1. Communication Networks and Services Research Conference
History Based Adaptive Backoff (HBAB) IEEE 802.11 MAC Protocol
Qassim Nasir (1) and Maali Albalt (2)
Department of Electrical and Computer Engineering
College of Engineering- University Of Sharjah
(1)
nasir@sharjah.ac.ae , (2) maali@sharjah.ac.ae
Abstract transmission without taking into consideration the
network conditions.
A Mobile Ad Hoc Network (MANET) is a collection Many researchers were motivated to enhance the
of mobile nodes interconnected by wireless media. performance of the IEEE 802.11 through modifying
Several protocols have been proposed to manage the BEB algorithm [1]-[11]. Most of the prior work in
multiple access to the shared wireless medium in this area have changed or modified the BEB algorithm
MANETs, with the IEEE 802.11 being the most such that it provides relative priority among two or
implemented protocol. In this work, a History-Based more traffic classes [1]-[4]. This solves the intra-class
Adaptive Backoff (HBAB) MAC Protocol is proposed contention problem since the class with the least
to provide a better QoS performance. HBAB modifies specified CW value would access the channel first.
the original IEEE 802.11Binary Exponential Backoff However, it does not solve the inter-class contention
(BEB) algorithm (which is used to control the problem since a number of stations wishing to send
contention window in the case of collisions) to reflect packets of the same priority class may still contend and
the network status and condition. Inspired by the collide (in case their backoff timers expire
adaptive delta modulation scheme that is used in simultaneously). The latter problem is solved by
communications, HBAB considers the history of past determining what to do until a successful transmission
trials for transmission. Simulations show a significant takes place, or simply how to increment the value of
improvement in performance measures (around 15% in the contention window in the case of a busy channel.
packet delivery fraction and up to 50% in average The suggested backoff algorithms were mostly slight
packet delay) with using HBAB compared to the variations or scales of the BEB algorithm for each
original backoff algorithm of IEEE 802.11. traffic class. However, the way how to decrement the
CW was unaddressed explicitly and hence assumed to
1. Introduction remain the same as the original BEB algorithm (the
CW would reset to CWmin upon a successful
Wireless network access of mobile devices such as transmission).
smart phones and laptops has recently become more It can be said that the above works solved the
feasible and popular due to the incredible contention problem from a priority point of view
developments in wireless technologies, with IEEE through determining which class should access the
802.11 Wireless Local Area Networks (WLANs) being medium first. But that is not enough since the sudden
one of the most implemented standards. Since the CW reset to CWmin may cause several collisions,
wireless medium is shared by all transmitting stations which requires addressing the contention problem from
in range, IEEE 802.11 has to control medium access a congestion point of view. More specifically, when a
among contending stations so as to minimize the effect station succeeds in transmitting a packet at a given
of collisions on the performance of the network. To CW, that doesn’t mean a decrease in congestion, but it
achieve that, 802.11 adopts a binary exponential means arriving at a convenient CW value [5].
backoff (BEB) algorithm that exponentially increases a This finding inspired some researchers to adopt a
station’s waiting time if the medium is found busy, and different approach of looking at the backoff algorithm,
resets to a minimum value right after a successful which is what to do after a successful transmission
transmission. The BEB algorithm is considered takes place, or simply how to decrement the CW.
“memory-less” since it resets the Contention Window Several works on slowly decreasing the CW value
(CW) value to the minimum right after a successful were proposed [5]-[11]. [6] and [7] suggested an
exponential decrease in the CW value upon a
978-0-7695-3135-9/08 $25.00 © 2008 IEEE 533
539
537
DOI 10.1109/CNSR.2008.20

2. successful transmission instead of resetting to CWmin, defines two medium access functions: a mandatory
but they assumed a fixed scale of decrease without distributed coordination function (DCF) and an
taking the network conditions into account. That might optional point coordination function (PCF) [2].
