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WiFi Models EE 228A Lecture 5 Teresa Tung and Jean Walrand Department of EECS University of California at Berkeley Overview: Contents WiFi models via an example of QoS over 802.11 • Overview • 802.11 DCF • Extension for 802.11e EDCF Overview: Scenario 802.11 Network D1 A1 … 5.5 Mbps S1 Am Dm AP 2 Mbps V1 11 Mbps H1 … H1 Vn Hn • What is the throughput? • Can we provide QoS? 5.5 Mbps Hn Sm 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 Overview: DCF review D1 A1 … 5.5 Mbps S1 Am Dm AP 2 Mbps V1 11 Mbps H1 … H1 Vn Hn V1 V’n 5.5 Mbps Hn V1 Dm A1 Dm Sm VoIP only V1 11 Mbps … AP H1 H1 Vn Hn V1 V’2 5.5 Mbps V1 Hn … 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 Vn VoIP only: approach Observation: Bottleneck at the AP # voice connections Bianchi’s model Pr(AP senses channel busy) M/G/1 model at the AP E[transmission delay] QoS criterion: ave delay < 20 ms Call capacity 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 Bianchi: 802.11b Markov chain 16 32 Bianchi: simplification Markov chains coupled Ex: 2 stations state (CW1,m1,CW2,m2) Simplification: Assume independence 1 p1 2 p2 … pn c1 = 1 – i 1 (1 – pi) Bianchi: background • Circuit switched networks [Erlang fixed point] A N1 B • • • • • N2 C 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] Bianchi: fixed point Node n Find fixed point solution (e.g. voice only) Markov chain M/G/1 review 802.11: Comparison with ns-2 • 802.11b network, G.711 codec (160 byte/D) 802.11: results Maximize throughput by • Limiting the number of contending stations • Using large packet payload Not suitable for VoIP 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 AIFS V AIFS D Backoff V Backoff D V1 AIFS D Backoff D D1 • However, may still be pre-empted by data 802.11e: approach Type A 0 AIFS D = AIFS V + 2 IDLE 1 Type B • Classify slots by two types • A reserved for VoIP transmissions • B for all types of transmissions • Changes fixed point equations e.g. AP 802.11e results • Cannot guarantee service Ex. Why 802.11e is not enough • Not enough transmission attempts for VoIP • AP admits too many data packets 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 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.