OSI Layer Description....

OSI Layers                    

Ø  OSI layers explain the complete network communication process

Ø  It explains how the systems interact with each other

Ø  OSI layered architecture was designed by ISO& ITU-T

ISO –International standards organization

ITU-T-International telecommunication union –Telecom standard sector

                               

Application, presentation, session layers are called software layers

Transport layer is called Heart of OSI

Network, data link, physical layers are called hardware layers

7.Application layer

Functions:

Ø  It provides user interface

Ø  It gives network services to the user

Ø  Identification of port no depends on service

Protocols:

DNS, DHCP, HTTP, FTP, SMPT, Telnet

6.Presentation layer

Functions:

Ø  It converts data from standard format to machine format

Ø  Encryption  and decryption

Ø  Compression and decompression

Protocols:

ASCII, EBCDIC, GIF, TIFF, BMP, JPEG, MPEG, AVI, WAV

Ø  ASCII – American standard code for information interchange

Ø  EBCDIC – Extended  binary coded decimal interchange code

Ø  JPEG – joint picture expert group

Ø  TIFF – Tagged image file format

Ø  GIF – Graphical image format

Ø  BMP – Bit map image

Ø  MPEG – Motion picture expert group

Ø  AVI – audio video interleave

Ø  WAV – Windows audio video

5. Session layer

                   Functions:

Ø  It establish , maintains and terminate a logical session

                             Protocols:

                      NFS – Network file system

                      RPC – Remote procedure call

4. Transport layer

                              Function:

Ø  Segmentation

Ø  Adding TCP/UDP header

Ø  Sequencing &reassembling

Ø  Multiplexing & Demultiplexing

Ø  Error connection

Ø  Flow control

Protocols:

TCP – Transmission control protocol

UDP – User datagram protocol

Segmentation:

Ø  It is not possible to handle whole data as a unit

Ø  TCP typically handles 64KB of data as pay load

Ø  The data is divided into smaller segments

Ø  No of segments =total size /64KB

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

Adding TCP/UDP header:

Ø  TCP/UDP header is added to the data fragment

Ø  TCP header size is 20 bytes

Sequencing and reassembling:

Ø  Segment will be rearranged if they arrive in different order

Ø  This can be done with the help of sequence number in TCP header

Multiplexing and Demultiplexing :

Ø  When a system communities with multiple systems , if send segments to all systems simultaneously

Error connection:

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

Flow control:

Ø  Speed is adjusted automatically between source and destination computers , if one of the computers is lower

TCP & UDP differences:

TCP	UDP
Transmission control protocol	User datagram protocol
Connection oriented	Connection less
Reliable and slow	Unreliable and fast
Eg : Telnet FTP , HTTP , SMTP	Eg : SNMP , TFTP , DHCP

3. Network layer:

Function:

Ø  It provides logical IP addressing scheme

Ø  IP header is added at network layer

Ø  It chooses best path to destination

Ø  Carries the data in the chosen path

Protocols:

Ø  Routing protocols : finds all possible paths and chooses the best path

Ø  RIP – Routing information protocol

Ø  IGRP – Interior gateway routing protocol

Ø  EIGRP – Enhanced IGRP

Ø  OSPF – Open shortest path first

Ø  ISIS – Intermediate system to intermediate system

Routed protocols: carries the data in the chosen path

Ø  IP – Internet protocol

Ø  IPX – Internet packet exchange

Ø  Apple talk

2. Data Link Layer:

Function:

Ø  It gives network services to the computer

Ø  It does error detection (No correction)-FCS

Ø  Data link Header and Tailor are added to the packet

Protocols/sub layers:

Ø MAC Sub Layer: Media Access control

        

·         LAN Protocols (LAN connectivity)

·         FDDI, token ring, Ethernet

·         FDDI – Fiber Distributed Data Interface

Ø LLC Sub Layer: Logical Link Control

   

·         WAN Protocols( WAN Connectivity)

·         HDLC, PPP, Frame-Relay, X.25

HDLC	PPP
High Level Data Link control	Point to Point Protocol
Cisco Proprietary	Open Standard
No compression	Supports compression
Doesn’t support Authentication	Supports authentication PAP - Password Authentication Protocol CHAP – Challenge handshake Authentication protocol

Adding data link Header and Tailor:

14 byte data link header is added to the packet at beginning

4 byte data link tailor is added to the packet at ending(FCS/CRS)

Data link tailor is used for error checking

Source computer generates one value by running CRC algorithm on the data and sends that value in the tailor. Destination computer also runs CRC and compares that value with original. Destination system accepts the data if it matches.

Layer 2 limitations:

Ø Connect to upper layers via logical link control(LLC)

Ø Uses addressing schemes to identify devices

Ø Uses frames to organize bits into groups

Ø Uses (MAC) to identify transmission sources

1.   Physical Layer

Functions:

Ø It deals with electrical and mechanical properties

Ø Cables, connectors, voltage levels

Ø Example:  Rj-45, Rj-11 connectors, transceiver, v.35 cables

Layer 1 limitations:

Ø  Cannot connect to upper layers

Ø  Cannot identify devices

Ø  Only recognizes streams of bits

Ø  Cannot determine the source of a transmission when multiple devices are transmitting