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CHAPTER 1

INTRODUCTION

1.1 OVERVIEW

For accessing computer networks and its services without cables,

wireless communications is a fast-growing technology which gives certain

advantages over wired network like the dynamic network formation, easy

deployment and cost reduction etc. The capabilities needed to deliver such

services are characterized by an increasing need of high throughput. However,

other applications in fields such as industrial, vehicular, and residential

sensors have more relaxed throughput requirements. With the emergence of

new Wireless Sensor Network (WSN) applications under timing constraints,

the provision of deterministic guarantees may be more crucial than saving

energy during critical situations. The IEEE 802.15.4 is a new Personal

Wireless Area Network (PWAN) (Jin-Shyan Lee et al 2006) standard

designed for applications like wireless monitoring and control of lights,

security alarms, motion sensors, thermostats and smoke detectors. The IEEE

802.15.4 Task Group (TG4), together with the ZigBee Alliance, has

developed an entire communication protocol stack for Low-Rate Wireless

Personal Area Networks (LR-WPAN).

The IEEE 802.15.4 ZigBee protocol is one potential protocol to

achieve predictable real-time performance for LR-WPAN. The physical layer

of the IEEE 802.15.4 protocol seems suitable for WSN applications, namely

in terms of data-rate, energy-efficiency and robustness. The IEEE 802.15.4

MAC protocol supports two operational modes: the Beaconless mode, in

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which nodes stay active all the time and the Beacon mode, in which Beacon

frames are periodically sent by coordinators to synchronize sensor nodes. The

advantage of this synchronization scheme is that all nodes can wake up and

sleep at the same time allowing very low duty cycles and hence save energy.

In addition, when the beacon mode is used, nodes can use Guaranteed Time

Slots (GTS) specifically designed to fulfill application’s QoS requirements.

The advantage of the non-beacon enabled mode, with regards to

WSN application requirements, is that it easily allows scalability and

self-organization. However, the non-beacon enabled mode does not provide

any guarantee to deliver data frames, within a certain deadline. For time-

critical applications, timeliness guarantees may be achieved with this beacon-

enabled mode. This mode offers the possibility of allocating/reallocating time

slots in a superframe, called GTSs and provides predictable minimum service

guarantees. Using minimum service guarantee, it is possible to predict a

worst-case timing performance of the network.

Recently, several analytical and simulation models of the IEEE

802.15.4 protocol have been proposed. OPNET Modeler, Network Simulator

2 (NS-2) and OMNeT++ are widely used and popular network simulators,

which include a simulation model of the IEEE 802.15.4 protocol. The

802.15.4/ZigBee simulation model in OPNET model library supports only

non-beacon-enabled mode, therefore, the star topology and GTS mechanism

cannot be simulated. In addition, the source codes of the network and

application layers are not available. The National Institute of Standards and

Technology (NIST) has developed its OPNET simulation model for the IEEE

802.15.4 protocol. However, while that model implements the slotted and the

unslotted CSMA/CA MAC protocols it does not support the GTS mechanism

as well. It also uses its own radio channel model rather than the accurate

OPNET wireless library. The NS-2 is an object-oriented discrete event

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simulator including a simulation model of the IEEE 802.15.4 protocol. The

accuracy of its simulation results are questionable since the Medium Access

Control (MAC) protocols, packet formats and energy models are different

from those used in real WSNs. Comparing the other network simulators

(Gilberto Flores Lucio et al 2003) OPNET Modeler provides real time

accuracy and has huge library. The ZigBee OPNET Modeler is the most

powerful simulator for analysis (Hammoodi et al 2009). Potential

improvements have been proposed to further develop OPNET Modeler to

compete with other well-known WSNs simulators. These improvements will

enhance OPNET Modeler to cover all aspects of WSNs simulations and

investigations for both researchers and network operators.

1.2 WIRELESS TECHNOLOGIES

Various wireless Technologies are compared based on different

parameters. Comparison is shown in Figure 1.1 and 1.2 and Table 1.1.

