If the 802.11 Station desires to change some Access Categories to be just Trigger enabled or to make other Access Categories as just Delivery enabled – then the mechanism employed by the 802.11 Station is the ADDTS action frame.

The Traffic Stream (TS) setup has a lot of parameters and the current article only tries to address how the particular Access category can be made as Trigger enabled or Delivery enabled or both by looking at the ADDTS Request and the ADDTS response.

The ADDTS request frame format is provided below

FIG COURTESY: 802.11-2012^TM Standard

• The Category field is set to 1 –> indicating QoS
• The QoS action field is set to 0 –> indicating ADDTS
• The TSPEC and the TCLAS Information elements define the Traffic Specification. The TSPEC field is mandatory and the other fields are optional

The ADDTS Response frame is provided below

FIG COURTESY: 802.11-2012^TM Standard

The Status code is two octets in length and has numerous values based on the particular failure. The Status code for SUCCESS is 0. If the status code in the ADDTS response is zero, then the Traffic specification is accepted by the Access Point.

Now to determine how to set the Traffic class as Trigger enabled or delivery enabled or both by looking at the TSPEC element and the TSINFO field

The parameter of interest is the TS info field and is provided below

• The APSD field is also termed as PSB (Power Save Behaviour) bit
• The Direction bits and the APSD (PSB) bits determine if the Access category is trigger enabled/Delivery enabled or both.

The Direction bits and their interpretation is provided below

According to the WMM U-APSD standard

• An access category is made to be a Trigger enabled Access category when
  • Direction bits set to up-link direction – Bits 5 and 6 are 0, 0
  • APSD (PSB) bit set to 1
• An access category is made to be a Delivery enabled Access category when à
  • Direction bits set to downlink direction – Bits 5 and 6 are 1, 0
  • APSD (PSB) bit set to 1
• An access category is made to be a Delivery enabled and Trigger enabled Access category when
  • Direction bits set to bi-directional link direction – Bits 5 and 6 are 1, 1
  • APSD (PSB) bit set to 1

Understanding Transmit Opportunity (TXOP)