The EDCA Parameter Set Information Element provides the access category information to the WLAN station. The EDCA Parameter set is not used widely in the real world but the WMM Parameter set element is used instead. The reason for the use of the WMM Parameter set element is that the WMM standard was ratified and adopted prior to the 802.11e QoS standard. The parameters in both formats are the same and differ only in the additional WMM OUI, version, type and sub-type fields.
The AC Parameters field is the same as the AC Parameter Record field for different access categories in the EDCA Element. The AC Parameters element QoS info field are shown below
The bits in the ACI/AIFSN field provide the following information
- ACI – Access Category index
- ACM – Admission control Mandatory bit – if this is set admission control is required
- AIFSN – Arbitrary Inter-Frame Spacing Number – indicates the number of slot times before contention window start for each access category
The mapping between Access Category Index and Access Category is given below
The ECWmin/max field is expanded below. The ECWmin/max field indicates the minimum and the maximum size of contention window for the access category.
The values provided are applied to as an exponent to the formula
The TXOP limit field provides the amount of time allowed to each Access Category during a transmit oppurtunity. Each unit value in the TXOP Limit field is equivalent to 32 micro-seconds
We will look at the QoS info field and a WMM capture example in the next article as a continuation of this article.
EDCA (WMM) information Elements – Part 2