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BSS Load: AP Loading Metric for QOS

May 2005. doc: IEEE 802.11-05/0079r1. BSS Load: AP Loading Metric for QOS. Joe Kwak InterDigital. TGe Load Metric: QBSS Load. Actually provides three elements all relating to load: Station Count Field, unsigned 16 bit integer indicating total number of STAs associated

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BSS Load: AP Loading Metric for QOS

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  1. May 2005 doc: IEEE 802.11-05/0079r1 BSS Load: AP Loading Metric for QOS Joe Kwak InterDigital Joe Kwak, InterDigital

  2. TGe Load Metric: QBSS Load • Actually provides three elements all relating to load: • Station Count Field, unsigned 16 bit integer indicating total number of STAs associated • Channel Utilization, 8 bit percentage of time the QBSS detects the medium as busy • Available Admission Capacity, 16 bit integer representing amount of medium time (32usec units) available via explicit admission control Joe Kwak, InterDigital

  3. Problems with TGe Metrics • Three metric components make comparative loading evaluations difficult/impossible. • No agreed way to combine the three variables into a summary metric for comparison purposes among QAPs and between APs and QAPs. • Three metric components not sensitive to traffic assymetry: • High downlink loads do not affect uplink access delays since dowlink traffic is “collsion free” within AP. • Three metric components not sensitive to distribution of traffic among prioritized access categories: • Access delay for BE not indicative of load and delay expected for VO traffic. Joe Kwak, InterDigital

  4. New Metric Definition • The Medium Access Delay (MAD) metric is defined as the average delay incurred from the time that any packet is ready for transmission (i.e. begins CSMA/CA access) to the actual packet transmission start time. • MAD is defined for each Tge access category and indicates the current loading in terms of the expected delay for any new EDCF traffic in that category. • For nonQAPs, MAD is defined for DCF traffic and indicates a summary loading in terms of the expected delay for any new DCF traffic. • AP Service Load (APSL) is defined as a summary laoding metric for either APs or QAPs. • For QAPs, APSL is the same as the MAD for the lowest priority BE access category. Joe Kwak, InterDigital

  5. CSMA/CA CSMA/CA MAD Timing for Single Access Next Packet from head of MAC Queue Packet from head of MAC Queue .... ACK Received TxRequest to PHY Tx Process Optional ACK Packet Transmission & ACK Time (Tpacket_TxAck) Transmit Access Delay == MAD MAD timing calculated by MAC = Total Time - TTxAck Joe Kwak, InterDigital

  6. CSMA/CA CSMA/CA MAD Timing with RTS/CTS Access Next Packet from head of MAC Queue Packet from head of MAC Queue .... ACK Received RTS CTS TxRequest for RTS to PHY Tx Process Optional ACK RTS/CTS & Packet Transmission & ACK Time (Tpacket_TxAck) Transmit Access Delay == MAD MAD timing calculated by MAC = Total Time - TTxAck Joe Kwak, InterDigital

  7. MAD Timing for Packet ReTransmissions Joe Kwak, InterDigital

  8. MAD Measurement Implemented by MAC • MAD is easily calculated for any measurement duration and for each access category. • Separate MAC queues exist for traffic in each access category. • MAC logs and timestamps queue status for each access category • MAC logs Tx/Ack time for each PHY access. • InterDigital has coded this to work in MAC using Brand A(theros) chipset. Joe Kwak, InterDigital

  9. Defer log Defer log Defer log Defer log MT log MT log MT log MT log Access cntr Access cntr Access cntr Access cntr TxAck Sum TxAck Sum TxAck Sum TxAck Sum MAD Timing and Priority Queues HI • Access counter counts number of accesses in this access category. • TxAck Sum accumulates total time for transmit/ack of each access in this access category. • When first packet enters access category queue: • MT logs “not empty” timestamp. • Defer logs “deferred” timestamp for all lower priority queues. • When last packet leaves access category queue: • MT logs “empty” timestamp. • Defer logs “not deferred” timestamp for all lower priority queues. AC = VO Priority AC = VI AC = BG LO AC = BE Joe Kwak, InterDigital

  10. MAD Calculation for AC where TQmt = time during which queue for this access category is empty and where TQdef = time during which queue for this access category is deferred to to higher priority queue Joe Kwak, InterDigital

  11. MAD Detects Loads at all TGe Priorities • MAD measurement at each access category include traffic loading effects from higher priority access categories which impact the performance of the measured access category. • The access delay experienced by BE traffic is impacted by the loading of higher priority access categories. • Since BE packets are lower priority than VO, VI or BG packets, the BE access delay values are sensitive to all VO, VI or BG channel loads. • While channel is busy for VO, VI or BG , BE backoff is deferred while higher priority accesses continue and thus affects the MAD measurement. Joe Kwak, InterDigital

  12. 3msec Access Delay BE 1. BE load with 20 users at rate A MAD Detects Loads at all TGe Priorities Each beacon interval example yields same MAD result for BE traffic 50% 3msec Access Delay 2. VI BE BE load with 10 users at rate A 75% 3msec Access Delay 3. HCF VI VO BE BE load with 5 users at rate A VI load with 10 users at rate A VO load with 5 users at rate A Joe Kwak, InterDigital

