Overview

In this chapter we present an overview of token ring networks, IEEE 802.5. Unlike Ethernet/802.3 LANs in which nodes contend for media access, token ring LANs use a tokenpassing scheme; that is, media access in token ring LANs is controlled by the possession of a token. We discussed general issues relating to token passing, including a comparison of random-access and token-passing protocols, in Chapter 5, which you might want to review before proceeding. In addition to token ring, the IEEE standards include a second token-passing protocol called token bus. Although token bus use is uncommon, we present a brief comparison of the two token-passing schemes for completeness.

1. What is a token ring network?

   A token ring network is a local area network technology based on a token-passing protocol for media access control. Data frames on a token ring network are transmitted from node to node, in either a clockwise or counter-clockwise direction, over a point-to-point link. A token ring LAN is implemented either as a logical ring using a physical ring topology (see Figure 9-1), or as a logical ring structure arranged in a physical star configuration (see Figure 9-2). It is also possible to extend the configuration in Figure 9-2 to include a ring consisting of several interconnected hubs. This is shown in Figure 9-3, which describes the arrangement in the special language of token rings.

   
   Figure 9-1: A token ring network consists of a logical ring implemented in a physical ring topology. A token, which is...