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WiFi Models

This lecture provides an overview of WiFi models for Quality of Service (QoS) over 802.11 networks, including the DCF and EDCF protocols. Topics covered include throughput, QoS provision, and capacity analysis. Examples and solutions for enabling QoS over WiFi are discussed.

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WiFi Models

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  1. WiFi Models EE 228A Lecture 5 Teresa Tung and Jean Walrand Department of EECS University of California at Berkeley

  2. Overview: Contents WiFi models via an example of QoS over 802.11 • Overview • 802.11 DCF • Extension for 802.11e EDCF

  3. Overview: Scenario 802.11 Network • What is the throughput? • Can we provide QoS? D1 S1 A1 5.5 Mbps … Am Sm Dm 2 Mbps AP V1 H1 H1 11 Mbps … Vn Hn Hn 5.5 Mbps

  4. Overview: 802.11 MAC • Point Coordination Function (PCF) • Not implemented • Simple to analyze TDMA • Distributed Coordination Function (DCF) • Implemented • More difficult to analyze CSMA/CA • Ex: 802.11b (11 Mbps) • Data only: 6 Mbps • VoIP: 12 connections  64 kbps/direction  1.5 Mbps

  5. Overview: DCF review V1 V1 V’n A1 D1 S1 A1 5.5 Mbps … Am Sm Dm 2 Mbps AP V1 H1 H1 11 Mbps … Vn Hn Hn 5.5 Mbps Dm Dm

  6. VoIP only V1 V1 V’2 V’1 • Hope to send V1,V2,…,Vn in 20 ms • Time depends on n and rates • Given rates, there is a maximum n feasible V1 H1 H1 11 Mbps AP … Vn Hn Hn 5.5 Mbps … Vn

  7. VoIP only: approach Observation: Bottleneck at the AP # voice connections Bianchi’s model M/G/1 model at the AP QoS criterion: ave delay < 20 ms Pr(AP senses channel busy) E[transmission delay] Call capacity

  8. Bianchi model • Discrete model with variable slot size • Idle slot • Success = VoIP + SIFS + ACK + DIFS • Collision = VoIP + EIFS • VoIP = (RTP + UDP + IP + MAC + payload)/rate

  9. Bianchi: 802.11b Markov chain 16 32

  10. Bianchi: simplification Simplification: Assume independence p1 1 p2 … pn c1 = 1 –  i 1(1 – pi) Markov chains coupled Ex: 2 stations state (CW1,m1,CW2,m2) 2 1

  11. Bianchi: background A N1 N2 B C • Circuit switched networks [Erlang fixed point] • Pr(A blocked) depends on (#A,#B,#C) • Simplification: Assume each call blocked independently by different links • Ex: Arrival rate at 1: 1 = A (1 – b2) + B Pr(blocked at 1): b1 = (N1) M/M/1/N1 • Packet switched network [Kleinrock independence approximation]: M/M/1 queuing model • Interacting particle systems [Gibbs]

  12. Bianchi: fixed point Node n Find fixed point solution (e.g. voice only) Markov chain

  13. M/G/1 review

  14. 802.11: Comparison with ns-2 • 802.11b network, G.711 codec (160 byte/D)

  15. 802.11: results Maximize throughput by • Limiting the number of contending stations • Using large packet payload Not suitable for VoIP

  16. 802.11e: EDCF review • Voice has edge over data (waits less) • Chooses random back-off from smaller interval • Waits less time after busy period to operate AIFS V = DIFS AIFS D = AIFS V + 2 IDLE • However, may still be pre-empted by data AIFS V Backoff V V1 D1 Backoff D Backoff D AIFS D AIFS D

  17. 802.11e: approach Type A • AIFS D = AIFS V + 2 IDLE Type B 0 1 • Classify slots by two types • A reserved for VoIP transmissions • B for all types of transmissions • Changes fixed point equations e.g. AP

  18. 802.11e results • Cannot guarantee service Ex.

  19. Why 802.11e is not enough • Not enough transmission attempts for VoIP • AP admits too many data packets

  20. Enabling QoS over WiFi Ideal solution: PCF • Requires changes of AP and wireless clients DCF solution using existing WiFi clients • Requires changes at the AP • Estimate capacity • Admission control for VoIP and video • Traffic shaping for TCP • PCF on downlink via NAV vector

  21. References • G. Bianchi, “Performance analysis of the IEEE 802.11 distributed coordination function,” IEEE J. Select Areas Communications, vol. 18, no. 3, pp. 535-547, 2000. • N. Hedge, A. Proutiere, and J. Roberts, “Evaluating the voice capacity of 802.11 WLAN under distributed control,” Proc. LANMAN, 2005.

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