What is token ring?

Although token ring can trace its origins back to the 1960s, IBM has been responsible for much of the development since 1981. IBM made an agreement with Texas Instruments to jointly develop the required chipsets to support a token-passing access methodology.

The IEEE became involved with the standardisation of token ring when it ratified the 802.5 standard for token passing. Token ring was originally based on shielded twisted pair media, but it can operate over unshielded twisted pair and fibre optic. Some manufacturers are hoping to develop token ring so it can use legacy coaxial installations. It is hoped that this will persuade organisations to migrate from older ethernet to token ring without having to change their wiring infrastructure.

A single computer has to act as a monitoring station for each ring. The monitoring station does not require any extra hardware or software, all other computers are designed to act as standby monitors if the primary device fails. Monitors are responsible for the destruction of frames not removed by their source computers. Any damaged tokens also have to be renewed by the monitor.

There is a great deal of discussion concerning the advantages of token ring over ethernet. Ethernet has a greater market share, but much of this is due to the higher cost of token ring hardware. There have been many studies that contrast the performance characteristics of ethernet and token ring. CSMA/CD causes a performance tail-off, under high network loading, due to the increased number of collisions and retransmissions. Conversely, token ring does not perform as well under low loading - the monitoring and managing overheads are proportionally greater. Overall though, there is not a great difference in the two architectures.

Token ring is a deterministic architecture that commonly runs at 16Mbps. It optimises its bandwidth through the token passing access methodology. Each network device can use the full bandwidth as soon as it possesses a special circulating 24 bit packet (token):

• When a computer wishes to transmit, it has to receive the circulating token.
  
  
• The computer modifies the token into a frame and transmits data.
  
  
• The frame circulates to each device on the way to its destination. Each device examines the address field of the frame to determine if the frame is destined for itself. No other station will attempt to transmit, they recognise that the frame is not the token.
  
  
• When a frame arrives at its destination, it is copied, processed and retransmitted slightly modified. The modification is designed to inform the source computer that the data has been received and processed.
  
  
• When the source computer receives its modified frame, it removes the frame from the ring and ,either completes its data transfer, or releases the token back onto the ring so that other devices may communicate.

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