Experimental Evaluation of TCP
Performance and Fairness in an 802.11e
Test-bed
David Malone
Anthony Ng & Doug Leith, Hamilton Institute
22nd August 2005
1
802.11b TCP
Performance
12 uploads
6 up, 6 down
12 TCP Uploads to AP for 180 sec
6 Uploads and 6 Downloads for 180 sec
1200
1200
1000
800
Throughput(kbits/sec)
Throughput(kbits/sec)
1000
600
400
200
0
800
600
400
200
1
2
3
4
5
6
7
8
STA id
9
10
11
0
12
1
2
3
4
5
Have proposed fixes, want to test in practice.
2
6
7
8
STA id
9
10
11
12
802.11(e) Summary
• After TX choose rand(0, CWmin − 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
transmissions.
In 11e have multiple queues. Each has own CWmin ,
DIFS(aka AIFS) and can have TXOP.
3
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?
4
Testbed setup
Number of identical stations (Linux) connection to AP
(Linux hostap).
1× AP
Dell GX 280
2.8Ghz P4
12× STA
Soekris net4801
266Mhz 586
WLAN
D-Link DWL-G520
Atheros AR5212
Cards have external antenna, PCI interface, Madwifi
driver with local patches for 11e parameter setting.
MGEN and iperf used for traffic generation.
5
6
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.
7
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
7000
Relative Throughput against CWmin
20
18
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)
4
0
0
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.
8
Proposed settings
AIFS
CWmin
(slots)
AP
TXOP
(packets)
Upload ACKs
0
4
1
Download data
4
32
nd
wireless
Download ACKs
0
32
1
station
Upload data
4
32
1
Derived using analytical modeling and ns.
Will they work in practice?
9
Before
12 uploads
6 up, 6 down
12 TCP Uploads to AP for 180 sec
6 Uploads and 6 Downloads for 180 sec
1200
1200
1000
800
Throughput(kbits/sec)
Throughput(kbits/sec)
1000
600
400
200
0
800
600
400
200
1
2
3
4
5
6
7
8
STA id
9
10
11
0
12
10
1
2
3
4
5
6
7
8
STA id
9
10
11
12
After
12 uploads
6 up, 6 down
12 TCP Uploads to AP for 180 sec
6 Uploads and 6 Downloads
600
700
600
500
Throughput(kbits/sec)
Throughput(kbits/sec)
500
400
300
200
400
300
200
100
0
100
1
2
3
4
5
6
7
8
STA id
9
10
11
0
12
11
1
2
3
4
5
6
7
8
STA id
9
10
11
12
Conclusions
• Small operational testbed.
• Hardware seems to behave as expected.
• Radio issues can be amplified by other issues.
• 11e can be used to combat MAC/TCP issues.
• Now looking at mixed voice/data networks.
12