| 3.2 | Layer 2 Protocol Overview - LAN Protocols | ||
| 3.2.5 | Token Ring access method |
| Token Ring and IEEE 802.5
are the primary examples of token-passing networks. Token-passing
networks move a small frame, called a token, around the network.
Possession of the token grants the right to transmit. If a node
receiving the token has no information to send, it simply passes the
token to the next end station. Each station can hold the token for a
maximum period of time.
If a station possessing the token does have information to transmit, it seizes the token, alters one bit of the token, turns the token into a start of frame (SOF) sequence, appends the information it wishes to transmit, and finally sends this information to the next station on the ring. While the information frame is circling the ring, there is no token on the network (unless the ring supports early token release), so other stations wishing to transmit must wait. Therefore, collisions cannot occur in Token Ring networks. If early token release is supported, a new token can be released when the information frame transmission is complete. The information frame circulates the ring until it reaches the intended destination station, which copies the information for further processing. The information frame continues to circle the ring and is finally removed when it reaches the sending station. The sending station can check the returning frame to see whether the frame was seen and subsequently copied by the destination. Unlike CSMA/CD networks, such as Ethernet, token-passing networks are deterministic. In other words, it is possible to calculate the maximum time that will pass before any end station will be able to transmit. This feature and several reliability features make Token Ring networks ideal for applications where delay must be predictable and robust network operation is important. Factory automation environments are examples of such applications. However, recent advances in Ethernet technology and Quality of Service (QOS) are making it very difficult for Token Ring to continue to compete in the marketplace. IBM Token Ring network stations are directly connected to MAUs, which can be wired together to form one large ring (as shown in the figure to the left). Patch cables connect MAUs to adjacent MAUs. Lobe cables connect MAUs to stations. MAUs include bypass relays for removing stations from the ring. Token Ring networks use a sophisticated priority system that permits certain user-designated, high-priority stations to use the network more frequently. Token Ring frames have two fields that control priority: the Priority field and the Reservation field. Only stations with a priority equal to or higher than the priority value contained in a token can seize that token. After the token is seized and changed to an information frame, only stations with a priority value higher than that of the transmitting station can reserve the token for the next pass around the network. When the next token is generated, it includes the higher priority of the reserving station. Stations that raise the priority level of a token must reinstate the previous priority after their transmission is complete. |