with no buffering at the repeater.

• The links are unidirectional; that is data are transmitted in one direction only and all are oriented in the same way.

• Thus, data circulate around the ring in one direction (clockwise or counterclockwise). • Each station is attached to the network at a repeater and can transmit data onto the network

through that repeater. • This topology is not very reliable, because when a link fails the entire ring connection is

broken. But reliability can be improved by using wiring concentrator, which helps in bypassing a faulty node.

Tree Topology:

• This topology can be considered as an extension to bus topology. • It is commonly used in cascading equipment. • For example, you have a repeater box with 8-port, as far as you have eight stations, this can

be used in a normal fashion. • But if you need to add more stations then you can connect two or more repeaters in a

hierarchical format (tree format) and can add more stations. • This tree topology is very good in an organization as incremental expansion can be done in

this way. • Main features of this topology are scalability and flexibility. • This is because, when the need arises for more stations that can be accomplished easily

without affecting the already established network. Data Transmission Media: Introduction:

• Transmission media can be defined as physical path between transmitter and receiver in a data transmission system.

• It may be classified into two types as shown in following figure: • Guided: Transmission capacity depends critically on the medium, the length, and

whether the medium is point-to-point or multipoint (e.g. LAN). Examples are co-axial cable, twisted pair, and optical fiber.

• Unguided: Provides a means for transmitting electro-magnetic signals but do not guide them. Example wireless transmission.

Twisted Pair:

• This is the least expensive and most widely used guided transmission medium. • In twisted pair technology, two copper wires are strung between two points. • The two wires are typically ``twisted'' together in a helix to reduce interference between the

two conductors. • Twisting decreases the cross-talk interference between adjacent pairs in a cable. • Typically, a number of pairs are bundled together into a cable by wrapping them in a tough

protective sheath. • They can carry both analog and digital signals. • It spans distances of several kilometers. • Data rate is determined by wire thickness and length. • Shielding is to eliminate interference from other wires impacts signal-to-noise ratio, and

ultimately, the data rate. • It provides for good, low-cost communication. • Indeed, many sites already have twisted pair installed in offices -- existing phone lines!

Typical characteristics:

• Twisted-pair can be used for both analog and digital communication. • To reduce interference, the twisted pair can be shielded with metallic braid. This type of wire

is known as Shielded Twisted-Pair (STP) and the other form is known as Unshielded Twisted-Pair (UTP).

Use:

• The oldest and the most popular use of twisted pair are in telephony. • In LAN it is commonly used for point-to-point short distance communication (say, 100m)

within a building or a room. Coaxial Cable:

• A: Protective Plastic Covering. • B: Braided Outer Conductor • C: Insulating material • D: Copper Wire

Coaxial cable consists of a stiff copper wire as the core, surrounded by an insulating material.

• A cylindrical conductor encases the insulator, often as a closely woven braided mesh. • This is enclosed in a protective plastic sheath. • The Signal is transmitted by the inner copper wire and is electrically shielded by the outer

metal sleeve. • Two kinds of coax are widely used:

• (Base band coaxial cable) used for digital transmission. • (Broad band coaxial cable) used for analog transmission.

Characteristics:

• It can be used to transmit both analog and digital signals. • Because of its shielded, concentric construction, it is less susceptible to interference and

crosstalk. • Base band cables give data rates upto 10 Mbps.

• Broad band refers to higher bandwidth and is used for analog transmission. • They are normally divided into multiple channels used for analog T.V., audio or data. • These cables need analog amplifiers to strengthen the signal periodically.

Two types of broadband systems have emerged:

• Single Cable System: It is the one in which different frequency bands are allocated for inbound and outbound communication on a single cable.

• Dual cable systems: It uses two cables, one for transmission in each direction: • One cable is used for receiving data. • Second cable used to send data to a device called headend.

• When a node wishes to transmit data, it sends the data to a special node called the headend.

The head end then resends the data on the first cable. Thus, the headend acts as a root of the tree, and all data must be sent to the root for redistribution to the other nodes.

Use:

• One of the most popular uses of co-axial cable is in cable TV (CATV) for the distribution of TV signals.

