Thread: Open Systems Interconnection (OSI)

OSI model itself is not a network architecture because it does not specify the exact services and protocols to be used in each layer. It just tells what each layer should do. It is known as OSR open system Reference model because it deals with connecting open system i.e. Systems that are open for communication with other systems.



The seven layers of OSI reference model's are:

1. The Application layer: The main task of the Application layer is to identify the communication partner, synchronization, determining resource availability, sending a receiving application. HTIP, FTp, SMTP like services are operated on this layer.

2. The Presentation layer: The Presentation layer performs certain function that are requested sufficiently often to warrant finding a general solution for them, rather then letting each user solve the problem. Task like data compression, decompression, encryption, and decryption are associated with this layer.

3. The Session layer: The session layer allows user on different machines to establish session between them. A session allows ordinary data transport as does the transport layer but it also provides enhance services useful in some application.

4. The Transport layer: the basic function of the transport layer is to accept data from session layer, split it up in to smaller units if need then pass them to the network layer and ensure that the pieces all arrive correctly at the other end. Under normal conditions transport layer creates a distinct network connection for each transport connection required by the session layer.

5. The network layer: The network layer is concerned with controlling the operation of subnet. A key design issue is determine how packets are routed from source to destination.

6. Data link layer: It divides the input data to frames then transmit these frames sequentially and process the acknowledge frames sent back by the receiver. It has two sub layers:

• Logical link control: It logically identify differeent protocol and

• Media access control: It provides physical ad¬dresses to the frames and also transmits frame on the network

7. Physical layer: This layer provides physical lin~ such as electrical and mechanical, procedural interfaces between the devices. This layer also manage how the initial connection is establish and it turn down when both sides are finished and how many pin the network connector have and working of each pin.