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Reference Model

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Layered network

• Why network are layered ?!

- Easy to change - Easy to troubleshoot- Give general idea of each layer

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OSI

Reference Model

Reference Model

TCP/IP

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OSI• History :- Developed By ISO (international standard

organization)- OSI shortened from ( open system Inter

connection)- OSI has 7 layers and there were principle to

arrive to these seven layers 1)- layers should be created where different

abstraction is needed.2) Each layer should perform specific function.

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OSI (cont)

3) The function of each layer should defined looking to protocol.

4) Minimize information flow5) Not too small not too large

6Figure 1-20: The OSI reference Model.

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Physical layer

• Where the actual connection happens

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Data link layer

• Transform row data into data frames and send it to receiver and the receiver will send acknowledgment frame .

• Flow control .

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Network Layer

• Routing packets from source to destination• Control congestion.• Solve problems of addressing.

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Transport Layer• Accept data from above split it up into smaller

units pass these to network layer and ensure all these pieces arrives correctly to other end.

• Error free point-to-point connection• End – to – end layer

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Session layer • Start or establish the connection between

users of different machines , and it offers:1-dialog control.2-token management.3-syncronization.

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Presentation layer• Concerned with data presentation.

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The application layer

• Contains protocol needed by users : HTTP _ SMTP _ FTP

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TCP/IPHistory:ARPANET “research network sponsored by

D.O.D to connect universities and governments institution using leased telephone line .

What led's to producing TCP/IP ?

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TCP/IP has 4 Layers

Figure 1-21: The TCP/IP reference Model.

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1) Internet layer

-Deliver packet to desired destination.-Defining official packet format and

protocol called (IP)“internet protocol”.

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2)Transport LayerMain job to allow peer entities in source and

destination to carry on conversation.

has two protocols:1) TCP _ transmission control protocol.- Reliable connection oriented protocol- Flow control2)UDP _ User Datagram Protocol- Unreliable connection protocol.

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The application layer

• Contains protocol needed by users : HTTP _ SMTP _ FTP

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Host_To_Network

• Main job to connect host to network.• Rarely discussed on TCP/IP papers.

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Comparison Between OSI and TCP/IP

• Similarities - both based on concept of stack of

independent protocols.- layers in both models have the same

function.

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Comparison Between OSI and TCP/IP

• Differences1)there are Three concepts that are centralizedto OSI Model:1- services 2- interfaces 3- protocols

2)number of layers.3)Connectionless and connection oriented

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Critique of OSI Model and protocols

1.Bad timing .2.Bad technology.3.Bad implementation.4.Bad politics.

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Figure 1-23: The apocalypse of the two elephants

Bad Timing

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Critique of TCP/IP reference model

• First: does not distinguish between services interface and protocol.• Second: it is not all general.• Third: host to network layer .• Fourth: physical and data link layer .• Last: many other protocol were ad hoc.