result in underutilizing the channel if it was idle or
returning to a congestion state if the network had not 2.1. DCF
yet relieved from a previous congestion. This function uses a carrier sense multiple access
While [5] and [8] suggested slow decrease backoff with collision avoidance (CSMA/CA) mechanism to
algorithms to adapt to the network load, there was control access to the shared wireless medium. Before
another proposal to assume a p-persistent MAC initiating a transmission, each STA is required to sense
protocol in which the station would transmit with a the medium for a time interval called DCF interframe
probability p and refrain form transmitting with a space (DIFS) and perform a binary exponential backoff
probability 1-p [9][10]. That p-value was calculated in procedure using an initial random Contention Window
runtime and updated after each transmission to reflect (CW) value. Only if the medium remains idle after the
the current number of active stations in [9] or the backoff timer expires, the station is allowed to
average time the channel is idle or busy in [10] among transmit. A positive acknowledgment (ACK) is used to
other conditions that affect the network load. In both notify the sender that the frame has been successfully
the slow decrease and p-persistent cases, complex received. If an ACK is not received within a time
computations were needed to update the p-value and to period of ACKTimeout, the sender assumes that there
estimate the network load, respectively. Complex is a collision and schedules a retransmission by
computations also mean high power consumption, entering the backoff process again until the maximum
which is in many cases considered unaffordable in the retransmission limit is reached [2].
wireless ad hoc networks context. A maximum of 7 retransmissions (4
This paper proposes a novel approach to slowly retransmissions) for short frames (long frames) are
increase and decrease the CW value based on the allowed before the frame is dropped. The basic access
busyness of the channel, i.e. MAC layer transmission procedure is depicted in Figure 1 (a). To further
retrials. The suggested algorithm to predict the decrease the overhead caused by frame collision and
contention window size, HBAB, is similar to the hidden terminal effects, Request-To-Send (RTS) and
adaptive delta modulation algorithm (which is used to Clear-To-Send (CTS) frames may also be exchanged
predict the step size in communications). before the data transmission.
The paper is organized as follows. Section 2 It is worth noting that collision avoidance is
reviews the IEEE 802.11 MAC protocol and backoff achieved by a virtual carrier sense mechanism.
algorithm. Section 3 presents the HBAB IEEE 802.11 Whether the channel is idle or busy is not solely
MAC Protocol. Section 4 discusses performance determined by the physical carrier sense result, but also
evaluation of the proposed HBAB against the standard by the value of the Network Allocation Vector (NAV)
BEB IEEE 802.11. Section 5 concludes the paper and timer maintained by the MAC. A duration value is
provides directions for future work. included in each frame that is transmitted by a station
and indicates how long the transmission will last,
2. IEEE 802.11 and Backoff including any subsequent acknowledgments and
fragments. Each station in the vicinity of the
To start with, let’s briefly review the IEEE 802.11 transmitting station receives the frame and uses the
standard [12]. It defines two basic network duration value to update its NAV. Therefore the NAV
configuration modes: the infrastructure mode, where value indicates how long another station has access to
all stations (STAs) have to go through a central access the wireless medium no matter what is the real activity
point (AP) device in order to talk to each other, and the of that station.
ad hoc mode, where any STA can directly talk to the
others without the need for a centralized device. The 2.1.2 Backoff Algorithms. The BEB algorithm is
IEEE 802.11 standard defines the specifications of used by the IEEE 802.11 to control access to the shared
both the physical (PHY) and medium access control wireless medium among contending stations. This is
(MAC) layers to construct a WLAN using either done through adjusting the contention window size
configuration mode. While the 802.11 PHY layer based on the current medium status. Figures 1 and 2
defines the signaling and modulation properties of the show how the DCF employs the BEB algorithm with
protocol, the 802.11 MAC layer controls access to the or without having CTS/RTS [2].
wireless medium, this is shared by all stations in the When a station has some data to send, it senses the
transmission range. To achieve that, the 802.11 MAC channel to determine whether it is idle. If the medium
534
540
538

3. remains idle for a specified time interval, called the
distributed inter-frame space (DIFS), the station is
allowed to transmit. If the medium is busy, the
transmission is postponed until the ongoing
transmission concludes. Meanwhile, a slotted binary
exponential backoff procedure takes place: each slot is
equal to DIFS, and the number of such slots is
determined by a random value uniformly chosen in [0,
CW -1], where CW is the current contention window
size.
