Jitter Analysis of the IEEE
802.11 DCF Aceess mode
Dimitrios J. Vergados
Dimitrios D. Vergados
School of Electrical and Computer Engineering
National Technical University of Athens
GR-15773 Zografou, Athens, Greece
email: djvergad@telecom.ntua.gr
Department of Information and Communication Systems Engineering
University of the Aegean
GR-83200 Karlovassi, Samos, Greece
email: vergados@aegean.gr
Abstract— Quality of service in WLANs has been a major
concern of both academia and industry for many years. A
number of methods have been published regarding delay analysis
of 802.11 networks, but little research has been carried out for
the jitter analysis, even though jitter can be more important than
delay for real time applications, like voice over IP, multimedia
streaming, etc. In this paper we introduce a new analytical
method for estimating the jitter caused by the distributed
coordination function of the 802.11 MAC protocol. This model is
also evaluated by a series of simulations.
Keywords; jitter analysis; 802.11;WLAN;
I.
INTRODUCTION
There have been a number of publications regarding delay
analysis of the MAC algorithm of 802.11 networks [1]-[5]. But
apart from delay, jitter is a very important quality of service
parameter for many applications with real time requirements
like voice over IP, multimedia streaming, etc. The buffer size
of a radio receiver for example must be larger for networks
with increased jitter; Furthermore, the percentage of out-ofprofile packets in interactive applications is larger, as jitter
increases.
At the same time, wireless networks are becoming more
and more popular. Most of the hotels, airports, office buildings,
and universities are covered by 802.11 networks for internet
access. The ongoing research in 802.11f intends to increase the
transmission rate at 100Mbps, whereas 802.11e promises to
provide quality of service for these networks, in order to
support numerous new demanding applications. The ad-hoc
case also seems as a promising network topology, for tactical
as well as commercial networks.
In this context, we developed a new analytical model for
investigating the performance of 802.11 ad-hoc networks in
terms of jitter. In the next session there is an outline of the
mathematic analysis, section III contains simulation results
validating the analytic method, and in section IV there are
some conclusions.
II. 802.11 DCF JITTER ANALYSIS
In the IEEE 802.11 distributed coordination function every
wireless station senses the channel prior to transmission. If the
channel is sensed idle for predefined durations (DIFS) then the
packet is transmitted, otherwise the transmission is differed and
the backoff procedure is initiated, to determine the backoff
time. During this backoff time the stations does not transmit
even though the channel is idle. The backoff counter is
decreased only when the channel is idle, and is paused when
transmissions are sensed.
In order to calculate the mean delay and the jitter in an
802.11 network, we make the following assumptions. First, we
denote as “tagged host” as the mobile that sends the traffic flow
under investigation. Then, we assume that each background
packet has a constant transmission time L, which duration
includes the time needed for RTS/CTS/ACK transmissions as
well. Also, we assume that background packet‘s arrival rate is
exponential distributed with average 1/λ and that each tagged
packet has a transmission time m with m<L. Moreover, we
assume that the tagged traffic only occupies a small portion of
the total utilization.
Thus the total channel utilization is equal to
U
L
L
1
 
U
L  LU

Adding all the above together
i
AC  L   L   ((L  1)b j  L)
j 1
In order to calculate the mean delay and the jitter in an
802.11 network, we first calculated the possibility of collision
for a given utilization. Then we calculated the delay
distribution caused by every retransmission attempt, the
possibility of each retransmission, and finally we calculated the
mean delay. The jitter was also calculated by considering the
probability and delay distribution of each retransmission.
A. Netwrok Model
The simulated network consisted of fifty wireless stations,
all within range of each other. All stations, except one, are
generating packets of fixed size at exponentially distributed
epochs, whereas the other station was receiving the packets.
The details of the simulation are in the following table.
This analysis produced the following equations, where
CWmin, CWmax, Tslot are parameters of the 802.11 algorithm, k
is the number of retransmissions until the CW reaches it’s
maximum value, L is the time needed for every transmission
(including RTS/CTS/DATA/ACK), m is the time required for
transmitting a packet, and p the probability of retransmission (it
is calculated from the channel utilization U). The calculated
mean delay is m’’ and the jitter is v’’.
k
TABLE I.
i
i 1 j 1
i
E ( AC )  m 
1  p m  2(2 p ) m  2 p m 1  (2 p ) m 1
1 2p
p m (m  1  mp)
L
L
)
 m
1 p
(1  p ) 2
m  Um  1  U m
The jitter in calculated by the following equations:


