1. International Symposium on Computer Science and its Applications
A Novel Pause Count Backoff Algorithm for Channel access
in IEEE 802.11 based Wireless LANs
Hao-Ming Liang1, Sherali Zeadally2, Naveen K Chilamkurti3, Ce-Kuen Shieh1
joeliang@hpds.ee.ncku.edu.tw;szeadally@udc.edu; n.chilamkurti@latrobe.edu.au
1
Institute of Computer and Communication Engineering,
Department of Electrical Engineering, National Cheng Kung University
3
Department of Computer Science and Computer Engineering, La Trobe University
Melbourne, Australia-3086
²Department of Computer Science and Information Technology
University of the District of Columbia
4200, Connecticut Avenue, N.W.
Washington, DC 20008
Abstract Sense Multiple Access with Collision Avoidance
(CSMA/CA) mechanism and contend for channel
This paper proposes a novel backoff algorithm to access. This contention behavior results in collisions
enhance the Distributed Coordination Function (DCF) among stations and therefore additional time is spent to
function in IEEE 802.11 based wireless networks. The recover from collisions. Many research [2-3] focuses
proposed algorithm, known as Pause Count Backoff on transmission quality and efficiency for DCF
(PCB) algorithm, observes the number of backoff functions because DCF is mandatory supported in
counter pauses during the channel access contention IEEE 802.11 devices.
and sets the appropriate contention window, based on As mentioned previously, there are unavoidable
the estimated results. We evaluate the performance of overheads such as header and collisions within the
the proposed PCB algorithm using simulation tests and network. In [4] the authors show that the efficiency of
we compare its performance with other proposed IEEE the network decreases when there are more active
802.11 backoff algorithms (Exponential Increase stations within the system. A Markov chain model is
Exponential Decrease (EIED), and Adaptive Enhanced used to represent the behavior of a DCF function and a
Distributed Coordination Function (AEDCF)). Our throughput formula is proposed. From the analysis in
results demonstrate that PCB outperforms other [4], the Binary Exponential Backoff (BEB) algorithm
backoff algorithms in various network conditions. is the key factor that influences system efficiency. In
Moreover, the fairness index and end-to-end delay of 802.11 standards, the size of the Contention Window
PCB are also much better than those obtained with (CW) is doubled when transmission fails and reset to
other channel access algorithms. initial value CWmin once transmission is successful.
This reset behavior becomes very unstable when
numerous stations are contending within wireless
1. Introduction channel. This can cause more collisions and decreases
the whole system utilization.
IEEE 802.11 [1] is the dominant technology used In prior research efforts [5-6], the authors discuss
in Wireless LANs. In IEEE 802.11 standard, a MAC techniques to reduce the collision rate by controlling
protocol supports two coordination functions. One of the contention window size in their backoff algorithms
them is Point Coordination Function (PCF) which to improve the throughput performance. The basic
provides a polling-based service and is only available concept central to these methods is the estimation of
in an infrastructure network mode. The Access Point the system load using the transmissions status and the
(AP) acts as the administrator and determines channel appropriate CW size is computed. The size of CW
access by sending a poll frame to each station in the could be additive or multiplicatively increased because
network. The other transmission function defined in a of collisions and additive or multiplicatively decreased
MAC protocol is the Distributed Coordination as a result of successful transmissions. However, the
Function (DCF) which is a contention-based service estimation method is based on partial observations,
and is available both in Ad Hoc and infrastructure such as that each station uses its own status of
network modes. In DCF, the stations adopt a Carrier transmissions to represent the whole system. The status
978-0-7695-3428-2/08 $25.00 © 2008 IEEE 163
DOI 10.1109/CSA.2008.38

2. of transmissions and system load may have a positive expected before the ACK timeout timer, otherwise the
correlation but is not sufficient to precisely set CW sender will consider this frame as lost and retransmits
value. As a result, such an imprecise calculation of CW it. For each failed transmission, the station doubles its
size for stations introduces a fairness issue. current CW size until CW reaches its maximum value
In this work, we propose a new method to estimate (CWmax) (the default value of CWmax is 1024 in IEEE
the number of active stations and find an appropriate 802.11b). The station then performs the backoff
value of CW within a wireless network. The procedure again and reduces the probability of
countdown procedure in backoff mode is paused when collision in the next transmission by using a larger CW
other stations use the wireless channel at the same (CWnew). After a successful transmission, the CW is
time. Therefore, each pause represents more than one reset to the initial value (CWmin), which is equal to 32
station using the wireless channel and the number of in IEEE 802.11b.
