Published on March 26, 2014
Impact of Network Scalability on Zigbee Physical Attributes A. Yadav1 , A. Kalra2 , A. Swami3 and P. Kaur4 Department of EECE, ITM University, Gurgaon, India Email:1 email@example.com, 2 firstname.lastname@example.org,3 email@example.com,4 firstname.lastname@example.org Abstract— ZigBee has been developed to support lower data rates and low power consuming applications. This paper targets to analyze various parameters of ZigBee physical (PHY). Performance of ZigBee PHY is evaluated on the basis of energy consumption in transmitting and receiving mode and throughput. Effect of variation in network size is studied on these performance attributes. Some modulation schemes are also compared and the best modulation scheme is suggested with tradeoffs between different performance metrics. Index Terms— ZigBee, Qualnet, throughput I. INTRODUCTION IEEE 802.15.4 (ZigBee) is recent standard that brought about lower data rate operations at low cost. It consumes less power, has lower duty cycle with high battery life & with low hardware complexity. It ranges from short to medium distance of 10 to 100 meters for personal area network (PAN). The main aim of designing IEEE 802.15.4 open standard is to support the wireless connectivity for various applications like for industry, home automation and automotive monitoring and control. This standard describes lower hardware complexity as modulation methods used are binary phase shift keying (BPSK), quadrature phase shift keying (QPSK) and orthogonal quadrature phase shift keying (O-QPSK). It uses a simple direct sequence spread spectrum (DSSS) which is robust enough to work in low power approximately at (1mW) transmitter output in unlicensed band. Lower power consumption requires efficient design that uses minimal hardware and clocking in efficient technology such as complementary metal oxide semiconductor (CMOS). In this paper we are analysing ZigBee physical layer. Evaluation of ZigBee physical layer is done on the basis of energy consumption in transmitting and receiving mode. We have used ZigBee 802.15.4 radio type and clear channel assessment (CCA) modes using constant bit rate (CBR) application, for ZigBee network designed in Qualnet simulator. This paper structured as follows. Section II describes basic structure of ZigBee which briefs the different layers of ZigBee wireless technology. Section III focusses on the ZigBee physical layer and describes the services provided by physical layer and its primitives. Section IV describes the simulation parameters used in this paper and presents an analysis of the results obtained. II. ZIGBEE OVERVIEW The reference model for ZigBee is shown in Table 1 depicting various layers of ZigBee wireless technology DOI: 02.ITC.2014.5.80 © Association of Computer Electronics and Electrical Engineers, 2014 Proc. of Int. Conf. on Recent Trends in Information, Telecommunication and Computing, ITC
153 architecture . These layers facilitate the features like reliable data transfer and easy implementation of Zigbee which makes it very attractive. It also provides features like interoperability and testing. TABLE I. ZIGBEE REFERENCE MODEL TABLE II. SIMULATION PARAMETERS The physical layer of this reference model specifies the network interface component, their parameters and operation. It offers the transmission and reception on the physical radio channel. The medium access control (MAC) layer manages network association and disassociation and regulates access to the medium. Its operation is achieved through two modes; beacon and non-beacon. The network layer provides the function to support network configuration and device discovery, topology management, MAC layer management, security and routing. The topologies like star, cluster tree and mesh are supported. The application layer has application support sublayer (APS), ZigBee device object (ZDO) and manufactured defined objects. The APS sublayer maintains table for binding devices together based on their services and need. III. ZIGBEE PHYSICAL LAYER ZigBee operates in unlicensed band. The standard offers two PHY options based on the frequency band. They are based on direct sequence spread spectrum (DSSS). The data rate is 250kbps at 2.4GHz, 40kbps at 915MHz and 20kbps at 868MHz . The higher data rate of 2.4GHz is attributed to a higher-order modulation scheme. The lower data rate can be translated into better sensitivity and large coverage area. It has been studied that the BER vs SNR values are affected by various communication parameters such as data rates and number of bits/symbol . Two types of services are provided by the physical layer. Firstly it controls the radio link i.e. the transmission & reception of physical payload data unit (PPDU). Secondly, it produces energy detection within the channel, clear channel assessment (CCA) before transmitting the message and produces link quality indication (LQI) for received packets. A. Primitives IEEE 802.15.4 standard describes 14 physical & 35 MAC primitives. It supports two types of devices i.e. full functional device (FFD) & reduced functional device (RFD). FFD supports all defined primitives and RFD supports 38 primitives. 1) Physical Primitives The physical primitives provide activation and deactivation of radio transceiver, energy detection, LQI measurement, clear channel assessment, data transmission & reception. a) Data transmission Whenever data is to be transmitted, MAC layer management entity calls upon physical layer with these primitives to transmit a data frame. PD_ DATA request: The data request is generated by MAC sub layer entity and issued to physical entity to request the transmission of a MAC-layer protocol data unit (MPDU). When the PD_DATA request is received the physical layer first constructs a physical protocol data unit (PPDU) with supported physical service data unit (PSDU). PD _DATA confirm: It is generated by physical entity & issued to its MAC sub layer. It returns a status of either success or an error code indicating that the transceiver is working in the receiver mode or the transceiver is switched off. PD_DATA indication: This primitive is generated by the PHY entity and issued to its MAC sub layer entity to transfer a received PSDU. It will not be generated if the received PSDU length field is zero or greater than a maximum PHY packet size. Application layer Network and security MAC layer Physical layer (915MHz, 2.4GHz) Parameters Specifications Channel frequency 2.4 GHz – OQPSK 915 MHz – ASK, BPSK Radio type 802.15.4 ZigBee Modulation schemes ASK, BPSK, OQPSK CCA modes a. Carrier sense b. Carrier sense with energy above threshold c. Energy above threshold