Figure 1.1 Complexity, Power, Cost Vs Data Rate

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Figure 1.2 Data Rate Vs Distance

Table 1.1 Wireless Technology Comparison

Name Wi-Fi Bluetooth ZigBee UWB

Bandwidth Upto 54 Mbps 1 Mbps 250Kbps 480Mbps

Power Consumption 400 +mA TX,Standby 20Ma

40 +mA TX, Standby 0.2mA

30 mA TX, Standby 356µA

200mw

Protocol Stack size 100 + KB ~ 100 + KB 34 KB /14 KB -

Typical Range 100 m < 10 m 70-300 m 10 -30 m

Modulation DSSS Adaptive FHSS DSSS OFDM or DS-UWB

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Table 1.1 (Continued)

Freq. Range 2.4 GHz 2.4 GHz 868/915MHz

2.4 GHz

3.1-10.6 GHz

Stronghold High Data Rate

Interoperability,CableReplacement

Long Battery Life, Low Cost

High data rate for short range

Battery Life

(Days)

0-5.5 1-7 days 100-1000 +-

Applications Internet browsing, PCnetworking, File Transfer

Wireless USB, Handset,Headset

Remote Control,batteryoperatedproducts,sensors

Sync, Transmission of video/Audio Data

1.3 WIRELESS SENSOR NETWORKS

WSN is a network which is used to gather relevant data from the

environment and subsequently to route the gathered data to Central

Processing Node. WSN consists of a large number of Sensor Nodes (SNs)

wirelessly connected to each other and Base Station (BS), which connects the

SNs with another network (Figure 1.3). WSNs are new field of research,

which is currently growing rapidly.

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Figure 1.3 Wireless Sensor Network

1.4 ZIGBEE

ZigBee takes its name from the zigzag flying of bees that forms a

mesh network among flowers. It is an individually simple organism that

works together to tackle complex tasks. ZigBee has built on the IEEE

802.15.4 low-rate, WPAN standard. The IEEE 802.15.4 defines the physical

layer (PHY) and Media Access Control (MAC) layer. The physical layer

supports three radio bands, those are individually defined 2.4 GHz ISM band

(Worldwide) with 16 channels, 915 MHz ISM band (Americas) with 10

channels, and 868MHz band (Europe) with single channel, the data rates are

individually defined as 250 Kbps at 2.4 GHz, 40Kbps at 915 MHz, and 20

Kbps at 868 MHz.

The MAC layer controls access to the radio channel using the

Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA)

mechanism. The transmission range is 1-100 meters. The ZigBee defines two

types of devices; those are Full Function Device (FFD) and Reduced Function

Device (RFD).

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The FFD can serve as a network coordinator or a regular device. It

can communicate with any other device. The RFD is intended for applications

that are extremely simple, such as a light switch or a passive sensor device. It

can communicate only with the FFD. Theoretically, ZigBee can support up to

65,536 nodes. For security, ZigBee uses 128-bit Advanced Encryption

Standard (AES) cryptography and trust-center based authentication.

1.4.1 ZIGBEE Applications

There are numerous applications (Figure 1.4) that are ideal for the

redundant, self-configuring and self-healing capabilities of ZigBee wireless

mesh networks. Key ones include:

Energy Management and Efficiency: To provide greater

information and control of energy usage. Also to provide

customers with better service and more choice, better

management of resources, and help to reduce environmental

impact.

Home Automation: To provide more flexible management of

lighting, heating and cooling, security, and home

entertainment systems from anywhere in the home.

Building Automation: To integrate and centralize management

of lighting, heating, cooling and security.

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Figure 1.4 ZigBee Applications

Industrial Automation: To extend existing manufacturing and

process control systems reliability.

The interoperable nature of ZigBee means that these applications

can work together, providing even greater benefits.

1.4.2 ZIGBEE Specifications

The specifications of ZigBee is shown in Table 1.2.

Table 1.2 ZigBee Specifications

Parameters ZigBee 802.15.4

Transmission range (meters) 1-100

Battery life (days) 100-1000

Network size (# of nodes) >64000

Throughput (Kbps) 20-250

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1.4.3 ZIGBEE Architecture

Figure 1.5 ZigBee Architecture

ZigBee Architecture is shown in Figure 1.5.

Physical Layer

The physical layer is provided by the IEEE 802.15.4 standard. This

standard manages the physical transmission of radio waves in different

unlicensed frequency bands around the world to provide communication

between devices within a WPAN. The frequency bands are specified in the

Table 1.2.

Physical layer provides,

Activation and deactivation of the radio transceiver,

Channel Frequency Selection,

Packet generation,

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Packet reception,

Data transparency and Power Management.