pauses could give a sense of the system status. The
proposed method, known as Pause Count Backoff
(PCB), counts pauses during the countdown procedure
and sets an appropriate CW size for the current
condition. Using simulation results, we show that the
proposed method improves system utilization and
reduces the end-to-end delay when compared with
other previous schemes such as the Distributed
Coordination Function (DCF), Exponential Increase
Exponential Decrease (EIED), and Adaptive Enhanced
Distributed Coordination Function (AEDCF) Figure 1 Illustration of the CSMA/CA mechanism
algorithms. The fairness index of PCB is 2.2 Related Backoff Algorithms
approximately 1 in most of the network simulation
scenarios. The size of the contention window in the backoff
This paper is organized as follows. Section 2 procedure can affect the overheads for network access
describes the DCF function and discusses the channels with contentions. A large CW results in a
overheads of transmissions in wireless channel. Brief long idle duration when there are only a few active
reviews of related studies are presented in section 2. In stations in the system (although a large CW could lead
section 3, we describe our proposed PCB algorithm. to a lower collision rate). A small CW can enhance the
Simulation results and a performance analysis of our channel utilization but the number of collisions could
proposed PCB algorithm are discussed in section 4. increase quickly if a small CW is used for many active
Finally, section 5 concludes the paper and presents stations. Thus, the backoff algorithm should adapt the
future works. correct value of CW to fit the system status. Many past
research efforts on enhanced algorithms have been
2. Background and Related work proposed taking into account various considerations
such as throughput maximization [5] and QoS
2.1 IEEE 802.11 DCF Algorithm requirements [6].
In In [5], a scheme named EIED has been proposed, the
Based on the 802.11 standard, DCF adopts a station sets the new contention window CWnew as
CSMA/CA mechanism. In this algorithm, each station CWold multiplied by the parameter ri, (where ri is the
needs to sense a wireless channel before sending increment backoff factor) as a successful transmission.
frames. Transmission is performed if the medium is When a collision occurs, a new value of CW is given
idle for a Distributed InterFrame Space (DIFS) period by CWold divided by the parameter rd where rd is the
as shown in Figure 1a – otherwise the backoff decrement backoff factor. The advantage of
procedure is activated. A backoff number is chosen exponential increase is that Exponential Increase (EI)
randomly from the interval [0,CW-1]. The backoff can reduce the probability of continuous collided
number is decremented by one for every idle timeslot frames when many stations contend a channel
during the countdown procedure. The station sends out concurrently. Moreover, ED (Exponential Decrease)
frames when the backoff number is counted down to keeps the collision history of the previous
zero as shown in Figure 1b. This frame may collide transmissions instead of resetting automatically to
with other frames or be discarded due to wireless error CWmin. ED could prevent numerous collisions from
during the transmission. To indicate a successful occurring, especially in a network with large number of
transmission, an acknowledgement from the receiver is stations. In [6], the authors propose an adaptive service
differentiation algorithm for IEEE 802.11 WLANs. In
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3. AEDCF each station calculates the collision rate it pauses during the countdown procedure in step 1 could
experiences during a given interval. A high collision observe the number of active stations in the system. In
rate often indicates a small CW under heavy system Figure 2, node A observes three pauses during its
loading. In AEDCF, a station does not reset the CW backoff procedure. From the observation, node A
after a successful transmission. The station sets a new paused resume paused resume paused
CW based on the current collision rate. In AEDCF, the node A backoff window
time
authors seek to determine the optimal size of the CW
that will minimize collisions and improve the system node B time
efficiency. The accuracy of the estimated system status node C time
determines whether the CW of the proposed backoff
algorithm is appropriate or not. In prior approaches, node D time
these algorithms estimate the system load from the Figure 2 Estimating number of active stations with the
number of collisions encountered. This is inadequate paused backoff counter
because the station only observes its own transmission determines that there are more than three mobile
status which does not represent the whole system stations concurrently contending for the wireless
condition. The imprecise information affects fairness channel. To keep the scheme stable, Exponential
among stations. In EIED and AEDCF algorithms, a Weighted Moving Average (EWMA) is applied to
station may inaccurately assign a small CW for a calculate an average pause count. The number of
highly contended channel. This will result in this pauses observed directly correlates to the backoff
station having a higher channel access opportunity than counter value. A high backoff counter records more
