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Performance Evaluation of IEEE 802.15.4 Ad Hoc Wireless Sensor Networks Simulation Approach

时间:2011-08-18


2006 IEEE Conference on Systems, Man, and Cybernetics October 8-11, 2006, Taipei, Taiwan

Performance Evaluation of IEEE 802.15.4 Ad Hoc Wireless Sensor Networks: Simulation Approach
Wilson T.H. Woon and T.C. Wan
Abstract— This paper presents a preliminary performance investigation of the recently released IEEE 802.15.4 standard focusing on multiple sources and multi-hop peer-to-peer wireless sensor networks. This standard was developed to work in all-wireless environment supporting either peer-to-peer or star network topology. Since the release of IEEE 802.15.4, several efforts were made to study its performance where simple star network topology was the main focus. This literature attempts to extend existing efforts but focuses on evaluating the performance of peer-to-peer networks on a small scale basis using ns2 simulator. We analyze the performance based on commonly known metrics such as throughput, packet delivery ratio, and average delay. In addition, we propose ad hoc wireless sensor networks (AD-WSNs) paradigm as part of the extension to the IEEE 802.15.4 standard. From the experiments conducted we identi?ed ‘hidden node’ problem in high-rate traf?c as a soruce for performance degradation and proposed slot-based channel access as the potential solution. Keywords—IEEE 802.15.4, ad hoc networks, wireless sensor networks, wireless personal area networks

I. INTRODUCTION Current trend in computer communication is gearing towards embedded ubiquitous computing. Advances in wireless communications and microelectronics (MEMS) enabled the development of low-cost sensors which are constantly being adopted for pervasive communication paradigm. Initially ad hoc wireless networks (AWNs) serve as an ideal platform towards achieving this objective due to its inherent ?exibility and low-cost implementation. It uses mobile devices such as laptops, PDAs, wearable gadgets, mobile phones, and many more with inbuilt circuitry or add-on cards to enable peerto-peer communication. The introduction of sensors opens up a new dimension for sensor-based AWN consisting of densely populated sensing apparatus that are comparatively inexpensive, energy ef?cient, compact in size, and can be manufactured in large quantities. These characteristics are ideal for the establishment of ad hoc wireless sensor networks (AD-WSN) that are simple, temporal, distributive, and collaborative. The absence of governing network such as ATM, MPLS, and WLAN greatly reduces the overall cost and complexity of the network. It allows mobile nodes (sensors) to be distributed without ?xed topology. Furthermore the network relies on the collaboration between sensors to perform a task, i.e. monitoring, data collection or surveillance while at the same time any nodes can be removed or become
This work is supported by a University short grant Wilson T.H. Woon is with the School of Computer Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia wilson@nrg.cs.usm.my T.C. Wan is with the School of Computer Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia tcwan@cs.usm.my

inactive (in a controlled manner) without affecting the overall network operation. The emergence of wireless sensor networks (WSNs) enabled the development and subsequently the release of IEEE 802.15.4 [1] or 802.15.4 (use interchangeably hereafter) standard for low-rate wireless personal area networks (LRWPANs). It offers simple, energy ef?cient, and inexpensive solution to a wide variety of applications ranging from industrial control, environment monitoring, home appliances, consumer electronics, security, military, healthcare, and wireless networking. It de?nes the medium access protocol (MAC) and physical (PHY) layer protocol for all-wireless network. It supports simple 1-hop star network and multi-hop peer-topeer network. In both, a PAN coordinator must be elected to start and manages the network. The former allows 1-hop communication between PAN coordinator and its associated devices and vice versa and peer-to-peer communication. The latter consists of a star network serving as the base network. This network is expanded through the election of one or more coordinator nodes to manage their respective cluster or WPAN. The coordinator nodes in turn elect other coordinators thus the formation of hierarchical tree network. In 802.15.4 networks, the PAN coordinator serves as the core node in handling network management and resource allocation duties. It is widely known that electing a virtual base station restricts network ?exibility and hinders selfcon?guration of nodes. In addition to potential bottleneck at the cluster head backbone, it also causes large power consumption due to concentration of traf?c at the neck. Though not speci?cally mentioned in the speci?cation, 802.15.4 offers an alternative AD-WSN topology to solve the above mentioned problems. We shall describe this networking paradigm further in Section V. Since the inception of 802.15.4, numerous efforts [2,3,4,5] were made to study its performance under various simulation environment. These works comprise of software, hardware, and mathematical analysis. Majority of them focused on simple star topology with the exception of [2] which has a small contribution on peer-to-peer network. These projects motivated us to initiate a performance study on AD-WSN. Figure 1 illustrates an example of ad hoc network. The remaining of this paper is organized as follows. We give a review of related literatures in section II. This is followed by an overview of 802.15.4 in Section III. We give detailed explanation of the IEEE 802.15.4 standard in Section IV which covers the PHY and MAC layer protocols. Our proposed AD-WSN is presented in section V. In section VI, we describe our simulation model and performance metrics

