Instructor Note   As described in Chapter 6, one of the primary Layer 2 data link issues is how access to the shared media is controlled. In Token Ring technologies, token passing is the Media Access Control (MAC) method. Have the students act out a token ring kinesthetically. The only person who may speak must possess the "talking stick", or token. This will help them visualize the graphic.

When a token is passed to a host that has information to transmit, the host seizes the token and alters 1 bit of it. The token becomes a start-of-frame sequence. Next, the station appends the information to transmit to the token and sends this data to the next station on the ring. There is no token on the network while the information frame is circling the ring, unless the ring supports early token releases. Other stations on the ring cannot transmit at this time. They must wait for the token to become available. Token Ring networks have no collisions. If early token release is supported, a new token can be released when the frame transmission has been completed.

The information frame circulates around the ring until it reaches the intended destination station, which copies the information for processing. The information frame continues around the ring until it reaches the sending station, where it is removed. The sending station can verify whether the frame was received and copied by the destination.

Unlike CSMA/CD networks, such as Ethernet, token-passing networks are deterministic. This means that you can calculate the maximum time that will pass before any end station will be able to transmit. This feature, and several reliability features, makes Token Ring networks ideal for applications where any delay must be predictable, and robust network operation is important. Factory automation environments are examples of predictable robust network operations.

Priority System
Token Ring networks use a sophisticated priority system that permits certain user-designated, high-priority stations to use the network more frequently. Token Ring frames have two fields that control priority - the priority field and the reservation field.

Only stations with a priority equal to, or higher than, the priority value contained in a token can seize that token. Once the token has been seized and changed to an information frame, only stations with a priority value higher than that of the transmitting station can reserve the token for the next network pass. The next token generated includes the higher priority of the reserving station. Stations that raise a token's priority level must reinstate the previous priority when their transmission has been completed.

Management Mechanisms
Token Ring networks use several mechanisms for detecting and compensating for network faults. One mechanism is to select one station in the Token Ring network to be the active monitor. This station acts as a centralized source of timing information for other ring stations and performs a variety of ring maintenance functions. The active monitor station can potentially be any station on the network. One of this station’s functions is to remove continuously circulating frames from the ring. When a sending device fails, its frame may continue to circle the ring and prevent other stations from transmitting their frames, which can lock up the network. The active monitor can detect these frames, remove them from the ring, and generate a new token.

The IBM Token Ring network's physical star topology also contributes to the overall network reliability. Active MSAUs (multi-station access units) can see all information in a Token Ring network, thus enabling them to check for problems, and to selectively remove stations from the ring whenever necessary. Beaconing - a Token Ring formula - detects and tries to repair network faults. When a station detects a serious problem with the network (e.g. a cable break) it sends a beacon frame. The beacon frame defines a failure domain. A failure domain includes the station that is reporting the failure, its nearest active upstream neighbor (NAUN), and everything in between. Beaconing initiates a process called autoreconfiguration, where nodes within the failure domain automatically perform diagnostics. This is an attempt to reconfigure the network around the failed areas. Physically, MSAUs can accomplish this through electrical reconfiguration.