The purpose of the IEEE 802.11 Quality of Service (QoS) feature is to assign different traffic types (voice, video, or best effort traffic) different priority levels of packet transmission. Packets belonging to delay-sensitive applications are assigned higher priorities and thus have a statistically higher chance of being transmitted before lower-priority packets.

When QoS is enabled a technique referred to as enhanced distributed channel access (EDCA) is utilized in the MAC layer to properly transmit packets based on their priority.

The levels of priority in EDCA are called access categories (ACs) and ACs with for higher priority traffic wait less time, on average, to be sent than the ACs for lower-priority traffic. The contention window (CW) can be set according to the traffic expected in each AC, with a wider window needed for categories with heavier traffic. The CWmin and CWmax values are calculated from aCWmin and aCWmax values, respectively, that are defined for each physical layer supported by 802.11e.

EDCA provide four different ACs (from lowest to highest priority):

• Background (AC_BK)
• Best Effort (AC_BE)
• Video (AC_VI)
• Voice (AC_VO)

CC33xx devices, in both AP and STA modes, supports EDCA in both software and hardware: while the software maintains the different queues within the AC categories, the hardware determines which packet is sent out from which AC queue in real-time. Table 2-3 shows the default EDCA values for the CW min and max, the AIFSN, and the max TXOP. The arbitration inter-frame spacing (AIFSN) is how long the transmitter defers before starting the backoff period based on the contention window, thus the lower the AIFSN number the higher the probability of the frame being transmitted. The TXOP (transmission opportunity) is a parameter that specifies the interval of time during which a client can initiate transmissions to an AP.

The actual EDCA parameters are published by the AP side. When running a CC33xx device as an AP role, you can configure the EDCA parameters in the TI configuration file. There is no option to disable QoS from the STA role (enabled by default), but there is an option in the hostapd.conf file to disable the QoS.

A frame is handled as a QoS frame only if it arrived from the network with QoS information. Each frame without QoS information is handled as a non-QoS frame. The default parameters of non-QoS frames are the same as best-effort frames (that is also the case when the AP does not support QoS).

The EDCA QoS is compatible with the Wi-Fi Alliance WMM Certification, with a small modification. WMM defines eight different TIDs (Traffic ID 0-7), while each traffic ID (TID) gets a specific AC handling.

For the CC33xx devices, four TIDs are supported for transmission while eight are supported for receiving. The eight TIDs outlined by the WMM are described in Table 2-4 and the ones supported for transmission are noted.