• Another importance use of co-axial cable is in LAN. • Long distance telephone transmission.

Fiber Optics:

• In fiber optic technology, the medium consists of a hair-width strand of silicon or glass, and the signal consists of pulses of light.

• For instance, a pulse of light means ``1'', lack of pulse means ``0''. • It has a cylindrical shape and consists of three concentric sections: the core, the cladding, and

the jacket as shown in following figure. • Optical fibers may be single mode or multimode. • Single mode fibers allow the light to travel in a straight line and require Laser as source. • Multimode fibers allow light to be incident at any angels and use multiple light paths.

Transmission in Optical Fiber:

• An optical transmission system has 3 components: the light source, the medium and the detector.

• The source is a laser which emits light when an electrical current is applied. • The detector converts light energy to electrical signal. • The core and cladding are designed such that the light signal is trapped inside the core and

does not escape outside. Applications:

• Very widely used in long distance telecommunications.

• Military applications. • LAN, WAN and MAN.

Unguided Transmission:

• Unguided transmission is used when running a physical cable (either fiber or copper) between two end points is not possible.

• For example, running wires between buildings is probably not legal if the building is separated by a public street.

• Infrared signals are typically used for short distances (across the street or within same room). • Microwave signals are commonly used for longer distances (10's of km). • Sender and receiver use some sort of dish antenna.

Difficulties in Unguided Transmission:

• Weather interferes with signals: For instance, clouds, rain, lightning, etc. may adversely affect communication.

• Radio transmissions easy to tap: A big concern for companies worried about competitors stealing plans.

• Signals bouncing off of structures may lead to out-of-phase signals that the receiver must filter out.

Satellite Communications:

• Satellite communication is based on ideas similar to those used for line-of-sight. • A communication satellite is essentially a big microwave repeater or relay station in the sky. • Microwave signals from a ground station is picked up by a transponder, amplifies the signal

and rebroadcasts it in another frequency, which can be received by ground stations at long distances.

• A satellite can be used for point-to-point communication between two ground-based stations or it can be used to broadcast a signal received from one station to many ground-based stations

Use:

• Now-a-days communication satellites are not only used to handle telephone, telex and television traffic over long distances, but are used to support various internet based services such as e-mail, FTP, World Wide Web (WWW), etc.

• New types of services, based on communication satellites, are emerging. Communication Protocols:

• A protocol is a set of formal operating rules, procedures or conventions that govern a given process.

• A communication protocol describes the rules that govern the transmission of data over communication networks.

• These rules provide a method for orderly and efficient exchange of data between the sender and receiver, and for the proper interpretation of controls and data, which are transmitted as raw bits and bytes.

• These rules are embedded in the data communication software. Roles of a communication protocol:

• Data sequencing: It refers to breaking a long message into smaller packets of fixed size. Data sequencing rules define the method of numbering (or sequencing) packets to detect loss or duplication of packets, and to correctly identify packets, which belong to the same message.

• Data routing: Routing algorithms are designed to find the most efficient paths between the source and destination nodes of a message. They can handle varying degree of traffic on the present network configuration with optimal time utilization.

• Data formatting: Data formatting rules define which group of bits or characters within a packet constitutes data, control, addressing or other information.

• Flow control: A communication protocol also prevents a fast sender from overwhelming a slow receiver. It ensures resource sharing and protection against traffic congestion by regulating the flow of data on the communication lines.

• Error Control: These rules are designed to detect errors in messages and to ensure transmission of correct messages.

• Precedence & order of transmission: These rules ensure that all nodes get a chance to use the communication lines and other resources of the network based on the priorities assigned to them.

• Connection establishment and termination: These rules define how connections are established, maintained and terminated when two nodes of a network want to communicate with each other.

• Data security: Providing data security and privacy is also built into most communication software packages. It prevents access of data by unauthorized users.

• Log information: Such information may be used for charging the users of the network based on their usage of network resources.

