ZigBee: Simulation and Investigation of Star and Mesh Topology by using different Transmission Bands

The AIUB Journal of Science and Engineering (AJSE), Vol. 14, No. 1, August 2015 ZigBee: Simulation and Investigation of Star and Mesh Topology by using different Transmission Bands Md. Mamunur Rashid and Rethwan Faiz Abstract ZigBee is based on an IEEE 802.15.4 standard and it is used to create Personal Area Networks (PAN) built for small, low-power digital radios. ZigBee operates in three different industrial, scientific and medical (ISM, unlicensed) radio bands: 868 MHz (EU and Japan), 915 MHz (ISM, US) and 2.4 GHz (Worldwide). The supported data rates of these three bands are 20 Kbps, 40 Kbps and 250 Kbps respectively. In this paper, three types of bands are experimented in star and mesh topology and performance metrics like throughput, delay etc. are measured using OPNET simulator. Index Terms WLAN, WPAN, WSN, PAN, OSI, ZigBee. I. INTRODUCTION Wireless personal area networks (WPANs) are used to convey information over relatively short distances. Unlike wireless local area networks (WLANs), connections effected via WPANs involve little or no infrastructure. This feature allows small, powerefficient, inexpensive solutions to be implemented for a wide range of devices. ZigBee is a wireless networking standard that is widely used at remote control and sensor applications which is suitable for operation in hazardous radio environments and in remote locations [1]. ZigBee technology builds on IEEE Standard 802.15.4 which defines the physical and MAC layers. The ZigBee wireless technology has powerful features such as low data rate, low power consumption, security, and reliability with low cost. It has a great contribution to the WPAN and WSN technologies. The ZigBee standard is organized under the auspices of the ZigBee Alliance. ZigBee Alliance defines two layers of the Open Systems Interconnection (OSI) model: The Application Layer and the Network Layer [2]. Each layer performs a specific set of services. For Network management procedures (e.g., nodes joining and leaving the network), security and routing, the ZigBee Network Layer is responsible. The layered architecture of Md. Mamunur Rashid is a Lecturer of Dept. of EEE, American International University- Bangladesh (AIUB), Banani, Dhaka-1213, Bangladesh. Email: mamunur@aiub.edu Rethwan Faiz is a Lecturer of Dept. of EEE, American International University- Bangladesh (AIUB), Banani, Dhaka-1213, Bangladesh. Email: rethwan_faiz@aiub.edu Zigbee has 4 layers: physical layer, MAC layer, network layer and application layer. The Zigbee protocol supports static, dynamic, and meshes network topologies. IEEE 802.15.4 offers star, tree, cluster tree, and mesh topologies; however, ZigBee supports only star, tree, and mesh topologies. It uses an association hierarchy; a device joining the network can either be a router or an end device, and routers can accept more devices. Various researchers shows the behavior of star, mesh and tree topologies for various parameters [3] [4][5]. ZigBee operates in the industrial, scientific and medical (ISM) radio bands: 2.4 GHz in most jurisdictions worldwide; 784 MHz in China, 868 MHz (1 channel) in Europe and 915 MHz (10 channels) in the USA and Australia. Data rates vary from 20 kbit/s (868 MHz band) to 250 kbit/s (2.4 GHz band) [6]. In this paper, three types of transmission bands will be experimented in different types of topologies such as Star and Mesh topology. The rest of the paper is organized as follows. In section 2, the specifications of IEEE 802.15.4 standard are provided. In section 3, the different topologies and devices of ZigBee technology has been presented. In section 4, the communication procedures of ZigBee are discussed. A brief description of performance metrics has been presented in Section 5. Finally, in section 5 and 6, the simulation results and conclusion have been discussed respectively. II. ZIGBEE / IEEE 802.15.4 Many working groups are sustained bythe Institute of Electrical and Electronics Engineers (IEEE) to develop and maintain wireless and wired communications standards as shown in Table 1. Table 1. Some IEEE wireless communications standards [7] Category Name of the standards 802.11 Wireless LANs (WLANs) 802.15 Wireless Personal Area Networks (WPANs) 802.15.1 Bluetooth 802.15.3 Ultra-Wideband (UWB) 802.15.4 Low-Data-Rate WPANs 115

