The Token Ring

The IEEE standard for Token Ring is IEEE 802.5. Token Ring was created by IBM for the forwarding of data on a logical unidirectional ring. Token Ring, like Ethernet, is a LAN technology that supports shared media access to several connected hosts and is applied in the Data-Link Layer.

Token Ring networks transmit a small frame, called a token, to the hosts on the network. A host that holds the token has the right to transmit a frame onto the ring. After a station has the token, it adjusts it into a data frame, attaches the data for transmission, and forwards the frame to the next station. A ring will not contain a token until the data frame is received by the source station and tagged as read and copied whereupon the token is returned back into the ring. One station at a time can transmit a token to avert collisions on the Token Ring network.

A Token Ring network has a bandwidth of 4 Mbps or 16 Mbps. At the higher end, hosts are allowed to present a new token once transmission of a frame is completed. This expedient token release boosts efficiency by allowing more than one host to transmit a frame during the original token’s round trip. One station is responsible for acting as a ring monitor to offer recovery from runaway frames or tokens. The ring monitor will remove frames that have circled the ring once, if no other station removes them.

Traditional Token Ring networks use Multistation Access Units (MSAUs) to offer connectivity between hosts. MSAUs have several ports that a host can link to, with a B connector for Type 2 cabling or an RJ-45 connector for Category 5 UTP cabling. One of the internal functions of MSAU is it allows host-to-host connections to form a ring segment.

The Ring-In and Ring-Out connectors of a MSAU can be connected to other MSAUs to form a complete ring topology. Token Ring also enables an optional priority system that permits stations that are given a higher priority value access to the network more often than permitted by the standard process. Eight levels of priority are implemented using a 3-bit reservation field and a 3-bit priority field. As the frame passes, a station sets a higher priority in the reservation field, reserving the token. The transmitting station then dispatches a token out with the higher priority set. After the high priority station completes sending its frame, it releases a token with the normal or previous priority.

Token Ring Operation

Another station on the Token Ring is chosen to be the Active Monitor (AM). This station extracts continuously circulating frames that are not removed by a defective transmitting station. As a frame passes the AM, the monitor count bit is set. If a frame passes with the monitor count bit set, the AM assumes that the original sender of the respective frame was unable to remove the frame from the ring. The AM then discards this frame, sends a Token Soft Error message to the Ring Error Monitor, and produces a new token.

The AM also supplies timing status to ring stations. It implements a 24-bit propagation delay to prevent the end of a frame from wrapping back to the beginning of the frame, and also verifies that a data frame or token is received every 10 milliseconds.

Standby Monitors and Ring Error Monitors are also on Token Ring networks. Standby Monitors will assume AM responsibilities if the main AM is extracted from the ring or no longer performs its functions. Ring Error Monitors can also be present on the ring to collect ring status and error information. If a station does not receive any additional frames either a data frame or a token – from its upstream neighbor, it transmits a beacon MAC frame, which contains the beaconing station’s MAC address and the address of the nearest active upstream neighbor (NAUN), and indicates that the problem lies between the two stations. An adapter will send continuous beaconing until frames are received again.

Early Token Release (ETR)

With ETR, a token is released right after the sending station transmits its frame. The sending station won’t wait for the data frame to circle the ring. Stations running ETR have compatibility with stations not running ETR. With ETR a free token can circle the ring with multiple data frames.

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