Hindawi Publishing Corporation
EURASIP Journal on Wireless Communications and Networking
Volume 2009, Article ID 478157, 12 pages
Qualityof VOD ServicesforWAVENetworks
1Department of Computing and Electronic Systems, University of Essex, Colchester CO4 3SQ, UK
2Department of Engineering Science, National Cheng Kung University, Tainan City 70101, Taiwan
3Department of Electronic and Electrical Engineering, University College London (UCL), London WC1E 7JE, UK
Correspondence should be addressed to Hsiao-Hwa Chen, email@example.com
Received 1 April 2008; Revised 21 August 2008; Accepted 14 November 2008
Recommended by Zhisheng Niu
Providing quality-of-service- (QoS-) guaranteed video on demand (VOD) services over wireless access in vehicular environments
(WAVEs) is a challenge as WAVE adopts enhanced distributive channel access (EDCA), a contention-based channel access
mechanism, for air interface access control. This paper proposes a selective downlink scheduling (SDS) algorithm to enhance
the quality of VOD for WAVE networks. According to the importance of video decoding, video packets are categorized into high
and low priorities. The categorized packets are put into different queues in roadside units (RSUs) to contend for transmission
opportunities. Aiming to improve video playback quality and reduce video playback delay, the proposed SDS algorithm schedules
video packets based on their importance, playback deadline, and their real-time parameters of receiving onboard units (OBUs),
such as velocity and remaining dwelling time. The effectiveness of SDS algorithm is verified by simulations.
Copyright © 2009 Shumao Ou et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Wireless access in vehicular environments (WAVEs)  is
a next generation intelligent transportation technology that
aims to improve transportation environments. The physical
layer in WAVE is defined in IEEE 802.11  and amended
by IEEE 802.11p . The spectrum of WAVE is allocated
at the range of 5.86 ∼ 5.92GHz, and it is divided into
seven channels each of 10MHz bandwidth. Apart from
improving transportation environment safety and intelligent
management, the WAVE systems also provide a convenient
means to provision data exchange services, such as video
on demand (VOD), when people are traveling in vehicles.
However, some specific features of WAVE networks, such as
fast movement of vehicles and topology dynamic changes,
render the provisioning of VOD services as a challenge. In
this paper, we endeavor to tackle this problem by proposing
a selective downlink scheduling (SDS) algorithm for VOD
services in WAVE networks.
Communications in WAVE systems are conducted
in two modes: vehicle-to-vehicle (V2V) and vehicle-to-
infrastructure (V2I) communications. There are two types
of devices in WAVE: roadside unit (RSU) and onboard unit
(OBU). Both RSU and OBU support at least one control
channel (CCH) and multiple service channels (SCHs). An
RSU is installed in a fixed position along roadside to support
communication with OBUs wirelessly in its cell coverage.
An OBU is a mobile device, normally fitted in a vehicle,
to support communications with RSUs and other OBUs.
Figure 1 illustrates a WAVE network that is adopted in this
moving in both directions communicate with RSUs in a V2I
mode. A VOD server is connected to the Internet. The OBUs
request VOD services through the RSUs and video streams
data flows from the VOD server to the requested OBUs (e.g.,
OBUs 1, 3, 5, 4, and 7 as shown in Figure 1).
Compressed video streams are able to tolerate small
amount of packet loss without degrading the quality of
video playback very much, since some packets are less
important for video decoding . This feature can be used
for schedule algorithms to deliver important packets and
discard unimportant packets when traffic is heavy, or a
wireless channel is unreliable. In this paper, the proposed
SDS algorithm is designed to improve video playback quality
2EURASIP Journal on Wireless Communications and Networking
OBU 1OBU 2OBU 3 OBU 4
OBU 5OBU 6 OBU 7
RSU 1RSU 2
Figure 1: A WAVE network for VOD services.
and reduce video playback delay, especially when traffic load
The existing works related to the issues concerned in
this paper can be classified into two categories. The first
category focuses on traffic scheduling of contention-based
wireless networks, such as [5–9]. The second category
concerns the issues on video streaming in wireless networks
[10–13]. The WAVE standard does not specify particular
scheduling algorithms for its media access control (MAC).
