Differentiation, QoS Guarantee, and
Optimization for Real-Time Traffic
over One-Hop Ad Hoc Networks
Yang Xiao, Senior Member, IEEE, and Yi Pan, Senior Member, IEEE
Abstract—Nodes having a self-centrically broadcasting nature of communication form a wireless ad hoc network. Many issues are
involved to provide quality of service (QoS) for ad hoc networks, including routing, medium access, resource reservation, mobility
management, etc. Previous work mostly focuses on QoS routing with an assumption that the Medium Access Control (MAC) layer can
support QoS very well. However, contention-based MAC protocols are adopted in most ad hoc networks since there is no centralized
control. QoS support in contention-based MAC layer is a very challenging issue. Carefully designed distributed medium access
techniques must be used as foundations for most ad hoc networks. In this paper, we study and enhance distributed medium access
techniques for real-time transmissions in the IEEE 802.11 single-hop ad hoc wireless networks. In the IEEE 802.11 MAC, error control
adopts positive acknowledgement and retransmission to improve transmission reliability in the wireless medium (WM). However, for
real-time multimedia traffic with sensitive delay requirements, retransmitted frames may be too late to be useful due to the fact that the
delay of competing the WM is unpredictable. In this paper, we address several MAC issues and QoS issues for delay-sensitive real-
time traffic. First, a priority scheme is proposed to differentiate the delay sensitive real-time traffic from the best-effort traffic. In the
proposed priority scheme, retransmission is not used for the real-time traffic, and a smaller backoff window size is adopted. Second,
we propose several schemes to guarantee QoS requirements. The first scheme is to guarantee frame-dropping probability for the real-
time traffic. The second scheme is to guarantee throughput and delay. The last scheme is to guarantee throughput, delay, and frame-
dropping probability simultaneously. Finally, we propose adaptive window backoff schemes to optimize throughput with and without
Index Terms— Distributed medium access control, IEEE 802.11, quality of service, real-time transmission, ad hoc networks.
hoc wireless networks, peer-to-peer nodes conduct initi-
alization, organization, and administration of networks.
Many challenges must be overcome to obtain the practical
benefits of ad hoc networks, including routing, medium
access control (MAC), mobility management, power man-
agement, security, and quality of service (QoS) issues .
The nodes of an ad hoc network communicate directly with
one another in a peer-to-peer fashion, and each node must
function as a router. Power capacity and transmission range
are further limited by the mobility of these nodes. Due to
the mobility, the network topology is dynamically changed.
Furthermore, the limited bandwidth of wireless channels
and hostile transmission characteristics impose additional
constraints. For ad hoc networks with a contention-based
MAC layer, the nature of contentions further imposes
challenges for QoS support.
Many researches , , , ,  on ad hoc networks
have been reported, and real-time transmissions for
Ethernet and cellular networks have been well studied ,
D hoc wireless networks consist of a collection of
mobile stations without a fixed infrastructure. In ad
, , , . However, most researches , , , ,
 focus on ad hoc routing protocols under the assumption
that some underline MAC protocols can provide good
services to higher layers. In this paper, we focus on
designing good MAC mechanisms for real-time transmis-
sions in ad hoc networks. Without the MAC layer’s support,
QoS support solely in higher layers is not possible.
Carefully designed distributed medium access techniques
must be used for channel resources so that mechanisms are
needed to recover efficiently from the inevitable frame
We are particularly interested in ad hoc networks with
the underneath IEEE 802.11 distributed MAC since it is
available. The IEEE 802.11 MAC employs a mandatory
contention-based channel access function called Distributed
Coordination Function (DCF), and an optional centrally
controlled channel access function called Point Coordina-
tion Function (PCF) . The popularity of the IEEE 802.11
market is largely due to the DCF, whereas the PCF is barely
implemented in current products due to its complexity and
inefficiency for normal data transmissions. The DCF adopts
a carrier sense multiple access with collision avoidance
(CSMA/CA) with binary exponential backoff. Functions of
the DCF and the PCF determine when a station/node,
operating within a Basic Service Set (BSS) or Independent
BSS (IBSS), is permitted to transmit. There are two types of
802.11 networks: Infrastructure Network (BSS) in which an
access point (AP) is present and ad hoc Network (IBSS) in
which an AP is not present. In this paper, we are
particularly interested in ad hoc networks formed by
multiple IBSSs in which no AP is present. There have been
538IEEE TRANSACTIONS ON PARALLEL AND DISTRIBUTED SYSTEMS, VOL. 16,NO. 6,JUNE 2005
. Y. Xiao is with the Computer Science Division, The University of
Memphis, 373 Dunn Hall, Memphis, TN 38152.