Table 1.3 Frequency Bands used in 802.15.4

PHY (MHz)

Frequency Band (MHz)

Spreading Parameters Data parameters

Chip rate (Kchip/s)

ModulationBit

Rate (Kb/s)

Symbol rate (Ksymbol/s)

Symbols

868/915868-868.6 300 BPSK 20 20 Binary

902-928 600 BPSK 40 40 Binary

2450 2400-2483.5 2000 O-QPSK 250 62.5 16-ary Orthogonal

Figure 1.6 Operating Frequency Bands

The standard offers two PHY options based on the frequency band.

Both are based on Direct Sequence Spread Spectrum (DSSS). The data rate is

250 Kbps at 2.4 GHz, 40 Kbps at 915 MHz and 20 Kbps at 868 MHz (Table

1.3).The higher data rate at 2.4 GHz is attributed to a higher-order modulation

scheme. Lower frequency provides longer range due to lower propagation

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losses. Low rate can be translated into better sensitivity and larger coverage

area. Higher rate means higher throughput, lower latency or lower duty cycle.

There is a single channel between 868 and 868.6 MHz, 10 channels between

902.0 and 928.0 MHz and 16 channels between 2.4 and 2.4835 GHz as shown

in Figure 1.6. Several channels in different frequency bands enable the ability

to relocate within spectrum. The physical layer of the IEEE 802.15.4 is in

charge of the following tasks:

Activation and deactivation of the radio transceiver :

The radio transceiver may operate in one of three states:

transmitting, receiving or sleeping. Upon the request of the MAC sub-layer,

the radio is turned ON or OFF. The turnaround time from transmitting to

receiving and vice versa should be no more than 12 symbol periods according

to the standard (each symbol corresponds to 4 bits).

Energy Detection (ED) within the current channel:

It is an estimation of the received signal power within the

bandwidth of an IEEE 802.15.4 channel. This task does not make any signal

identification or decoding on the channel. The energy detection time should

be equal to 8 symbol periods. This measurement is typically used by the

network layer as a part of Channel Selection algorithm or for the purpose of

Clear Channel Assessment to determine if the channel is busy or idle.

Link Quality Indication (LQI) :

The LQI measurement characterizes the Strength/Quality of a

received packet. It measures the quality of a received signal on a link. This

measurement may be implemented using receiver ED, a signal to noise

estimation or a combination of both techniques. The LQI result may be used

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by the higher layers (Network and Application layers), but this procedure is

not specified in the standard.

Clear Channel Assessment (CCA) :

This operation is responsible for reporting the medium activity

state: busy or idle. The CCA is performed in three operational modes:

Energy Detection mode: the CCA reports a busy medium if

the detected energy is above the ED threshold.

Carrier Sense mode: the CCA reports a busy medium only is it

detects a signal with the modulation and the spreading

characteristics of IEEE 802.15.4 and which may be higher or

lower than the ED threshold.

Carrier Sense with Energy Detection mode: this is a

combination of the above mentioned techniques. The CCA

reports that the medium is busy only if it detects a signal with

the modulation and the spreading characteristics of IEEE

802.15.4 and with energy above the ED threshold.

Channel Frequency Selection: The IEEE 802.15.4 defines 27

different wireless channels. A network can support only part

of the channel set. Hence, the physical layer should be able to

tune its transceiver into a specific channel request by a higher

layer.

There are already commercially available sensor motes that are

compliant with the IEEE 802.15.4. The MICAz mote from Crossbow Tech.

provides a partial implementation of IEEE 802.15.4, operating at 2.4 GHz and

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250 Kbps. This mote uses 5 MHz for channel spacing conforming to the

standard.

Medium Access Control Layer

This layer extracted from the IEEE 802.15.4 standard provides

services to the network layer above, which is part of the ZigBee stack level.

The MAC layer is responsible for the addressing of data to determine the

frames source and destination and also provides multiple access control such

as CSMA/CA allowing for reliable transfer of data. It provides two modes of

operation, namely Beacon enabled and non-beacon enabled (Figure 1.7).

The features of MAC sub layers are beacon management, channel

access, GTS management, frame validation, acknowledged frame delivery,

association and disassociation.

Figure 1.7 MAC Protocol - Two Modes of Operation

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CSMA/CA Mechanism

The IEEE 802.15.4 defines two versions of the CSMA/CA

mechanism:

The slotted CSMA/CA version – used in the beacon-enabled

mode.

The unslotted CSMA/CA version – used in the non-beacon-

enabled mode.

