hJam: Attachment Transmission in WLANs
Kaishun Wu, Member, IEEE, Haochao Li, Student Member, IEEE, Lu Wang, Student Member, IEEE,
Youwen Yi, Student Member, IEEE, Yunhuai Liu, Member, IEEE, Dihu Chen, Member, IEEE,
Xiaonan Luo, Member, IEEE, Qian Zhang, Fellow, IEEE, and Lionel M. Ni, Fellow, IEEE
Abstract—Effective coordination can dramatically reduce radio interference and avoid packet collisions for multistation wireless local
area networks (WLANs). Coordination itself needs consume communication resource and thus competes with data transmission for
the limited wireless radio resources. In traditional approaches, control frames and data packets are transmitted in an alternate
manner, which brings a great deal of coordination overhead. In this paper, we propose a new communication model where the control
frames can be “attached” to the data transmission. Thus, control messages and data traffic can be transmitted simultaneously and
consequently the channel utilization can be improved significantly. We implement the idea in OFDM-based WLANs called hJam,
which fully explores the physical layer features of the OFDM modulation method and allows one data packet and a number of control
messages to be transmitted together. hJam is implemented on the GNU Radio testbed consisting of eight USRP2 nodes. We also
conduct comprehensive simulations and the experimental results show that hJam can improve the WLANs efficiency by up to
200 percent compared with the existing 802.11 family protocols.
Index Terms—Wireless network, interference, coordination, OFDM
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1INTRODUCTION
C
OORDINATION among stations can effectively reduce
radio interference and avoid packet collisions in
multistation wireless local area networks (WLANs).
Coordination needs communication and stations have to
exchange control messages to well coordinate. The control
messages can be delivered in an explicit, implicit, or
hybrid manner. However, all control messages will
consume valuable communication resources such as the
communication channel and transmission air time.
In a practical WLAN, the tran smissions of control
messages and data traffic often interleave. As illustrated in
Fig. 1a, the current CSMA/CA protocols (e.g., 802.11 a/g/n)
transmit the control messages and data traffic in an alternate
manner. Between data traffic there are always fractions of
air time for coordination purposes such as DIFS, SIFS,
backoff, and packet acknowledgment. It is well known that
such mechanism is quite inefficient when data frames are
small [1]. When higher physical layer (PHY) data rates are
supported, the efficiency becomes even worse because of the
shortened data traffic air time. Off-the-shelf 802.11n
products now support up to 300 Mbps PHY data rate, while
the effective throughput is only 60 Mbps [1].
To deal with this issue, a direct way in traditional
approaches is to separate the control messages and data
traffic. In this approach (e.g., [22]), a dedicated PHY channel
is allocated for coordination. This approach consumes an
entire channel for control purposes only, which is also too
expensive. The separation can also be done in other
dimensions. Side Channel [15] transmits the control
messages in the code space. It is a customized design for
direct sequence spread spectrum (DSSS) modulation only
and does not have general applicability.
Rather than interleaving or separating the control
messages and data traffic, serving them together at the
same time is more desirable. As illustrated in Fig. 1b, in this
model the data traffic and the control messages are
transm itted si multaneously in the same channel. D ata
traffic accounts for the entire fraction of transmission air
time and is allocated the same bandwidth as in traditional
systems. In the meanwhile, control messages are trans-
mitted in an attached manner with the data traffic. As such
the coordination overhead can be dramatically reduced.
This idea is simple but very challenging to realize. It is
mainly because in the Fig. 1b scenario the control
messages and data traffic are transmitted from indepen-
dent transmitters. These transmitters will have no extra
coordination and thus are very likely to collide with each
other. It becomes even more challenging when there are
several control messages from different transmitters. In a
typical WLAN, it is common that when one node is
transmitting the data, all others may have the demands to
transmit their requests.
Recently, interference cancellation (IC) technique [5],
[14], [30] has been developed well which brings a new hope.
Since a practical rate adaptation scheme is unlikely to
operate at the ideal bitrate, there will always be a slack for
2334 IEEE TRANSACTIONS ON MOBILE COMPUTING, VOL. 12, NO. 12, DECEMBER 2013
. K. Wu is with College of Computer Science and Software Engineering,
Shenzhen University, and also with Guangzhou Fok Ying Tung Research
Institute. E-mail: kwinson@ust.hk.
. H. Li, L. Wang, Y. Yi, Q. Zhang, and L.M. Ni are with the Department of
Computer Science and Engineering, HKUST Fok Ying Tung Graduate
School, Hong Kong University of Science and Technology, Hong Kong
SAR, China. E-mail: {stevenli, wanglu}@ust.hk, euwen.17@gmail.com,
{qianzh, ni}@cse.ust.hk.
. Y. Liu is with the Third Research Institute of Ministry of Public Security,
China. E-mail: yunhuai.liu@gmail.com.
. D. Chen is with the School of Physics and Engineering, Sun Yat-sen
University, Guangzhou 510006, China. E-mail: stscdh@mail.sysu.edu.cn.
. X. Luo is with the National Engineering Research Center of Digital Life,
State-Province Joint Laboratory of Digital Home Interactive Applications,
School of Information Science and Technology, Sun Yat-sen University,
Guangzhou 510006, China. E-mail: lnslxn@mail.sysu.edu.cn.
Manuscript received 29 Nov. 2011; revised 23 Aug. 2012; accepted 30 Aug.
2012; published online 12 Sept. 2012.
For information on obtaining reprints of this article, please send e-mail to:
tmc@computer.org, and reference IEEECS Log Number TMC-2011-11-0639.
Digital Object Identifier no. 10.1109/TMC.2012.194.
1536-1233/13/$31.00 ß 2013 IEEE Published by the IEEE CS, CASS, ComSoc, IES, & SPS