1-4244-0100-3/06/$20.00 ?2006 IEEE

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used. The results and analysis are presented in section VII. The conclusion and future research directions are presented in the ?nal section. II. LITERATURE REVIEW J. Zheng and M.J. Lee [2] implemented the IEEE 802.15.4 standard on ns2 simulator and subsequently produced the ?rst performance evaluation on 802.15.4. The literature comprehensively de?nes the 802.15.4 protocol as well as simulations on various aspects of the standard. This paper has a minor evaluation on the performance of peer-to-peer networks. Our paper focuses on small scale networks while introducing additional settings and performance metrics. We will explain this further in Section VI. Other works [3,4] focused on simple 1-hop star network. G. Lu et. al. [3] implemented their own ns2 version of 802.15.4 and studied its performance in beacon-enabled mode while J.S. Lee [4] performed a realistic experiment using hardware devices. Finally, Timmons and Scanlon [5] presented an analytical analysis of the protocol in body area networking (BAN). III. BRIEF OVERVIEW OF IEEE 802.15.4 The 802.15.4 standard de?nes physical (PHY) and medium access control (MAC) layer protocols for supporting relatively simple sensor devices that consume minimal power and operate in an area of 10m or less. The point of service (POS) may be extended beyond 10m but this requires additional energy to operate. It also allows two types of topologies such as a simple one hop star or a self con?guring peer-to-peer network to be established. We have brie?y explained these two in the earlier section. In terms of wireless links, 802.15.4 operates in three license free industrial scienti?c medical (ISM) frequency bands, i.e. data rates of 250 kbps in the 2.4 GHz band, 40 kbps in the 915 MHz band, and 20 kbps in the 868 MHz band. The ?rst band has 16 channels while the second has 10. The latter was allocated one channel. Though only one channel is used at a time, the additional channels allow the ?exibility of switching to another in case the existing becomes not conducive. There are two categories of devices in 802.15.4. One of them is called full-function device (FFD) while the other is reduced-function device (RFD). RFD is crude device supporting simple application such as a switch or sensor. It is usually controlled by FFD device. RFDs can be used to communicate among themselves and with FFDs. The former is desired in this paper because it can take on the role of a router that enables peer-to-peer communication. In terms of addressing, the protocol assumes the use of either 16bit short or extended 64-bit IEEE addresses. The latter is available in all devices by default and is commonly known as physical (MAC) address while the previous is allocated by the PAN coordinator which the device is associated with. In the following section we shall describe brie?y the IEEE 802.15.4 standard particularly the MAC and PHY layer.

IV. MAC AND PHY L AYER P ROTOCOLS OF IEEE 802.15.4 A. PHY Layer The physical (PHY) layer is an essential component in computer communication. It is generally used for data transmission and reception, channel sensing, link quality determination, channel selection, and node state setting. It interacts directly with the wireless channel, thus supplying information to and from the upper layers. While the ?rst function is obvious, the second is essential to the carrier sense multiple access with collision avoidance (CSMA/CA) mechanism. Further description is provided in the next subsection. Before a particular node reserves the channel for communication it needs to determine whether the channel is free or not. In particular, the PHY protocol performs energy detection (ED) scan and clear channel assessment (CCA) on the channel to detect any ongoing activities and relay the results to the MAC layer. A channel is considered busy if the activity levels detected exceed certain threshold value. Another important assessment is link quality. Upper layers protocols (MAC and network) depend on this information before deciding on using a particular channel because external interferences such as noise and electromagnetic signal could affect the network performance. If a particular channel is not feasible, there are 26 other channels available under 802.15.4 to be selected. As part of 802.15.4 effort in preserving energy, the radio transceiver can be turned off if inactive (not receiving or transmitting). However this function is only possible in beacon-enabled network which is beyond the scope of this paper. B. MAC Layer This layer provides an interface between upper layers and the PHY layer. It handles channel access, link management, frame validation, security, and nodes synchronization. In our approach, we adopt beaconless mode which implies unslotted CSMA/CA mechanism. For this mode, the PAN coordinator is responsible of handling only device association/disassociation and (short) address allocation in case the 64-bit IEEE addressing is not used. The CSMA/CA protocol is an important mechanism for channel access but does not include the RTS/CTS handshake, considering low data rate adopted in 802.15.4. This mechanism evaluates the channel and allows data packets to be transmitted if the condition is suitable (free of activities). Otherwise the algorithm shall backoff for certain periods before assessing the channel again. Without the RTS/CTS handshake, it would appear to encourage packet collisions due to hidden nodes [6]. Nodes are considered hidden if they are out of signal range of each other. Therefore when these nodes attempt to send data to a similar destination at the same time, collisions happen. However, with low data rate, the probability of collisions could be reduced as proven by [2]. To further improve link reliability between two nodes, the MAC layer employs various mechanisms such as frame acknowledgment