. Y. Pan is with the Department of Computer Science, Georgia State
University, University Plaza, Atlanta, GA 30303.
Manuscript received 29 Dec. 2003; revised 16 Sept. 2004; accepted 19 Sept.
2004; published online 21 Apr. 2005.
For information on obtaining reprints of this article, please send e-mail to:
firstname.lastname@example.org, and reference IEEECS Log Number TPDS-0243-1203.
1045-9219/05/$20.00 ? 2005 IEEE Published by the IEEE Computer Society
many performance studies for the DCF. Bianchi 
proposed a simple and accurate analytical model to
compute saturation throughput. Bianchi and Xiao enhanced
Bianchi’s original model in . Calı ` et al.  studied an
optimization method for a p-persistent WLAN MAC.
Several priority studies have been reported in the
literature for the DCF. Deng and Chang  proposed a
priority scheme by differentiating the backoff window: the
higher priority class uses the window ½0;2jþ1? 1? and the
lower priority class uses the window ½2jþ1;2jþ2? 1?, where j
is the backoff stage. Aad and Castelluccia  proposed a
priority scheme achieved by differentiating interframe
spaces (IFS). Ahn et al.  proposed priority schemes by
differentiating the initial backoff window size and the
maximum window size. Pallot and Miller  proposed a
prioritized backoff time distribution mechanism, in which
the backoff time is chosen in the current window range with
different distributions for different priorities. All the
priority schemes , , , ,  were based on
simulations. Related recent work also includes Sheu et al.
 about priorities for Ad hoc networks, and Ge and Hou
 about priority analysis for p-persistent WLANs.
In the DCF, error control adopts positive acknowl-
edgment and retransmission to improve transmission
reliability in the wireless medium (WM). In other words,
every transmitted frame needs a positive acknowledgment.
If an acknowledgement for a transmitted frame has not
been received for a timeout period, the transmitted frame is
assumed corrupted, and the frame will be retransmitted for
many times until a positive acknowledgement is received or
the number of retransmissions reaches a limit. In the later
case, the frame is dropped. Therefore, the DCF is a very
robust protocol for the best-effort service in the WM.
However, the current DCF is unsuitable for real-time
applications with QoS requirements. In the DCF a station
might have to wait an arbitrarily long time to send a frame
so that real-time applications such as voice and video may
suffer . Furthermore, for real-time multimedia traffic
with sensitive delay requirements, retransmitted frames
may be too late to be useful due to the unexpected delay.
In this paper, we consider two classes of traffic: delay
sensitive real-time (RT) traffic and best-effort (BE) traffic.
The RT traffic can be voice or video. The BE traffic is normal
data transmission. A priority scheme is proposed to
differentiate RT and BE classes. For the RT class, retrans-
mission is not used and a smaller backoff window size is
adopted. The BE class still follows the original DCF. Such a
scheme is similar to video/multimedia over UDP. The
rationale is the same as the rationale of UDP. An analytical
model for the proposed priority scheme (differentiating the
RT and the BE priority classes) is proposed to evaluate
system performance, and validated via simulations. The
proposed real-time differential mechanisms are a little
similar to recently IEEE 802.11e draft . However, the
proposed mechanisms are much simpler than IEEE 802.11e
as well as other priority schemes in the literature, and more
likely to be used in real products in which the IEEE 802.11e
has not been implemented. In other words, they can be
implemented in the original IEEE 802.11a/.11b/.11g with
very little effort.
We further consider several QoS and optimization issues.
the first scheme is to guarantee frame-dropping probability
for the real-time traffic; the second scheme is to guarantee
throughput and delay; the last scheme is to guarantee
throughput, delay and frame-dropping probability simulta-
neously. Finally, we propose adaptive window backoff
schemes to optimize throughput with and without QoS
constraints, based on the fact that there exists an optimal
initial backoff window size for a fixed traffic load.
The rest of the paper is organized as follows: The IEEE
802.11 CSMA/CA is introduced in Section 2. Service
differentiation, its analytical model, and results are studied
in Section 3. QoS guarantee mechanisms are presented in
Section 4. Optimization schemes with adaptation and QoS
guarantee are proposed in Section 5. We conclude this
paper in Section 6.
