Token Passing

Token Passing

Token passing utilizes a frame called a token, which circulates around the network. A computer that needs to transmit must wait until it receives the token, at which time the computer is permitted to transmit. When the computer is finished transmitting, it passes the token frame to the next station on the network. Figure 4.2 shows how token passing is implemented on a Token Ring network. Token Ring networks are discussed in greater detail later in this chapter in the section titled “Token Ring.”

Several network standards employ token passing access control:

Token Ring. The most common token-passing standard, embodied in IEEE standard 802.5
IEEE standard 802.4. Implemented infrequently; defines a bus network that also employs token passing
FDDI. A 100 Mbps fiber-optic network standard that uses token passing and rings in much the same manner as 802.5 Token Ring

Token-passing methods can use station priorities and other methods to prevent any one station from monopolizing the network. Because each computer has a chance to transmit each time the token travels around the network, each station is guaranteed a chance to transmit at some minimum time interval.

Token passing is more appropriate than contention under the following conditions:

When the network is carrying time-critical data. Because token passing results in more predictable delivery, token passing is called deterministic.
When the network experiences heavy utilization. Performance typically falls off more gracefully with a token-passing network than with a contention-based network. Token-passing networks cannot become gridlocked due to excessive numbers of collisions.
When some stations should have higher priority than others. Some token-passing schemes support priority assignments.