That random value is used to initialize the backoff Figure 2: IEEE 802.11 DCF without RTS-CTS.
timer, which keeps running as long as the channel is
sensed idle, paused when data transmission (initiated
by other stations) is in progress, and resumed when the
channel is sensed idle again for more than DIFS. The 3. History Based Adaptive Backoff (HBAB)
time immediately following an idle DIFS is slotted, IEEE 802.11 MAC Protocol
with each slot equal to the time needed for any station
to detect the transmission of a frame (in the IEEE This paper proposes a novel backoff algorithm, the
802.11 term, MAC Service Data Unit (MSDU)) from History Based Adaptive Backoff (HBAB) algorithm, in
any other station. When the backoff timer expires, the which the history of the past trials for transmission is
station attempts to transmit a data frame at the taken into account. This approach is similar to what
beginning of next slot. adaptive delta modulators do when deciding the step
Finally, if the data frame is successfully received, size [13-15]. The HBAB algorithm checks the last N
the receiver transmits an acknowledgment frame after a states of the medium (N=2 in this implementation), and
specified interval, called the short inter-frame space decides whether to increment or decrement the CW
(SIFS), that is less than DIFS. If an acknowledgment is value based on the channel's tendency to being free or
not received, the data frame is presumed to be lost, and busy. In other words, if the channel tends to be free
a retransmission is scheduled. The value of CW is set (the most recent state(s) indicate(s) a free channel),
to CWmin in the first transmission attempt, and is then the CW value is decreased; if the channel tends to
doubled at each retransmission up to a pre-determined be busy (the most recent state(s) indicate(s) a busy
value CWmax. Retransmissions for the same data channel), then the CW value is increased. The HBAB
frame can be made up to a pre-determined retry limit,
algorithm fixes two parameters, α and β, which are
L, times. Beyond that, the pending frame will be used to increase or decrease the new CW based on the
dropped. In the case that the floor acquisition RTS- old CW value (that will automatically increase or
CTS mechanism is used, the same procedure is decrease the backoff time). The paper does not discuss
conducted except that an RTS-CTS handshake the optimization of those two parameters and could be
operation precedes the DATA-ACK exchange (Figure a good future work. Table 1 shows the suggested CW
1). [2] values per state check (0 indicates a busy channel and
1 indicates a free channel):
State CW value Ex: CW value
(with α=1 β=2)
00 CW=CWold* (α β) 2 CWold
01 CW=CWold* (α / β) 1/2 CWold
10 CW=CWold* (β / α) 2 CWold
11 CW=CWold* (1/ α β) 1/2 CWold
Table 1: CW estimation algorithm in HBAB.
Figure 1: IEEE 802.11 DCF with RTS-CTS.
Figure 3 shows the operation of HBAB. The node
tries to transmit a packet and in case of failure then a
Channel Status Bit (CSB) will be set to ‘0’, otherwise
will be set to ‘1’ and send the packet. The last three
CSB's are collected to form the Channel Status (CS)
535
541
539

4. word, e.g. if CS=001 then it represents that current Simulation time 600 s
transmission trial is successful while the previous trials Packet size 512 bytes
was failed. According to the value of this word, the Packet rate 4 packets/s
contention window (CW) is estimated according to Mobility pattern Random waypoint with
Table 1. The estimated CW can not be set below a max speed of 10 m/s and
minimum CW or beyond the maximum configured by pause time of 50 s
the interface card hardware (the values of 31and 1023 Traffic type Constant bit rate (CBR)
respectively are used in this paper). CWmin, CWmax 31, 1023
α,β 1.1, 1.9
Table 2: Simulation Parameters.
For the mobile scenarios, the random waypoint
model is used to model node mobility. In this model, a
node chooses a random point in the network, and
moves towards that point at a constant speed. The
speeds are uniformly chosen between the minimum
and maximum speeds set to 0 m/s and 10 m/s,
respectively. When the node reaches its destination, it
stays there for a certain pause time (fixed to be 50
seconds in this paper), after which it chooses another
random destination point and repeats the process.