Rashid and Faiz: ZigBee: Simulation and Investigation of Star and Mesh Topology The largest standard for low-data-rate WPANs is the 802.15.4 category which includes many subcategories. It was developed for low-data-rate monitor and control applications and extended-life low-power-consumption uses. The media access control (MAC) and the physical layer (PHY) of the Open System Interconnection (OSI) model of network operation is defined by the 802.15.4 standard. The frequency, power, modulation, and other wireless conditions of the link are defined by the PHY. MAC defines the format of the data handling. The remaining layers define the other procedures for managing the data and relate protocol enhancements including the final application. OSI communications model is utilized by the majority of the networking systems. Most systems use at least the first four layers, but many do not use all seven layers. More specifically, the details of 802.15.4 are shown in figure 1: Fig 1: The layer 1 and layer 2 details of 802.15.4 [7] As defined by additional standards The 802.15.4 standard uses the first two layers plus the logical link control (LLC) and service specific convergence sublayer (SSCS) additions to communicate with all upper layers [7].The options for frequency assignments are given below: Table 2. Options for Frequency Assignments [7] Geographic al Regions Frequency Assignments Number of Channels Channel Bandwidth Europe Americas Worldwide 868 MHz 915 MHz 2400 MHz 1 10 16 600 khz 2 MHz 5 MHz Data Rate 20 kbits/s 40 kbits/s 250 kbits/s Modulation BPSK BPSK Q-QPSK ZigBee, which is a standard of the ZigBee Alliance, is the most widely deployed enhancement to the 802.15.4 standard [2]. For advanced applications, the organization maintains, supports, and develops more sophisticated protocols. It uses layers 3 and 4 to define additional communications features (Fig. 2). These improvements include authentication with valid nodes, encryption for security, and a data routing and forwarding capability that enables mesh networking. ZigBee is mostly popular for wireless sensor networks that utilize the mesh topology. A. Devices Fig 2: ZigBee Layer 3 and 4 [7] III. ZIGEBEE DEVICES & TOPOLOGIES The ZigBee standard has the capacity to address up to 65535 nodes in a single network [6]. However, there are only three general types of node: 1) Coordinator It is responsible to start the network, choosing appropriate channel and link up other devices in the network. The end devices and the routers can be directly connected with this device. In time critical application, it allocates time slots to other devices. 2) Router It provides the interfacing between coordinator and end devices. It keeps the record of the routes and relays the message among the devices. 3) End Devices End Devices are always located at the extremities of a network. The main tasks of an End Device at the network level are sending and receiving messages. Note that End Devices cannot relay messages and cannot allow other nodes to connect to the network through them. B. Topologies A ZigBee network can adopt one of the three topologies: Star, Tree and Mesh. These are illustrated and briefly described below. 1) Star topology The Coordinator is responsible for the operations of the network. It is the disadvantage of this topology. All the packets must travel through the coordinator [4]. Another disadvantage is that there is no alternative path from source to destination. The configuration of this topology is simple and all the packets travel through at most two hops to reach their destination (Fig. 3). 116

The AIUB Journal of Science and Engineering (AJSE), Vol. 14, No. 1, August 2015 IV. COMMUNICATION PROCEDURES A. Data Transmission There are three distinctive mode of data transmission. They are: Fig 3: Star Topology 2) Tree topology This type of network consists of a coordinator which is central node, routers and End devices. The end devices can be connected with a router or coordinator. In this case, the end devices can be called children. The routers or coordinators can be parents. Each end devices can communicate with its parents. The end devices cannot be parents [4]. The main disadvantage of this topology is, if the parent nodes become disabled, the children node within the parents will not be able to communicate with others even if there are two geographically close nodes (Fig. 4). Fig 4: Tree Topology 3) Mesh topology Mesh topology usually consists of a Coordinator, several routers, and end devices. In a Mesh topology, packets pass through multiple hops to reach their destination. By adding more devices in the network, the coverage area can be increased, and thus the dead zones can be eliminated. Addition or removal of a device is simple. Any source can communicate with any destination of the same network [4]. Therefore, if a path fails, the node will find an alternate path to the destination. Mesh topology requires greater overhead and uses complex routing protocol than the other topologies (Fig. 5). 