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.
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.
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