other stations. When the station with a small CW pauses than a small backoff counter during the
collides with others, the station will set the CW to a countdown procedure. We propose the solution by
small value once again due to the small CW used in the setting CW size to active stations in step 2. In step 2, a
last transmission and a lower collision rate. policy with successful transmission results sets the
proper contention window size for the backoff
3. Pause Count Backoff (PCB) Algorithm procedure. While transmission is a success, CW is set
to the average pause value multiplied by β which
The main objective of our proposed algorithm is to relates to the collision rate. The method used to
improve the system performance of DCF with fairness compute β is given below:
into consideration as well. Previous efforts have 2N (1)
Pc ≅
focused on improving system efficiency by adjusting CW
the size of CW from transmission results obtained. β = 1 / Pc (2)
Although the transmission result correlates with the Pc represents the probability of collision for each
system status, it cannot precisely determine the system transmission in a station. N is the number of active
status from a partial transmitting event. Inaccurate stations in the system. The new CW size can be
estimation results in inefficiency and unfairness among derived from the number of active stations and β (as
stations. Thus, our proposed PCB algorithm takes shown above). To minimize oscillations in the pause
global observations to estimate the system status, and count, the new CW is applied after an observation
then PCB sets an appropriate CW that matches the period which is determined by the number of
global system status. There are two steps in our transmission attempts. In this paper, we set the
proposed PCB algorithm. The first step is the observation period to 10 transmission attempts in the
estimation of CW and the second step involves setting station. PCB sets a proper contention window size
CW. according to the system status when transmission is
In step 1 of our proposed PCB approach, a method successful instead of resetting CW to CWmin
estimates the number of active stations. In DCF, the immediately or additively/ multiplicatively decreasing
backoff counter pauses if other stations transmit frames CW by a single transmission status. When transmission
at the same time and resumes countdown whilst a fails, the new CW should prevent further collisions in
channel is idle for a DIFS period. The concept is the next transmission to reduce the overhead of
illustrated in Figure 2. The proposed method observes retransmissions. In our proposed PCB algorithm, the
the number of pauses until the counter becomes zero. station sets a new CW to a large contention window to
Each pause represents another station transmitting its avoid further collisions. The station could avoid the
frames or more than two stations incurring a collision. next collision effectively by choosing a large backoff
The backoff counter is uniformly distributed and the counter, although such a large CW may result in more
parameter avg_paused_count for average number of idle timeslots during the backoff procedure. The idea
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4. here is that the overheads of collided frames are
usually larger than waiting for idle timeslots. The Control rate 1 Mbits/s
parameter rd (where rd is a number for division) used in Data rate 11 Mbits/s
the pseudocode for the PCB algorithm below
Slot_time 20 μs
determines the size of the new CW. The new CW
computed using rd and CWmax addresses the fairness SIFS 10 μs
issue among stations in step 1 when a station does not DIFS 50 μs
calculate the system status correctly. The new CW CWmin 32
could reset the inappropriate CW and adjust CW again
CWmax 1024
after an observation period.
Packet size 1000 bytes
Pseudocode of PCB algorithm
Step 1 : estimation
• Goodput:
avg _ pause _ count = (1 − α ) * avg _ pause _ count + α * current _ pause _ count Goodput is the most common performance metric
that calculates total data amount received in a period
Step 2 : setting CW
by a station. In general, a higher goodput always
when collision occurs
set CW new = CW max / rd
indicates better efficiency in a system. In this paper,
we use aggregate goodput to evaluate PCB and
when transmissi on succeeds related backoff algorithms.
if during observatio n period
CW new = current CW
• Fairness Index:
else Fairness among stations is an important problem
CW new = avg _ pause _ count * β in BEB study and has been discussed by many
restart a new observatio n period
researchers. Fairness index could show if resource is
fairly allocated to each stations. Authors in [8]
4. Performance evaluation derived a fairness index using the formula given
below:
This section evaluates the system performance of
the proposed PCB algorithm with other existing
(∑ G )
i i
2
(3)
Fairness Index =
n * ∑ (G )
2
backoff algorithms under different system loads. The i i
simulation is performed using NS-2 version 2.28 Where n is the number of stations and Gi is the
simulator [7]. We use IEEE 802.11b based WLAN goodput of station i achieved. The value of the
setup and we assume transmissions without the fairness index is bounded to the interval [0, 1]. The
Request to Send/Clear to Send (RTS/CTS) mechanism index is equal to 1 when all stations obtain the same
in an ideal channel. Each mobile station establishes a goodput.