All simulations last for 600 seconds. The data
traffic is generated by Constant Bit Rate (CBR)
sessions initiated between random source and
destination pairs. To prevent the network from being
flooded with RREQs when the simulation starts, the
start of each of the CBR sessions is time-staggered
with a 5 second delay between sessions. Each session
lasts until the end of the simulation.
The metrics used in comparing protocol
performance are 1) the Packet Delivery Fraction
(PDF), which represents the ratio of the number of the
successfully delivered data packets to their destinations
versus the number of all data packets being sent; and
2) the average end to end delay, which measures the
average required time in seconds to receive a packet .
The following graphs (Figures 4 to 7) show the
Figure 3: Operation of HBAB PDF and average delay in seconds of both the original
and the modified IEEE 802.11 protocol (HBAB) with
varying the number of connection (connections load),
4. Performance Analysis and number of nodes (network density) respectively.
The results clearly show the improvement of the delay
The simulation is carried out in GloMoSim (Global
with the implementation of the HBAB algorithm
Mobile system Simulator) 2.03 [16]. The GloMoSim
(around 15% increase in PDF and up to 50% decrease
library is a scalable simulation environment for
in average packet delay). In the light of these results, it
wireless network systems using the parallel discrete-
can be strongly suggested to consider modifying the
event simulation capability provided by PARSEC. Our
backoff algorithm in order to provide QoS in the MAC
simulation modelled a network of 50 mobile hosts
layer.
placed randomly within a 1000 X 1000 meter area.
Figure 4 shows the PDF against the number of
Radio propagation range for each node was 250 meters
connected nodes. This test studies the behavior of the
and channel capacity was 2 Mbits/sec.
two algorithms under different network load condition.
It is clear that HBAB has better PDF (around 15%
Parameter Value
better) compared with the original 802.11 which uses
Area 1000x1000 m an ordinary exponential Backoff.
Number of nodes 50
536
542
540

5. delay and shows almost the same improvement as the
90.00%
85.00% previous test case.
80.00%
75.00% 6
Average Delay (Sec)
70.00%
PDF
802.11
65.00% 5
60.00% HBAB
55.00% 4
50.00% 802.11
3
45.00% HBAB
40.00% 2
0 10 20 30 40 50 60
1
# of Connections
0
Figure 4: Packet Delivery Fraction (PDF) when 0 50 100 150 200 250
varying the number of connections from 10 to 60. # of Nodes
The average end to end delay has been also Figure 7: Average delay (in seconds) with varying
measured for both algorithms under different network the number of nodes from 10 to 250.
load condition as it can be seen from figure (5). Again
HBAB has less average delay (up to 50%) compared to 5. Conclusion
standard BEB 802.11 MAC layer.
This paper proposes the History-Based Adaptive
5 Backoff algorithm (HBAB), a simple-to-implement
Average Delay (Sec)
4.5 backoff scheme that improves the performance of the
4
3.5 IEEE 802.11 DCF. The HBAB algorithm dynamically
3
802.11 adjusts the contention window based on the history of
2.5
2 HBAB past trials for transmission, thus adapting to the current
1.5 congestion level of the network.. It can be used in
1 MANETs to help achieve lower end-to-end delay and
0.5
0 better packet delivery fraction. The most important
0 10 20 30 40 50 60 characteristic of the HBAB scheme is its simplicity of
# of Connections implementation in the widely deployed IEEE 802.11
WLANs. Simulation results show that HBAB achieves
Figure 5: Average delay (in seconds) when varying better performance (around 15% increase in packet
the number of connections from 10 to 60. delivery fraction and up to 50% decrease in average
packet delay) compared to the standard BEB,
90.00% especially when basic access scheme is employed for
80.00% high congested environment. The expense of deploying
70.00% the HBAB algorithm depends on the desired depth in
PDF
60.00%
802.11 channel status history (1 bit for each saved channel
HBAB state). HBAB algorithm is currently being tested using
50.00%
a Linux-based testbed.