  13. New BSS Load IE • Similar to TGe QBSS Load but modified for Radio Measurement capable APs • Four component elements: • New AP Service Load (total load metric for AP) • New Access Category Service Load (MAD for each of 4 ACs) • Station Count Field, same as TGe • Channel Utilization, same as TGe • Included in Beacons and Probe Response, like TGe • Conditional inclusion of Station Count and Channel Utilization in Beacon and Probe Response to prevent redundant information • Included as new Statistics Group in Statistics Report to provide upper layer MIB interface and radio interface Joe Kwak, InterDigital

  14. New AP Service Load Properties • Quantized to 8 bits like RSSI & RCPI • Based on MAD measurements in AP downlink during contention periods. • logarithmic scaling over meaningful range • Scaled so min value 1 represents 50usec (DIFS), and max value 253 represents 5.5msec covering a 20.4 db range • Special values: • 0 = All capacity available (no STAs associated) • 254 = no capacity available (not accepting new associations) • 255 = AP Service Load not available • AP measures MAD over thirty second window; accuracy of +/-200 usec specified with minimum of 200 packets in average • Combines AP loading effects of #STAs, Channel Utilization at all priorities and traffic assymetry into SINGLE metric to permit comparison of BSS loading. Joe Kwak, InterDigital

  15. New AP Service Load Definition • "The AP Service Load shall be a scalar indication of the relative level of service loading at an AP. A low value shall indicate more available service capacity than a higher value. The value 0 shall indicate that this AP is not currently serving any STA. The value 255 shall indicate that the AP Service Load is not available. • If dot11QoSOptionImplemented is true: the values between 0 and 254 shall be set equal to the subfield value for the Average Access Delay for the Best Effort (AADBE) within the Access Category Service Load field. • If dot11QoSOptionImplemented is false: the values between 0 and 254 shall be a logarithmically scaled representation of the average medium access delay for DCF transmitted packets measured from the time the DCF packet is ready for transmission (i.e. begins CSMA/CA access) until the actual packet transmission start time. A value of 1 shall represent a 50 us delay while a value of 253 shall represent a 5.5 ms delay or any delay greater than 5.5 ms. The value 254 shall indicate that DCF services are currently blocked. The AP shall measure and average the medium access delay for all transmit packets using DCF access mechanism over a continuos thirty second measurement window. The accuracy for the average medium access delay shall be +/- 200 usec or better when averaged over at least 200 packets.” Joe Kwak, InterDigital

  16. New Access Category Service Load Definition • “The Access Category (AC) Service Load field shall be included in the BSS Load only if dot11QoSOptionImplemented is true. The AC Service load field is formatted as four subfields as shown in Figure kx-2. The AC Service Load shall be a scalar indication of the Average Access Delay (AAD) at a QAP for services for each of the indicated Access Categories. A low value shall indicate shorter access delay than a higher value. If the QAP is not currently providing services at the indicated AC, the AAD for this AC shall be set equal to the AAD of the following AC (located adjacent and to the right) within the Access Category Service field. The value 0 shall indicate that this QAP is not currently providing services of the indicated AC or of any higher priority AC. The values between 0 and 254 shall be a logarithmically scaled representation of the average medium access delay for transmitted packets in the indicated AC measured from the time the EDCF packet is ready for transmission (i.e. begins CSMA/CA access) until the actual packet transmission start time. A value of 1 shall represent a 50 us delay while a value of 253 shall represent a 5.5 ms delay or any delay greater than 5.5 ms. The value 254 shall indicate that services at the indicated AC are currently blocked. The value 255 shall indicate that the AC Service Load is not available. The QAP shall measure and average the medium access delay for all transmit packets of the indicated AC using EDCF access mechanism over a continuos thirty second measurement window. The accuracy for the average medium access delay shall be +/- 200 usec or better when averaged over at least 200 packets.” Joe Kwak, InterDigital

  17. Conclusions • TGe QBSS Load does not provide adequate information for comparative load evaluation among neighbor APs. • Meda Access Delay measurement does provide useful, comparative metric for traffic loading for each TGe Access Category and for all AP traffic. • MAD calculation in MAC is straightforward and does not affect PHY implementations. • TGk should incorporate this new BSS Load information into TGk draft. Joe Kwak, InterDigital

  18. Motion for Improved Normative Text • Move to instruct the editor to incorporate text from document 11-05-0012-02-000k-BSS_Load_norm_text.doc into next TGk draft specification document • Moved by Joe Kwak • Seconded by: _______________ • Vote YEA _______ • Vote NEA _______ • ABSTAIN _______ • Vote Passes/Fails at ___% Joe Kwak, InterDigital

  19. Background: MAD Simulations • Simulation results show how AP Service Load metric relates to Channel Utilization, number of STAs associated and traffic asymmetry. • New AP Service load metric permits AP loads to be compared between APs operating under different conditions. Joe Kwak, InterDigital

  20. Simulation Results 1 Joe Kwak, InterDigital

  21. Simulation Results 2 Joe Kwak, InterDigital

  22. Simulation Results 3 Joe Kwak, InterDigital

  23. Joe Kwak, InterDigital

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