In both cases, the CSMA/CA algorithm is based on backoff

periods, where one backoff period is equal to aUnitBackoffPeriod= 20

Symbols. This is the basic time unit of the MAC protocol and the access to the

channel can only occur at the boundary of the backoff periods. In slotted

CSMA/CA the backoff period boundaries must be aligned with the

superframe slot boundaries where in unslotted CSMA/CA the backoff periods

of one device are completely independent of the backoff periods of any other

device in a PAN.

The CSMA/CA mechanism uses three variables to schedule the

access to the medium:

NB is the number of times the CSMA/CA algorithm was

required to backoff while attempting the access to the current

channel. This value is initialized to zero before each new

transmission attempt.

CW is the contention windows length, which defines the

number of backoff periods that need to be clear of channel

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activity before starting transmission. CW is only used with the

slotted CSMA/CA version. This value is initialized to 2 before

each transmission attempt and reset to 2 each time the channel

is assessed to be busy.

BE is the backoff exponent, which is related to how many

backoff period a device must wait before attempting to assess

the channel activity.

The Slotted CSMA/CA Mechanism

The slotted CSMA/CA (Koubaa et al 2006, Hui Jing Aida et al

2011) can be summarized in five steps:

Step 1 - initialization of NB, CW and BE: NB is initialized to 0

and the contention window CW is initialized to 2. Then the MAC protocol

checks if the macBattLifExtattribute is set to true. In this case, the Backoff

exponent BE is set to set to the minimum value of 2 or macMinBE attribute,

otherwise BE is set to macMinBE. MacMinBEattribute specifies the minimum

of the backoff exponent, which is set to 3 by default. Note that when

macMinBEis set to zero, collision avoidance is disabled during the first

iteration of the algorithm, as it could be understood from step 2 in Figure 1.8.

After the initialization, the algorithm locates the boundary of the next backoff

period.

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The Slotted CSMA/CA Mechanism

Figure 1.8 Slotted CSMA/CA Mechanism

NB=0, CW=0

BE =macMinBE

Locate Backoff Period Boundary

BE=lesser of (2, macMinBE)

Delay for random (2BE-1) unit

backoff period

Perform CCA on backoff period

boundary

Channel idle?

CW=2, NB=NB+1, BE=min(BE+1,

amax BE) CW=CW-1

CW=0? NB>macMaxCSMABackoff

Battery lifeext?ens idle?

Slotted CSMA

Failure Success

Y

N

(1)

(2)

(3)

Y

N

Y Y

N

N

(5) (4)

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Step 2- random waiting delay for collision avoidance: the algorithm

attempts to avoid collision by waiting during a given delay randomly

generated in the range of [0, 2BE 1] backoff periods. To disable the collision

avoidance procedure at the first iteration, BE must be set to 0 and thus the

waiting delay is null and the algorithm directly goes to step 3.

Step 3- Clear Channel Assessment (CCA): the CCA must be started

at a boundary of a backoff period just after the expiration of the waiting delay

timer and repeatedly performs CW times a clear channel assessment before

the access to the channel. If the channel is detected in a busy state, the

algorithm goes to step 4, otherwise, i.e. the channel is idle, the algorithm goes

to step 5.

Step 4 - busy channel: if the channel is assessed to be busy, CW

value is reset to 2 and the values of NB and BE are increased by one.

However, BE cannot exceed aMaxBE, which is a constant defined in the

standard, and with a default value equal to 5. If the number of retries exceeds

macMaxCSMABackoffs, whose the default value is 5, the algorithm

terminates with a channel access failure status, otherwise, i.e. the number of

retries is below or equal to macMaxCSMABackoffs, the algorithm returns

to step 2.

Step 5 - idle channel: if the channel is assessed to be idle, the value

of the contention window CW is decreased by one. If the contention window

has expired (CW = 0), the MAC protocol may start successfully its

transmission, otherwise, i.e. CW 0, the algorithm returns to step 3. It is

important to note that the transmission of the current frame is started only if

the remaining number of backoff periods in the current superframe is

sufficient to handle both the frame and the subsequent acknowledgement

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transmissions. Otherwise, the transmission of the frame is deferred until the

next superframe.

The Unslotted CSMA/CA Mechanism

Figure 1.9 Unslotted CSMA/CA Mechanism

UnSlotted CSMA

NB=0, BE=macMinBE

Delay for random (2BE-1)unit backoff periods

Perform CCA

NB=NB+1, BE=min(BE+1,aMaxBE)

NB>macMaxCSMABackof

ChannelIdle?

