propose an individual beacon order adaptation (IBOA) algorithm for IEEE 802.15.4 networks, which can individually adapt the beacon interval and duty cycle of ...
An Individual Beacon Order Adaptation Algorithm for IEEE 802.15.4 Networks Bo Gao, and Chen He Department of Electronic Engineering Shanghai Jiao Tong University Shanghai, China
Abstract—IEEE 802.15.4 is one of the most important protocols for wireless sensor networks. To improve the performance of this protocol in consideration of the trade-off between power consumption and end-to-end delay, duty cycle adaptation algorithms are effective solutions. In this paper, we propose an individual beacon order adaptation (IBOA) algorithm for IEEE 802.15.4 networks, which can individually adapt the beacon interval and duty cycle of each node at the same time to the node’s individual performance requirements. The simulations show that the IBOA algorithm makes IEEE 802.15.4 focus on the low level of power consumption under light traffic loads, while try to ensure the performance of end-to-end delay rather than power consumption under heavy traffic loads. In addition, the throughput is also guaranteed.
Figure 1. Superframe structure [1].
the low-latency applications requiring specific data bandwidth are not our focus. As for the optional inactive portion, the nodes may enter a low-power sleep mode in it. According to Fig.1, the beacon interval (BI) can be derived from the beacon order (BO, 0BO14). Similarly, the superframe duration (SD) can be controlled by the superframe order (SO, 0SOBO14).
Keywords—wireless sensor network; IEEE 802.15.4; duty cycle adaptation; beacon order.
I.
INTRODUCTION
Ever since the initial release of IEEE 802.15.4 [1], it has aroused wide interest in the academic world, since this standard is regarded as a set of protocols well suitable for the requirements of wireless sensors networks (WSNs). IEEE 802.15.4 supports either single-hop star or peer-topeer topology. Because the peer-to-peer topology can be formed into star-based one using such as tree-based clustering algorithms, we only consider the star topology in this study, which is typical and practical enough for low-power WSN applications. In addition, either nonbeacon-enabled or beaconenabled WSNs are supported by IEEE 802.15.4. A typical beacon-enabled WSN is controlled by central coordinator, which broadcasts regular beacons for node synchronization and association control. Since the duty cycle can be defined by user in this way, only the beacon-enabled WSN is considered in this study to compare the performance of the proposed IBOA-based IEEE 802.15.4 with that of the traditional one. Coordinator can bind its channel time using superframe structures. It is illustrated in Fig.1 that a superframe is bounded by two successive beacons, and can have an active portion and an optional inactive portion. The active portion can be subdivided into a contention access period (CAP) and an optional contention-free period (CFP). During the CAPs, any node should compete for channel access with other nodes using a slotted carrier sense multiple access/collision avoidance (CSMA/CA) algorithm. For the simplicity of algorithm validation, we neglect every CFP in this study, since
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BI = aBaseSuperframeDuration × 2 BO symbols
(1)
SD = aBaseSuperframeDuration × 2 symbols
(2)
SO
In terms of (1) and (2), the duty cycle (DC) of each node can be determined by the difference between BO and SO.
SD = 2 SO − BO (3) BI For easy study, a ns-2 simulator for IEEE 802.15.4 has been developed [2]. Recently, the performance trade-off between power consumption and end-to-end delay has aroused attention, and several different kinds of adaptation methods have been proposed [3]-[6]. Duty cycle adaptation can be achieved by fixing SO and adapting BO to traffic [3] or fixing BO and adapting SO [4]. Additionally, for burst or real-time traffic, references [5], [6] improve the performance of end-toend delay by introducing extended active portions. However, the BOAA [3] does not work effectively enough when the traffic of all the nodes is not uniformly distributed, because all of the nodes have to work under the same duty cycle conditions. In addition, the simulation tool used in [3] is not typical. The DCA [4] fixes BO, so that the considerable power consumption of sensor nodes for beacon receptions can not be further reduced. Moreover, the active portion extension methods [5], [6] are not as flexible as duty cycle adaptations. In this paper, we propose an individual beacon order adaptation (IBOA) algorithm for IEEE 802.15.4 networks, DC =
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ICCS 2008
which can individually and effectively adapt the beacon interval and duty cycle of each node at the same time. The rest of this paper is organized as follows. The IBOA algorithm and related changes applied to IEEE 802.15.4 are described in Section II. Simulations used to compare the performance of the IBOA-based IEEE 802.15.4 with that of the traditional one are provided in Section III. Conclusion and future work are summarized in Section IV. II.
