Token Release Timing Differences Explained | Generated by AI
Question: Although both FDDI and IEEE 802.5 use the token passing protocol, what is the key difference in how they handle token regeneration after frame transmission?
Answer:
The correct answer is C. In the FDDI protocol, the sending station can immediately send a new token after finishing transmission of its frame, while in IEEE 802.5, the new token is sent only after the frame’s leading edge returns to the sending station.
Detailed explanation:
Both FDDI and IEEE 802.5 are token ring protocols, but they implement token release timing differently, which significantly affects performance, especially at high speeds.
- IEEE 802.5 (classic Token Ring)
- Uses “delayed token release” or “token after frame return” policy.
- The transmitting station keeps the token (does not release a new one) until it receives its own frame back (specifically, until it sees the beginning of its own transmitted frame returning to it).
- Only after the frame header returns does the station release a new free token.
- This ensures that only one frame is circulating on the ring at a time (single-token mode).
- This conservative approach reduces the risk of frame collisions but limits throughput, especially on large rings or at high speeds.
- FDDI (Fiber Distributed Data Interface)
- Uses “immediate token release” or “early token release”.
- As soon as the transmitting station finishes sending its frame (i.e., after transmitting the last bit of the frame trailer), it immediately generates and releases a new free token.
- It does not wait for the frame to come back around the ring.
- This allows multiple frames to be circulating on the ring simultaneously (multiple-token operation), greatly improving efficiency and throughput — especially important for FDDI’s 100 Mbps speed and large possible ring size (up to 100 km and 500 stations per ring, 1000 with dual ring).
- FDDI can support synchronous and asynchronous traffic and uses a timed token rotation protocol (TTRT) to guarantee bandwidth and latency.
Summary of the key difference:
Why the other options are incorrect:
- A: Wrong — IEEE 802.5 does not release the token immediately.
- B: Wrong — FDDI does not wait for the frame to return; only 802.5 does.
- D: Wrong — FDDI does not generate the new token while still sending the frame; it releases immediately after finishing the frame.
Memory tip: FDDI = Fast → “Finish frame → Free token immediately” 802.5 = Slow & safe → “Wait until frame comes back → then free token”
References:
- None (standard comparison between IEEE 802.5 Token Ring and ANSI X3T9.5 / ISO 9314 FDDI specifications)