Data Link Layer

In digital telecommunication systems the link layer is responsible for the transmission of data streams over point to point links. It performs framing of higher layer packets into link layer packet data units (PDUs) and can perform packet error recovery by exploiting ARQ mechanisms. In addition, the link layer also implements channel access procedures, which are often tightly coupled with the adopted ARQ scheme. Operations at this layer are essential for reliable and efficient transmissions over noisy and possibly shared links. We observe that the QoS at the application layer is highly dependent on the combination of techniques that we use at this layer as well as on the way we configure them. In particular, the following techniques play a fundamental role in the link layer: 1) the selected retransmission policy, 2) the feedback strategy that is used, at the receiver, to request the retransmissions of lost packets, 3) the channel access mechanism, 4) adaptive behavior in terms of data frame length (as a function of the error rate) as well as 5) advanced features such as combined ARQ and coding (for throughput enhancement). This chapter will give a solid introduction to points 1-4, will discuss some techniques related to point 5 and will cover the data link layer techniques used in Ethernet, IEEE~802.11 and Bluetooth.