Introduction to DataLink Layer

The data-link layer is responsible for node-to-node communication. While the network layer provides host-to-host communication, a packet at the data-link layer travels from one node to the next directly connected node along the path.

For example:Figure 5.1 shows communication between Alice and Bbo. Communication at the data-link layer, however, is made up of five separate logical connections between the data-link layers in the path.The data-link layer at Alice’s computer communicates with the data-link layer at router R2. The data-link layer at router R2 communicates with the data-link layer at router R4, and so on. Finally, the data-link layer at router R7 communicates with the data-link layer at Bob’s computer. Only one data-link layer is involved at the source or the destination, but two data-link layers are involved at each router. The reason is that Alice’s and Bob’s computers are each connected to a single network; each router, however, takes input from one network and sends output to another network.

Although communication at the application, transport, and network layers is end-toend, communication at the data-link layer is node-to-node. A data unit from one point in the Internet needs to pass through many networks (LANs and WANs) to reach another point. Theses LANs and WANs are connected by routers. It is customary to refer to the two end hosts and the routers as nodes and the networks in between as links. The following is a simple representation of links and nodes when the path of the data unit is only six nodes..


  • Nodes → Source host, destination host, and routers
  • Links → Networks connecting two consecutive nodes
  • At the source and destination, one data-link layer is involved.
  • At a router, two data-link layers are involved because the router receives data from one network and forwards it to another.

Two Types of Links

The data-link layer can operate over:

  1. Point-to-point link – dedicated connection between two devices.
  2. Broadcast link – a shared link used by multiple devices.

Two Sublayers

The data-link layer is divided into two sublayers:

SublayerMain responsibility
Data Link Control (DLC)    Functions common to both point-to-point and broadcast links
Media Access Control (MAC)    Controls access to a shared/broadcast link

Thus, the data-link layer provides reliable and controlled communication between directly connected nodes, using DLC and MAC functions depending on the type of link.



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