Performance Evaluation of Adaptive Network Tree Construction Mechanism for ZigBee Grid Router Network Lukman Rosyidi, Riri Fitri Sari Electrical Engineering Department University of Indonesia Depok, Indonesia
[email protected],
[email protected] Abstract— This paper presents a performance evaluation of the adaptive network tree construction mechanism for ZigBee grid router network. ZigBee is a communication protocol for Wireless Personal Area Network (WPAN) which is built on the IEEE 802.15.4 standard. The low power characteristic of ZigBee is one of the main reasons in the selection of this protocol. An adaptive network tree construction mechanism can be implemented to support this characteristic. The mechanism is intended to form a ZigBee network topology which always minimizes the number of active ZigBee routers in a grid router topology. This mechanism will save energy consumption by turning the ZigBee routers to sleep condition when there is no end device within range to be served. A predefined routing map is used to determine the minimum number and the position of routers to be active based on the position of end devices. The performance evaluation of the mechanism is carried out using NS2 network simulator software tool. The simulation result showed that the energy consumption can be proportional to the number of active routers. In the simulation, maximum energy saving is up to 87% compared to the condition that all router always active in a 4x2 ZigBee grid router topology. Keywords— ZigBee network, tree construction, adaptive network, network simulator.
I. INTRODUCTION ZigBee is a popular protocol, which is used in many sensor and embedded system applications to establish low power and low cost wireless data transmission. It is built on the IEEE 802.15.4 standard for Wireless Personal Area Network (WPAN), which is designed for low data rate communication. ZigBee network is a scalable network. A new node can join to network automatically if there are other nodes support its connection to the network coordinator. The ZigBee routers can be arranged to form a grid, in order to cover certain communication service area. Router is a type of ZigBee node which is always active and ready to give communication service to other type of ZigBee devices. It stores and relays transmitted data in a multihop communication. This way will make possible for other type of ZigBee device to use most of its time in sleep condition to save power. The ZigBee grid router is used in many application areas, especially for Machine-to-Machine (M2M) communication. For example, the ZigBee grid router is used in an exhibition place, themes park or convention center to provide communication for sensors and actuators. It covers certain
communication service area, so that the network ready to serve any active end device node in its range. In many applications, the ZigBee end devices may have certain work schedule which is different from each other. Each of ZigBee end devices can be active in a specific time, for only a few hours, and then become inactive again. In this kind of application, the ZigBee routers do not need to be active all the time. An adaptive mechanism can be implemented to save more power by automatically minimizing the number of active ZigBee routers according to the number and the position of the end devices to be served. The remainder of this paper is organized as follows: Section II gives an overview of ZigBee network, including the underlying standard, the technology, and the characteristics. Section III explains about the ZigBee grid router topology. Section IV explains the adaptive network tree construction mechanism for ZigBee grid router network. The mechanism validation and simulation results are shown in section V. Our conclusion is drawn in section VI. II. ZIGBEE TECHNOLOGY A. IEEE 802.15.4 Standard The IEEE 802.15.4 standard defines communication in the second layer (data link / MAC layer) of Open Systems Interconnection (OSI). In this layer, units of digital information arranged as impulses of electromagnetic signals and transmitted to the lower layer (physical layer). Role of this layer is similar to other standards that are more commonly known, e.g. IEEE 802.3 (Ethernet) and IEEE 802.11 (Wi-Fi). The frequency used in IEEE 802.15.4 is spread into 27 channels, which are classified into 3 primary bands, 868.0868.6 MHz (1 channel) for use in Europe, 902.0-928.0 MHz (10 channels) for use in US, and 2.40-2.48 GHz (16 channels) for worldwide use [1]. The IEEE 802.15.4 standard uses Direct Sequence Spread Spectrum (DSSS) to modulate the information signal before it is sent through the physical layer. Every bit of information is coded into four different signals (bits alphabet). This process causes information transmissions occupy a larger bandwidth, but use lower spectral power density [1],[2]. There are two techniques which can be used to avoid all nodes start emissions at the same time, i.e. Carrier Sense Multiple Access Collision Avoidance (CSMA-CA) and
