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.
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