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But none of them concentrated on the effect of the node
density due to the mobility of the coordinator. The goal
of this paper is to study the analysis of mobility of coor-
dinator on the Load of the network.
This paper is organized as: Section 2 gives a brief
overview of the IEEE 802.1 5.4 and the reason for adopt-
ing Zigbee, Section III discusses about OPNET modeler,
Section IV explains about the types of ZigBee devices
and their network topologies, Section 5 describes the
arrangement of nodes in the network and the trajectories
for the coordinator motion, Section 6 gives the assump-
tion and layout of network field, Section 7 analyses the
simulations performed using OPNET modeler and Sec-
tion 8 concludes the paper giving the results.
2. Overview of IEEE 802.15.4 (Zigbee)
ZigBee takes its name from the zigzag flying of bees that
forms a mesh network among flowers. It is an individu-
ally simple organism that works together to tackle com-
plex tasks. Zigbee (IEEE 802.15.4) standard intercon-
nects simple, low power and low processing capability
wireless devices. The Zigbee devices facilitate numerous
applications such as pervasive computing, national secu-
rity, monitoring and control etc. In recent years, the
technology has been gaining use in industrial and com-
mercial acceptance, this is clear from the wide spread use
in defence, monitoring and control, commercial use etc.
As shown in Figure 1 Zigbee architecture comprises
of 4 layers—Physical Layer, MAC Layer, Network and
Security Layer and Application Layer. Physical and
MAC Layer are defined as IEEE 802.15.4 standards
while the higher layers follow standards set by Zigbee
Alliance. Our simulation model implements the physical
layer of the IEEE 802.15.4 standard running at 2.4 GHz
Frequency band with 250 kbps d ata rate. The MAC layer
supports the beacon-enabled mode and implements slotted
Figure 1. Zigbee architecture.
CSMA/CA and GTS mechanism according to the stan-
dard specification. There is also a battery module that
computes the consumed and remaining energy levels.
The network layer implements hierarchical tree routing
according to the ZigBee standard. The application layer
can generate best effort and/or real-time unacknowledged
and/or acknowledged frames transmitted during Conten-
ti on Access Period (CAP) or Con tention Free Period (CFP)
(contains GTSs) of t he s u per fr ame, respectively.
3. Overview of OPNET
The OPNET Modeler environment includes tools for all
phases of a study, including model design, simulation,
data collection, and data analysis. OPNET Modeler pro-
vides a comprehensive development environment sup-
porting the modelling of communication networks and
distributed systems. Both behaviour and performance of
a model can be analysed by performing discrete event
simulations [10]. A Graphical User Interface (GUI) sup-
ports the configuration of the scenarios and the develop-
ment of network models.
According to our personal experience, we strongly be-
lieve that the current version of the WPAN implementa-
tion in the network simulator (ns-2) simulator is not ac-
curate for the simulation of wireless sensor networks.
OPNET simulation model implements more accurately
the IEEE 802.15.4/ZigBee protocols without these un-
necessary overheads. This is mainly due to the amount of
additional overheads introduced by the ns-2 simulator,
since it imposes the use of a UDP (User Datagram Pro-
tocol) agent in each node for generating data, and also
the generation of ARP (Address Resolution Protocol)
frames.
Three hierarchical levels for configuration are differ-
entiated: Network level, Node level and Process level.
Network level creating the topolog y of th e networ k und er
investigation, Node level defining the behaviour of the
node and controlling the flow of data between different
functional elements inside the node and Process level
describing the underlying protocols, represented by finite
state machines (FSMs) and are created with states and
transitions between states. The source code is based on
C/C++.
4. Network Topologies in Zigbee
There are three types of devices defined by the Zigbee
standards [9]—coordinators, routers and end-devices. Zig-
bee coordinator is respon sible for initializing the netwo rk,
selecting the appropriate channel, and permitting other
devices to connect to its network. It can also be responsi-
ble for routing traffic in a ZigBee network. For every
ZigBee network, there can be only one coordinator. Zig-
Bee routers are the intermediate devices in a network
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