OSI Model:

• OSI model is an international standard on network architectures. • OSI model is a framework for defining standards for linking heterogeneous computers in a

packet switched mode. • OSI protocol made it possible for any two heterogeneous computer systems located anywhere

in the world, to easily communicate with each other. • OSI model is designed in a highly structured way. • It is a seven layer architecture in which a separate set of protocols is defined for each layer. • Each layer has an independent function and deals with one or more specific aspects of the

communication. Physical Layer:

• The physical layer is responsible for transmitting raw bit streams between two nodes. • It converts the sequence of binary digits into electric signals, light signals or electromagnetic

signals depending on whether the nodes are on cable circuit, fiber optic or microwave respectively.

• It decides about details as whether the transmission can take place in one direction or in both directions, how many bits can be sent per second.

• Devices associated with physical layer are hubs, cables, connectors, repeaters, multiplexers, transmitters, receivers.

Data Link Layer:

• The physical layer simply transmits the data from the sender’s node to the receiver’s node as raw bits.

• It is the responsibility of the data link layer to detect and correct any errors in the transmitted data.

• Data link layer partitions data into frames so that error detection and correction can be performed independently for each frame.

• It performs flow control of frames between two sites. • It ensures that a sender does not overwhelm a receiver by sending frames at a rate faster than

the receiver can process. • Devices operated at this layer are bridges & intelligent hubs.

Network Layer:

• It is responsible for setting up a logical path between two nodes for communication to take place.

• It encapsulates frames into packets, which can be transmitted from one node to another. • Routing is the primary job of the network layer. • Congestion control is another function of this layer. • It handles billing of customers by keeping track of the number of packets or bits sent by each

customer. • This network has to handle internetworking i.e. if two end systems are in different networks. • It has to provide for heterogeneous networks to be connected.

Transport Layer:

• Transport layer accepts messages of arbitrary length from session layer. • It segments them into packets. • It submits packets to the network layer for transmission. • It reassembles the packets at the destination. • The transport layer includes mechanisms for handling lost and out-of-sequence packets. • For this it records a sequence number in each packet, and uses this number for detecting lost

packets and for ensuring that messages are reconstructed in the correct sequence. • Device used by transport layer is Gateways.

Session Layer:

• The session layer provides means of establishing, maintaining and terminating a dialogue or a session between two end users.

• It allows the two parties to authenticate each other before establishing a dialog session between them.

• It specifies dialog type-one way, two way alternate, or two way simultaneous. Presentation Layer:

• It provides facilities to convert message data into a form, which is meaningful to the communicating application layer entities.

• Presentation layer may perform transformations as encoding and decoding, code conversion, compression and decompression, encryption and decryption on the message data.

Application Layer:

• The application layer provides services that directly support the end users of the network.

• It is a collection of protocols for commonly used applications as electronic mail, file transfer, remote login, remote job entry.

Components of LAN: A LAN is a combination of several components working together to transfer data from one machine to another. The components of LAN are:

• Workstations. • Servers.

• File • Database • Print • Communication • Fax

• Transmission Media. • Network interface cards. • Network connecting devices (Hub, Repeater, Bridges, Routers and Gateways) • Network operating system.

Workstation:

• A workstation is any device connected to the LAN and is capable of communicating with other devices connected to the LAN i.e. computer, printer etc.

• Workstations are PCs or terminals connected to the server. • Workstations are also referred to as Clients.

Servers:

• The basic engine of a LAN is the server. • A server is simply software running on a particular computer that provides services to LAN

users. • It can be a personal computer, minicomputer, mainframe, or specialized computer.

Different types of Servers:

1. File Server: • A file server is a combination of hardware and software that permits LAN users to share

computer programs and data. • Many LANs have a dedicated server whose only purpose is dealing with shared files on a

LAN. • They have extensive hard disk capacity, large memory, and a powerful processor so they

can handle the needs of many users.

2. Database Server: • A database server is a computer that stores a database on disk and makes the data

accessible by multiple users. • Specialized database software using “client/server” architecture makes the best use of a

database server. • The software is made of two components the client component and the server

component. • The client component runs on one or more of the LAN workstations where it provides an

interface for users to request information or modify records in database. • The server component runs on a central machine where it stores and controls the data.

• Some of its activities include protecting the data, permitting multiple users to access data, updating and deleting records etc.