The conventional first-come-first-service (FCFS) algorithm
adopted by  is still recommended to be used in WAVE
in coordination with IEEE 802.11e enhanced distributed
channel access (EDCA) technology [1, 14]. The earliest
deadline first (EDF) algorithm is widely used in traffic
scheduling on wireless channels for multimedia [7, 9]. EDF
can improve the QoS of multimedia services and overall
system utilization when the deadline of each packet in the
traffic is known. In , Chang et al. proposed a maximum
freedom last (MFL) scheduling algorithm specifically for
dedicated short-range communication (DSRC) networks.
MFL is an enhancement of EDF which achieves a low-
handoff rate by taking DSRC network parameters, such as
cell size and vehicle dwelling time, into account.
Video streaming over IEEE 802.11 networks attracted
much research interest. The authors of [11–13] proposed
mechanisms to transport H.264 and MPEG-4 over IEEE
802.11e/a networks. However, their mechanisms are based
on hybrid coordination function controlled channel access
(HCCA) . HCCA is a polling-based channel access
mechanism which is different from contention-based EDCA
used in WAVE. Furthermore, these mechanisms are only
suitable for scalable video streams. Ksentini et al.  pro-
posed a cross-layer architecture for H.264 over IEEE 802.11e.
They utilized a specific feature of H.264 data partitioning
(defined in the extended profile of H.264 standard). In their
work, the bitstreaming of a frame is classified into three
data partitions with different importance levels. The data
partitions are delivered by different access categories (ACs)
in IEEE 802.11e MAC. However, they only dealt with the
different importance of data partitions within a frame, and
they did not distinguish the importance between different
Inspired by EDF, MFL, and aforementioned video
streaming schemes over wireless mechanisms, the SDS
algorithm proposed in this paper aims to improve video
playback quality and reduce video playback delay in WAVE
networks. The SDS algorithm schedules video packets based
on their importance, playback deadline, and their real-
time parameters of receiving OBUs, such as velocity and
remaining dwelling time. The SDS algorithm selectively
drops unimportant video packets when it is not possible
to transmit all packets due to limited dwelling time, heavy
load, or undesired channel conditions. This selective packet
In the SDS algorithm, video packets destined to different
OBUs are coordinately scheduled by their playback deadline.
This feature can be used to reduce the video playback delay
of all OBUs. The proposed mechanism in this paper is not
designed for only a specific video codec scheme (such as
H.264), but it is also applicable to other popular video
compression formats, such as H.263, MPEG-2.
The rest of the paper is outlined as follows. Section 2
presents some preliminaries, and Section 3 discusses the
system model. In which the system architecture and system
is presented in Section 4. In Section 5, the scheduling
algorithm is evaluated by simulations, and performance of
the algorithms is analyzed. Finally, Section 6 concludes the
2.1. Video Frame and Packet. In video compression tech-
nologies, such as MEPG and H.264 , encoded pictures
(or frames) are arranged in groups of pictures (GOPs). An
encoded video stream consists of successive GOPs. A GOP
12EURASIP Journal on Wireless Communications and Networking
the quality of VOD services. Video packets are marked into
high and low priorities before they are fed into WAVE MAC.
Marked video packets are put into different access category
queues. Our proposed SDS algorithm schedules the video
packets according to their playback deadline and received
OBU’s network parameters, such as the time remained
to communicate with current RSU and so on. The SDS
algorithm is evaluated through simulations, showing its
effectiveness and efficiency for video streaming over WAVE
networks. In this work, we only focus on increasing the
delivery ratio of high-priority packets and reducing video
playback delay. We believe that these two parameters are
fundamental for the quality of video streaming over WAVE.
Other video quality evaluations, such as peak signal-to-noise
in our future work.
The authors would like to gratefully acknowledge the
UK Engineering and Physical Sciences Research Coun-
cil (EPSRC) Project PANDA (EP/D061881/1) and Taiwan
National Science Council Grant NSC 97-2219-E-006-004.
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