Transcript Communication Networks - University of California, Berkeley

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.