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> 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)
