In token-ring networks, each station connects to the ring via an active, signal-regenerating circuit, which takes the incoming data from the station upstream of it and forwards it with minimal delay to the station downstream of it. The stations are connected together in a logical ring, and the data always travels in the same direction around the ring. See Figure 1.
Figure 1: Example of a token-ring network
The functioning of a token-ring network depends on the token - a special packet which gives a station the right to transmit a frame. The token is 24 bits long. It continuously circulates around the ring until it reaches a station that wants to transmit data. In 4Mbit/s rings, only one token or frame can be on the ring at any one time. This ensures orderly, predictable, and collision-free access to the ring.
When a station wants to transmit, it waits until it receives the token. Then, instead of forwarding the token, it changes one bit of the token, transforming the token into a data frame. The station then adds addressing information, appends its data, and terminates the frame with a 4-byte checksum.
As the data frame travels around the ring, each station forwards the frame to the next station in the ring. At the same time, it checks the destination address of the frame, and if it matches the station's own address, the station copies the frame into its buffer memory. The station sets a marker to indicate receipt of the frame. When the frame arrives back at the station that sent it, it is removed from the ring and a new token is issued and passed to the next station, thus giving other stations an opportunity to transmit.
A token-ring network incorporates a system of priority reservation, which permits stations with a more urgent need to send data, such as bridges and file servers, to obtain the token before a station with a lower priority.
A 4Mbit/s token-ring network allows only one token or frame to be on the ring at any one time. However, the more recently introduced 16Mbit/s token ring implements a concept called early token release, which allows one or more frames to be on the ring at the same time as the token. A station can send a token immediately after it has transmitted a frame, rather than waiting for the frame to completely circulate around the ring before it releases a token. The next station can therefore transmit a frame before the first frame has been removed from the ring. Early token release improves throughput, especially for small frames.