Evaluate IEEE 802.11e EDCA Performance

Tyler Ngo

CMPE 257

EDCA vs. DCF

EDCA classifies traffic flows in different access categories (AC). Modifiable MAC parameters include:

– Arbitration Interframe Space (AIFS) replaces the DIFS in IEEE 802.11.– Minimum Contention Window (CWmin).– Maximum Contention Window (CWmax).– Transmission Opportunity (TXOP).

Shorter CW and AIFS for higher-priority traffic.

AC CWmin CWmax AIFSN Max TXOP Background (AC_BK) 31 1023 7 0 Best Effort (AC_BE) 31 1023 3 0

Video (AC_VI) 15 31 2 3.008ms Voice (AC_VO) 7 15 2 1.504ms

Legacy DCF 15 1023 2 0

Analytical Modeling Transmission Probability

Let W = CWmin, then CWmax = W * 2m, where m is the maximum backoff stage.

Let ρc be the probability that a packet of class c encounters a collision on the channel. Let τc be the probability that a station of class c transmit in a random

chosen slot. Then:

Analytical Modeling Throughput

Let ρtr be the probability that there is at least one transmission in the considered slot time. Then:

Let ρs,i be the probability that a transmission of a packet of node i occurring on the channel is successful. Let τj be the probability that a node j transmit data (j ≠ i, j = [1, n]). Then:

Analytical Modeling Throughput, Cont.

Let Ts,c be the average time that a node of class c senses the channel busy because of a successful transmission, TC,c be the average time that a node of class c senses the channel busy during a collision. Let E[P] be the expected packet length, H = PHYhdr + MAChdr be the packet header, δ be the propagation delay, and α be the length of a slot time. Then:

The throughput of node i, Si is then:

Simulation Modeling

Controlled Parameters:– Loss Model: Log Distance

Exponent = 3 Reference Distance = 1 Reference Loss: 46.67

– Delay Model: Random, Uniform variable; Constant speed.– Nist Error Rate Model– Transmission Range

Energy Detection Threshold: -96.0 CCA Mode1 Threshold: -99.0 Tx Power End/Start: 16.0206

– Routing Protocol: OLSR– TCP Protocol: New Reno– Data Rate: 5MB/s– Run time = ~100s– TCP Packet Size = 1024– UDP Packet Size = 120

Topology 1

Traffic: 6 7; 9 1; 3 8; 5 2 Total run duration: 100s

Topology 1

-150

-100

-50

0

50

100

150

-150 -100 -50 0 50 100 150

X

Y

Topology 1: N6to7 TCP/BE; Others UDP/BE or AC_VO

0

0.05

0.1

0.15

0.2

0.25

0 200 400 600 800 1000 1200

Packet ID

Del

ay

Stationary:BE

Stationary:BE/AC_VO

Mobility:BE

Mobility:BE/AC_VO

Topology 1: N6to7 TCP/AC_VO; Others UDP/BE

0

0.005

0.01

0.015

0.02

0.025

0.03

0 200 400 600 800 1000 1200 1400 1600

Packet ID

Del

ay (

s)

Mobility

Stationary

Topology 2

Traffic:– 6 7; – 9 1; – 3 8; – 5 2;

Total run duration: 100s

Topology 2

-200

-150

-100

-50

0

50

100

150

200

250

300

350

-200 -150 -100 -50 0 50 100 150 200 250 300

X-Pos

Y-Po

s

Topology 2: N6to7 TCP/BE; Others UDP/BE or AC_VO

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0 50 100 150 200 250 300 350 400

Packet ID

Del

ay (

s)

Stationary: BE

Mobile: BE

Stationary: BE/AC_VO

Mobility: BE/AC_VO

Topology 2: N6to7 TCP/AC_VO; Others UDP/BE

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 200 400 600 800 1000 1200

Packet ID

Del

ay (

s)

Stationary

Mobility

Topology 3

Traffic: – 6 7; – 9 1; – 3 8; – 5 2;

Total run duration: 100s Mobile environment only

– Gaussian Markov Mobility Model

Topology 3: N6to7 TCP/BE or AC_VO; Others UDP/BE or AC_VO

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 100 200 300 400 500 600 700 800

Packet ID

Delay

(s)

BE

TCP: BE; UDP: AC_VO

TCP: AC_VO; UDP: BE

Conclusion

Higher-priority tagging improves throughputs. But…

What are the rules for tagging? TCP starvation is the main issue.