OSI Reference Model

   

The model is called the ISO OSI (Open System Interconnection) model because it deals with connecting open systems, that is, system that are open for communication with other systems.
The following figure shows the overview of OSI Reference Model:

Fig. OSI Reference Model

The OSI model has seven layers. The principles that were applied to arrive at the seven layers can be summarised as follows:

• A layer should be created where a different abstraction is needed.
• Each layer should perform a well-defined function.
• The function of each layer should be chosen with an eye toward defining an intentionally standardised protocol.

1. Physical Layer.

• The physical layer is concerned with transmitting raw bits over a communication channel.
• The design issues have to do with making sure that when one side sends a 1 bit, it is received by the other side as a 1 bit, not as 0 bit.
• The design issues here largely deal with mechanical, electrical and timing interfaces and the physical transmission medium which lies below the physical layer.

2. Data Link Layer.

• The main task of the data link layer is to transform a raw transmission facility into a line that appears free of undetected transmission errors to the network layer.
• It accomplishes the task by having the sender break up the input data into data frames (typically a few hundred or thousand bytes) and the transmit the frame sequentially.
• If the service is reliable the receiver confirms correct receipt of each frame by sending back an acknowledgement frame.

3. Network Layer.

• The network layer handles routing among nodes within a packet switched network. 
• At this layer, the unit of data exchange among nodes is typically called a packet.

4. Transport Layer.

• The transport layer then implements what we have to do up to this point been calling a process-to-process channel.
• Here, the unit of data exchanged is commonly called a message rather than a packet or a frame.
• It runs on the end hosts and not on intermediate switches or routers.

5. Session Layer.

• The session layer allows users on different machines to establish a session between them.
• Session after various services includes dialog control (keeping track of whose turn is to transmit), token management (preventing two parties from attempting the same critical operation at the same time), Synchronization (checkpointing long transmission to allow them to continue from where they were after a crash).

6. Presentation Layer.

• Unlike lower layer, which is mostly concerned with moving bits around, the presentation layer is concerned with the system and semantics of the information transmitted.
• It manages abstract data structures and allows higher level data structures to be defined and exchanged.

7. Application Layer.

• The application layer contains a variety of protocols that are commonly needed by users.
• One widely used application protocol is HTTP (Hyper Text Transfer Protocol) which is the basis for the World Wide Web(www).