As discussed in Chapter 1, the data link layer is responsible for organizing data in frames and for detecting errors that occur in a frame. It is also responsible for hop-by-hop (or link-by-link) flow control. In this chapter, we examine the structure of a frame, the data link layer flow control schemes, and two of the protocols defined for this layer, namely the high-level data link control (HDLC) and the point-to-point protocol (PPP).
The data link layer breaks up the bitstream it receives from the network layer into discrete frames and computes the checksum for each frame. In the past, many schemes were used to create frames, especially when character-oriented transmission was still used. Today, the common method is the bit-oriented scheme in which a frame consists of the following:
The format of a frame is shown in Figure 3.1. The exact structure of the header depends on the type of data link control (DLC) protocol in use. The checksum is used for error detection in the frame at the receiver.
The flag is a special bit pattern 01111110, which is 1 byte (or octet) long. A process called bit stuffing is used to preserve the uniqueness of the flag in ...