154 B. Services Provided by Physical Layer The physical layer (PHY) provides the data transmission service, as well as the interfacing to the physical layer management entity, which offers access to every layer management function and maintains a database of information on related personal area network. Thus, the PHY manages the physical RF transceiver and performs channel selection, energy and signal management functions. The data service enables the transmission and reception of PHY protocol data units (PPDU) across the physical radio channel. The main features are activation and deactivation of the radio transceiver, energy detection (ED), link quality indication (LQI), channel selection, clear channel assessment (CCA) and transmitting as well as receiving packets across the physical medium The receiver energy detection is used by network layer as a part of channel selection algorithm. It calculates the received signal power within the specified range of frequencies. Then channel assessment is processed through clear channel assessment algorithm which operates in three modes in accordance to some threshold level. When the energy is above threshold CCA reports that the medium is busy. In the second mode; it only senses the carrier irrespective of the energy level. In the third mode; the carrier is sensed with energy above threshold and reports that the channel is busy. Furthermore, the link quality is indicated using receiver energy detection and signal to noise ratio estimation. LQI is the characterization of quality and strength of the received packet. IV. SIMULATION SETUP We created different scenarios with varying network size of 2 nodes to 50 nodes using Qualnet . The sensor network in our scenario follows ZigBee (IEEE 802.15.4) standard with PAN coordinator, RFDs and FFDs. The figure 1 shows the qualnet scenarios for 10 and 40 nodes and similiarly scenarios for 20, 30 and 50 nodes were formed using constant bit rate (CBR) application and results were analysed. Different parameters like throughput and energy consumption have been analysed for these different scenarios. Various simulation parameters used in our analysis are given in Table 2. A. Result Analysis Evaluation for different parameters like energy consumed in receiving mode and transmitting mode, throughput has been done with varying number of nodes in wireless sensor networks. Simulations have been done for different CCA modes i.e. carrier sense, carrier sense with energy above threshold and energy above threshold. Various plots have been obtained using different modes for varying network size and are discussed below. (a) (b) Fig. 1. Scenario and simulation for (a) 10 nodes and (b) 40 nodes Figures 2 and 3 shows the comparison between different modulation schemes for energy consumption in transmitting mode and receiving mode respectively. Result shows that the energy consumption is least in ASK and OQPSK modulation scheme whereas BPSK consumes the maximum energy. This is the reason why OQPSK is considered efficient for higher data rates. In this plot, we have used the scenarios
155 Fig. 2. Energy consumption in transmitting mode with varying number of nodes using ASK, BPSK and OQPSK modulation schemes Fig. 3. Energy consumption in receiving mode with varying number of nodes using ASK, BPSK and OQPSK modulation schemes Fig. 4. Throughput of PAN cordinator with varying number of nodes using ASK, BPSK and OQPSK modulation schemes with10,20,30,40 and 50 nodes and it shows that as the number of nodes are increasing, energy consumption is also increasing. Figure 4 is a plot between number of nodes and throughput for various modulation schemes. This plot shows that the OQPSK modulation scheme has comparatively higher throughput than other modulation schemes. It also shows that ASK modulation scheme has higher throughput than BPSK modulation scheme although
156 BPSK is employed mostly in ZigBee standard. So this study also gives us an option of employing another modulation scheme in form of ASK which can be used in ZigBee sensor network. Fig. 5. Energy consumption in transmitting mode (PAN cordinator) with different CCA modes and varying number of nodes Figure 5 is a plot between number of nodes and energy consumption in transmitting mode for various CCA schemes. The result shows that there is a minor difference between these schmes in energy consumption. This is the reason why we mainly employ carrier sense scheme as it has higher throughput than other CCA schemes. Fig. 6. Energy consumption in receiving mode (PAN cordinator) with different CCA schemes and varying number of nodes Fig. 7. Energy consumption both for transmitting and receiving mode for all the three CCA schemes. CS: carrier sense, CSEAT: carrier sense with energy above threshold and EAT: energy above threshold
157 Figure 6 shows the plot between number of nodes and energy consumption in receiving mode for various CCA schemes. As it was indicated previously in the transmitting mode there is a minor difference between energy consumption in these CCA schemes. In the receiving mode also, there is a minute change in the energy consumption of CCA schemes. Figure 7 shows the plot between the energy consumption in all the three CCA schemes. It is shown that the energy consumption in carrier sense with energy above threshold is maximum and least in case of energy above threshold but carrier sense is preferred because it has higher throughput among all the CCA schemes. V. CONCLUSION In this paper various modes of operations of IEEE 802.15.4 ZigBee have been studied for different modulations and CCA schemes with varying number of nodes. Energy consumption and throughput have been considered for comparing various modulation schemes and modes of operation. We have observed the OQPSK is the best modulation scheme in terms of energy consumption and throughput. Further it has been observed that carrier sense scheme outperforms all other CCA schemes. In future we can analyse impact of different routing protocols on ZigBee physical layer and can also devise a technique to evaluate LQI for different CCA mechanisms and modulation schemes. REFERENCES  ZigBee Alliance ZigBee Specification[EB/OL]. http://www.zigbee.org/en/spec-download,2007  IEEE Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks[S] IEEE Std 802.15.4-2003, October 2003.  M. Alnuaimi, K. Shuaib and I. Jawhar, “Performance evaluation of IEEE 802.15.4 physical layer using matlab/simulink,” in innovations in information technology, Nov 2006.  Qualnet simulator 5.02 purchased from scalable networks. web.scalable-networks.com/content/qualnet
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