Mumbai university > Electronics and telecommunication Engineering > Sem 6 > Computer Communication and Telecom Network

Marks: 05

Years: May 2016

Data link layer is responsible for reliable node to node delivery of the data. It accepts packets from the network layer and forms frames and gives it to the physical layer as shown in the below figure:

Following are the functions of data link layer:

1. Framing: It divides the stream of bits received from the network layer into manageable data units called frames.

2. Physical addressing: It adds to the frame to define the physical address of the sender and/or receiver of the frame.

3. Flow control: It provides a flow control mechanism to avoid a fast transmitter from over-running a slow receiver by buffering the extra bits.

4. Error control: Error control is achieved by adding a trailer at the end of the frame. It also uses a mechanism to prevent duplication of frames.

5. Access control:

   • The data link layer protocol determines which device has control over the link at any given time, when two or more devices are connected to the same link.

   • The Institution of Electrical and Electronics Engineers (IEEE) felt the need to define the data link layer in more details, so they split it into two sub-layers:

    Logical Link Control (LLC): It establishes and maintains links between the communicating devices.

    Media Access Control (MAC):

   o It controls the way multiple devices share the same media channel.

   o The logical link control sub-layer provides Service Access Points (SAPs) that the other computers can refer to and use to transfer information from LLC to the network layer.

   o The MAC sub-layer provides for shared access to the network adapter and communicates directly with the network interface cards.

   o Network interface cards (NIC) have a unique 12 digit hexadecimal MAC address assigned before they leave the factory where they are manufactured.

   o The MAC addresses are used to establish logical link between two computers on the same LAN.

   o Bridges, intelligent hubs and network interface cards are devices associated with the data link layer.

   o The data link layer is responsible for moving frames from one hop (node) to the next.

   o Figure shows the node delivery by the data link layer.

o Figure illustrates that the communication at data link later takes place between two adjacent nodes.

o The data is being sent from end system-1 to end-system-4. To do so, three partial deliveries are made.

o First the data link layer of 1 sends a frame to the data link layer of 2 (router). Then the data link layer of 2 sends a new frame to that of 3 (again a router) and finally the data link layer of 3 send a new frame to the data link layer of the end-system-4.