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Fig. 2. Fig. 1. Proposed Ad Hoc Wireless Sensor Network

Simple 1-hop Star Network (without PAN coordinator)

and retransmission and data veri?cation using 16-bit CRC, in addition to CSMA/CA. V. P ROPOSED A D H OC W IRELESS S ENSOR N ETWORKS (AD-WSN S ) As mentioned earlier the IEEE 802.15.4 standard supports simple star network and multi-hop peer-to-peer network. In both topologies a PAN coordinator must exist. However the PAN coordinator is generally responsible of handling nodes association/disassociation and allocating addresses in nonbeacon networks. The latter can be easily replaced by the default 64-bit IEEE address held uniquely by each device. The former can also be handled by any FFD nodes. Therefore we assume an AD-WSN consisting of FFDs or router-enabled devices. Each node could scan the channel (assuming a default channel for all nodes) to locate their immediate neighbors. Once the neighbor identi?cation process is completed peer-to-peer communications could take place. All nodes shall remain active for as long as they are participating in the network. For every data packet received, the destination may return an optional acknowledgment (ACK) message. Figure 1 depicts the AD-WSN where all nodes are homogeneous FFDs. VI. S IMULATION PARADIGM AND P ERFORMANCE M ETRICS We conduct our performance experiments on ns2 simulator [7]. We assume beaconless mode (unslotted CSMA/CA) and all devices have the capabilities of a coordinator (FFD) that is to handle association and relay data packets. The simulations primarily study the effect of varying number of traf?c sources and number of communication hops (multi-hop) in small scale networks. In the previous, we begin with one source and one destination node and then gradually increase the traf?c sources to four nodes. As shown in Figure 2, the shaded node serves as the destination while the others are source nodes. Nodes are positioned in such that they are within range of each other. Therefore the problem of ‘hidden node’ [6] does not exist. Nevertheless we dedicate a small sub-section for investigating the impact of packet collisions and the ‘hidden node’ problem. As for the latter, we establish 3-hop network consisting of four nodes. Unlike star network

Fig. 3.

Multi-hop Peer-to-Peer Data Transmission

simulations, only the sending and receiving nodes are within range of each other. Figure 3 depicts this simulation topology. The general simulation parameters are summarized in Table 1. The performance metrics are the following:
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Throughput (kbps) It measures the amount of data received by the destination node within certain period of time. In multi-hop environment, the throughput is computed at the ?nal destination as the intermediate nodes are responsible of relaying the packets. Packet Delivery Ratio (%) It measures the ratio of total packets received against total packets sent in the MAC sublayer. Average Packet Delay (seconds) It refers to the amount of time taken by a data packet to reach its destination.