Constant Bit Rate (CBR) flow with a 2 Mbps link to • Collision Rate:
the base station. The number of stations is varied from Collision rate gives a probability that packets be
5 to 50 and the duration of simulation is set to 30 discarded due to collisions in each transmission. A
seconds. The parameters used in the simulation are higher collision rate usually indicates heavy system
listed in Table 1. The parameters ri and rd in EIED load and implies more overheads.
algorithm are set to 2 as suggested in [5]. For AEDCF, • Average end-to-end delay:
we set the observation period of the estimated collision End-to-End delay is the time it takes for a packet
rate to 0.5 seconds and α to 0.8 used in [6]. In the to travel from sender to receiver. For some time-
proposed PCB algorithm, we set the weight α to 0.9 to constraint applications, end-to-end delay is the most
obtain a smooth pause count. The parameter β is set to concern than other metrics. In this paper, we
5 which mean the expected collision rate Pc of a station calculate the average end-to-end delay in the system
is around 20% in heavy network load by PCB by various backoff algorithms.
algorithm. The parameter rd is set to 4 to get a large
size of new CW that is value 256 in IEEE 802.11b 4.1 Goodput Performance
after collisions.
We use the following performance metrics to Figure 3 shows the goodput performance results of
evaluate the performance of PCB with other previously various backoff algorithms for IEEE 802.11 WLANs.
proposed algorithms: The efficiency of standard DCF performs worse (as
Table 1 IEEE 802.11 b MAC and network parameters expected) when more stations contend for the channel.
used in simulation Although the EIED algorithm takes an exponential
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5. decrease CW policy instead of resetting to CWmin when
there is a successful transmission, the curve decreases
when there are more active stations in the system. This
means that stations applying EIED and DCF
algorithms make decisions with unclear system status
and adjust the CW quickly from the result of a single
transmission. In contrast to EIED and DCF, the
goodput of AEDCF and PCB algorithms remains high
with respect to various system loads. These
improvements mean that stations using AEDCF and Figure 3. Aggregate goodput vs. number of stations
PCB algorithms adjust CW value appropriately
according to the load variation within the network. In
case of a light system load, when there is a collision,
PCB wastes some idle timeslots due to the large new
value of CW. The graph of PCB has maximum
goodput at around 20 stations. When the number of
stations increases, the overheads of collision decreases
the efficiency in PCB. Overall, the PCB algorithm
obtains a high efficiency compared with other backoff
algorithms in various network conditions. Although
Figure 3 shows that AEDCF performs consistently
better compared to PCB, we demonstrate further below Figure 4. Fairness index vs. number of stations
other additional performance (e.g.,fairness, collision
rate, average end-to-end delay) benefits provided by
PCB over AEDCF.
4.2 Fairness Index
The aggregate goodput in Figure 3 can represent
the efficiency of a system. However, when designing a
backoff algorithm, we should also consider another
important criterion: fairness among stations. The worst
case scenario is when one station sets a very small CW
due to inaccurate calculations of the system status and Figure 5. Collision rate vs. number of stations
as a result always occupies the channel. The aggregate
goodput is high but it is unfair to other stations. In simulation enlarges the goodput of stations achieved at
Figure 4, we present the fairness index of each backoff the end of the experiments.
algorithm among stations. Using the simulation setup
described in prior section, we executed the simulation 4.3 Collision Rate
for 30 iterations and we calculated a 95% confidence
interval. From Figure 4, the proposed PCB algorithm Figure 5 presents the collision rate of various
has the most stability and is close a fairness index of backoff algorithms. As expected, a high collision rate
one when compared with other contention algorithms. usually indicates additional overheads and longer end-
We also observe that the fairness index of AEDCF and to-end delays. From the simulation results, more frame
EIED are low and oscillatory. This phenomenon means collisions occurred as the number of active stations in
some stations occupy more channel capacity than other the system grows. The collision rate of a standard DCF
stations due to a different understanding of the system is the highest among the compared algorithms due to
status among stations. In the case of the AEDCF the resetting behavior for each successful transmission.
