Token Ring

Token Ring

Token Ring uses a token-passing architecture that adheres to the IEEE 802.5 standard, as described earlier. The topology is physically a star, but Token Ring uses a logical ring to pass the token from station to station. Each node must be attached to a concentrator called a multistation access unit (MSAU or MAU).

In the earlier discussion of token passing, it may have occurred to you that if one computer crashes, the others will be left waiting forever for the token. MSAUs add fault tolerance to the network, so that a single failure doesn’t stop the whole network. The MSAU can determine when the network adapter of a PC fails to transmit and can bypass it.

Token Ring network interface cards can run at 4 Mbps or 16 Mbps. Although 4 Mbps cards can run only at that data rate, 16-Mbps cards can be configured to run at 4 or 16 Mbps. All cards on a given network ring must run at the same rate.

As shown in Figure 4.15, each node acts as a repeater that receives tokens and data frames from its nearest active upstream neighbor (NAUN). After the node processes a frame, the frame transmits downstream to the next attached node. Each token makes at least one trip around the entire ring and then returns to the originating node. Workstations that indicate problems send a beacon to identify an address of the potential failure.

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