In our communication model we disabled packet retransTABLE I G ENERAL S IMULATION S ETTINGS Network dimension Simulation duration No. of node Traf?c type Max. queue length No. of simulation trial Statistical error rate No. of communication hop 50m x 50m 300s Vary according to experiment (two to ?ve) cbr 50 20 0.002% Vary according to experiment (one to four)

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mission therefore ACK message is not sent. VII. S IMULATION R ESULTS AND A NALYSIS In this section, we present the results and analysis of our simulations on multi-source and multi-hop networks. For the previous, we shall conduct two sets of simulation scenarios, i.e. with and without background traf?cs. The latter is a straightforward multi-hop communication along a chain of nodes. A. Effect of Varying Traf?c Sources without Background Traf?cs In the ?rst experiment, the transmission rate for each device is ?xed at 3 ms while the payload varies between 20, 60, and 102 bytes. The second experiment involves varying the traf?c loads between 20, 60, and 100 kbps. A practical application could be such that a command station receives emergency messages (data or voice) from one or more sensors in a rescue mission. We present the experiment results in Figure 4, 5, and 6. From Figure 4, the amount of information received at the destination centre increases as the number of traf?c sources increases for application traf?c of 20 bytes. As for larger payloads like 60 and 102 bytes, the performance degrades when three or more sources transmitting. From this analysis, we conclude that packet collision is more severe when the packet size is large. In addition the average delay per packet as shown in Figure 6 indicates that large data packets incure more processing delays as compared to small application traf?c, e.g 20 bytes payload. In the second experiment, we introduced various traf?c loads into the simulation. They include 10 kbps (low load), 50 kbps (medium load), and 100 kbps (high load). The result is depicted in Figure 5. Unsurprisingly low traf?c load performed consistently in the event of increasing number of traf?c sources. For medium and high traf?c loads, the performance degrades when two or more traf?c sources are involved in the communication. At 100 kbps traf?c, a combination of 400 kbps traf?c load in four traf?c sources utilizes approximately 140 kbps traf?c which is only about 35% utilization rate. However for 10 kbps traf?c, four sources with a total of 40 kbps traf?c load delivers more than 60 kbps of information! These results con?rmed that IEEE 802.15.4 is meant for low traf?cs and simple applications. B. Effect of Varying Traf?c Sources with Background Traf?cs In this section, we established a main traf?c source and various background traf?cs. Again this section consists of two experiments. The ?rst involve varying the source application size between 20, 60, and 100 bytes while the background traf?c(s) are ?xed at 100 kbps. In the second test we vary the background traf?cs while maintaining the source interval at 3 ms and 20 bytes payload. One application scenario could be such that a gateway collects information from a particular source while ?ltering out data from others. The results are shown in Figure 7, 8, 9, and 10. We ?rst focus on Figure 7. The experiment indicates that throughput is at optimum
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without the existence of background traf?c. However when background traf?c(s) are introduced the throughput utilization decreases due to packet collisions and channel access delays. On the effect of packet collisions, the result in Figure 9 indicates that the posibility of collisions increases as the number of background traf?cs increases in all application scenarios. We note that in the absence of background sources the delivery ratio is not 100% due to the introduction of packet error rate in the traf?cs. However this value is rather small and does not affect the simulation results much. In the second test, we vary the background loads and the results are shown in Figure 8 and 10. Similar to Figure 7, the throughput utilization slumps as the number of background traf?c increases. However the performance of 10 kbps traf?c is better than the others. This suggest that throughput limiting factors such as collisions, channel access delays, and processing delays are minimal. The result in Figure 10 con?rms our previous ?ndings that packet collisions are imminent in increasing number of traf?cs. Again this experiment strengths the argument that IEEE 802.15.4 is not suitable for complex and high traf?c loads applications. C. Effect of Peer-to-Peer and Multi-hop Communications In this section, we will investigate two issues, namely the optimal offered load in a multi-hop (3-hop) topology and

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Fig. 9. Packet Delivery Ratio with Varying Number of Background Nodes and Payload Sizes

the effect of packet collision on throughput utilization. For the previous, we establish a multi-hop network as depicted in Figure 3 where the source and destination nodes are at the two end-points respectively of the chain. We ran the simulations on various traf?c loads to determine the optimum load. For the latter experiment we have a topology as depicted in Figure 11. We clasify the distance of y as (1) contention free and collision free region, (2) collision and contention free region, and (3) contention and collision region. The latter allows all nodes to be within transmission range of each other while the earliest is collision free. The second region is meant to investigate the effect of collisions due to ‘hidden node’ [6]. In this test, we disabled the packet error rate, thus the reason for packet drop is restricted to collision. Two traf?c loads of 50 kbps are established on link 1 and link 2. Figure 12 shows that the end-to-end throughput stabilizes when the offered load is beyond 70 kbps. This ?gure also shows high throughput utilization for low application traf?cs, i.e. 10 and 20 kbps. However as the traf?c load increases the performance slumps, delivering less than 50% for traf?cs beyond 70 kbps as shown in Figure 13. Finally in Figure 14, the maximum throughput recorded is for interference free (and error free traf?c) which is about 40 kbps. One interesting point to discuss is that packet collisions due to

