wifi models ee 228a lecture 5 teresa tung and jean walrand department of eecs university of...
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![Page 1: WiFi Models EE 228A Lecture 5 Teresa Tung and Jean Walrand Department of EECS University of California at Berkeley](https://reader030.vdocuments.site/reader030/viewer/2022032801/56649d545503460f94a317f9/html5/thumbnails/1.jpg)
WiFi ModelsEE 228A Lecture 5
Teresa Tung and Jean WalrandDepartment of EECSUniversity of California at Berkeley
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Overview: Contents
WiFi models via an example of QoS over 802.11
• Overview
• 802.11 DCF
• Extension for 802.11e EDCF
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Overview: Scenario802.11 Network
• What is the throughput?• Can we provide QoS?
AP
H1
Hn
V1
Vn
H1
Hn
11 Mbps
5.5 Mbps
…
S1A1
D1
5.5 Mbps
…
SmAm
Dm 2 Mbps
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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
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Overview: DCF review
AP
H1
Hn
V1
Vn
H1
Hn
11 Mbps
5.5 Mbps
…
S1A1
D1
5.5 Mbps
…
SmAm
Dm 2 Mbps
Dm
V1
V’n Dm
V1
A1
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VoIP only
• Hope to send V1,V2,…,Vn in 20 ms• Time depends on n and rates• Given rates, there is a maximum n feasible
AP
H1
Hn
V1
Vn
H1
Hn
11 Mbps
5.5 Mbps
…VnV1
V’2 V’1
V1 …
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VoIP only: approach
Observation: Bottleneck at the AP
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
# voice connections
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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
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Bianchi: 802.11b Markov chain
16
32
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Bianchi: simplification
Markov chains coupled
Ex: 2 stations state (CW1,m1,CW2,m2)
1 2
c1 = 1 – i 1 (1 – pi)
pn
p2…1
p1
Simplification: Assume independence
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Bianchi: background• 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]
N2N1
A
CB
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Bianchi: fixed point
Markov chain
Find fixed point solution (e.g. voice only)
Node n
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M/G/1 review
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802.11: Comparison with ns-2• 802.11b network, G.711 codec (160 byte/D)
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802.11: results
Maximize throughput by
• Limiting the number of contending stations
• Using large packet payload
Not suitable for VoIP
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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
V1AIFS V Backoff V
D1AIFS D Backoff D AIFS D Backoff D
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802.11e: approach
• Classify slots by two types• A reserved for VoIP transmissions• B for all types of transmissions
• Changes fixed point equationse.g. AP
Type B
Type A
0 1
AIFS D = AIFS V + 2 IDLE
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802.11e results• Cannot guarantee service
Ex.
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Why 802.11e is not enough• Not enough transmission attempts for VoIP
• AP admits too many data packets
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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
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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.