1. TXOP Request: in Time vs. in Queue Size?
doc.: IEEE /xxxr0
November 2001
May 2002
TXOP Request: in Time vs. in Queue Size?
Sai Shankar, Javier del Prado and Sunghyun Choi
Philips Research USA
Briarcliff Manor, New York
Sai Shankar et al., Philips Research
BobbyJose, Hazel Ranch

2. Outline QoS Control Field - Overview
May 2002
Outline
QoS Control Field - Overview
Problem Statement: Making a TXOP Request
TXOP Requests in time
TXOP Requests in Queue Size
Conclusions
Sai Shankar et al., Philips Research

3. May 2002
References
Bobby Jose, “Contention Free TXOP Request and Allocation Issues,” IEEE /248r0
Bobby Jose, “Updated QoS Control Field,” IEEE /290r0
IEEE e QoS draft D2.0a
Sai Shankar et al., Philips Research

4. QoS Control Field - Overview
May 2002
QoS Control Field - Overview
16 bit field
Bits 0-8 represent either:
TXOP limit in the Poll Frame
TXOP duration request or queue size information (in QoS data, QoS Null and RR frames)
Bit 9, 10, 11 identities FEC, non final and no Ack respectively
Bits identifies TID
Sai Shankar et al., Philips Research

5. May 2002
Making a TXOP Request
A QSTA can request for Bandwidth Allocation using the QoS control field:
Request can be in time: TXOP duration in units of 16 seconds
Or based on queue size (units of 128 octets)
What are the advantages of each request method?
Do we need both methods?
Sai Shankar et al., Philips Research

6. May 2002
TXOP Requests in Time
Time requests simplifies the problem of the scheduler at the HC/QAP - just grant the bandwidth in time to QSTA if available
May not be optimal as requests can be arbitrary
However, an scheduling algorithm based on “time requests” can not be optimal
Sai Shankar et al., Philips Research

7. May 2002
TXOP Requests in Time
Increases computational complexity at the QSTA as it has to determine the time required depending on the queue size. Need to consider:
Data rate
Frame length
Fragmentation
Additional problem: Is the TXOP duration requested for aggregated frames or for a single frame?
If TXOP duration requests are for aggregated frames, how does one evaluate the minimum TXOP duration?
Need mechanism to determine if the TXOP duration request is for a single frame
Parameters that vary during time. Even within the same TXOP
Sai Shankar et al., Philips Research

8. TXOP Requests in Time - Scales
May 2002
TXOP Requests in Time - Scales
9 bits available for the request. Bit 9 indicates if it is time or queue size. Only 8 available
The maximum TXOP duration that can be requested up to day is milliseconds (very less)
We need two scales:
Of the 8 bits 1 bit should set aside to clarify the scale of the request being made:
If bit 8 equal to 0: scale of 32 sec (32 sec -> 4096 sec)
If bit 8 equal to 1: scale of 256 sec (256 sec -> sec)
Sai Shankar et al., Philips Research

9. TXOP Requests based on Queue Length
May 2002
TXOP Requests based on Queue Length
QSTA need NOT determine the time required, so simple at QSTA side.
Scheduler at HC needs to determine the TXOP time to be allocated using the queue size information, so the complexity is passed to the HC/QAP side.
An scheduling algorithm based on queue size may be optimal if the HC has some information regarding the traffic characteristics
Main Problems at HC side are
Determining Rate (Transmission rate from QSTA to HC)
Determining Fragment Size (Based on Queue length as there may be more than one fragments. This may vary frame to frame)
Sai Shankar et al., Philips Research

10. TXOP Requests based on Queue Length
May 2002
TXOP Requests based on Queue Length
Problem for Prioritized QoS:
If Parameterized QoS, the HC knows information regarding the stream (minimum rate, nominal MSDU/MPDU size). This is necessary to compute TXOP duration based on queue size information
If Prioritized QoS, the HC does not know which is the nominal MSDU size - so difficult to determine TXOP duration. However the HC can “monitor” the traffic from a QSTA an easily determine the frame/fragment size and the data rate used.
Sai Shankar et al., Philips Research

11. May 2002
Conclusions
An scheduling algorithm based on “queue size” can be optimal.
TXOP requests in time can be arbitrary. The scheduling algorithm will always be sub-optimal.
Change all the requests to bytes. Having two methods to request a TXOP can be confusing and complicated to implement.
Sai Shankar et al., Philips Research

12. May 2002
Motion
Move to remove “TXOP duration request” in the QoS Control field and all the references from the e draft
Sai Shankar et al., Philips Research