802.11 Medium Access

Below is my attempt to lay out what happens when an 802.11 Wireless device (STA) wants to send a frame over the medium (air). Whether the STA is a client, or an Access Point, the process is the same. I appreciate any feedback (via comments, Twitter or email) to correct inaccuracies.

First, some definitions to help understand the acronyms referenced in the drawings.

• Duration/ID feild (present in MAC header) -- Represents the time, in microseconds, it will take to transmit a frame, and may include time it will take to receive subsequent ACK plus any DIFS and/or SIFS
• DIFS/SIFS/RIFS/AIFS/EIFS -- Interframe spaces, a time (in microseconds) that exists between frame transmissions
• Network Allocation Vector (NAV) -- Stations set their NAV to the previously mentioned Duration value, upon hearing a frame. They then count down to 0.
• Clear Channel Assessment (CCA) -- A station listens for RF energy, at the Physical layer. Channel is either "Busy" (STA can demodulate 802.11 bits) or "Clear". Note, non-802.11 RF energy does not equal "Busy".
• Slot Times -- A value, in microseconds, representing quiet periods. How long ea. one is, depends on protocol, modulation, and preamble being used. See bottom of post for various slot times.*

The first explanation is assuming this is not an 802.11e/WMM QoS WLAN. Every single Data frame requires an ACK. If an ACK is not received, for whatever reason, the Data frame is retransmitted .
Next, I'll detail an 802.11e (QoS) BSS. The major differences, with regard to medium contention include:

• IFS -- AIFS is used instead of DIFS. AIFS time varies based on Priority level (Access Category)
• Contention Window -- The Random Backoff-Timer Slot Times are now influenced by Access Category
• Frame Transmissions -- a Transmit Opportunity (TXOP) is used, for Voice & Video Access Categories, so more than a single frame can be transmitted in succession without Backoff timer or AIFS. This is called a Contention Free Burst(CF).
• Block Ack -- Not every frame needs to be ACK'd. Multiple frames (part of a "frame-burst") can have a single [Block]ACK. This is mandatory in 802.11n. It was optional in 802.11e (not implemented in 802.11a/b/g).