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UNIT IIINETWORK

LAYER

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Network Layer Deals With

1.Routing(data transfer)2.Deals with the congestion control

3.To make the data transfer compatible for

two systemLowest layer which deals with the end to endtransmission.

When transmitting any pkt from source todestination it requires many routers.

2

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The network layer is concerned withgetting packets from the source all the way

to the destination.Goals which n/w layer has to achieve:

It must know about the topology of the

communication subnet and choose appropriatepaths through it.

It must take care to choose routes to avoidoverloading some of the communication lines

while leaving others idle. When the source and destination are in different

networks, it must deal with these differences

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Goals which designer should keepin mind while designing n/w layer

The services provided should beindependent of the underlying technology.

The transport layer (that is the hostcomputer) should be shielded from thenumber, type and different topologies of

the subnets he uses. That is, all the transport layer want is a

communication link, it need not know how

that link is made.

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Design issues for the network layer

Services provided to the transport layer

With these goals in mind, two differenttypes of service emerged:

1.Connection oriented and

2.Connectionless

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Connection-less service

In a connection-less service, the user simplybundles his information together, puts anaddress on it, and then sends it off, in the hopethat it will reach its destination.

There is no guarantee that the bundle will arrive.So - a connection less service is one reminiscentof the postal system.

A letter is sent, that is, put in the post box. It isthen in the "postal network" where it getsbounced around and hopefully will leave thenetwork in the correct place, that is, in theaddressee's letter box.

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A connection-oriented service

A connection-oriented service is one in whichthe user is given a "reliable" end to endconnection.

To communicate, the user requests aconnection, then uses the connection to hishearts content, and then closes the connection.

A telephone call is the classic example of aconnection oriented service.

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COMPARISON BETWEEN

CONNECTION ORIENTED AND

CONNECTION LESS SERVICE With a connection oriented service, theuser must pay for the length (ie theduration) of his connection.

Usually this will involve a fixed start up fee.Now, if the user intends to send a constantstream of data down the line, this is great -

he is given a reliable service for as long ashe wants.

CO SO

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COMPARISON BETWEEN

CONNECTION ORIENTED AND

CONNECTION LESS SERVICE However, say the user wished to send only apacket or two of data - now the cost of setting upthe connection greatly overpowers the cost of

sending that one packet. Consider also the case where the user wishes to

send a packet once every 3 minutes. In aconnection-oriented service, the line will thus be

idle for the majority of the time, thus wastingbandwidth.

COMPARISON BETWEEN

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COMPARISON BETWEEN

CONNECTION ORIENTED AND

CONNECTION LESS SERVICE So, connection-oriented services seem tobe useful only when the user wishes tosend a constant stream of data.

COMPARISON BETWEEN

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COMPARISON BETWEEN

CONNECTION ORIENTED AND

CONNECTION LESS SERVICE Its interesting to note here that it is easy toprovide a connection oriented service over aninherently connectionless service, so in fact

defining the service of the network layer asconnectionless is the general solution.

However, at the time of defining the networklayer, the controversy between the two camps

was (and still is) unresolved, and so instead ofdeciding on one service, the ISO allowed both.

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EXAMPLE OF CL AND CO

These two camps are represented by ourtwo running examples:

The Internet has a connectionless networklayer.

ATM networks have a connection-orientednetwork layer.