OSI MODEL

key points...................................
1.It is the certified model of ISO.
2.It is the reference model of data communication.
3.It divides the communication in smaller parts
4.It provide compatibility between different network vendor products.
5.It is divides the task into layers(seven layer)
6.Each layer does the specific task without affecting other layers
7.Each upper layer request the service from adjacent lower layer,in return lower layer provide service to upper layer.
8.Each layer after completing task add the header(encapsulation) which tells what is done by particular layer.called PDU.
9.Encapsulation is process of wrap the data in suitable technology which is understand by particular device.
10.Upper layer(7,6,5),makes data header.
11.layer 4 add segment header
12.layer 3 add packet header
13.layer 2 add frame header
14.layer 1 add bit &byte header.
Working of layer...........................
Application layer:-(7)
1.send data header
2.use service like http,ftp,telnet,smtp,dns,dhcp.
3.provide availability of resources
4.interface between user and application.
Presentation layer:-(6)
1.Maintain data format(jpeg,doc,xml etc)
2.provide service like encryption and compression.
Session Layer:-(5)
1.start and end the connection between application
2.use method like simplex,half duplex,full duplex.

Transport layer:-(4)
1.provide host to host communication
2.terminate virtual circuit
3.use tcp and udp service
4.provide error detection and correction.
5.add segment header.
Network layer:-(3)

1.provide network to network communication
2.provide path determination and selection
3.does routing
4.add packet header.
Data link Layer:-(2)
1.encapsulate the packet with frame header

2.provide error notification but not detection
3.provide upper layer interaction.(802.2)
4.provide physical addressing(802.3)
5.devices like switch nic,bridge work on it.
Physical Layer:-(1)
1.carry data over electronics signals
2.devices like hub repeater work on it.