Failure Success

(1)

(2)

(4)

(3)

(5)

Y

N

N

Y

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The unslotted CSMA/CA Mechanism(Figure 1.9) is similar to the

slotted version with some few exceptions.

Step 1- A first exception, the CW variable is not used in the

unslotted CSMA/CA. This is because the unslotted CSMA/CA has no need to

iterate the CCA procedure after detecting an idle channel. Hence, in step 3, if

the channel is assessed to be idle, the MAC protocol immediately starts the

transmission of the current frame. Second, the unslotted CSMA/CA does not

support macBattLifeExtmode and hence, BE is always initialized to the

macMinBEvalue.

Step 2 and Step 3 are exactly the same as those in the slotted

CSMA/CA version. The only difference is that the CCA starts immediately

after the expiration of the random backoff delay generated in step 2.

Step 4 is the same than that in the slotted CSMA/CA with the

exception that the algorithm does not increase the value of CW. If ever NB

exceeds the value of macMaxCSMABackoffs, the algorithm terminates in a

failure state, otherwise, it returns to step 3.

In Step 5, the MAC sub-layer starts immediately transmitting its

current frame just after a channel is assessed to be idle by the CCA procedure.

Network Layer

A feature of ZigBee such as the self-healing mechanism is acquired

through this layer. As Figure 1.5 shows, this layer provides network

management, network message broker, routing management and network

security management. This layer is defined by the ZigBee Alliance, which is

an association of companies united to work for a better ZigBee standard.

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Application Layer

Applications running on the ZigBee network are contained here.

For example, applications to monitor temperature, humidity, or any other

desirable atmospheric parameters can be placed on this layer for agricultural

use. This is the layer that makes the device useful to the user.

ZigBee Device Object (ZDO)

A special application is on every ZigBee device, and this is the

ZigBee Device Object (ZDO). This application provides key functions such

as defining the type of ZigBee device (end device, router, and coordinator) a

particular node is, initializing the network and to also participate in forming a

network.

Security Plane

The security plane spans across both the network layer and the

application layer. It is here; that security measures such as Advanced Standard

Encryption (AES) based encryption is implemented.

1.4.4 Network Topologies

ZigBee networks can contain a mixture of three potential

components (Figure 1.10). These Components are a ZigBee coordinator, a

ZigBee router, and a ZigBee end device. Different types of nodes will have

different roles within the network layer, but all various types can have the

same applications.

ZigBee Coordinator – For every ZigBee network, there can be

only one coordinator. This node is responsible for initializing the network,

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selecting the appropriate channel, and permitting other devices to connect to

its network. It is also responsible for routing traffic in a ZigBee network.

ZigBee Router – A router is able to pass on messages in a network

and is also able to have child nodes connected to it, whether it to be another

router, or an end device. Router functions are only used in a tree or mesh

topology, because in a star topology, all traffic is routed through the center

node, which is the coordinator.

ZigBee End Device– The power saving features of a ZigBee

network can be mainly credited to the end devices. Because these nodes are

not used for routing traffic, they can be sleeping for the majority of the time,

expanding battery life of such devices. In the following sections, we go into

detail about the three different types of topology possible for a ZigBee

network. The legend to all topology figures are shown below and each type of

device is given a color code for easy viewing.

Figure 1.10 ZigBee Device Type

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Star Topology

In the star topology (Figure 1.11), the communication paradigm is

centralized, i.e. each device (Full Function Device (FFD) or Reduced

Function Device (RFD)) joining the network and willing to communicate with

other devices must send the data to the ZC, which dispatches it to the

adequate destination node. The star topology is not adequate for most WSN

due to the lack of scalability. This lack of scalability does not result from the

allowable number of nodes (maximum addressing space of 65535) but from

the limitation in terms of covered region, since all nodes in the cluster must be

within the radio coverage of the ZC. Star network can operate both in beacon-

enabled and non-beacon-enabled modes). This type of topology is attractive

because of its simplicity, but at the same time presents some key

disadvantages. The IEEE 802.15.4 standard recommends the star topology for

applications such as home automation, personal computer peripherals, toys

and games.

Figure 1.11 Star Topology

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Tree Topology

In a Tree network (Figure 1.12), a coordinator initializes the

network, and is the top (root) of the tree. The coordinator can now have either

routers or end devices connected to it. For every router connected, there is a

possibility for connection of more child nodes to each router. Child nodes

cannot connect to end devices because it does not have the ability to relay

messages.

