1. The document discusses optimizing the capacity of IEEE 802.11 wireless local area networks (WLANs) for multiplayer network games with many wireless clients. 2. It presents a model to calculate the maximum number of stations that can play an online game over a WLAN before throughput drops or delay increases significantly. 3. The model shows throughput efficiency and delay as a function of the number of stations, with simulations indicating a WLAN can support around 30 stations for a good quality online gaming experience.

1. IEEE 802.11 WLAN Capacity and Optimization for
Multiplayer Network Games
Hanghang Qi David Malone Dmitri Botvich
Hamilton Institute, National University of Hamilton Institute, National University of TSSG, Waterford Institute of Technology,
Ireland, Maynooth, Ireland. Ireland, Maynooth, Ireland. Ireland.
hanghang.qi@nuim.ie david.malone@nuim.ie dbotvich@tssg.org
1 Problem define:network game within an WLAN 1 stations 0.014
stations
game server
0.012 AP
AP
0.8 game server
Throughput efficiency
0.01
0.6
Delay (s)
0.008
AP
nλs 0.006
0.4
n(λc + λs ) S 0.004
0.2
0.002
0 0
λc λc λc 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35
Number of stations Number of stations
C C ... C (a) Throughput efficiency (b) Delay
n
0.04 4.5
stations DCF
0.035
AP 4
Figure 1: How many players can play a good game within a 802.11 WLAN? 0.03 game server
3.5
0.025
Jitter (s)
MOS
0.02 3
0.015
2.5
2 Network games traffic 0.005
0.01
2
We did many experiments in our 4 PCs wireless game network testbed and got 0 1.5
0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35
the game traffic in packet transmission level. The key characteristics are shown Number of stations Number of stations
in Fig. 2 (c) Jitter (d) Mean Opinion Score
AtoS StoA
Figure 4: Performance of a basic 802.11 DCF network
14000 500
450
12000
400
10000
8000
350
300
4 AP and Server with larger TXOP n
6000
250
200
Larger TXOP, one of the 802.11e parameters, are given to AP and Server as n
4000 150
100
increases (n is the number of clients; normal client’s TXOP is 1) to give AP
2000
0
50
0
and Server larger transmission opportunity, the network performance can be
0 0.02 0.04 0.06 0.08
interarrival time (s)
0.1 0.12 0 0.02 0.04 0.06 0.08
interarrival time (s)
0.1 0.12
improved for the games.
(a) Client to Server (b) Server to Client 1 0.02
AtoS StoA game server game server
5000 700
0.9 stations stations
4500 AP AP
600
0.8 0.015
Throughput efficiency
4000
3500 500
0.7
Delay (s)
3000
400
2500 0.6 0.01
300
2000
0.5
1500 200
1000 0.4 0.005
100
500
0.3
0 0
0 50 100 150 200 0 50 100 150 200
packet size (bytes) packet size (bytes) 0.2 0
0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35
Number of stations Number of stations
(c) Client to Server (d) Server to Client
(a) Throughput efficiency (b) Delay
Figure 2: Quake 4 game traffic characteristics 0.05 4.5
game server TXOP
stations 4
0.04 AP
3 802.11 network model and performance 0.03
3.5
Jitter (s)
MOS
IEEE 802.11 MAC DCF uses a CSMA/CA with exponential backoff scheme 3
0.02
which can be modelled with a 2-D Markov Chain. 2.5
0.01
2
0 1.5
0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35
Number of stations Number of stations
0,0e 1−q 0,1e 0,2e ... 0,w0−1e
1−q 1−q
q q q
(c) Jitter (d) MOS
q
0,0 0,1 0,2 ... 0,w0−1
Figure 5: AP and Server priority with TXOP
1−p 1 1 1
... ... ... ...
1−p
i−1,0 ...
5 Conclusions
Our analytical model suggests that a basic DCF 802.11b WLAN can support
1−p i,0
1
i,1
1
i,2 ...
1
i,wi−1 maximum 10 players of Quake 4. By using 11e parameter TXOP to give AP
... ... ... ...
and Game Server higher priority to access the channel, the network perfor-
mance can be improved to 15 players.
1−p m,0 1
m,1 1
m,2 ... 1
m,wm−1
References
[Bianchi, 2000] Bianchi, G. (2000). Performance analysis of the ieee 802.11
Figure 3: Markov chain model of 802.11 MAC distributed coordination function. Selected Areas in Communications, IEEE
Journal on, 18(3):535–547.
[Cricenti and Branch, 2007] Cricenti, A. and Branch, P. (2007). Arma(1,1)
The Bianchi’s 2-D Markov chain model together with the traffic arrival modeling of quake4 server to client game traffic. In NetGames ’07, NY,
model are used to calculate the throughput, delay and jitter. Then Frank’s em- USA. ACM.
pirical mapping model is used to get Mean Opinion Score (MOS) from delay
and jitter. [Wattimena et al., 2006] Wattimena, A. F., Kooij, R. E., van Vugt, J. M., and
Ahmed, O. K. (2006). Predicting the perceived quality of a first person