Data Fragmentation Scheme in IEEE 802.15.4
Wireless Sensor Networks
Jongwon Yoon, Hyogon Kim and Jeong-Gil Ko
Department of Computer Science and Engineering, Korea University
{yoonj, hyogon, jgko}@korea.ac.kr
Abstract— The IEEE 802.15.4 Medium Access Control (MAC)
protocol is designed for low data rate, short distance and
low power communication applications such as Wireless Sensor
Networks (WSN). However, in the standard 802.15.4 MAC, if the
remaining number of backoff periods in the current superframe
are not enough to complete data transmission procedure, the sen-
sor nodes hold the transmission until the next superframe. When
two or more sensor nodes hold data transmission and restart the
transmission procedure simultaneously in the next superframe,
it causes a collision of data packets and waste of the channel
utilization. Therefore, the MAC design is inadequate to deal with
high contention environments such as densely deployed sensor
networks. In this paper, we propose a data fragmentation scheme
to increase channel utilization and avoid inevitable collision. Our
proposed scheme outperforms the standard IEEE 802.15.4 MAC
in terms of collision probability and aggregate throughput. The
proposed scheme is easily adapted to the standard IEEE 802.15.4
MAC without any additional message types.
I. INTRODUCTION
The IEEE 802.15.4 low-rate wireless personal area network
(LR-WPAN) [1] is one of the candidates for standard wireless
sensor networks (WSNs). Previous works on the 802.15.4
MAC are mostly centered around the performance studies on
the original 802.15.4 MAC [2]–[4]. In Misic et al [5] the
problem of the small backoff range is discussed and analyzed.
We note that WSNs are ideal for applications such as secu-
rity systems, environmental monitoring, industrial automation,
and consumer electronics that operate periodically [6]. For
example, the IEEE 802.15.4 is widely used in applications
of consumer electronics, vital monitoring applications and
security systems, such as smoke detectors operate with a small
beacon interval. In this paper, we deal with such applications
that operate in small beacon intervals. Previous works, such
as [7], [8] were related to the modification of BO value itself
to increase performance. However, our proposed scheme deals
with small beacon interval situations, and we do not require
modification to the BO.
The IEEE 802.15.4 MAC standard wastes the number of
backoff periods at the end of a superframe. This is because
typical data frame is too large to be transmitted during
the small number of remaining backoff periods. This causes
inevitable collision and waste of channel utilization. Therefore,
we propose to fragment large data frames into smaller frames
to transmit in small number of backoff periods that are not
being used in the IEEE 802.15.4 standard. Previous works
[9], [10] use the data fragmentation scheme to increase the
performance in the legacy of 802.11, however we adapt
GTSGTS
Inactive
0 1 2 3 4 5 6 7 8 9 10 1211 13 1514
SD = aBaseSuperframeDuration x 2
SO
symbols
(Active)
BI = aBaseSuperframeDuration x 2
BO
symbols
CFPCAP
Beacon Beacon
Fig. 1. An example of the superframe structure
the fragmentation scheme to 802.15.4 MAC to solve these
problems. Using the fragmentation scheme, we intend to avoid
the inevitable collision and achieve high channel utilization
compare to the MAC of the IEEE 802.15.4 legacy.
II. IEEE 802.15.4 MAC
The IEEE 802.15.4 WSNs consist of sensor device nodes
and a controlling coordinator. The coordinator manages all
device nodes and handles the superframe structure. The su-
perframe is bounded by the transmission of a beacon frames
and have active and inactive portions. The coordinator interacts
with its devices only during the active portion and enters sleep
mode during the inactive portion to save the power consump-
tion. Fig. 1 shows the superframe structure which consists of
active and optional inactive portions. The beacon frame is used
for time synchronization and system configuration between
the coordinator and sensor nodes. The active portion consists
of a contention access period (CAP), where the sensor nodes
equally access the channel using contention, and an optional
contention free period (CFP). The length of a superframe is
controlled by the value of beacon order (BO) and the length
of CAP is represented by superframe order (SO). The values
of BO and SO are determined by the coordinator [1].
The standard of 802.15.4 states that, channel detection in
802.15.4 MAC is based on the CSMA-CA procedure [1]. The
CSMA-CA algorithm is used before the transmission of data or
MAC command frames transmitted within the CAP. The back-
off in IEEE 802.15.4 MAC is processed within the CSMA-
CA algorithm. The initial value is given as macM inBE and
the system randomly selects a backoff time from a number
between [0::2
BE
− 1] [1]. After the random backoff, the
remaining CSMA-CA operations can be undertaken and the
data transmission can be performed until the end of the CAP.
In cases where the randomly selected backoff period is smaller
1550-2252/$25.00 ©2007 IEEE
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