According to section in the IEEE 802.11e standard,

one and only one of the following actions is allowed at slot boundaries:
 - decrement of backoff timer, or
 - initiate transmission, or
 - backoff procedure due to internal collision, or
 - do nothing.


Backoff counter is resumed one slot time before the AIFS expiration.

However, since a single MAC operation per slot is permitted
(backoff decrement or packet transmission),

when the backoff counter decrements to 0, the station cannot
transmit immediately, but has to wait for a further backoff
slot if the medium is idle, or a further AIFS expiration if
the medium is busy.

I think that 802.11e TKN code is a good implementation of IEEE 802.11e.

However, it seems that the implementation conforms to the requirement
1, but do not to the requirement 2.

Am I correct?

The following paper can be a good guide.

Giuseppe Bianchi, "Understanding 802.11e Contention-Based
Prioritization Mechanisms and Their Coexistence with Legacy 802.11
Stations", IEEE Network, 2005

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