2 CSMA/CA IN THE DCF
The IEEE 802.11 MAC employs a mandatory DCF and an
optional PCF. In a long run, time is divided into repetition
intervals called superframes. Each superframe starts with a
beacon frame, and the remaining time is further divided
into a contention-free period (CFP) and a contention period
(CP). The DCF works during the CP and the PCF works
during the CFP. If the PCF is not active, a superframe will
not include the CFP.
In the DCF, a station with a frame to transmit monitors
the channel activities until an idle period equal to a
distributed interframe space (DIFS) is detected. After
sensing an idle DIFS, the station waits for a random backoff
interval before transmitting. The backoff time counter is
decremented in terms of slot time as long as the channel is
sensed idle. The counter is stopped when a transmission is
detected on the channel, and reactivated when the channel
is sensed idle again. The station transmits its frame when
the backoff time reaches zero. At each transmission, the
backoff time is uniformly chosen in the range ½0;CW ? 1?,
where CW is the current backoff window size. At the very
first transmission attempt, CW equals the initial backoff
window size CWmin. After each unsuccessful transmission,
CW is doubled until a maximum backoff window size
value CWmaxis reached. Once it reaches CWmax, CW shall
remain at the value CWmaxuntil it is reset. CW shall be reset
to CWminafter every successful attempt to transmit, or the
retransmission counter reaches the retry limit Lretry. In the
later case, the frame will be dropped. After the destination
station successfully receives the frame, it transmits an
acknowledgment frame (ACK) following a short interframe
space (SIFS) time. If the transmitting station does not
receive the ACK within a specified ACK Timeout, or it
detects the transmission of a different frame on the channel,
it reschedules the frame transmission according to the
previous backoff rules.
The above mechanism is called the basic access mechan-
ism. To reduce the hidden station problem, an optional
four-way data transmission mechanism called Request-To-
Send (RTS)/Clear-To-Send (CTS) is also defined in DCF. In
the RTS/CTS mechanism, before transmitting a data frame,
a short RTS frame is transmitted. The RTS frame also
follows the backoff rules introduced above. If the RTS frame
succeeds, the receiver station responds with a short CTS
frame. Then, a data frame and an ACK frame will follow.
All four frames (RTS, CTS, data, and ACK) are separated by
an SIFS time. In other words, the short RTS and CTS frames
XIAO AND PAN: DIFFERENTIATION, QOS GUARANTEE, AND OPTIMIZATION FOR REAL-TIME TRAFFIC OVER ONE-HOP AD HOC...539
limit. The frame-dropping probability of the RT class
decreases little whereas the frame-dropping prob-
ability of the BE class decreases much more and goes
near zero when the retry limit is near 12.
Simulation results match pretty well with the
We have proposed and studied some QoS guarantee
mechanisms for the real-time traffic:
We have proposed a simple scheme to provide an
upper bound on frame dropping probability. Our
results show that the QoS requirements (a bound on
the frame dropping probability) can be guaranteed,
and a stringent QoS requirement needs a larger
initial window size. Therefore, frame dropping
probability for the real-time traffic can be controlled
with a reasonable value.
We have proposed an admission control scheme to
provide guaranteed throughput and/or delay. We
have observed that when the required delay is
loosen (the required delay increases), more stations
are accepted. For a fixed required delay, the scheme
with a larger initial window size can accommodate
We have proposed an algorithm to guarantee
throughput, delay, and/or frame dropping prob-
ability. We have obtained the admission control
region, in which QoS can be guaranteed.
We have further studied how to optimize the throughput
with and without guaranteeing QoS parameters via adapta-
tion of initial window size:
We have studied how to optimize the throughput
without QoS considerations. The optimal initial
window size increases as the number of active
stations increases. The optimal initial window size is
496 when the number of active stations is 30. The
adaptive scheme achieves the optimal throughput
that bounds the fixed scheme.
