Modeling 802.11e for data traffic parameter design
P. Clifford, K. Duffy, J. Foy, D. Leith and D. Malone
April 2006
Hamilton Institute, NUIM, Ireland
TCP Uploads
1.8
1.6
TCP throughput (Mbps)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
1
2
3
4
5
6
7
8
9
10
Number of the upstream connection
Figure 1: Competing TCP uploads, 10 stations (NS2 simulation, 802.11 MAC,
300s duration).
Hamilton Institute, NUIM, Ireland
1
Existing literature
Empirical studies.
• Q. Ni and L. Romdhani and T. Turletti, Wirel Commun Mob Com 5:4 (2004).
• P. Gopalakrishnan, D. Famolari and T. Kodama, Proc. IEEE GLOBECOM
(2004).
• Y. Xiao and H. Li and S. Choi, Proc. IEEE INFOCOM (2004).
Saturated 802.11e multi-class models.
• R. Battiti and Bo Li, University of Trento Technical Report DIT-03-024 (2003).
• J.W. Robinson and T.S. Randhawa, IEEE JSAC 22:5 (2004).
• Z. Kong, D. H.K. Tsang, B. Bensaou and D. Gao, IEEE JSAC 22:10 (2004).
Hamilton Institute, NUIM, Ireland
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Contribution
•
•
•
•
•
multi-class 802.11e EDCF finite-load model,
simple enough to be solvable,
accurately predicts the throughputs of unsaturated traffic,
can model all of TXOP, AIFS and CWmin,
is applied to choose parameters to restore TCP upload fairness.
Hamilton Institute, NUIM, Ireland
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The 802.11e MAC
• After transmission counter initialized to uniform integer in [0, CW − 1].
• Medium determined idle if idle for period DIFS.
• Counter decremented once for each slot that the medium is idle.
• Countdown halts if medium busy; resumes after DIFS.
• If packet arrives before counter reaches zero, station transmits for max duration
TXOP (one packet without 802.11e).
• If packet arrives after countdown completed, the station senses the medium. If
medium idle, the station transmits; if busy, another back-off counter is chosen.
• If > 2 stations transmit simultaneously, a collision occurs. Colliding stations
double their CW (up to a maximum value), select a new back-off counter
uniformly and the process repeats.
• After successful transmission, CW is reset to its minimal value CWmin and a
new countdown starts regardless of the presence of a packet at the MAC.
Hamilton Institute, NUIM, Ireland
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The 802.11e MAC enables the values of DIF S (called AIFS in 802.11e), CWmin
and TXOP to be set on a per-class basis for each station. That is, traffic is
directed to up to four different queues at each station, with each queue assigned
different MAC parameter values.
The standard suggests parameter values for four classes: best effort, voice, video
and background. It also allows the AP to assign parameters to queues on stations.
Hamilton Institute, NUIM, Ireland
5
Finite load 11b
K.D., D.M., D.L., RAWNET (2005), Comms Letters (2005), ToN (2007).
Complications:
• Post-backoff.
• Relating real offered load to non real time Markov chain.
• Queueing issues due to non real time service model.
Can cope with CWmin and T XOP , but not different AIF S.
Hamilton Institute, NUIM, Ireland
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Matching Offered Load
0.35
normalised throughput
0.3
8 stations (sim)
(model unif)
(model Pois)
(model Cond)
6 stations (sim)
(model unif)
(model Pois)
(model Cond)
4 stations (sim)
(model unif)
(model Pois)
(model Cond)
2 stations (sim)
(model unif)
(model Pois)
(model Cond)
0.25
0.2
0.15
0.1
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
normalised total offered load
Figure 2: Throughput as the traffic arrival rate is varied. Results for the three
load relationships.
Hamilton Institute, NUIM, Ireland
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Finite load 11e
Added complications:
• Need hold states for AIF S.
• New, different coupling equations.
(1 −
Q n1
Ph =
1 + (1 −
(1)
j=1(1 − τj )
Qn1
j=1(1 −
Q n2
(2)
j=1(1 − τj ))
(1) Qn2
τj ) j=1(1
(2)
D
X
PS−i
1
i=1
D
X
− τj ))
.
