 symbols, an FC field, and an ED field. Stations that have data to transmit must first acquire a token. However, unlike a Token Ring LAN where the token is part of the frame and is transmitted within the formed frame, under FDDI the token is held by the transmitting station, a process referred to as token absorption. The actual period of time that the token is held is specified by a timed-token protocol (TTP). A station can transmit as much data as it has during its token hold time or until the hold time expires. Actually, each station on an FDDI ring uses three timers to regulate its operation. These timers are described in the following three paragraphs. TOKEN ROTATION TIMER The token rotation timer (TRT) is used to time the duration of operations permitted by a station. The value assigned to the TRT depends on the state of the ring. For instance, during steady- state operations the TRT will expire when the target token rotation time (TTRT) is exceeded. Stations negotiate the value for the TTRT during the claim process, which determines the station that will initialize the FDDI ring. This process represents an auction in reverse, where the station with the lowest bid for the TTRT time acquires the right to initialize the ring. TOKEN HOLDING TIMER The token holding timer (THT) controls the period of time during which a station can operate asynchronously. FDDI supports both asynchronous and synchronous services; the for- mer designed to support bandwidth-intensive operations when the station does not need the guaranteed bandwidth provided by synchro- Figure 5.7 Preamble FDDI token frame. Legend: Start Delimiter (2 symbols) Frame Control (2 symbols) End Delimiter (2 symbols) Fiber in the LAN nous services. Thus, asynchronous frames are transmitted only when bandwidth reserved for synchronous operations is not used. In compar- ison, synchronous frames can be transmitted at any time as long as the negotiated synchronous bandwidth allocation is not exceeded. The token holding time for a station is initialized with th