4. INTORDUCTION OF OSI LAYERS

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OSI LAYERS  

> OSI layers explain the complete network communication process.
> It explains how the systems interact with each other.
> OSI layered architecture was designed by ISO & ITU-T.
        ISO- International standards Organization.
        ITU-T- International Telecommunication Union- Telecom standard sector.

 There are seven layers in OSI model. 

>Application, Presentation, Session layers are called Software Layers.

>Transport layer is called Heart nof OSI.

> Network, Data link, physical layers are called Hardware layers.

Application Layer 

Functions: -

> It provides user interface.

>It gives network services to the user.

> Identification of port No depends on service.

Presentation Layer

Function: -

> It converts data from standard format to machine format.

> Encryption and decryption.

>Compression and decompression.

Protocols

ASCII, EBCDIC, GIF, TIFF, BMP, ETC

Session layer

Functions: -

> It establish , maintains and terminates a logical session.

Protocols

NFS- Network file system.

RPC- Remote Procedure call

Transport Layer

Functions -:

>Segentaton.

> Adding TCP/UDP header.

>Sequencing & Reassembling.

>Multiplexing & Demultiplexing.

> Error correction.

>Flow control.

 Protocols

TCP- Transmission Control Protocol.

UDP- User datagram Protocol.

Segmentation

> It is not possible to handle whole data as a Unit.

> TCP typically handles 64KB of data as payload.

>The data is divided into smaller segments.

> No of Segments = Total size / 64KB

> Example: 1MB of data is made into 16 segments

Adding TCP/UDP Header

> TCP/UDP Header is added to the data fragment.

> TCP Header size is 20 Bytes.

Sequencing & Reassembling :

> Segments will be rerranged if they arrive in different order.

> This can be done with the help of sequence number in TCP Header.

Multiplexing & Demultiplexing:

>When a system communicates with multiple systems, it sends segments to all systems simultaneously.

Error Correction:

> Destination system queries the source for missing segments. Source needs to resend them.

Flow Control:

>Speed is adjusted automatically between source and destination computers, if one of the computers is slower.

 Network Layer :

Functions:-

    >It provides logical IP Addressing Scheme.

    > IP Header is added at Network layer.

    > It chooses best path to destination.

    > Carries the data in the chosen path.

Protocols: 

     >Routing Protocols:- Finds all possible paths and choose the best path.

               * RIP- Routing information Protocol.

               * IGRP-  Interior Gateway Routing Protocol.

               * EIGRP- Enhanced IGRP.

               * OSPF- Open Shortest Path First.

               * ISIS- Intermediate system to Intermediate System.

   >Routed Protocols:- Carries the data in the chosen path.

               * IP- Internet protocol.

               * IPX- Internet packet exchange.

               * Apple  talk.

Data Link Layer:

Functions:-

   > It gives network services to the computer.

   > It does error detection (No correction) -FCS

   > Data link Header and Tailor are added to  the packet.

Protocols/sub layers:

    > MACSub Layer : Media Access Control

                       * LAN protocol ( LAN connectivity)