Data Communications Systems: Token Ring

Token Ring
Token ring is a LAN system developed by IBM that passes a token to each computer connected to the network. Holding of the token permits the computer to transmit data. The token ring specification is IEEE 802.5 and token ring data transmission speed range from 4 Mbps or 16 Mbps. 100 Mbps and higher token ring speeds are in development.

Token ring networks are non-contention based systems, as each computer connected via the token ring network must have received and hold a token before it can transmit. This ensures computers will not transmit data at the same time. Token ring systems provide an efficient control system when many computers are interconnected with each other. This is the reason token ring systems will not see data traffic degradation when many new users are added compared to Ethernet systems. However, passing tokens does add overhead (additional control messages) that reduces the overall data transmission bandwidth of the system.

The token ring LAN architecture was invented by IBM and touted to be the standard for clients of IBM mainframes who sought to replace aging 3270 terminals with LAN’s. IBM also developed cabling standards along with hub-like devices called multi-station access units (MAU’s). The original MAU’s formed a star network with the client PC’s and simulated the ring internally. The PC’s were connected to the MAU via IBM category type 1, 2, or 3 cable.

Figure 1 shows a typical token ring LAN. This diagram shows that the network is logically setup in a ring and each computer in the token ring network must receive a token before it can transmit. Since the token is relatively small compared to the packets of data that are sent, the token can rapidly move from computer to computer. When a computer receives a token, it can transmit data for a limited amount of time before it is required to forward the token.


Figure 1: Token Ring