Figure 1.12 Tree Topology

This topology allows different levels of nodes, with the coordinator

being at the highest level. In order the messages to be passed to other nodes in

the same network, the source node must pass the messages to its parent,

which is the node higher up by one level of the source node and the message

is continually relayed higher up in the tree until it is passed back down to the

destination node. Because the number of potential paths a message can take is

only one, this type of topology is not the most reliable topology.

Mesh Topology

In the mesh topology (Figure 1.13) the communication paradigm is

decentralized; each node can directly communicate with any other node

within its radio range. The mesh topology enables enhanced networking

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flexibility, but it induces an additional complexity for providing end-to-end

connectivity between all nodes in the network. Basically, the mesh topology

operates in an ad-hoc fashion and allows multi-hop routing from any node to

any other node. The mesh topology may be more energy efficient than the star

topology since communications do not rely on one particular node, but does

not allow efficient duty-cycle management due to the lack of synchronization

(only operates in non-beacon enabled mode), thus leading to limited network

lifetime. A mesh topology is the most flexible topology of the three.

Flexibility is present because a message can take multiple paths from source

to destination. If a particular router fails, then ZigBee’s self-healing

mechanism (aka route discovery) will allow the network to search for an

alternate path for the message to take.

Figure 1.13 Mesh Topology

1.5 OPNET Modeler 14.5

OPNET referred as Optimizing Network Engineering Tool,

The OPNET Modeler 14.5 environment includes tools for all

phases of a study, including model design, simulation, data

collection, and data analysis,

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OPNET Modeler 14.5 (OPNET Technologies, Inc., 2009)

provides a comprehensive development environment

supporting the modeling of communication networks and

distributed systems,

Both behavior and performance of a model can be analyzed by

performing discrete event simulations,

A Graphical User Interface (GUI) supports the configuration

of the scenarios and the development of network models.

Figure 1.14 OPNET Hierarchy

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OPNET Hierarchy is shown in Figure 1.14. Three hierarchical

levels for configuration are differentiated:

i) The network level creating the topology of the network under

investigation.

ii) The node level defining the behavior of the node and

controlling the flow of data between different functional

elements inside the node.

iii) The process level, describing the underlying protocols,

represented by Finite State Machines (FSMs) and is created

with states and transitions between states. The source code is

based on C/C++.

1.5.1 Network Domain

Network domain (Figure 1.15) specifies the overall scope of the

system to be simulated,

• It is a high level description of the objects contained in the

system.

• Network model specifies the objects in the system as well as

their physical locations, interconnections and configurations.

Network models consist of nodes, links and subnet.

Nodes represent network devices and groups of devices,

• Servers, workstations, routers, etc.,

• LAN nodes, IP clouds, etc.,

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Figure 1.15 Network Domain

Links represent point-to-point and bus links,

Icons assist the user in quickly locating the correct nodes and

links,

Vendor models are distinguished by a specific color and logo

for each company.

Figure 1.16 Network Devices

1.5.2 Node Domain

The Node domain defines the behavior of each network object.

Behavior is defined using different modules, each of which models some

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internal aspect of node behavior such as data creation, data storage, etc.

Modules are connected through packet streams or statistic wires. Node model

editor is shown in Figure 1.17.

Basic building blocks (modules) include processors, queues, and

transceivers,

• Processors are fully programmable via their process model,

• Queues also buffer and manage data packets,

• Transceivers are node interfaces.

Interfaces between modules,

• Packet streams,

• Statistic wires.

Figure 1.17 Node Domain

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1.5.3 Process Domain

Process model specifies object in node domain (Figure 1.18). The

Process Editor creates process models, which control the underlying

functionality of the node models created in the Node Editor. Process models

are represented by FSMs and are created with icons that represent states and

lines that represent transitions between states. Operations performed in each

state or for a transition are described in embedded C or C++ code blocks.

Process model components:

• State transition diagrams,

• Blocks of C code,

• OPNET Kernel Procedures (KPs),

• State variables,

• Temporary variables.

A process is an instance of a process model,

Processes can dynamically create child processes,

Processes can respond to interrupts.