algorithm, the estimated collision rate dominates the In the case of n=50, the collision rate is around 50%.
new CW size. A slight difference in the estimated The PCB algorithm achieved a low collision rate when
collision rates among stations at the start of the compared with other backoff algorithms (as shown in
Figure 5). According to the parameter β setting, the
collision rate is around 20% as we expected in heavy
system load in Figure 5. A low collision rate indicates
that the station effectively reduces the retransmission
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6. overheads in the system. However, a low collision rate
does not mean the least overhead in the system. Even a
large CW can result in a low collision rate. In the mean
time, the duration of idle channels causes the efficiency
of the system to decrease. Therefore, a good backoff
algorithm should also examine another metric such as
end-to-end delay.
Figure 7 Normalized delay vs. number of stations
4.4 End-to-End Delay 6. References
The variation of the mean end-to-end delay with [1] IEEE WG, “Part 11: Wireless LAN Medium Access
Control (MAC) and Physical Layer (PHY)
number of active stations is presented in Figure 6. As
Specifications,” IEEE 802.11 Standard, 1999.
expected, the delay increases with the number of [2] Andras Veres, Andrew T. Campbell, Michael Barry, and
stations. The objective of the PCB algorithm is that it Li_Hsiang Sun, “Supporting Service Differentiation in
precisely estimates the actual network status and sets Wireless Packet Networks Using Distributed Control,”
the corresponding CW to minimize overheads in the IEEE Journal on Selected Areas in Communications, vol.
system. In Figure 6, the PCB shows the advantage of 19, no. 10, October 2001.
overhead reduction and obtains the lowest delay [3] Chonggang Wang, Bo Li, and Lemin Li, “A New
among these backoff algorithms. In Figure 7, we Collision Resolution Mechanism to Enhance the
present the gain on end-to-end delay by normalizing Performance of IEEE 802.11 DCF," IEEE Transaction on
Vehicular Technology, vol. 53, no. 4, pp. 1235-1246,
end-to-end delay of a standard DCF. The delay of the
July 2004.
PCB is around 20% less than that of a standard DCF in [4] Bianchi G., “Performance analysis of the IEEE 802.11
the case of n = 50. However, the end-to-end delay is distributed coordination function,”IEEE Journal on
too large for multimedia services. The reason for the Selected Areas in Communications, vol. 18(3), pp. 535-
results is that 802.11b is throughput-oriented without 547, 2000.
considering QoS requirements. [5] N.-O. Song, B.-J. Kwak, J. Song, and L. E. Miller,
“Enhancement of IEEE 802.11 distributed coordination
5. Conclusion and Future Work function with exponential increase exponential decrease
backoff algorithm,” in Proc. IEEE VTC-Sprin, 2003, vol.
4, pp. 2775-2778.
In this paper, we have proposed a PCB backoff [6] Lamia Romdhani, Qiang Ni, and Thierry Turletti,
algorithm to improve the efficiency of IEEE 802.11 “Adaptive EDCF: Enhanced Service Differentiation for
DCF function. Our PCB algorithm estimates system IEEE 802.11 Wireless Ad-Hoc Networks”, in Proc. IEEE
status by using a pause count backoff counter and WCNC 2003, vol. 2, pp.1373-1378
determines a proper contention window size that [7] Network Simulator 2, http://www.isi.edu/nsnam/ns .
accurately matches current network conditions. We [8] R. Jain, D. Chiu, and W. Hawe, “A Quantitative Measure
compared the performance of PCB with past proposed of Fairness and Discrimination for Resource Allocation
in Shared Computer Systems,” DEC Research Report
algorithms such as IEEE 802.11 DCF, EIED, and
TR-301, 1984.
AEDCF. Our simulation results demonstrate that PCB
outperforms these previously proposed algorithms for
various performance metrics and dynamically adapts to
the variations in a network. In the future, we plan to
consider an extension of this work to consider
RTS/CTS mode and wireless transmission errors.
Figure 6 End-to-end delay vs. number of stations
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