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

Marks: 05

Years: May 2016

Data link layer:

• The 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 figure below:

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

• Figure below shows the node delivery by the data link layer.

• Figure illustrates that the communication at data link layer takes place between two adjacent nodes.

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

• 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.

Network layer:

• The main function of this layer is to deliver packets from source to destination across multiple networks (links).

• If two systems are connected on the same link, there is no need for a network layer.

• The relationship of the network layer to the data link layer and transport layer is shown in the figure below:

• The network layer carries out the end to end delivery and routing. This is illustrated in the figure below:

• As shown in the figure we are expected to make a source to destination (end to end delivery).

• The sequence of events takes place as follows:

a) Network layer of end system 1 sends the packet to the network layer of intermediate system 2 which is a router.

b) The router (2) makes a decision based on the final destination of the packet. This router uses its routing table to find that the next hop is the router (3). The network layer of 2 sends packet to the network of router 3.

c) The network layer of 3 will then send the packet to the network layer of end system 4.