Token Ring

Token Ring refers to both IBM's Token Ring and IEEE 802.5 network implementations. IBM originally developed the Token Ring network in the 1970s and Token Ring is still IBM's primary LAN technology. IEEE 802.5 is a related specification and is completely compatible with IBM's Token Ring network implementations.

NOTE

The IEEE 802.5 specification was modeled after IBM's Token Ring specification, and the IEEE continues to follow IBM's research and developmental work with Token Ring.

Token Ring is considered a half-duplex network implementation because only one host can transmit at any given time. Token Ring's full-duplex network implementation is known as Dedicated Token Ring (DTR). In DTR implementations, LAN hosts connect to a DTR concentrator or switch and have all available link bandwidth to use for data transmission and reception.

Token Ring and IEEE 802.5 are two of the three chief examples of token passing networks; the third being FDDI. Token passing networks move a small frame, the token, around the network. Possession of the token by a host gives that host the right to transmit data. If a host receiving the token has nothing to send, it passes the token to the next host downstream in the ring. Each host can hold the token for a maximum period of time, called the Token Holding Time (THT), and the default is 10 ms.

If a host possessing the token has something to send, it seizes the token, alters one bit of the token to turn it into a start-of-frame (SOF), adds the data, and sends the complete frame (SOF and data). This frame is sent to the next host on the ring, known as the downstream neighbor, as illustrated in Figure 5-7.

Figure 5-7. Token Ring Host B is the Downstream Neighbor of Host A

While the data frame is circling the ring, no token is on the network, unless the ring supports early token release. If the ring does not support early token release, other hosts wanting to transmit must wait. If early token release is supported, a new token can be released when frame transmission is completed. If early token release is not in use, collisions cannot occur in Token Ring network implementations.

The data frame circles the ring until it reaches the intended destination host, which copies the information for further processing. The data frame continues to circle the ring and is removed when it reaches the originating host. The originating host then checks the returning frame, determining whether the frame was seen and copied by the destination.

Unlike Ethernet, Token Ring networks are deterministic, meaning that it is possible to calculate the maximum time that will pass before any host will be capable of transmitting. This deterministic nature makes Token Ring networks ideal for applications in which delay must be predictable, such as SNA-based (mainframe) applications.