1. Transmission from a device to the coordinator 2. Transmission from the coordinator to the device 3. Transmission between any two devices In a star topology only the first two transmission procedures are possible and the transmission between any two devices is not supported in this system. In a peer to peer network all the three types of transmissions are possible. The transmissions can be executed again in either of two ways, depending on if the beacon transmissions are permitted or not. [7] 1) Transmission from a Coordinator to a Device a. The coordinator comprises the data to be transmitted to the device. b. The pending address fields of its beacon specify the condition. c. Devices then tracks the beacons, hence it decodes the pending address fields. d. If a device locates its address listed amid the pending address fields, it comprehends that it has data to be received from the coordinator. e. Hence a Data-Request Command is issued to the coordinator. f. An acknowledgement is send from the coordinator as a feedback. g. Data will be sent to the device if there are any accumulated data. h. If acknowledgements are not optional, the device would respond with an acknowledgement. Fig 6: Transmission from a device to the coordinator [7] 2) Transmission from a device to a coordinator Fig 5: Mesh Topology a. The device first takes notice to the beacon. b. The device synchronizes first to the superframe structure after finding the beacon. This process lets it distinguish the start and end time of the contention access period. 117

Rashid and Faiz: ZigBee: Simulation and Investigation of Star and Mesh Topology c. A competition between the device peers will take place for a share of the channel. d. On its turn, it will transmit the data to the coordinator. e. Depending on whether it is optional or not the coordinator will send a feedback in the form of an acknowledgement. Similarly the throughput for the network can be defined as: Fig 7: Transmission from the coordinator to the device [7] B. MAC Delay The delay is the time taken for a data packet to reach the destination node. The delay for a packet is the time taken for it to reach the destination. And the average delay is calculated by taking the average of delays for every data packet transmitted. The parameter comes into play only when the data transmission has been successful [9]. 3) Transmission between two devices There is no preordained approach through which there can be a direct communication between two devices in the network. Though, the appropriate techniques of transmission can be done by mutual synchronization techniques, or direct transmission using unslotted CSMA-CA [8]. Both the techniques have got their own flaws. The synchronization technique is harder to implement even though it appears uncomplicated. Correspondingly, direct transmission might degrade the throughput performance of the PAN. This can be an interesting topic for further research. V. PERFORMANCE METRICS There are several metrics to define the grade of the performance of a technology against the elements of wireless networking. These metrics are chosen to have an idea of behavior and the reliability of the ZigBee networks. A detailed explanation of these metrics follows: A. Network Throughput It is a measure of the amount of data transmitted from the source to the destination in a unit period of time (second). The throughput is measured in total bits received per second. Also to be noted is that this metric only measures the total data throughput, ignoring all other overhead, over the network. The throughput of a node is measured by first counting the total number of data packets successfully received at the node, and computing the number of bits received, which is finally divided by the total simulation runtime. The throughput of the network is finally defined as the average of the throughput of all nodes involved in data transmission. Therefore, throughput