3. Print Server:

• A print server can be a computer and/or program that provide LAN users with access as a centralized printer or a shared printer of a workstation.

• Print servers use a spool, which is hardware, software, or a combination of both to control a buffer that holds data before it is sent to the printer.

• Here files are stored and placed in a queue in a first in, first out order until it’s their turn to be printed.

• Print servers also have a chunk of RAM as a buffer that holds data while the printer and server communicate.

4. Communication Server:

• Communication server gives all nodes on a LAN access to its modems. • It provides links to external data networks and to corporate minicomputers and

mainframes.

5. Internet Information Server/web server: • A web server provides the workstation users access to the Intranet web site of the

company or organization. • This web site if registered can be accessed by Internet.

6. Mail Server:

• Mail servers provide e-mail, chat etc services to the users. Example includes Microsoft Exchange 2000 server and outlook express being used in the client side.

7. Transmission Media:

• The transmission media is the stuff through which signals travel. It can be guided as in the case of a wire, or unguided as in the case of air (wireless).

Network Interface Cards:

• A network interface card (NIC) is a computer circuit board or card that is installed in a computer so that it can be connected to a network.

• Personal computers and workstations on a local area network (LAN) typically contain a network interface card specifically designed for the LAN transmission technology, such as Ethernet or Token Ring.

• Network interface cards provide a dedicated, full-time connection to a network. • A network interface card, more commonly referred to as a NIC, is a device that allows

computers to be joined together in a LAN. • Networked computers communicate with each other using a given protocol for transmitting

data packets between the different machines, known as nodes. • The network interface card acts as the liaison for the machine to both send and receive data

on the LAN. Network Connecting Devices:

1. Repeater: • All transmission media weaken the signal traveling through them. • This limits the distance any medium can carry data. • A repeater is a device that amplifies the signal allowing it to travel farther.

• They fall in two categories: o Amplifiers: These simply amplify the entire incoming signal. They amplify both

the signal and the noise. o Regenerative Repeaters: They identify the incoming data admidst the noise,

reconstruct the original data and retransmit it. 2. Hub:

• Hub refers to a multiport repeater. • It can be used to create multiple levels of hierarchy of stations. • This type of interconnected set of stations is easy to maintain and diagnose. • Figure shows how several hubs can be connected in a hierarchical manner to realize a

single LAN of bigger size with a large number of nodes. • Hub as a multi-port repeater can be connected in a hierarchical manner to form a single

LAN with many nodes.

3. Bridges: • The device that can be used to interconnect two separate LANs is known as a bridge. • It is commonly used to connect two similar or dissimilar LANs.Unlike repeater, which

blindly passes on all the signals it receives, a bridge reads the address of all the signals it receives and determine the appropriate network segment to which it has to be passed.

• They use a process of learning , filtering and forwarding to direct traffic to its proper destination.

• They are useful in network partitioning. • When it is found that a network segment has excessive traffic, it can be broken into two

segments with a bridge. • The bridge does not allow any traffic to segment unless it is intended for it, thus limiting

traffic flow. • Use of bridges offers a number of advantages, such as higher reliability, performance,

security, convenience and larger geographic coverage.

4. Routers: • Router is more intelligent than bridge and is basically used to connect two networks using

different medium or access protocols. • They consists of a combination of hardware and software. • The hardware includes physical interface to various network. • The software consists of routing protocols. • A router operates at a level higher than the bridge. • It can get a lot of information about the network. • When multiple networks are interconnected, there may be more than one path from a

given source to destination machine. • A router maintains a table of available routes and their status and uses this information

along with some routing algorithm to determine the best route for the packet. • The route tables may be static or dynamic. • Static route tables once created are not changed and thus all packets must follow a

predetermined path. • Dynamic route tables change according to changing network conditions and hence allow

the best routes to be selected. Difference between a bridge and a router:

• Bridge is simpler and less expensive. A router is more intelligent and more expensive. • A bridge makes simple decision whether or not to send the packet to the other side. A router

uses route tables and algorithms to determine the best path for a packet through a complex network of many LAN’s.

• Router processing is slower than bridge processing because it has to read the network and device address use the algorithm along with the router table to determine the most efficient path.

