CS402: Computer Communications and Networks

Chapter 6: The datalink layer and the Local Area Networks

The datalink layer is the lowest layer of the reference model that we discuss in detail. As mentioned previously, there are two types of datalink layers. The first datalink layers that appeared are the ones that are used on point- to-point links between devices that are directly connected by a physical link. We will briefly discuss one of these datalink layers in this chapter. The second type of datalink layers are the ones used in Local Area Networks (LANs). The main difference between the point-to-point and the LAN datalink layers is that the latter need to regulate the access to the Local Area Network which is usually a shared medium.

This chapter is organised as follows. We first discuss the principles of the datalink layer as well as the services that it uses from the physical layer. We then describe in more detail several Medium Access Control algorithms that are used in Local Area Networks to regulate the access to the shared medium. Finally we discuss in detail important datalink layer technologies with an emphasis on Ethernet and WiFi networks.

6.1 Principles

The datalink layer uses the service provided by the physical layer. Although there are many different implementations of the physical layer from a technological perspective, they all provide a service that enables the datalink layer to send and receive bits between directly connected devices. The datalink layer receives packets from the network layer. Two datalink layer entities exchange frames. As explained in the previous chapter, most datalink layer technologies impose limitations on the size of the frames. Some technologies only impose a maximum frame size, others enforce both minimum and maximum frames sizes and finally some technologies only support a single frame size. In the latter case, the datalink layer will usually include an adaptation sublayer to allow the network layer to send and receive variable-length packets. This adaptation layer may include fragmentation and reassembly mechanisms.

Figure 6.1: The datalink layer and the reference model

The physical layer service facilitates the sending and receiving of bits. Furthermore, it is usually far from perfect as explained in the introduction:

• the Physical layer may change, e.g. due to electromagnetic interferences, the value of a bit being transmitted
• the Physical layer may deliver more bits to the receiver than the bits sent by the sender
• the Physical layer may deliver fewer bits to the receiver than the bits sent by the sender

The datalink layer must allow endsystems to exchange frames containing packets despite all of these limitations. On point-to-point links and Local Area Networks, the first problem to be solved is how to encode a frame as a sequence of bits, so that the receiver can easily recover the received frame despite the limitations of the physical layer.

If the physical layer were perfect, the problem would be very simple. The datalink layer would simply need to define how to encode each frame as a sequence of consecutive bits. The receiver would then easily be able to extract the frames from the received bits. Unfortunately, the imperfections of the physical layer make this framing problem slightly more complex. Several solutions have been proposed and are used in practice in different datalink layer technologies.

Source: Olivier Bonaventure, https://s3.amazonaws.com/saylordotorg-resources/wwwresources/site/wp-content/uploads/2012/02/Computer-Networking-Principles-Bonaventure-1-30-31-OTC1.pdf
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