can be calculated as [9]: C. Load It represents the total load (in bits/sec) submitted to 802.15.4 MAC by all higher layers in all WPAN nodes of the network [10]. VI. SIMULATION & RESULTS Simulation and modeling are very much important approaches in order to develop and evaluate a system in terms of time and costs. The simulation shows the expected behavior of the network based on its simulation model under different conditions. Hence, the purpose of this simulation model is to determine the exact model and predict the behavior of the real environment. In order to investigate the performances of the ZigBee networks by using different Bands, the OPNET Modeler simulation tool was used [10]. The OPNET Modeler supports ZigBee technology. Two types (Mesh and Star) of topologies of ZigBee are considered in the simulation. As there are three working Bands for the ZigBee technology, total six scenarios (3 for Mesh and 3 for Star) are created to build the project in OPNET. The total area of the network coverage is 500 * 500 Meters. In the Mesh topology, One ZigBee Coordinator, six ZigBee Routers and ten ZigBee End Devices are used. In the Star topology, One ZigBee Coordinator and ten ZigBee End Devices are used only. The parameters used for ZigBee Coordinator are shown in table 3: 118

Load (bits/sec) Delay (sec) ZigBee_Mesh_868MHz: MAC.Delay (sec) ZigBee_Mesh_915MHz:MAC.Delay (sec) ZigBee_Mesh_2450MHz: MAC.Delay (sec) ZigBee_Mesh_868MHz: MAC.Load (bits/sec) ZigBee_Mesh_915MHz: MAC.Load (bits/sec) ZigBee_Mesh_2450MHz: MAC.Load (bits/sec) Delay (sec) ZigBee_Star_868MHz: MAC.Delay (sec) ZigBee_Star_915MHz: MAC.Delay (sec) ZigBee_Star_2450MHz: MAC.Delay (sec) The AIUB Journal of Science and Engineering (AJSE), Vol. 14, No. 1, August 2015 Table 3. The parameters of Coordinator Parameters Selected Values Set Transmission Bands 2450 MHz, 915 MHz & 868 MHz Minimum Backoff Exponent 3 Maximum Number of Backoffs 4 Packet Reception- Power Threshold -85 Transmit Power 0.05 Beacon Order 6 Mesh Routing Enabled Route Discovery Timeout 10 100 10 1 0.1 0.01 The parameters used for ZigBee End Device are shown in table 4: Table 4. The parameters of End Device Parameters Selected Values Set Transmission Bands 2450 MHz, 915 MHz & 868 MHz Minimum Backoff Exponent 3 Maximum Number of Backoffs 4 Packet Reception- Power Threshold -85 Transmit Power 0.05 Number of Retransmissions 5 The Simulation scenarios are designed according to the communication procedures discussed on section 4 of this article. The parameters shown in the tables above are selected in the simulation. The simulation was set for 2000 seconds. While doing simulation results analysis, we focused on three parameters namely Delay, Load and Throughput. The results are graphically presented from the obtained data from the simulation. 1E-3 Fig 9: Delay (Star topology) analysis of different bands The data (Delay Comparison) obtained from fig. 8 and fig. 9 are represented in the table 5. In case of Mesh topology, the messages can be transmitted usually after more than single hop, and in the case of Star topology, the messages are transmitted after a single hop. So it is usual to have lower delay in Star topology. From table 5, it can be seen that the shortest delay is 0.0099 in case of Star topology using 2450MHz. It can be said that the shorter the delay, the better the network. The results for Mesh using 915MHz, Mesh and Star using 868 MHz are unacceptable. Bands Table 5. Delay Comparison Mesh Delay (Sec) Star 2450 MHz 0.011 0.0099 915 MHz 200 0.08 868 MHz 400 150 100 10 1 0.1 0.01 1E-3 Fig 8: Delay (Mesh topology) analysis of different bands 50000 40000 30000 20000 10000 0 Fig 10: Load (Mesh topology) analysis of different bands 119

Throughput (bits/sec) ZigBee_Mesh_868MHz: MAC.Throughput (bits/sec) ZigBee_Mesh_915MHz: MAC.Throughput (bits/sec) ZigBee_Mesh_2450MHz: MAC.Throughput (bits/sec) Load (bits/sec) ZigBee_Star_868MHz: MAC.Load (bits/sec) ZigBee_Star_915MHz: MAC.Load (bits/sec) ZigBee_Star_2450MHz: MAC.Load (bits/sec) Throughput (bits/sec) ZigBee_Star_868MHz: MAC.Throughput (bits/sec) ZigBee_Star_915MHz: MAC.Throughput (bits/sec) ZigBee_Star_2450MHz: MAC.Throughput (bits/sec) Rashid and Faiz: ZigBee: Simulation and Investigation of Star and Mesh Topology 25000 25000 20000 20000 15000 15000 10000 10000 5000 5000 0 Fig 11: Load (Star topology) analysis of different bands The data (Load Comparison) obtained from fig. 10 and fig. 11 are represented in the