Deliver enabled and trigger enabled

There are different methods to make particular frame
1.Both delivery enabled and trigger enabled
2.Neither delivery enabled nor trigger enabled
3.only trigger enabled
4.only delivery enabled

Before continuing with the above discussion we will look into the frames which are used in APSD in WLAN

WMM Information element:

This information element used to announce its (AP or STA) capability of WMM(QoS).



QOS Info Field

When It is sent from the AP.It needs to tell whether it supports the U-APSD(UN-scheduled) or Scheduled power save mode by setting and resetting U-APSD flag bit in this info field.

If U-APSD =1 then it supports U-APSD
and if U-APSD =0 then it does not supports U-APSD



The format of the QoS info field ,when sent from the STA



AC_BE U-APSD,
AC_BK U-APSD,
AC_VI U-APSD,
AC_VO U-APSD

Sending of (Re)association request by setting all the above flags makes those frames to both trigger and delivery enabled.If not all are neither delivery enabled nor trigger enabled

Management Action Frame:

The management action frame has the following frame format


action codes are defined in the following details




ADDTS(req and response),DELTS management frame used for admission control and power management must contain TSPEC element










General operations:

1.Procedure applies to only unicast QOS data or QOS-NULL frame to be delivered when WMM STA is in PS mode by the AP
2.BC/MC frames are delivered in legacy mode only
3. WMM APs capable of supporting U-APSD (“U-APSD-capable WMM AP”) shall signal this capability through the use of the U-APSD subfield (b7) in the QoS Info Field in Beacon, Probe Response and (Re)Association Response management frames
4.A WMM STA may configure a WMM AP to use U-APSD using two methods.

First:
A WMM STA may set individual U-APSD Flag bits (b3~b0) in the QoS Info field of the
WMM Information element carried in (re) association request frames (see §2.2.1). When a
U-APSD Flag bit is set to 1, it indicates that the corresponding AC is both a delivery-enabled
AC and trigger-enabled AC. When a U-APSD Flag bit is set to 0, it indicates that the
corresponding AC is neither a deliver-enabled AC nor a trigger-enabled AC. When all four
U-APSD Flag subfields are set to 1 in the most recent (re) association request frames, all the
ACs associated with the WMM STA are trigger-enabled ACs and delivery-enabled ACs upon
successful (re) association. When all four U-APSD Flag subfields are set to 0 in (re) association
request frames, the ACs associated with the WMM STA are neither trigger-enabled ACs nor
delivery-enabled ACs upon successful (re) association.

second:(ADDTS req frame from the WMM STA to AP)
A WMM STA may request one or more AC as a trigger-enabled AC and one or more AC as delivery-enabled ACs by sending an ADDTS request per AC to the WMM AP with
the PSB subfield (b10) in the TS Info field in the TSPEC element.

And whether it is delivery enabled or trigger enabled decided by the direction field present in the direction field(b5-b6) in TS info field in the TSPEC element

delivery enabled :
PSB=1 and direction =downlink
trigger enable:
PSB=1 and direction =uplink
both delevery enabled and trigger enabled
PSB=1 and direction =bi directional
neither delivery nor trigger enabled
PSB=0


[NOTE:APSD settings in an admitted TSPEC take precedence over the static U-APSD settings carried in the WMM Information element in the most recent (re) association request and An acknowledged DELTS for an AC reverts that AC to the static UAPSD settings carried in the WMM Information element in the most recent (re) association request]

5.If, for a particular WMM STA, an AC is not a delivery-enabled AC, then all downlink frames
destined to that WMM STA that map to that AC are buffered and delivered using the legacy power save mode

6.WMM STA must remain awake as long as an USP is still in progress