DESCRIPTION OF IBOA ALGORITHM
A. General Description In an IEEE 802.15.4 WSN, low power consumption or long lifetime can be achieved by low duty cycles. However, low duty cycle can bring large end-to-end delay, since any unfinished or newly added operation in the current active portion has to wait until the next active portion to perform. For the efficiency of low-power WSN applications, the IBOA algorithm is proposed to effectively deal with such longlasting large end-to-end delay. According to (3), the duty cycle can be determined by the difference between BO and SO. For the simplicity of each node’s operations, we choose to fix one of the two parameters, and adapt the other one to the traffic queue of that node. In respect that the power consumption of a node in the transmit or receive mode is several tenfold higher than that in the idle mode [7], so the node can benefit more from reducing the beacon receptions than just from shortening the active portions. Therefore, we set SO to a constant and leave BO to the adaptation performed by IBOA, taking the BOAA [3] for reference. That is to say, both the beacon interval and duty cycle of each node can be changed by BO at the same time. Trying to meet the needs of each node working in a WSN, the IBOA adapts BO of each node individually. Unlike a unique BO applied to all of the nodes every superframe, the IBOA assigns a BOi for each node(i) (ię[1, N], N is the number of nodes working in the network). Thus the beacon interval and duty cycle of each node may not be as same as that of another node at one time, and can be adapted to the node’s individual performance. Another benefit brought by the individually adapted beacon intervals is that the active portions of any two different nodes, which are used to compete for channel access, may not begin at the same time. In another words, a part of nodes can access channel during their active portions while the other nodes are sleeping in their inactive portions. Therefore, the IBOA can prevent the case, which happens a lot in the traditional IEEE 802.15.4 networks, that all of the nodes with pending frames wake up at one time and involve themselves into the fierce channel access competition in a same active portion. If such competition is alleviated, both the performance of throughput and end-to-end delay can be improved.
Figure 2. Operations of coordinator with IBOA for node(i).
beacon intervals, the coordinator must make sure that every node can receive beacons whenever it wants. Then we set a common minimum value BOmin for any BOi. In terms of (1), the minimal beacon interval BImin, when the BOi is set to BOmin, is the period of any possible value of beacon interval BIi. Therefore, the coordinator should keep broadcasting beacon every BImin symbols. Moreover, we also set a common maximum value BOmax for any BOi. If the BIi is increased to a too large value without any limitation on BOi, the extreme large end-to-end delay brought by transmitting a frame from node(i) can be unbearable, and it is also possible that the coordinator considers the node(i) as a dead node. Therefore, any BOi should be limited in the range of [BOmin, BOmax]. For node(i), the operations of coordinator with the IBOA algorithm are illustrated in Fig.2, and are described as follows: (S-1) The MAC first sets the constants, including SO, BOmin, and BOmax, and initializes BOi to BOmin. We set BOmin to SO here. Then the coordinator begins to broadcast beacons. (S-2) The MAC calculates the next beacon interval BIi for node(i) by using the current BOi. (S-3) According to the timer of the coordinator, if there is a data frame from node(i) received before the end of the current beacon interval (within BIi symbols), the IBOA moves to (S-4). Otherwise, the IBOA moves to (S-6). (S-4) The MAC takes a check on the newly added traffic queue flag Fq in the received frame. If the Fq is ‘1’, which indicates that there is still more than one frame waiting in the traffic queue of node(i), the IBOA moves to (S-5). On the contrary, if the Fq is ‘0’, which indicates that there is no frame waiting in the traffic queue, the IBOA returns to (S-2). (S-5) If BOi is still bigger than BOmin, the MAC decreases BOi by one, and adds the update information on BOi to the next beacon. If BOi has reached BOmin, there is no information needed to be updated. Then the IBOA returns to (S-2). (S-6) If BOi is still smaller than BOmax, the MAC increases BOi by one, and adds the update information on BOi to the next beacon. If BOi has reached BOmax, there is no information needed to be updated. Then the IBOA returns to (S-2). Because we still focus on the low-power WSN applications, judging Fq before deciding whether to update BOi is reasonable. Many typical and practical low-power WSN applications, such as traffic or medical monitoring, just collect
B. Operations of Coordinator The coordinator, which serves as the central controller in a WSN, is responsible for beacon broadcast every after a beacon interval. Because of the difference in the lengths of individual
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Figure 4. Beacon frame format of IBOA-based IEEE 802.15.4.