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Fig. 11. Topology Setting for Investigating the Impact of Packet Collision

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VIII. C ONCLUSION AND F UTURE W ORK From the experiments conducted we conclude that IEEE 802.15.4 is suitable for simple and non-complex applications. It has the potential of supporting high-end applications but requires extensive enhancements. We proposed AD-WSN network paradigm as an extension to the standard as well as slot-based (mentioned slightly) channel access to solve the ‘hidden node’ problem. In multi-hop environment the end-to-end throughput utilization saturates at 70 kbps traf?c load and delivers less than 50% of information. This network environment also requires further improvements. In particular we would also investigate whether our proposed slot-based channel access could ?t in well. Our future plans include experimenting the IEEE 802.15.4 wireless networks on hardware motes. R EFERENCES
[1] IEEE 802.15.4, “Wireless Medium Access Control (MAC) and Physical Layer (PHY) Speci?cations for Low-Rate Wireless Personal Area Networks (LR-WPANs), IEEE NY, August 2003. [2] J. Zheng and M.J. Lee, A Comprehensive Performance Study of 802.15.4, IEEE Press Book, 2004. [3] G. Lu, B. Krishnamachari, and C.S. Raghavendra, “Performance Evaluation of the IEEE 802.15.4 MAC for Low-Rate Wireless Networks”, in Proc. IEEE Int. Performance Computing and Communication Conf. (IPCCC’04), Phoenix, AZ, April 2004, pp. 701-706. [4] J.S. Lee, “An Experiment on Performance Study of IEEE 802.15.4 Wireless Networks”, in Proc. IEEE Int. Conf. on Emerging Technologies and Factory Automation, Catania, Italy, 19-22 September 2005. [5] N. F. Timmons and W. G. Scanlon, “Analysis of the Performance of IEEE 802.15.4 for Medical Sensor Body Area Networking”, in Proc. IEEE SECON 2004, Santa Clara, CA, Oct. 2004. [6] P.C. Ng, S.C. Liew, K.C. Sha, and W.T. To, “Experimental Study of Hidden Node Problem in IEEE 802.11 Wireless Networks”, in Proc. IEEE SIGCOMM 2005, Philadelphia, PA, Aug. 2005. [7] The Network Simulator 2 (ns2). Available from http://www.isi.edu/nsnam/ns/ [8] C. Busch, M.M. Ismail, F. Sivrikaya, and B. Yener, “Contention-Free MAC Protocols for Wireless Sensor Networks”, in Proc. DISC 2004, Trippenhuis, Amsterdam, Netherlands, Oct. 2004, pp. 245-259. [9] F.A. Tobagi and L. Kleinrock, Packet Switching in Radio Channels: PArt II- the Hidden Terminal Problem in Carrier Sense Multiple Access Modes and the Busy-Tone Solution, IEEE Transactions on Communications, vol. 23, pp. 1417-1433, 1975. [10] Z.J. Hass and J. Deng, “Dual Busy Tone Multiple Access (DBTMA) - Performance Results”, in Proc. IEEE WCNC 1999, Sept. 1999 .

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‘hidden node’ is more severe than ordinary collision. We mentioned earlier that IEEE 802.15.4 does not include the mechanism for preventing collision caused by ‘hidden node’. As for preventing packet collision due to two or more nodes transmitting at the same time, the 802.15.4 protocol includes several measures such as energy detection scan, random backoff, carrier sense, 16-bit CRC veri?cation, and frame retransmission and acknowledgement (not applicable here). With the removal of RTS/CTS handshake, the standard does not provide any solution to the ‘hidden node’ particularly when in high traf?c load. Though there are existing methods to solve this problem [9,10], they are not tested on realistic experiments and not many are designed for wireless sensor networks. This would remain an interesting topic for future research. We are currently developing a time division multiple access (TDMA) based handshake which is motivated by [8] on real sensors. The basic idea is to divide time into frames with equal size and number of slots. Each node attempts to determine a collision-free slot for communication. This approach could well be extended into a contention-free MAC protocol for IEEE 802.15.4. Furthermore we would also investigates whether this proposed solution would improve the performance in a multihop environment.

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