3  p k  44 p   4 p k 1  4 p 
361  4 p 
k
k 1
p k 1  k  kp
121  p 

  1  p  p  2

i  k 1

802.11 DCF
CWmin
32
CWmax
1024
572 bytes

i 1
i
1
CWmax 1


L 

2
c3  c4  
v   L  1Tslot b1  b2   L  1Tslot
L2
12
v  Uv
20 μsec
2DIFS + PACKET + ACK
0.00245
Exponential
4kbps – 400kbps
Traffic Rate (per station)
B. Simulation Results
The simulation results are plotted in Figure 1 and 2. Figure
1 illustrates the mean delay as a function of the channel
utilization, whereas Figure 2 illustrates the jitter as a function
of the channel utilizations. In both figures the dashed lines are
the simulation results, whereas the continuous lines are the
analytical results. As we can see, the simulation results are
relatively close to analytic results, even though there not
always an exact match.
SIMULATIONS
In order to validate the above analysis, a number of
simulations have been carried out with the Network Simulator
2 [6].
CONCLUTIONS
In this paper a new model for jitter analysis of 802.11
networks is presented. The simulation results validated the
analytical results. These results clearly show that the DCF
access mode can provide low mean delay (very close to the
transmission delay) for low utilization, but the mean delay
increases rapidly, as the utilization approaches the value of one.
On the other hand, the value calculated for the jitter can be
considered fairly high (it approaches 0.01 for relatively low
utilizations), much higher than the value produced by the PCF
access mode [7]. The mathematic analysis as well as the
simulation will be presented in detail in the full paper.
V.
III.
572 * 8 / 2*108 = 0.002288 sec
Tslot
IV.
k
i


c3    1  p  p i 1  2 CWmin  j 2  L 
i 1 
1

c4 
50
Traffic Generator
(L  1) * T * (2W min 1
b2  2 CWmax 2
Number of wireless stations
Transmission Duration (m)
i  k 1 j 1
b1  2 CWmin
Value
Packetsize (with headers)
a2  2 CWmax 1   1  p  p i 1
 2 w max 1
Attribute
Mac protocol
a1  2 CWmin 1  2 j 1 1  p  p i 1

SIMULATION PARAMETERS
[1]
[2]
[3]
REFERENSES
IEEE std. 802.11, “Wireless LAN Medium Access Control (MAC) and
Physical Layer (PHY) Specifications“, 1999.
Giuseppe Bianchi, "Performance analysis of the IEEE 802.11 distributed
coordination function", IEEE Journal on Selected Areas in
Communications, no. 3, March 2000 pp. 535-547
Andras Veres, Andrew T. Campbell, Michael Barry, Li-Hsiang Sun,
"Supporting service differentiation in wireless packet networks using
distributed control", IEEE Journal on Selected Areas in
Communications, no. 10, October 2001 pp. 2081-2093
[4]
[5]
[6]
Stefan Mangold, Sunghyun Choi, Guido R. Hiertz, Ole Klein, Bernhard
Walke, "Analysis of IEEE 802.11e for QoS Support in Wireless LANs",
IEEE Wireless Communications, no. 6, December 2003
Wasan Pattara-Atikom, Prashant Krishnamurthy, Sujata Banerjee,
“Distributed mechanisms for quality of service in wireless LANs”, vol.
10, no. 3, Jun 2003, pp. 26 – 34
The Network Simulator 2, www.isi.edu/nsnam/ns
[7]
Dimitrios J. Vergados, and Dimitrios D. Vergados, "Synchronization of
multiple access points in the IEEE 802.11 Point coordination Function",
IEEE 60th Vehicular Technology Conference 2004-Fall "Wireless
Technologies for Global Security" (2004 VTC – Fall VTS), Los
Angeles, CA, USA 2004
0.04
0.04
0.03
0.03
0.02
0.02
0.01
0.01
0.2
0.4
0.6
0.8
Figure 1. Mean Delay as a function of channel utilization
1
0.2
0.4
0.6
0.8
Figure 2. Jitter as a function of channel utilization
Figure 3.
1