WiFi ModelsEE 228A Lecture 5

Teresa Tung and Jean WalrandDepartment of EECSUniversity 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: 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

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

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

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 …

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

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)

1 2

c1 = 1 – i 1 (1 – pi)

pn

p2…1

p1

Simplification: Assume independence

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

Bianchi: fixed point

Markov chain

Find fixed point solution (e.g. voice only)

Node n

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

• However, may still be pre-empted by data

V1AIFS V Backoff V

D1AIFS D Backoff D AIFS D Backoff D

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

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.