Presentation Layer

The presentation layer is concerned with the syntax and semantics of information exchanged between two system.
The presentation layer has following responsibilities;

•  Translation; The process of changing information in two systems is usually in the form of character strings,numbers and so on. The information is changed into a bit stream by encoding on presentation layer.
• Compression; Data compression reduces the number f bits to be transmitted. It is important mostly in the transmission of multimedia such as text,audio and video.

                                           

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Session Layer

The session layer is te network dialog controller. It establish,maintain and synchronize the interaction between communicating systems.
The responsibilities of session layer are:

• The session layer allows two systems to enter into a dialog. It allows the communication between the two in half or full duplex. It called dialog control.
• It make synchronization checkpoints into a stream of data. The checkpoints can be specified by time limit or data limit.

                                          

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Transport Layer

The transport layer is responsible for source to destination delivery of the entire message. The transport layer ensures that the whole message arrive intact and in order and check all the packets relation to each other. Also check the error control and flow control at source to destination level.
For additional security, network layer may create the connection between two end ports. It involves the etablishment, data transfer and connection release.
Transport layer have following responsibilities;

• Service point addressing; As compters run many programs at the same time. So here source to destination delivery is not from one computer to other but it is froma specific program on one computer to a specific program on the other. Therefore transport layer header must include a service point address or port address.The transport layer gets the entire message to the correct program running on computer.
• A mesasge is divided into transmittable segmentrs, each segment contains a sequence number. These number used in reassembling.
• The transport layer can be connectionless or connection oriented. In connectionless it simply deliver packet to network layer at destination but in connection oriented it makes connection with the transport layer and then deliver packets.
• Like data link layer it is responsible for flow control. Here flow control is end to end not on a single link.
• Like data link layer it is responsible for error control. It makes sure that the entire message is delivered correctly at the destination without any error.

                                                          

                                   

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Network Layer

The network layer is responsible for the source to destination delivery of a packet possibly across multiple networks. It also determines the delivery of packet from origin to destination in a network.
If two systems connected to same network then network layer is not needed. But for different network from source to destination delivery it is needed.
Some responsibilties of network layer are below:

• For different networks the network layer adds a header to the packet coming from upper layer including logical address of sender and receiver.
• When the independent networks are connected together to create an internetwork, the connecting devices ( routers and gateways ) route the packets to their destination. 

                                            

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Data Link Layer

The data link layer transforms the physical layer a a raw transmission facility to a reliable link and is resposible for nod eto node delivery. It makes the physical layer error free for the network layer.
This layer includes the following responsibilities :

• It divides the stream of bits received from network layer into manageable data units called frames.
• If frames are to be distributed to different systems on network, it adds a header to the frame to define the physical address of the sender(source address) and/or receiver(destination address) of frame.
• It also control that the data rate absorbed by reveiver must match the data rate at which is sent by sender called flow control.
• It also control the retransmitting of data if any frame is lost or damaged and the duplication of frames called error control.
• If two or more devices are connected to a same link. Then data link layer determines that which device has control over the link at any given time.

                                           

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