table 6. It represents the total load (in bits/sec) submitted to 802.15.4 MAC by all higher layers in all WPAN nodes of the network. It can be seen that the Loads are almost same in case of Star topology using different Bands. The Maximum Load is found in Mesh topology using 915 MHz Band. Bands Table 6. Load Comparison Load (Kbits/Sec) Mesh 2450 MHz 38 24 915 MHz 44 24 868 MHz 34 22 50000 40000 30000 20000 10000 0 Star Fig 12: Throughput (Mesh topology) analysis of different bands Fig 13: Throughput (Star topology) analysis of different bands The data (Throughput Comparison) obtained from fig. 12 and fig. 13 are represented in the table 7. Throughput means the number of successful transmissions. The throughput will vary widely depending upon channel size. It can be seen from the fig. 12 and fig. 13 that the throughputs obtained are varied with the change of frequencies. The maximum throughput are obtained from Mesh topology using 2450 MHz and 915 MHz. The left results are not satisfactory as their throughputs are quite low to be accepted. Table 7. Throughput Comparison Bands Throughput (Kbits/Sec) Mesh 2450 MHz 38 22 915 MHz 43 22 868 MHz 20 16 VII. CONCLUSION Star In this paper, Mesh and Star topology of ZigBee technology have been investigated using three different frequency Bands. The results shows that 2450 MHz Band outperformed the other two bands though in some cases 915 MHz also showed better performance than 2450 MHz. The load is less in case of 868 MHz among the other bands, but for the higher delay this band is not acceptable in many cases. The tree topology is not investigated in this paper. The power consumption of the End Devices is not also considered in this paper which is left for further research. REFERENCES [1] Niteen Deshmukh Pravin and N.Matte, An application or ZigBee for Machine Health Monitoring, International Journal of Innovative Research in Science, Engineering and Technology, vol. 3, Issue 3, (2014). 120

The AIUB Journal of Science and Engineering (AJSE), Vol. 14, No. 1, August 2015 [2] Zigbee Alliance, Zigbee Overview, (2003). [3] B. Mihajlov and M. Bogdanoski, Overview and analysis of the performance of Zigbee based wireless sensor networks,international Journal of Computer Application, vol. 29, no. 12, (2011). [4] M. Hammoshi and A. Sayed, An analysis for a cluster tree Zigbee topology, Journal of Theoretical and applied Information Technology, vol. 64, no. 3, (2014). [5] K. Vats, P. Jain, L. Jaiswal and S. Singh, Zigbee based WSN topology Simulation Investigation and performance analysis using OPNET, International Journal of Advanced Scientific Research and Technology, vol. 3, no. 2, (2012). [6] J. Shyan Lee, Y. W. Su and C. C. Shen, A Comparative Study of Wireless Protocols:Bluetooth, UWB, Zigbee, and Wi-Fi, The 33rd Annual Conference of IEEE Industrial Electronics Society (IECON),(2007); Taipei, Taiwan. [7] Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs), IEEE Standard 802.15.4, 2003. [8] Amritpal Kaur, Jaswinder Kaur and Gurjeevan Singh, Modeling and Simulation of CSMA/CA Slotted and Unslotted Mode in Zigbee Routing Schemes, International Journal of Computer Applications, vol. 103, no.7, (2014). [9] Pan Li, Yuguang Fang and Jie Li, Throughput, Delay, and MobilityinWireless Ad Hoc Networks, in INFOCOM, Proc. 2010. [10] Opnet-Riverbed, Available: http://www.riverbed.com/products/performancemanagement-control/opnet.html He joined AIUB as a Lecturer in 2012 at the Department of Electrical and Electronic Engineering. He was awarded Summa Cum Laude for his outstanding result in his bachelor studies. His research interest includes Wireless Sensor Network and Nanoelectronics. Md. Mamunur Rashid was born in Bogra, Bangladesh, in 1987. He received his B.Sc in EEE and M.Engg. in Telecommunications from American International University- Bangladesh (AIUB), in 2010 and in 2012 respectively. He joined AIUB as a Lecturer in 2013 at the Department of Electrical and Electronic Engineering. Jointly, he has been working as an Assessor in AIUB- Institutional Quality Assurance Cell (AIUB-IQAC) at 2015. His research interests include Wireless Sensor Network, Software Defined Network, and Internet of Things. Rethwan Faiz was born in Dhaka, Bangladesh, in 1987. He received his B.Sc in Electrical and Electronic Engineering and MBA from American International University- Bangladesh (AIUB), in 2011 and 2014 respectively. 121

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