Experimental Evaluation of 802.11e
EDCA for Enhanced Voice over WLAN
Performance
Ian Dangerfield, David Malone, Douglas J. Leith
Hamilton Institute, NUI Maynooth, Ireland.
3 April 2006
1
802.11(e) MAC
Summary
• After TX choose rand(0, CW − 1).
• Wait until medium idle for DIFS(50µs),
• While idle count down in slots (20µs).
• TX when counter gets to 0, ACK after SIFS (10µs).
• If ACK then CW = CWmin else CW∗ = 2.
Ideally produces even distribution of packet TX.
In 11e have multiple queues. Each has own CWmin ,
DIFS(aka AIFS) and can have TXOP.
2
Why use a testbed?
• Can we believe ns?
Bugs: aCCATime, virtual collisions.
• Can we believe the standard?
• Can we believe models?
• What are the practical issues?
3
Testbed setup
Multiple STA (Linux) connected to AP (Linux hostap).
Hardware
model
spec
1× AP
Dell GX 260
2.66Ghz P4
18× STA
Soekris net4801
266Mhz 586
1× STA
Dell GX 270
2.8Ghz P4
WLAN NIC
D-Link DWL-G520
Atheros AR5212
External antenna, PCI interface, Madwifi driver with
local patches for 11e parameter setting.
MGEN and iperf used for traffic generation.
4
5
Practical Issue:
Calibration
UDP up
TCP up
11 STAs Upload To AP UDP
11 STAs Upload To AP
700
1500
600
Throughput(kbits/sec)
Throughput(kbits/sec)
500
400
300
1000
500
200
100
0
1
2
3
4
5
6
STA id
7
8
9
10
0
11
Small changes until well behaved.
6
1
2
3
4
5
6
STA id
7
8
9
10
11
Validation
Relative Throughput against TXOP parameter
Relative Throughput against AIFS parameter
5.5
30
Analytical Model
Experimental Results
5
25
4
Relative Throughput
Relative Throughput
4.5
3.5
3
2.5
20
15
10
2
5
1.5
1
0
1000
2000
3000
4000
TXOP in unit of us
5000
6000
0
7000
Relative Throughput against CWmin
Analytical Model
Experimental Results
16
Relative Throughput
14
12
10
8
6
4
2
0
−3
−2
−1
0
1
2
3
CWmin Shift (0 corresponds to the default CWmin 32)
5
10
15
AIFS in unit of 20us timeslot
20
25
Measure relative performance of two saturated
flows
while
varying
TXOP,
AIFS
and
CWmin .
Compare to
well-known models.
20
18
0
4
7
Voice
• Has a loss and delay requirement.
• Low rate vs. high rate.
• Aim to protect voice from saturated sources.
• AIFS is the obvious parameter.
• For sake of argument, target loss of 10%.
• (simulation says 4, model says 6 to be safe).
8
Unprioritised Voice
Throughput
Delay
70
30000
60
Average Delay (seconds x 10-6)
25000
50
Throughput (kbps)
Unpriotised Station
Delay bound for queue stability
40
30
20
20000
15000
10000
5000
10
Unpriotised Station
Ideal Throughput
90% Throughput
0
0
2
4
6
0
8
10
12
14
16
18
0
Number of competing stations
2
4
6
8
10
12
Number of competing stations
Rapid drop in throughput.
Linear growth in delay.
9
14
16
18
Measuring Delay
• Want to measure one-way MAC delay.
• NTP slow and insufficiently accurate.
• Simultaneously observable TX better, largish noise.
0.1
station 1
station 2
station 3
station 4
Offset (s)
0.05
0
-0.05
-0.1
0
500
1000
1500
Time (s)
10
2000
2500
Delay Technique
• Transmission not complete until MAC ACK.
• Hardware supports interrupt after ACK.
Interface TX
Queue
1. Driver notes
enqueue time.
Driver
4. Driver notes
completion time.
Driver TX
Discriptor
Queue
2. Hardware
contends until
ACK received
Packet transmitted
Hardware
ACK received
11
3. Hardware
interrupts
driver.
Validation
1600
Median delay (seconds x 10-6)
1400
1200
1000
800
600
400
Long preamble
Fit line: 11.0012Mbps x + 484.628us
Short preamble
Fit line: 11.0016Mbps x + 292.565us
200
0
0
200
400
600
800
1000
1200
Packet size (bytes, excluding headers)
12
1400
4000
16000
1400
1400
3500
14000
600
1500
400
1000
Mean delay (us)
800
2000
Throughput x delay (bytes)
Mean delay (us)
1000
2500
1200
12000
0
500
0
1000 1500 2000 2500 3000 3500 4000 4500
1400
Mean delay (us)
1000
25000
800
20000
600
15000
400
10000
200
5000
Throughput x Mean delay (Station 2)
Mean delay (Station 2)
300
400
CWmin value second station
500
0
600
Throughput x delay (bytes)
1200
30000
200
2
4
6
0
8
10
AIFS value of second station
35000
100
200
Throughput x Mean delay (Station 2)
Mean delay (Station 2)
0
40000
0
400
0
TXOP value for station 1
0
600
6000
2000
Throughput x Mean delay (Station 1)
Mean delay (Station 1)
0
800
8000
4000
200
500
1000
10000
Throughput x delay (bytes)
1200
3000
Measure relative performance of two saturated
flows while varying TXOP,
AIFS and CWmin . Check
Throughput * delay has
expected value.
13
AIFS Impact
Throughput
Delay
70
30000
60
Average Delay (seconds x 10-6)
25000
50
Throughput (kbps)
Unpriotised Station
Competitors AIFS = 4
Competitors AIFS = 6
Delay bound for queue stability
40
30
20
Unpriotised Station
Competitors AIFS = 4
Competitors AIFS = 6
Ideal Throughput
90% Throughput
10
0
0
2
4
6
20000
15000
10000
5000
0
8
10
12
14
16
18
0
Number of competing stations
2
4
6
8
10
12
Number of competing stations
Delay now practically bounded.
AIFS 4 is enough, but on boundary.
14
14
16
18
Delay Distribution
AIFS 0
AIFS 6
100
Percentage of packets (less than associated delay)
Percentage of packets (less than associated delay)
100
80
60
40
1 competing station
6 competing stations
12 competing stations
18 competing stations
Interpacket time
20
0
0
20000
40000
60000
80000
80
60
40
1 competing station
6 competing stations
12 competing stations
18 competing stations
Interpacket time
20
0
100000
Delay (seconds x 10-6)
0
20000
40000
60000
Delay (seconds x 10-6)
15
80000
100000
Autocorrelation
1
Unpriotized, 1 competing station
Unpriotized, 6 competing stations
Unpriotized, 12 competing stations
Unpriotized, 18 competing stations
AIFS = 4, 1 competing station
AIFS = 4, 6 competing stations
AIFS = 4, 12 competing stations
AIFS = 4, 18 competing stations
AIFS = 6, 1 competing station
AIFS = 6, 6 competing stations
AIFS = 6, 12 competing stations
AIFS = 6, 18 competing stations
0.8
autocorrelation
0.6
0.4
0.2
0
-0.2
0
5
10
15
number of packets
16
20
25
30
Conclusions
• Small 11e testbed.
• Hardware seems to behave as expected.
• Instrumented to fine-grained MAC delay.
• 11e can be used to help voice out.
• Look at mixed voice/data and voice only networks.
17
Thanks
David.Malone@nuim.ie
http://www.hamilton.ie/
18