Guarantee Time Slots (GTS). In CSMA-CA, each node listens to the medium before transmitting. If the transmitting node detects any energy level which is higher than a certain threshold, it will have to stop and wait for a random time before trying again. In GTS, a central node (PAN coordinator) gives timeslot to each node so that every node will know when to transmit. There are 16 timeslots that may be used. To be able to do the transmission, a node must send a GTS request message to the PAN coordinator, and as the response, the PAN coordinator will send beacon message containing the information of allocated slots [3]. B. ZigBee Standard The ZigBee standard was first developed in 1999, when many engineers realized that Wi-Fi and Bluetooth technology were still complicated and not quite suitable for many applications that have low power and low cost requirement, which is implemented for low data rate communication. The IEEE 802.15.4 standard is chosen as physical and datalink layer for ZigBee [4]. The ZigBee transceiver is designed to be able to sleep in most of time, while sending and receiving data can be set to use very low energy consumption. It can connect up to 65,535 nodes in one network and range of its radio can cover up to hundreds of meters. Its maximum data rate is only 250 Kbps. It is suitable for periodic or intermittent data communication [5]. The ZigBee has mesh networking capability. It forms multihop wireless communication network. In multihop wireless networks, communication between two end nodes is carried out through a number of intermediate nodes whose function is to relay information from one point to another. There are two types of ZigBee devices, i.e. full-function devices (FFD) and reduced function devices (RFD). FFD can send, receive and forward the data to another node. This type of ZigBee has a buffer for temporary storage of data before it is passed to other node. The other type is RFD. It can only send and receive data. It cannot pass data to another node. This module is relatively cheaper and consumes less power than the FFD. There are three roles of nodes in a ZigBee network. Coordinator is the master device that builds the network. In a ZigBee network, there must be one coordinator. Coordinator should be a FFD. End devices or also known as motes, are nodes which send and receive information. The end device can be FFD or RFD. Routers are the nodes that serve end devices by routing the information. The router must be a FFD. When an end device node is already joined to a network, it can send information to other nodes via router. When a router receives a packet with a specific destination, it will see whether the destination is in awake or sleep state. If the destination node is awake, then the packet will be sent directly. However, if the destination is in sleep state, the router will store the packets in the buffer until the destination is awake. The ZigBee network has two working modes, i.e. beaconenabled mode and non-beacon enabled mode. In non-beacon mode is enabled, the ZigBee routers will always be active, thus requiring a robust power supply. In this mode, routers are ready
to serve data communication at any time, while the end devices are active only when they want to send or receive data. In beacon-enabled mode, the ZigBee router periodically send beacon to notify its presence to other nodes in the network, capture data transmission request, and notify if there is pending data for the end device. The end device node can enter sleep state while it is not in beacon period. This procedure will decrease the duty cycle of the devices and save power. However, it needs a very precision timing, increases the complexity of implementation and increases the cost of ZigBee product. C. ZigBee Characteristics ZigBee technology has characteristic of low power and low cost implementation. In many applications, low power is the main reasons in the selection of this protocol. Battery powered ZigBee device can last more than a year, while other technology will not last more than a week. Keeping ZigBee devices in low power consumption on battery is an important issue on the implementation. Table I shows the comparison between ZigBee and other technology. TABLE I.
Application area Objective of use Max number of node Bandwidth Memory used Range Battery life
COMPARISON ZIGBEE TO OTHER TECHNOLOGY[5] ZigBee 802.15.4 Monitoring and control Low power, low cost
Bluetooth 802.15.1 Short range wireless Convenient use
255/65.535
7
Wi-Fi 802.11b Data transfer High transfer rate 30
GSM/GPRS
250 Kbps 4-32 KB
720 Kbps >250KB
54 Mbps >1MB
2 Mbps >16MB
> 100 meter > 1 year
10-100 m < 1 week
50-100 m < 1 week
Several km < 1 week
Voice/data communication Long range 1.000
The ZigBee network is a self-organized network. Before establishing communication, a ZigBee network will build the network tree construction. At this stage, nodes that want to join the network can request a network address to the coordinator, either directly or through other nodes (routers) with its routing mechanism [6],[7]. All nodes in the network will perform this request. The ZigBee nodes will automatically make association among the coordinator, the routers and the end devices. A router that serves end device is called parent node, and the end device is called child node. The end device node that cannot be associated to any router is called orphan node, which can happen because all routers in range already have maximum capacity of child nodes or no more address available [8]. The end device that cannot be associated because there is no router in range will become unconnected node. After performing tree construction, all of associated nodes form the network topology. These associations and topology are automatically updated because of the changing condition of network nodes. New nodes can join the network, while inactive nodes will be disconnected.