40.00%
30.00%
0 50 100 150 200 250
# of Nodes
10. References
[1] D. Deng and R. Chang, "A priority scheme for IEEE
802.11 DCF access method ", IEICE Transaction on
Figure 6: Packet Delivery Fraction (PDF) with
Communications, vol.E82-B, No.1 Jan 99, pp. 96-102.
varying the number of connections from 10 to 60.
[2] M. Barry, A. Campbell, A. Veres, "Distributed Control
Figures 6 and 7 show the performance of both Algorithms for Service Differentiation in Wireless Packet
algorithms under different network densities (the Networks", Proc. INFOCOM 2001, Anchorage, AK, USA,
number of active nodes in the working area). HBAB Apr 2001.
algorithm behave better in terms of PDF and average
[3] V. Kanodia, C. Li, A. Sabharwal, B. Sadeghi, and E.
Knightly, “Distributed Multi-Hop Scheduling and Medium
537
543
541

6. Access with Delay and Throughput Constraints”, Proc. IEEE Transactions on Circuits and Systems II, Vol. 48, No.
MobiCOM 2001, Rome, Italy, Jul 2001. 3, 2001, pp. 233–244.
[4] I. Aad and C. Castelluccia “Differentiation mechanisms [15] E. A. Prosalentis and G. S. Tombras, “A 2-bit Adaptive
for IEEE 802.11”, Proc. INFOCOM 2001, Anchorage, AK, Delta Modulation System with Improved Performance",
USA, Apr 2001. EURASIP Journal on Advances in Signal Processing,
Hindawi Publishing Corporation, Vol 2007, Issue 1, Jan
[5] Q. Ni, I. Aad, C. Barakat, and T. Turletti , “Modeling 2007, p. 3
and Analysis of Slow CW Decrease for IEEE 802.11
WLAN”, Proc. PIMRC 2003, Beijing, China, Sep 2003. [16] X. Zeng, R. Bagrodia, and M. Gerla, “GloMoSim: a
Library for Parallel Simulation of Large-Scale Wireless
[6] P. Chatzimisios, et al, "A simple and effective backoff Networks,” Proc. PADS 1998, Banff, Canada, May 1999.
scheme for the IEEE 802.11 MAC protocol ", Proc. CITSA
2005, Orlando, FL, USA, Jul 2005.
[7] J. A. Moura and R. N. Marinheiro, “MAC approaches
for QoS Enhancement in Wireless LANs”, Proc. JETC 2005,
Lisbona, Portugal, Nov 2005.
[8] B Li, R. Battiti, “Achieving optimal performance in
IEEE 802.11 wireless LANs with the combination of link
adaptation and adaptive backoff”, Computer Networks
Journal, Elsevier Science BV, Vol 51, Issue 6, Apr 2007, p.
1574–1600.
[9] F. Calì, M. Conti and E. Gregori, “IEEE 802.11
Protocol: Design and Performance Evaluation of an Adaptive
Backoff Mechanism”, IEEE Journal on Selected Areas in
Communications, Vol. 18, No. 9, Sept 2000, pp. 1774-1786.
[10] R. Bruno, M. Conti, E. Gregori A simple protocol for
the dynamic tuning of the backoff mechanism in IEEE
802.11 networks”, Computer Networks Journal, Elsevier
Science BV, May 2001, p.1-12.
[11] T. B. Reddy, J. P. John , C. S. R. Murthy,
"Providing MAC QoS for multimedia traffic in
802.11e based multi-hop ad hoc wireless networks",
Computer Networks: The International Journal of
Computer and Telecommunications Networking, Vol
51, Issue 1, Jan 2007, pp.153-176
[12] Q. Ni. “Performance Analysis and Enhancements
for IEEE 802.11e Wireless Networks”, IEEE
Networks, Vol. 19, Issue 4, Jul-Aug. 2005, pp. 21- 27.
[13] M. A. Aldajani and A. H. Sayed, “A stable adaptive
structure for delta modulation with improved performance”,
Proc. ICASSP 2001, Salt Lake City, Utah, USA, May 2001,
Vol. 4, pp. 2621–2624.
[14] M. A. Aldajani and A. H. Sayed, “Stability and
performance analysis of an adaptive sigma-delta modulator,”
538
544
542