The user of computer network are located over the wide physical range.

Therefore to ensure that for communication system can be developed and are compatible

To each other, and international group of standards has been developed.

These standards will fit into a framework which has been developed by the ISO (international organization of standardization).

It is called as ISO-OSI reference model because it is designed to deal with open system.

i.e. the system which are open for communication with other system.

There are seven layers of OSI model:

1. Physical layer - For make and break connection. It convert data bits into electrical signal.

2. Data link layer - To assemble outgoing messages into frames. For synchronization, error detection & correction.

3. Network layer - Routing of the signals, divide the outgoing message into packets, to act as network controller for routing data.

4. Transport layer - decide whether transmission should be parallel or single path, multiplexing, to break data into smaller units for efficient handling.

5. Session layer - to manage and synchronize conversation between between two system. It control logging on and off, user identification.

6. Presentation layer - it works as a translating layer.

7. Application layer - retransferring files of information, login, password checking.