III. ZIGBEE GRID ROUTER NETWORK In many applications, the ZigBee end devices are scattered and each of them may not have a fixed position. To provide communication services in a certain area, ZigBee routers are arranged in a grid. ZigBee grid router is a form where ZigBee routers are arranged in columns and rows which have certain distance between nodes, to cover an area of communication service [9],[10]. It is implemented in exhibition places, themes parks or convention centers to provide communication service for sensors and actuators (the end devices). The example is shown in Figure 1.
transceiver itself. The diagram block of the node’s system is shown in Figure 3.
Fig. 3. Diagram block of end device system
Embedded controller is a central processing module that controls the work of all nodes’ system including the ZigBee transceiver. It gets information from the sensor and sends to other node via the ZigBee transceiver. It also receives information from other node via ZigBee transceiver and then gives command to the actuator for necessary action. IV. ADAPTIVE NETWORK TREE CONSTRUCTION Chiang et al. in [9] proposed Grid-Based Data Aggregation Scheme (GBDAS) for WSN, where the sensor field is partitioned into a grid of cells, each with a cell head. Shih et al. in [11] proposed a novel ZigBee tree construction framework, which consists of three phases i.e. 1) the router node deployment; 2) the coordinator selection; and 3) the routing tree construction. They exploited the regularity in node mobility patterns to reduce the frequency of route reconstructions and ensure that the transmission of data to mobile nodes is efficient.
Fig. 1. Example of ZigBee Grid Router
The distance between nodes in a ZigBee grid router is usually chosen to support star or tree network connection. As shown in Figure 2, a router node can have connection to other router nodes in left and right of it. Furthermore, the router node also may have many connections to end device nodes in its radius, forming a tree network topology.
The main reason for implementing an adaptive network tree construction mechanism for ZigBee is the behavior of communication usage by end devices in many application areas which has different work schedule from each other. For example, in an exhibition place, the management deploys the ZigBee grid router to serve machine-to-machine (M2M) communication for all devices in all exhibition booths. In an event, it may happen that not all booths in coverage area are operating in the same time. The work schedules between the devices may be very different of each other. In this kind of application, not all ZigBee routers need to be active all the time. An adaptive mechanism is implemented to save more power by automatically minimizing the number of active ZigBee routers according to the number and the position of the end devices to be served at the time. The mechanism also supports the changing of the network tree construction when an end device is disconnected from the network or a new end device joins the network [12].
Fig. 2. Example of nodes formation in a ZigBee grid router
A ZigBee transceiver is used in a node to send or receive information. Usually, the node consists of three main parts, i.e. an embedded controller, a sensor or actuator, and the ZigBee
The implementation of adaptive network tree construction mechanism on ZigBee network needs the role of embedded controllers, particularly the one in the ZigBee coordinator and those in the ZigBee router nodes. The procedure to run adaptive construction mechanism is programmed in the firmware of embedded controllers, which is described in steps as follow. 1.
At first, all embedded controllers in network nodes will activate their ZigBee transceivers. These ZigBee
routers will detect request signals from all active end devices in its coverage area. 2.
The routing mechanism of ZigBee protocol will automatically associate all end devices into the network with network address registered in the coordinator.
3.
After all nodes are associated, the coordinator can identify all active end devices and their parent routers.
4.
The program in the embedded controller at the coordinator will calculate and determine the minimum number of routers that must be activated to serve end devices based on predefined routing map.
5.
Based on the calculation, the embedded controller at the coordinator sends commands to each embedded controller in the router that does not serve any end device or other router to deactivate the ZigBee transceiver for a specified period of time.
6.
The ZigBee network will automatically form a new network tree construction based on the existing active routers and end devices.
7.
During the specified period of time, the ZigBee network will remain in the form of its network tree construction to serve the end devices, before repeating back to step 1.
The duration of the period can be set in the embedded controller program which is based on the nature of the application requirement and tolerable data transmission latency in the communication. Figure 4 shows an example of an end device distribution in a ZigBee grid router. There are four active end devices that need communication service. At first, all routers will be active to detect request signal from end devices and form ZigBee network association, as shown in Figure 5. The coordinator will minimize the number of active routers as shown in Figure 6 and then remain in the form of its network tree construction for a specified period of time, before adapting to new end device requirement.