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Figure 1.18 Process Domain

Recently, several analytical and simulation models of the IEEE

802.15.4 protocol have been proposed. Nevertheless, currently available

simulation models for this protocol are both inaccurate and incomplete, and in

particular they do not support the GTS mechanism, which is required for

time-sensitive WSN applications. OPNET Modeler, NS-2 and OMNeT++ are

widely used and popular network simulators, which include a simulation

model of the IEEE 802.15.4 protocol. The 802.15.4/ZigBee simulation model

in OPNET model library supports only non beacon-enabled mode, therefore,

the cluster-tree topology and GTS mechanism cannot be simulated. In

addition, the source codes of the network and application layers are not

available. The National Institute of Standards and Technology (NIST) has

developed own OPNET simulation model for the IEEE 802.15.4 protocol.

However, while that model implements the slotted and the unslotted

CSMA/CA MAC protocols it does not support the GTS mechanism as well. It

also uses its own radio channel model rather than the accurate OPNET

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wireless library. The Network Simulator 2 (NS-2) is an object-oriented

discrete event simulator including a simulation model of the IEEE 802.15.4

protocol. The accuracy of its simulation results are questionable since the

MAC protocols, packet formats and energy models are very different from

those used in real WSNs . This basically results from the facts that NS-2 was

originally developed for IP-based networks and further extended for wireless

networks. Moreover, the GTS mechanism was not implemented in the NS-2

model. OMNeT++ (Objective Modular Network Test bed in C++) is another

discrete event network simulator supporting unslotted IEEE 802.15.4

CSMA/CA MAC protocol only. Finally, note that while NS-2 and OMNeT++

are open-source projects, the OPNET Modeler is commercial project

providing a free of charge university program for academic research projects.

1.6 NEED FOR PERFORMANCE ANALYSIS

Performance of the networks mainly depends on the various

parameters like Throughput, End-to-End delay, Signal to Noise Ratio, Bit

Error Rate and the Utilization of the channel. Throughput is the data quantity

transmitted correctly starting from the source to the destination within a

specified time (seconds). The importance of analyzing this QoS parameter is

because the increased number of users of the wireless medium leads to

increased possibility of interference. Throughput usually depends on many

aspects of networks such as power control, scheduling strategies, routing

schemes, packet collision, acknowledgment, obstructions between nodes and

network topology.

End-to-End delay is a measurement of the network delay on a

packet and is measured by the time interval between when a message is

queued for transmission at the physical layer until the last bit is received at the

receiving node. As the number of nodes in the WPANs increases the delay

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obviously will increase. Minimum end-to-end delay is required for

applications like the smoke detector, accident detector and carbon monoxide

detector.

1.7 OVERVIEW OF IEEE 802.15.4

The IEEE 802.15.4 protocol has recently been adopted as a

communication standard for low data rate, low power consumption and low

cost WSNs.

The IEEE 802.15.4 MAC protocol supports two operational modes

that may be selected by a central node called PAN coordinator:

The non-beacon enabled mode, (Chiara Buratti et al 2009) in

which the MAC is ruled by non-slotted CSMA/CA.

The beacon enabled mode, (Chiara Buratti et al 2010) in

which beacons are periodically sent by the PAN coordinator to

identify it’s PAN and synchronize nodes that are associated

with it.

In beacon-enabled mode, the Beacon Interval (BI) defines the time

between two consecutive beacons, and includes an active period and

optionally an inactive period. The active period called superframe. The

superframe structure is an optional part of a WPAN. It is the time duration

between two consecutive beacons. The structure of the superframe is

determined by the coordinator. The coordinator can also switch off the use of

a superframe by not transmitting the beacons. The superframe duration is

divided into 16 concurrent slots. The beacon is transmitted in the first slot.

The remaining part of the superframe duration can be described by the terms,

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Contention Access Period (CAP) (Ashrafuzzaman et al 2011), Contention

Free Period (CFP) and Inactive portion. The superframe is used to provide

vital statistics like synchronization, identifying the PAN and the superframe

structure, to the devices connected in a Wireless PAN. This information is

critical for the operation of the PAN in a Beacon enabled network.

Figure1.19 Superframe Structure

The lengths of the Beacon Interval and the Superframe Duration

(SD) are determined by two parameters, the Beacon Order (BO) and the

Superframe Order (SO), respectively. The Beacon Interval is defined as

follows:

BI=aBaseSuperframeDuration*2BO, for 0 BO 14 (1.1)

The Superframe Duration, which determines the length of the

active period, is defined as follows:

SD=aBaseSuperframeDuration*2SO, for0 SO BO 14 (1.2)