• Many routers can also function as bridges but is not possible other way round. • Bridges are suitable for smaller, simpler networks. Routers are used with large complex

networks.

5. Brouters: • A brouter is a network bridge and a router combined in a single product. • They are referred as multi-protocol routers. • A brouter first tries to deliver a packet based on network protocol information. • If the packet uses a protocol that is routable, it routes the packet otherwise the packet is

bridged. • It is complex and difficult to install.

6. Gateways:

• Gateways operate at the top layers of the OSI model. • It is a powerful device that can interconnect networks built on totally different

communication architecture. Multiplexers:

• There are many applications in which several terminals are connected to a computer. • If a terminal is operating at a speed less than communication line it is an inefficient

operation. • A channel is an expensive resource. • For its optimal utilization, the channel can be shared to simultaneously transmit multiple

signals over it. • The method of dividing a physical channel into multiple logical channels to enable a number

of independent signals to be simultaneously transmitted on it is known as multiplexing. • The electronic device that performs this task is known as a multiplexer. • A multiplexer takes several data communication lines or signals, and converts them into one

data communication line or signal at the sending location. • For eg. If there are 4 terminals connected to a multiplexer. • The multiplexer takes the signal from 4 terminals and convert them into one large signal,

which can be transmitted over one communication line. • At the receiving location an multiplexer takes one large signal and breaks it into the original

4 signals. Concentrators:

• A concentrator performs the same function as a multiplexer, but concentrator has the ability to actually reduce the number of signals.

• Example 100 signals from different devices coming into the concentrator could leave as onto 70 or 80 signals.

• They are concentrators to form a fewer number of signals. • A concentrator is an intelligent multiplexer.

Front End Processors:

• In early network days, the communication processing job was handled by the main computer.

• This began to place heavy additional loads on the CPU. • Hence the ability of the computer to do information processing was degraded. • Front end processors were designed to solve this problem.

• The primary purpose of front end processor is to offload communications processing task from the main computer.

• Main computer can be dedicated to applications and data processing jobs.

Switching Techniques: • Data communication takes place between two devices which are directly connected by some

form of transmission medium. • However it is impractical or uneconomical for two devices to be directly connected. • Communication is achieved by transmitting data from source to destination through a

network of intermediate nodes. • These nodes provide a switching facility which moves data from node to node until the

destination is reached. • There are 3 switching techniques used to establish communication links between sender &

receiver in communication network: – Circuit Switching – Message Switching – Packet Switching

Circuit Switching:

• This is the simplest method of data communication. • A dedicated physical path is established between the sending and receiving stations through

the nodes of the network. • Eg. Method used to connect two subscribers for a telephonic conversations. • There are number of nodes between the sending station and receiving station. • When a message is to be communicated, aphysical path is established between the sending

station and receiving station by physically connecting the incoming and outgoing line of each of the intermediate switching nodes which fall on the path.

• Once a circuit is established between two stations it is used by two parties and link continues to exist until the connection is terminated either by sender or the receiver.

• As soon as the connection is terminated by one of the two stations, the dedicated resources are deallocated and can be used by other stations.

• Circuit switching involves 3 phases: • Circuit establishment • Data transfer • Circuit disconnection

Message Switching:

• A message is a logical unit of information. • In this method before sending a message from one station to another first destination

address is appended to the message. • After this the message is transmitted from the source to its destination either by store &

forward or broadcast method. Store & Forward Method:

• The message is transmitted from source node to an intermediate node. • Intermediate node stores the complete message temporarily, inspects it for errors and

transmits it to the next node based on an available free channel. • When the message reaches a node, a channel on which it came is released for use by another

message. Broadcast Method:

• Message is broadcast over a common medium.

• All stations check the destination address of each message as they pass by and accepts only those addressed to them.

Packet Switching:

• A message is split up into ‘packets’ of fixed size. • A packet consists of:

– Block of data to be sent, – Header of a packet with source & destination address. – Control information, – Message number, – Number of current & last packet, – Acknowledgement bytes.

• The packet is routed either by store & forward method or by broadcast method.

Ankit Gupta [email protected]