QoS Control is a 16-bit field that identifies the Quality of Service (QoS) parameter of a data frame (only in data frame type QoS-Data).QoS Control field is comprised of five subfields
1. [bit 0-3 ] Traffic Identifier (TID)
2. [bit 4 ] End of Service Period (EOSP)
3. [bit 5-6 ] ACK Policy
4. [bit 7 ] Reserved
5. [bit 8-15 ] TXOP limit, TXOP duration, AP PS buffer state, Queue Size
Below chart list which type of devices use bit 8-15 information. (CWAP study guide-page 104)
Here is a QoS Data type wireless frame capture shows the above subfields in QoS control field.WiFi uses EDCA- Enhanced Distributed Channel Access, a wireless access method that provides differentiated access for stations using 8 user priorities & 4 QoS Access categories (AC_VO, AC_VI, AC_BE, AC_BK). These UP values of a wireless frame map to QoS field (CoS/802.1D) of a 802.1q header when it translated to Ethernet frame(Page 105 – CWAP Official Study Guide).
WiFi alliance QoS certification called WMM-WiFi Multimedia also defined those 4 access categories. So WMM cetified end client should classified its traffic on to one of those classes prior to transmit them over the air.
Below shows the relationship between WMM Access categories & 802.1D tag values (Page 105 – CWAP Official Study Guide)
Here is the brief description of the QoS subfields
1st subfield (TID – Traffic Indicator)
4 bit value used to identify the user priority (UP) and traffic Access Category(AC) of a QoS data frame. 802.11 WMM clients use WMM-PS (power save) to indicate to an AP that STA is awake. Unlike in legacy PS, WMM-PS client can ask to deliver more than 1 frame.
2nd subfield (EOSP- End of Service Period)
1 bit value to indicate the end of a service period. If this bit set to 1, then client can go back to asleep.
3rd subfield (Acknowledge)
Specify the 2-bit Acknowledgement policy. There are four different options available
No Explicit ACK :
4th subfield (Reserved)
Allocated for future use
TXOP Limit: Indicate the transmitt opportunity granted by the AP
AP PS Buffer State : AP use this to indicate PS buffer state for a given client station.
TXOP Duration Requested: Client use this to tell AP how much time client station wants for its next TXOP. AP may choose to assign shorter TXOP as well.
Queue Size: Client station use that to inform AP how much buffered traffic it has to send. AP can use this information to determine duration for next TXOP to that client
Here is a two sample packet capture of QoS data frames when 7921 phone is communicating with 7960 phone in the given setup.
This is a RTP packet going from wireless client to wired phone. As you can see bit 4 set to 0 (TXOP Duration Requested in bit 8-15). Also TID is 6 indicating it is Access Class of Voice traffic.Below show a wireless packet coming from wired phone to the wireless client. In here bit 4 is used for EOSP. Also in this case bit8-15 (5th subfield) is AP PS Buffer state to tell client whether any buffer data available for 7921 phone.
1. CWAP Official Study Guide – Chapter 3
one place observed EOSP is written like ESOP. Please correct that
Thank you, I have corrected it now.
Who will set EOSP? STA or AP?
Every Tx packets send from STA will have EOSP set as ‘0’?
My understanding is AP set it to indicate end of service period for clients.
Here is what IEEE 802.11-2016 standard says
“18.104.22.168.3 EOSP (end of service period) subfield
The EOSP subfield is 1 bit in length and is used by the HC to indicate the end of the current service period
(SP) and by a DMG STA to indicate the end of the current SP or the end of the current allocated CBAP with
individually addressed destination AID. The HC sets the EOSP subfield to 1 in its transmission and
retransmissions of the SP’s final frame to end an SP and sets it to 0 otherwise. To end an SP allocation or a
CBAP allocation with individually addressed destination AID, the DMG STA sets the EOSP subfield to 1 in
its final frame transmission and retransmissions within the allocation; otherwise, the DMG STA sets the
EOSP subfield to 0.
The mesh STA uses the EOSP subfield to indicate the end of the current mesh peer service period in which
it operates as the owner. The mesh STA sets the EOSP subfield to 1 in its transmission and retransmissions
of the mesh peer service period’s final frame to end a mesh peer service period, and sets it to 0 otherwise.
See 22.214.171.124 for details.
If dot11RobustAVStreamingImplemented is true, then the HC sets the EOSP subfield to 1 in a GCR-SP
group addressed frame in order to indicate that no more GCR-SP frames of that group address are to be
transmitted by the AP until the next scheduled SP for this GCR-SP stream.
NOTE—As GCR-A frames are sent outside of any SP, the EOSP subfield is set to 0 in a group addressed frame
delivered using the GCR-A procedures described in 126.96.36.199.8.”
Is there an y use of EOSP bit in the transmissions form AP to STA?
And is there any provision that station can end the triggered service period? If so, can you tell how?