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where

aBaseSuperFrameDuration = aBaseSlotDuration × aNumSuperframeSlots

aBaseSlotDuration = 60symbols

aNumSuperFrameSlots = 16

aBaseSuperFrameDuration = 60 × 16symbols = 960symbols

InactivePortion = BeaconInterval SuperframeDuration

In Equation (1.1) and Equation (1.2) (Koubaa et al 2006),

aBaseSuperframeDurationdenotes the minimum length of the superframe,

corresponding to SO = 0. The IEEE 802.15.4 standard fixes this duration to

960 symbols (a symbol corresponds to 4 bits). This value corresponds to

15.36ms, assuming a 250 Kbps in the 2.4 GHz frequency band. By default,

the nodes compete for medium access using slotted CSMA/CA (Koubaa et al

2006) within the CAP during SD. In case of a busy channel, a node computes

its backoff period based on a random number of time slots.

The IEEE 802.15.4 protocol also offers the possibility of having a

CFP within the superframe (Figure 1.19). The CFP, being optional, is

activated upon request from a node to the PAN coordinator for allocating time

slots depending on the node's requirements. Upon receiving this request, the

PAN coordinator checks whether there are sufficient resources and if

possible, allocates the requested time slots. These time slots are called GTSs

(Jurcik et al 2007) and constitute the CFP. If the available resources are not

sufficient, the GTS request fails. The corresponding node then sends its data

frames during the CAP.

In a Non-Beacon mode, MAC uses un-slotted CSMA/CA

mechanism in which device could start transmission procedure at any time. It

does not provide any GTS mechanism, but it has the advantage of lower

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complexity, high routing redundancy and scalability when compared to the

beacon-enabled mode, since the former doesn’t require any synchronization.

1.8 MOTIVATION

The Literature survey shows that research has been carried out in

the area of IEEE 802.15.4 ZigBee WSN. The research work is started based

on the mobility concept in WSN and Load Density is analyzed for the

Hexagonal Configuration with ACK Enabled and Disabled Scenario for

different network size which results in better reliability. Beacon Enabled

Mode doesn’t have mobility model. Hence extensive work has not been

carried out on the analysis of mobility model in Beacon Enabled Mode.

Acknowledgment plays a critical role in analyzing the network

parameter. Performance of the non-beacon enabled network is simulated and

analyzed for ACK Enabled Scenario and ACK Disabled Scenario. In this

research Beacon Enabled mode is analyzed to support in OPNET Modeler

and then performance of the beacon enabled network is simulated and

analyzed for ACK Enabled Scenario and ACK Disabled Scenario. Finally

network performance is compared with Beacon Enabled and Non Beacon

Enabled Mode.

1.9 OBJECTIVE

The objective of the thesis is to introduce the beacon enabled mode

in OPNET Modeler and analyze the performance of the network by increasing

the nodes in beacon enabled and non-beacon enabled mode with

acknowledgement enabled and disabled. The following are the set objectives

to realize the goal.

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To compare 5 different number of nodes i.e., 10, 20, 30, 40

and 50 and analyzed their load density in hexagonal

configuration by enabling and disabling acknowledgment.

To study the performance of the ZigBee based WSNs in

beacon enabled and non-beacon enabled mode.

To compare the performance by increasing the nodes from 10

to 50 in both the modes.

To analyze which mode is suitable for the reliable

communication.

To compare the performance of the beacon enabled and non-

beacon enabled mode by enabling and disabling the

acknowledgment.

To study the parameters of Throughput, End-to-End delay,

Load and Utilization.

1.10 THESIS ORGANIZATION

Chapter 1 presents an introduction to the different wireless

Technologies and overview of IEEE 802.15.4 and WSNs. The main objective

of the research work is presented.

Chapter 2 presents the literature survey on wireless networks, Non

Beacon Enabled Mode of IEEE 802.15.4, Beacon Enabled Mode of IEEE

802.15.4, Mobility and OPNET Modeler.

Chapter 3 deals with the load density analysis in hexagonal

configuration by enabling and disabling the acknowledgment in mobile

coordinator.

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Chapter 4 presents the reliability of the IEEE 802.15.4 network i.e.,

Load is analyzed in Beacon enabled and Non Beacon enabled mode by

enabling and disabling acknowledgment.

Chapter 5 deals with the performance analysis of the IEEE 802.15.4

network parameters like Throughput, End-to-End delay and Utilization of

both beacon enabled and non-beacon enabled modes by enabling and

disabling acknowledgment and also compared the Load, Throughput, End-to-

End delay and Utilization of IEEE 802.15.4 network in all scenarios.

Chapter 6 concludes the research work with scope for future work.