(1)
PS−i
1
i=1
Hamilton Institute, NUIM, Ireland
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Some equations
(1)
1 − pi
=
Y
(1)
(1 − τj )(Ph + (1 − Ph)
j6=i
1−
(2)
pi
n2
Y
(2)
(1 − τj ))
(2)
j=1
=
n1
Y
(1 −
j=1
(1)
τj )
Y
(2)
(1 − τj ).
(3)
j6=i
Similar expressions for calculating expected state lengths.
8Eλ =
Hamilton Institute, NUIM, Ireland
− log(1 − q)8E
Mbps.
Es
(4)
9
How good is it?
0.25
0.2
0.15
0.1
0.05
0.25
model class 1
model class 2
simulation
0.2
Per station throughput (Mbps)
model class 1
model class 2
simulation
Per station throughput (Mbps)
Per station throughput (Mbps)
0.25
0.15
0.1
0.05
0
5
10
15
20
Total offered load (Mbps)
(a) D = 0
25
30
0.2
0.15
0.1
0.05
0
0
model class 1
model class 2
simulation
0
0
5
10
15
20
Total offered load (Mbps)
(b) D = 2
25
30
0
5
10
15
20
Total offered load (Mbps)
25
30
(c) D = 4
Throughput for a station in each class vs. offered load. 10 class 1 stations offering
one quarter the load of 20 class 2 stations. Range of D values, the difference
in AIFS between class 2 and class 1 (NS2 simulation and model predictions,
802.11e MAC, 11Mbps PHY, 100s duration. ).
Hamilton Institute, NUIM, Ireland
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How good is it?
0.25
0.2
0.15
0.1
0.05
0.25
model class 1
model class 2
simulation
0.2
Per station throughput (Mbps)
model class 1
model class 2
simulation
Per station throughput (Mbps)
Per station throughput (Mbps)
0.25
0.15
0.1
0.05
0
5
10
(1)
(d) W0
15
20
Total offered load (Mbps)
(2)
= 32, W0
25
30
= 16
0.2
0.15
0.1
0.05
0
0
model class 1
model class 2
simulation
0
0
5
(1)
(e) W0
10
15
20
Total offered load (Mbps)
(2)
= 32, W0
25
30
= 64
0
5
(1)
(f) W0
10
15
20
Total offered load (Mbps)
(2)
= 32,W0
25
30
= 256
Throughput for a station in each class vs. offered load. There are 10 class
1 stations each offering one quarter the load of 20 class 2 stations. Range of
CWmin values (NS2 simulation and model predictions, 802.11e MAC 11Mbps
PHY, 100s duration).
Hamilton Institute, NUIM, Ireland
11
How do you use it?
6
0.9
W1=1
W1=2
W1=4
W1=8
W1=16
W1=32
5
W1=1
W1=2
W1=4
W1=8
W1=16
W1=32
0.8
Ratio of lost traffic to offered load
Throughput of Data Stations (Mbps)
0.7
4
3
2
0.6
0.5
0.4
0.3
0.2
1
0.1
0
0
0
5
10
Difference in AIFS
(g) Data throughput
15
20
0
5
10
Difference in AIFS
15
20
(h) ACK loss
10 stations (1500 byte packets) and AP transmitting (60 byte packets) at half
achieved data rate (Model predictions, 802.11e MAC, 300s duration).
Aim for 1%: (CWmin, AIFS) = (1,2), (2,4), (4,7), (8,12), . . .
Hamilton Institute, NUIM, Ireland
12
1.2
0.6
1
0.5
0.8
0.4
TCP Throughput (Mbps)
TCP Throughput (Mbps)
Does it work?
0.6
0.4
0.2
0.3
0.2
0.1
0
0
1
2
3
4
5
6
7
8
9
Number of the upstream connection
10
11
12
1
2
3
4
5
6
7
8
9
Number of the upstream connection
10
11
12
Competing TCP uploads, 12 stations experiment without and with prioritization
(802.11e MAC, 300s duration).
Hamilton Institute, NUIM, Ireland
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Conclusions
• 802.11e CSMA/CA model that is simple, solvable, yet complex enough to
predict data throughput.
• Model gives insight into effect of 802.11e parameters.
• Prioritization schemes can now be designed analytically.
• Have applied model to voice.
• Future: applications to mesh.
Hamilton Institute, NUIM, Ireland
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