T1: A Survival Guide

Chapter 8. High-Level Data Link Control Protocol (HDLC)

A precedent embalms a principle.

—Benjamin Disraeli

Up until now, this book has focused on strictly the physical layer of the OSI model: volts, pulses, timing, plugs, and so on. Stretching it, we can see as far as bits, flowing together in an unending stream of zeros and ones. Nothing, however, has touched upon creating packets, and without packets, there is no data network.

Packets are lumps of data with addresses that are independent of any physical medium. Putting packets on the wire (or on the fiber, or on the air) requires a lower-level construct. Below the medium-independent addressing at layer 3 of the OSI model is the frame, which lives down at layer 2. Frames are a network-dependent wrapper for packets. Because physical networks may be used for several different network protocols, distinguishing between them is the most important function of the link layer. To accomplish this, frame headers include tags for network protocols. Each individual link layer decides how its frames are tagged.

LAN technology presents a relatively homogenous interface to network protocols, in part due to the dominance of IEEE 802 methods. WAN technologies, however, present a wide field of diverse requirements. As a result, several link layer protocols are in widespread use on serial links. Riding at the link layer, framing protocols are well positioned to monitor link quality, report problems to higher-layer protocols, and discard ...