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TXOP Request: in Time vs. in Queue Size?

TXOP Request: in Time vs. in Queue Size?. Sai Shankar, Javier del Prado and Sunghyun Choi Philips Research USA Briarcliff Manor, New York sunghyun.choi@philips.com. Outline. QoS Control Field - Overview Problem Statement: Making a TXOP Request TXOP Requests in time

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TXOP Request: in Time vs. in Queue Size?

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  1. TXOP Request: in Time vs. in Queue Size? Sai Shankar, Javier del Prado and Sunghyun Choi Philips Research USA Briarcliff Manor, New York sunghyun.choi@philips.com Sai Shankar et al., Philips Research

  2. 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. References • Bobby Jose, “Contention Free TXOP Request and Allocation Issues,” IEEE 802.11-02/248r0 • Bobby Jose, “Updated QoS Control Field,” IEEE 802.11-02/290r0 • IEEE 802.11e QoS draft D2.0a Sai Shankar et al., Philips Research

  4. 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 12-15 identifies TID Sai Shankar et al., Philips Research

  5. 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. 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. 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 • 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 4.096 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 -> 32768 sec) Sai Shankar et al., Philips Research

  9. 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 • 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. 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. Motion • Move to remove “TXOP duration request” in the QoS Control field and all the references from the 802.11e draft Sai Shankar et al., Philips Research

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