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AP Service Load: Improved Definition

This document presents a new metric definition for AP service load and details MATLAB simulation setup for IEEE 802.11 DCF access method modeling. Key metrics such as medium access delay, throughput, and MAC transmit delay are analyzed. The document also discusses new properties for quantized AP service load measurements based on medium access delay.

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AP Service Load: Improved Definition

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  1. May 2004 doc: IEEE 802.11-04/550r0 AP Service Load: Improved Definition Joe Kwak InterDigital Joe Kwak, InterDigital

  2. Existing MIB definition for AP Service Load • "This counter 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 128 shall indicate that the AP is operating at its target service loading. The value 255 shall indicate no additional AP service capacity is available or that this STA is not an AP." Joe Kwak, InterDigital

  3. New Metric Definition • The Medium Access Delay (MAD) metric is defined as the average delay incurred from the time the packet is ready for transmission (i.e. begins CSMA/CA access) to the actual packet transmission start time. Joe Kwak, InterDigital

  4. MATLAB Simulation Setup • The simulator models a single BSS with one AP and multiple associated STAs. • Traffic modeling: • Packets arrive for transmission at each STA according a Poisson process. • New Packets are queued for transmission at the AP every time a successful packet is received at the AP from a transmitting STA (i.e. the AP responds to the STAs transmission). The number of packets that are transmitted by the AP per received packet is equal to asymmetry_factor. For example, asymmetry_factor=1 corresponds to symmetric upstream/downstream traffic. When asymmetry_factor=2, there are 2 downstream packets for every upstream packet. • Different offered loads are achieved by varying the alpha_STA, N_STA and asymmetry_factor parameters. Joe Kwak, InterDigital

  5. MATLAB Simulation Setup (cont.) • The 802.11 DCF access method is modeled, i.e. carrier sense (deferral) and collision avoidance (random back-off procedure) are modeled. • All STAs "hear" each other’s transmissions, i.e. no hidden terminals. • Transmission errors due to collisions are modeled. The effects of collisions are accurately modeled by doubling the size of the Contention Window (CW) following transmission errors (i.e. collisions), up to the maximum allowed CW. The CW is reset to the minimum CW upon successful transmission (i.e. without collision). • All collision-free transmissions have sufficient SNR (i.e. perfect channel) to be error free. Joe Kwak, InterDigital

  6. MATLAB Simulation Setup (cont.) • Packets are dropped once the maximum number of retransmissions is attained (default value of 4 is assumed), after which point CW is reset to CWmin. • When a packet is transmitted, the PLCP header, the transmission of the MPDU, the SIFS wait-time (waiting for ACK), the PLCP header of the ACK and the transmission of the ACK MPDU are modeled. • The following metrics are logged individually for the AP and each STA during a simulation run: • Offered load (number of packets and bits/second) • Throughput (number of packets and bits/second) • Medium Access Delay (MAD, in seconds per packet) • MAC Transmit Delay ( MTD, in seconds per packet) where MTD= queueing delay + MAD Joe Kwak, InterDigital

  7. Simulation Results 1 Joe Kwak, InterDigital

  8. Simulation Results 2 Joe Kwak, InterDigital

  9. Simulation Results 3 Joe Kwak, InterDigital

  10. Simulation Results 4 Joe Kwak, InterDigital

  11. Simulation Results 5 Joe Kwak, InterDigital

  12. 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 0 represents 50usec (DIFS), and max value 255 represents 5.5msec covering a 20.4 db range • AP measures MAD over minimum of 200 packets with an accuracy of +/-200 usec. Joe Kwak, InterDigital

  13. New AP Service Load Definition • "This counter 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 no additional AP service capacity is available or that this STA is not an AP. The values between 0 and 255 shall be a logarithmically scaled representation of the average medium access delay incurred from the time the packet is ready for transmission (i.e. begins CSMA/CA access) to the actual packet transmission start time. Value 1 represents 50 usec, Value 254 represents 5.5msec. The AP shall average medium access delay during DCF access over at least 200 packet measurements with an accuracy of +/- 200 usec or better." Joe Kwak, InterDigital

  14. TGe Load Metric • 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

  15. Problems with Tge Metrics • Three metric components make comparative evaluations difficult/impossible. • No agreed way to combine the three variables into a summary metric for comparison purposes. • Simulation results show how new AP Service Load metric relates to Channel Utilization and number of STAs associated. • New AP Service load metric permits AP loads to be compared between APs operating under different conditions. Joe Kwak, InterDigital

  16. Motion for Improved Normative Text • Move to instruct the editor to incorporate text from document 11-04-0551-00-000k-Improved_AP_Service_Load.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

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