Chapter List

Overview

When discussing network technologies, the first thing that must be realized is that there are a number of different technologies and access methods from which to choose. In this volume alone, we have discussed the CSMA/CD access method used in Ethernet and 802.3, and Token Passing methods used in Token Ring/802.5 and the Fiber Distributed Data Interface. Also recall that since the initial introduction of the 7 Layer ISO Reference Model, it has had to be refined in order to accommodate LANs. In short, since upper layer protocols must be able to be transported across any network technology, we can say that the Network Layer requires a consistent interface regardless of which MAC we decide to use. It is the task then of the Logical Link Control sub-layer to ultimately provide this consistency as figure 30-1 shows.

Figure 30-1: The LLC Sub-Layer

Logical Link Control (LLC) is a derivative of High-Level Data Link Control (HDLC) and, as its name implies, controls the link. As we saw in the early chapters, and indeed throughout this text, there are a number of different Medium Access methods. Each of these provides broadly the same functionality, in that they allow host systems to access the network medium for the purpose of data transmission. Each also has its own methodology defining whether access is by a connectionless or connection-oriented mechanism, depending upon the overall reliability required.

In addition, when looking at the data to be transferred,...