We have studied how to optimize the throughput
with QoS considerations. Results show that QoS
requirements are guaranteed and throughput is
The authors are grateful to the three referees for their
careful reading and suggestions which have greatly
improved the readability of the paper, and to the associate
editor, Professor Wei Zhao, for timely and professional
handling of our manuscript. Yi Pan’s research was
supported in part by the National Natural Science Founda-
tion of China (NSFC) under Grant No. 60440420451 (“two
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548IEEE TRANSACTIONS ON PARALLEL AND DISTRIBUTED SYSTEMS, VOL. 16,NO. 6,JUNE 2005
Yang Xiao received the PhD degree in compu-
ter science and engineering from Wright State
University, Dayton. He had been a software
engineer, a senior software engineer, and a
technical lead working in the computer industry
for five years in the early 1990s. He worked at
Micro Linear-Salt Lake City Design Center as an
MAC architect involving the IEEE 802.11 (Wire-
less LAN) standard enhancement work before
he joined The University of Memphis as an
assistant professor of computer science. Dr. Xiao is a voting member of
the IEEE 802.11 Working Group, a senior member of the IEEE and the
IEEE Computer Society, and a member of the ACM. He is an associate
editor of EURASIP Journal on Wireless Communications and Network-
ing, and he currently serves on the editorial boards of the (Wiley) Journal
of Wireless Communications and Mobile Computing, the International
Journal of Wireless and Mobile Computing, and the International Journal
of Signal Processing. He serves a lead guest editor for the (Wiley)
Journal of Wireless Communications and Mobile Computing, special
issue on mobility, paging, and quality of service management for future
wireless networks in 2004, a lead guest editor for the International
Journal of Wireless and Mobile Computing, special issue on Mmdium
access control for WLANs, WPANs, ad hoc networks, and sensor
networks in 2004, and an associate guest editor for the International
Journal of High Performance Computing and Networking, special issue
on parallel and distributed computing, applications and technologies in
2003. He serves as a symposium cochair for the Symposium on Data
Base Management in Wireless Network Environments in IEEE
VTC’2003 Fall. He serves as a TPC member for many conferences
such as ICC, GLOBECOM, ICDCS, WCNC, ICCCN, PIMRC, WMASH,
etc. Dr. Xiao’s current research interests include wireless local area
networks, wireless personal area networks, and mobile cellular net-
works. He has published many papers in major journals and refereed
conference proceedings related to these research areas, such as the
IEEE Transactions on Mobile Computing, IEEE Transactions on
Wireless Communications, IEEE Transactions on Parallel and Distrib-
uted Systems, IEEE Transactions on Vehicular Technology, IEEE
Communications Letters, IEEE Communications Magazine, IEEE
Wireless Communications, ACM/Kluwer MONET, etc.
Yi Pan received the BEng and MEng degrees in
computer engineering from Tsinghua University,
China, in 1982 and 1984, respectively, and the
PhD degree in computer science from the
University of Pittsburgh in 1991. Currently, he is
a chair and a full professor in the Department of
Computer Science at Georgia State University.
Dr. Pan’s research interests include parallel and
distributed computing, optical networks, wireless
networks, and bioinformatics. Dr. Pan has
published more than 80 journal papers with 26 papers published in
various IEEE journals. In addition, he has published more than 90 papers
GLOBECOM). He has also coedited 13 books (including proceedings)
and contributed several book chapters. His pioneer work on computing
using reconfigurable optical buses has inspired extensive subsequent
work by many researchers, and his research results have been cited by
more than 100 researchers worldwide in books, theses, journal and
conference papers. He is a coinventor of three US patents (pending) and
five provisional patents, and has received many awards from agencies
such as the US National Science Foundation, AFOSR, JSPS, IISF, and
Mellon Foundation. His recent research has been supported by the US
National Science Foundation, NIH, AFOSR, AFRL, JSPS, IISF, and the
states of Georgia and Ohio. He has served as a reviewer/panelist for
many research foundations/agencies such as the US National Science
Foundation, the Natural Sciences and Engineering Research Council of
Canada, the Australian Research Council, and the Hong Kong Research
member foreightjournals,including threeIEEETransactions and aguest
editor for seven special issues. He has organized several international
conferences and workshops and has also served as a program
committee member for several major international conferences such as
INFOCOM, GLOBECOM, ICC, IPDPS, and ICPP. Dr. Pan has delivered
more than 50 invited talks, including keynote speeches and colloquium
talks, at conferences and universities worldwide. Dr. Pan is an IEEE
Distinguished Speaker (2000-2002), a Yamacraw Distinguished Speaker
(2002), a Shell Oil Colloquium Speaker (2002), and a senior member of
the IEEE and the IEEE Computer Society. He is listed in Men of
Achievement, Who’s Who in Midwest, Who’s Who in America, Who’s
Who in American Education, Who’s Who in Computational Science and
Engineering, and Who’s Who of Asian American.
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