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Token and Token-Passing Access Methods

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A token is a special control frame on token ring, token bus, and FDDI (Fiber Distributed Data Interface) networks that determines which stations can transmit data on a shared network. The node that has the token can transmit. Unlike contention-based networks, such as Ethernet, workstations on token-based networks do not compete for access to the network. Only the station that obtains the token can transmit. Other stations wait for the token rather than trying to access the network on their own. On Ethernet networks, "collisions" occur when two or more workstations attempt to access the network at the same time. They must back off and try again later, which reduces performance-especially as the number of workstations attached to a network segment increases.

In token ring networks, a station takes possession of a token and changes one bit, converting the token to a SFS (start-of-frame sequence). A field exists in the token in which workstations can indicate the type of priority required for the transmission. The priority setting is basically a request to other stations for future use of the token. The other stations compare a workstation's request for priority with their own priority levels. If the workstation's priority is higher, the other stations will grant the workstation access to the token for an extended period.

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