802.11e defines EDCA (Enhanced Distributed Channel Access) mechanism for medium access for QoS frames and WMM is a Wi-Fi Alliance certification that verifies if an 802.11 station implements EDCA as stated in the protocol.
It is important to give more priority for medium access to high priority traffic like Voice and Video; unlike DCF where same priority is given for all kinds of frames. Since Voice and Video are latency sensitive, focus should be given for fast delivery of correct data.
EDCA is implemented using the below techniques.
1. QoS data is classified into 4 access categories: AC_BK for background traffic, AC_BE for best effort, AC_VI for video traffic and AC_VO for voice data based on the type of data and its priority. Below table mentions the queues in the increasing order of priority.
| User Priority | Access Category |
| 0,3 | AC_BK |
| 1, 2 | AC_BE |
| 4,5 | AC_VI |
| 6,7 | AC_VO |
2. AIFS timer in EDCA varies with the modulation technique that is used instead of DIFS timer for DCF medium access. Higher the priority of the queue, lower is the AIFS timer.
AIFS = SIFS + AIFSN * slot time
AIFSN is a configurable variable whose default values are shown in the table below.
| AC | AIFSN |
| AC_BK | 7 |
| AC_BE | 3 |
| AC_VI | 2 |
| AC_VO | 2 |
3. Contention Window in EDCA varies is shorter for higher priority queue. For the initial transmission, a random value is chosen between 0 and CWmin (i.e. upper boundary of the window) and for every retransmission of frames, CWmin is doubled and incremented by 1 to until CWmax is reached. aCWmin is a variable that depends on the type of modulation used with values 31 for DSSS, HR/DSSS, ERP for backward compatibility with dot11b and 15 for ERP, OFDM, HT, VHT.
| CWmin | CWmax | |
| AC_BK | aCWmin | aCWmax |
| AC_BE | aCWmin | aCWmax |
| AC_VI | (aCWmin+1)/2-1 | aCWmin |
| AC_VO | (aCWmin+1)/4-1 | (aCWmin+1)/2-1 |
4. TxOP is the transmission opportunity for QoS stations to use the medium to transmit burst of frames; instead of only frame. TxOP =0 indicates that only one frame can be transmitted. This is a configurable value and can be set according to the network requirements.
5. WMM Power save mechanism is introduced for QoS stations to implement power save more efficiently compared to the traditional method. This is explained in another post – “Power Save Mechanisms”.
All the above mentioned WMM parameters can be seen in beacons, probe response and assoc response frames. WMM capable STA should match their settings to the ones that AP is broadcasting using these frames – mainly the Assoc Response frame. It should also check for any further WMM changes in the beacons of the APs and update accordingly.
As a note, end to end QoS implementation should be performed and not just on specific devices in the network. QoS will not work if any of the device in the network does not tag the traffic properly. Wired devices should convert wireless QoS marking to the appropriate Wired QoS tags.
References:
CWAP – 403 – Chapter 6
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