CHAPTER 1 INTRODUCTION

1 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

2 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 timecritical applications, timeliness guarantees may be achieved with this beaconenabled 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

3 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

4 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, 40 +ma TX, 30 ma TX, 200mw Standby Standby 0.2mA Standby 20Ma 356µA 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

5 Table 1.1 (Continued) Freq. Range 2.4 GHz 2.4 GHz 868/915 MHz 2.4 GHz 3.1-10.6 GHz Stronghold High Data Interoperability, Long High data rate Rate Cable Battery Life, for short Replacement Low Cost range Battery Life (Days) 0-5.5 1-7 days 100-1000 + - Applications Internet Wireless USB, Remote Sync, browsing, Handset, Control, Transmission PC Headset battery of video/audio networking, operated Data File Transfer products, sensors 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.

6 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).

7 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.

8 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

9 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,

10 Packet reception, Data transparency and Power Management. Table 1.3 Frequency Bands used in 802.15.4 Spreading Parameters Data parameters PHY (MHz) Frequency Band (MHz) Chip rate (Kchip/s) Modulation Bit Rate (Kb/s) Symbol rate (Ksymbol/s) Symbols 868/915 868-868.6 300 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

11 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

12 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

13 250 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

14 CSMA/CA Mechanism mechanism: The IEEE 802.15.4 defines two versions of the CSMA/CA The slotted CSMA/CA version used in the beacon-enabled mode. The unslotted CSMA/CA version used in the non-beaconenabled 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

15 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.

16 The Slotted CSMA/CA Mechanism Slotted CSMA Delay for random (2 BE -1) unit backoff period (2) NB=0, CW=0 (1) Battery lifeext?e ns idle? Y BE=lesser of (2, macminbe) Perform CCA on backoff period boundary (3) N Channel idle? Y BE =macminbe (4) N (5) CW=2, NB=NB+1, BE=min(BE+1, amax BE) CW=CW-1 Locate Backoff Period Boundary N NB>mac MaxCS MABack off CW=0? N Y Y Failure Success Figure 1.8 Slotted CSMA/CA Mechanism

17 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, 2 BE 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

18 transmissions. Otherwise, the transmission of the frame is deferred until the next superframe. The Unslotted CSMA/CA Mechanism UnSlotted CSMA NB=0, BE=macMinBE (1) Delay for random (2 BE -1) unit backoff periods (2) Perform CCA (3) Channel Idle? N Y (5) (4) NB=NB+1, BE=min(BE+1,aMaxBE) N NB>macMax CSMABackof Y Failure Success Figure 1.9 Unslotted CSMA/CA Mechanism

19 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.

20 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,

21 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

22 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 beaconenabled 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

23 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

24 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,

25 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

26 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) iii) The node level defining the behavior of the node and controlling the flow of data between different functional elements inside the node. 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.,

27 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. 1.5.2 Node Domain Figure 1.16 Network Devices The Node domain defines the behavior of each network object. Behavior is defined using different modules, each of which models some

28 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. transceivers, Basic building blocks (modules) include processors, queues, and 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

29 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.

30 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

31 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

32 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,

33 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*2 BO, for 0 BO 14 (1.1) The Superframe Duration, which determines the length of the active period, is defined as follows: SD=aBaseSuperframeDuration*2 SO, for0 SO BO 14 (1.2)

34 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

35 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.

36 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 nonbeacon 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.

37 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.