(a) End to end reliability

(b) Link to link reliability

Or Draw the OSI layer architecture. Explain function of each layer and show the path of actual and virtual communication between the layers.

OSI is the reference model designed to divide the entire communication operation into multiple processes. This model divides basic communication functions into seven different layers as shown in the figure below. Here two end communication devices are named Application A and Application B. Each layer communicates with their adjacent layer while adding or removing header and trailer.

When the data is being transmitted it goes from bottom to top in the OSI layer architecture. Each layer follows its own function:

1.Physical layer:

• This layer deals with the transfer of bits over a communication channel, for example copper wire pairs, coaxial cable, optical fibre.
• This layer is concerned with the particular choice of system parameters such as voltage level and signal duration, also setup and release of physical connection as well as mechanical aspect such as socket type and number of pins.

2.Data link layer:

• This layer converts bits into group of bits called frames. It also adds additional information related to frame such as address and correction bits and bits to define the boundaries of frames.
• This control is practically important when the transmission link is prone to transmission error. Check bits ensures the detection of error if any happens during transmission also address bits help in the flow control frames.

3.Network Layer:

• This layer combines multiple frames forming a packet while sending the data and when it receives a data it separates frames from the packet.
• This layer provides routing of packet across the communication network and when doing so it is required that the nodes through which packets are passing are always connected to the network. This makes the network layer most complex layer in the OSI model.
• This layer also deals with the congestion that occurs from time to time due to surge in the traffic in the network.

4.Transport Layer:

• The role of this layer is to provide appropriate address information so that the messages can be delivered to the appropriate destination session layer.
• It also performs various functions such as error free transfer of information by the error detection and recovery techniques and sequence and flow control.
• This layer also does the segmentation and reassembly or blocking and unblocking to match the size of the message produced by the session layer to the packet sizes that can be handled by the network layer.

5.Session Layer:

• This layer enhances the reliability of transfer of data over transport layer by providing means of dialogue between two applications. Such as in some communication manners data transfer is half duplex where both applications takes turn in transferring the data. This facility is supported by session layer.
• This application of session layer becomes useful while transferring large files to provide synchronization points from which error recovery can be initiated instead of transferring large files again.

6.Presentation Layer:

• This layer provides presentation of data to the application layer. Different computers use different coding technique to represent different symbols. It is the function of this layer to convert the data into presentable form in the receiving application.
• In other words this layer converts machine dependent information into machine independent and vice versa at the receiving end.

7.Application Layer:

• This layer closet to the user. It provides user services that are frequently required by the application in communication. These services include e-mail, name services, login service, network management etc.