FUNCTIONS OF TRANSPORT LAYER :
Segmentation and Reassembling : A message is divided into segments; each segment contains sequence number, which enables this layer in reassembling the message. Message is reassembled correctly upon arrival at the destination and replaces packets which were lost in transmission.
Connection Control : It includes 2 types :
Connectionless Transport Layer : Each segment is considered as an independent packet and delivered to the transport layer at the destination machine.
Connection Oriented Transport Layer : Before delivering packets, connection is made with transport layer at the destination machine.
Flow Control : In this layer, flow control is performed end to end.
Error Control : Error Control is performed end to end in this layer to ensure that the complete message arrives at the receiving transport layer without any error. Error Correction is done through retransmission.
FUNCTIONS OF DATA LINK LAYER :
Framing: Frames are the streams of bits received from the network layer into manageable data units. This division of stream of bits is done by Data Link Layer.
Physical Addressing: The Data Link layer adds a header to the frame in order to define physical address of the sender or receiver of the frame, if the frames are to be distributed to different systems on the network.
Flow Control: A flow control mechanism to avoid a fast transmitter from running a slow receiver by buffering the extra bit is provided by flow control. This prevents traffic jam at the receiver side.
Error Control: Error control is achieved by adding a trailer at the end of the frame. Duplication of frames are also prevented by using this mechanism. Data Link Layers adds mechanism to prevent duplication of frames.
Access Control: Protocols of this layer determine which of the devices has control over the link at any given time, when two or more devices are connected to the same link.