The model is proposed by the ISO (International Standards Organization). It is called ISO-OSI (Open Systems Interconnection) Reference Model because it deals with connecting open systems. The OSI Reference Model has 7 layers.

a. Physical Layer :

• It is concerned with the actual physical attachment to the network i.e. it deals with the means of connecting two nodes in a network.

• It deals with transmitting raw bits over the communication channel.

• The design issues here deal with mechanical, electrical, timing interfaces and the physical transmission medium which lies below the physical layer.

b. Data Link Layer: It breaks the data into frames and passes it to the network layer. It also does:

• Error Control: To control transmission errors.

• Flow Control: To prevent the drowning of slow receiver by fast transmitter.

• Access Control: Control access to the shared channel. A special section of the DLL called the Medium Access Control sub layer deals with this.

c. Network Layer: It has the responsibility of preforming source to destination delivery of packets. It focuses on:

• Dynamic routing

• Congestion control

• Quality of service

• Addressing

• Integration of heterogeneous networks.

d. Transport Layer: It deals with:

• Control of data flow in the network.

• Ensuring no loss of data.

• Ensuring that destination is not inundated with data.

• Ensuring that all pieces arrive correctly at the other end.

• It is a true end to end layer.

e. Session Layer: Its features are:

• Dialogue Control: Keeping track of whose turn it is to transmit.

• Token management: Preventing two parties from attempting the same critical operation at the same time.

• Synchronization: Check pointing long transmissions to allow them to continue from where they were after a crash.

f. Presentation Layer: It is concerned with the following:

• Syntax of information.

• Semantics of information.

• Compression

• Encoding of information.

g. Application Layer: Application layer provide user interface and support for services like:

• E-mail.

• Remote file access.

• File transfer

• Shared database management.

Advantages of OSI Reference Model:

i. OSI Model distinguish between the services, interfaces and protocols.

ii. Protocols of OSI Model are very well hidden.

iii. They can be replaced by new protocols as technology changes.

iv. Supports connection oriented as well as connectionless service.

Disadvantages of OSI Reference Model:

i. Model was devised before the invention of protocols.

ii. Fitting of protocols is a tedious task.