Fig. 5. The adaptive mechanism activates all routers
Fig. 6. The adaptive mechanism minimize the number of active routers
In the communication activity, the energy consumption of ZigBee nodes can caused by event transmitting, receiving, checking channel for incoming transmission and idle. Energy consumption can be calculated as below [10]. Erx = Prx * dsize / S ……………………………………... (3) Etx = Ptx * dsize / S……………………………………... (4) Emin = Echeck + Eidle …………………………………….. (5) Econs = Emin + drx * Erx + dtx * Etx …………………….. (6) Ere = Einit - Econs ………………………………………. (7) where
Fig. 4. Example case of ZigBee end device distribution
Etx = energy consumed to transmit a data packet (J) Ptx = Tx power (W) Erx = energy consumed to receive a data packet (J) Prx = Rx power (W) dsize = data packet size (bit) dtx = number of data packet transmitted (packet) drx = number of data packet received (packet) S = communication speed (bits per second) Emin = minimum consumed energy (J) Echeck = energy for checking channel (J) Eidle = energy in idle time (J) Econs = total energy consumed (J) Einit = initial energy (J) Ere = remaining energy (J)
V. SIMULATION RESULT To evaluate the adaptive network tree construction mechanism, we use the open source Network Simulator (NS2) tools. NS2 is used because it is the open source tools that widely used by academics and reseachers in network simulation, including the 802.15.4 standard and ZigBee technology topics [14]. The topology is based on a 4x2 point of ZigBee grid router. Figure 7 shows the topology, where the circle-in-square node is the coordinator, the round nodes are routers and the solid square nodes are the end devices. Figure 8 shows the topology of the network that is visualized in the Network Animator NS2.
TABLE II. Parameters Number of Nodes Simulation Area Channel type Radio propagation Physical protocol Mac protocol Link layer protocol Queueing Antenna Routing method Traffic Rx power Tx power
THE NS2 PARAMETER SIMULATION Selected Parameters Value 23 800x500 m2 Wireless Channel TwoRayGround IEEE 802.15.4 IEEE 802.15.4 LL (standard) Queue/Drop Tail/PriQueue Omni Antenna ZigBee Routing AODV CBR 30 mW 30 mW
In the simulation, we calculate the average energy consumption for various numbers of active routers. Figure 9 shows the graph for the average energy consumption over time. The result shows that the energy consumption is proportional to the number of active routers. For a 4x2 ZigBee grid router, energy saving is up to 87% compared to the condition where all routers are always active, as shown in Figure 10.
Fig. 7. Topology of the simulated ZigBee grid router
Fig. 9. The average energy consumption over time
Fig. 8. Visualisation in Network Animator NS2
Table II shows the parameters used in NS2 simulation. In simulation, several scenarios of end device number and position are applied to form various network tree constructions, which are validated through Network Animator tool of NS2. Fig. 10. The percentage of adaptive network energy consumption to the condition where all routers are always active
The energy consumption is successfully reduced by minimizing the number of active routers. The fewer number of active routers, the less energy is consumed in the network. The reduction of energy consumption is linear to the reduction of the number of active routers. For 20 seconds of communication activity, each active router contributes approximately 0.2 Joule of energy consumption. When the data transmission activity is completed, it remains to consume some small amount of energy for scanning and coordination activity. VI. CONCLUSION This paper has presented the performance evaluation of the adaptive network tree construction mechanism for ZigBee grid router network. It is implemented for the condition that work schedules between end devices may be very different of each other. In this kind of application, not all ZigBee routers need to be active all the time. The adaptive mechanism aims to save power by automatically minimizing the number of active ZigBee routers according to the number and the position of the end devices to be served. The adaptive mechanism has a periodic activity. At first, the network activates all routers for rescanning end device requests for association. The coordinator will perform calculations and send command to deactivate unnecessary routers to get minimum number of active router. Then the network will maintain the topology for a certain period of time before repeating again the activity. The simulation result showed that the energy consumption is proportional to the number of active routers. In the simulation, maximum energy saving can reach 87% compared to the condition that all router always active in a 4x2 ZigBee grid router topology.
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ACKNOWLEDGMENT Author Lukman Rosyidi thanks to LPDP (Indonesia Endowment Fund for Education) and STT NF (Sekolah Tinggi Teknologi Terpadu Nurul Fikri) for supporting the writing of this paper.
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