When STA B transmits an ACK frame, the discrepancy in the end time of ACK transmission and reception is equal to the propagation delay δ and hence, the start times of both the backoff counters are inconsistent.
The conventional model assumes that STAs A and B reset the start timings of the backoff counters when a DIFS period has elapsed after the reception of the ACK frame.
The start timings of the backoff counters of both STAs A and B are then synchronized by eliminating δ.
On the other hand, in the unsynchronized slot model shown in Fig. 3, no slot alignment for synchronizing the start timings of the backoff counters to eliminate δ is performed; that is, the start timing of