Figure 3. Working process of IBOA.
a small amount of data every several minutes or a longer time. In such cases, judging Fq can prevent the coordinator from raising the duty cycle of a node unnecessarily for receiving only one frame from the node at a time. In addition, updating BOi one by one is to minimize the operation cost and the fairness problem brought by the individually adaptation of duty cycle, which offers some nodes more active portions and thus higher possibility to access channel. A sample of how the IBOA works is illustrated in Fig.3. The SO is set to zero, and the BOi is also initialized to zero. During the first beacon interval for node(i), there is no data frame from node(i) received, thus the BOi is increased by one at the end of the period. Then the duty cycle of node(i) is further reduced after the second beacon interval. In the third period, a frame from node(i) with Fq set to ‘1’ is received by the coordinator, and then the BOi is decreased by one. After that, another frame from node(i) is received within the fourth beacon interval, whose Fq has been reset to ‘0’, then the BOi is kept to its former value. Because of the fifth period without a frame from node(i) received, the duty cycle of node(i) goes down again. As mentioned in (S-5) and (S-6), the coordinator adds the update information on BOi to the next beacon. Therefore the beacon frame format of IEEE 802.15.4 should be changed. According to Fig.4, we introduce a BO Adaptation Fields in the MAC payload of the traditional beacon frame, which is similar to the Pending Addressing Fields next to it. In detail, the newly added BO Adaptation Fields consists of two parts: BO Adaptation Specification (1 byte) and Address List. In the BO Adaptation Specification, the BO Increase (4 bits) indicates the number of nodes whose BOs have been increased by the coordinator during the former minimal beacon interval (within BImin symbols). Similarly, the BO Decrease (4 bits) indicates the number of nodes whose BOs have been decreased. To minimize the operation cost, the Address List only contains the addresses of the nodes whose BOs have been changed. Moreover, if the number of nodes with newly updated BOs within a minimal beacon interval is more than that the Address List can contain, the coordinator can leave the BO of every excess node unchanged at the time and update it later.
Figure 5. Data frame format of IBOA-based IEEE 802.15.4.
C. Operations of Sensor Node The sensor nodes working in the WSN are responsible for data collection and transmission. According to the timer of node(i) itself keeping pace with that of its coordinator, it should get ready to receive new beacon at the end of its every beacon interval, and get the update information or no update on BOi from the received beacon before its following period to determine the length of its following beacon interval BIi. As mentioned above, each node provides the information on its traffic queue for its coordinator by updating the traffic queue flag Fq in its regular data frames. Before a node begins to send pending frame, it should check its traffic queue first. If there is still more than one frame waiting in the traffic queue, the Fq should be set to ‘1’. Otherwise, the Fq should be ‘0’. The new data frame format is illustrated in Fig.5. We just make use of one of the reserved bits in the MAC header of the traditional data frame to inform coordinator of the Fq (1 bit). III.
SIMULATIONS AND COMPARISON
Our proposed algorithm is validated by using ns-2.32 simulator. We have adapted the code to the IBOA algorithm. In our simulation scenario, a coordinator serves as the central controller and sink node of the star network, and six sensor nodes are set around it to send Poisson traffic to it through uplink channel. The simulation parameters are set as follows: Frame Payload=100 bytes, MAC Header=13 bytes, PHY Header=6 bytes, Channel Bit Rate=250 kbps. The values of power consumption in the four radio states, including idle, transmit, receive, and sleep state, are set according to the study results of Chipcon CC2420 [7]: Pidl=712 uW, Ptx=31.32 mW,
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Figure 6. Comparison on power consumption of sensor nodes between IBOA-based IEEE 802.15.4 and traditional one.
Figure 7. Comparison on power consumption of coordinator between IBOAbased IEEE 802.15.4 and traditional one.
Prx=35.28 mW, and Pslp=144 nW. Both the SO and BOmin are set to zero, and the BOmax is set to six, thus the maximal beacon interval is next to one second in terms of (1). It should be noticed that setting BOmin to an extreme small value is not always necessary, since the low-power WSN applications, like the above-mentioned traffic or medical monitoring whose lifetime can be several months or years, do not require frequent operations of synchronization and communication. To a certain extent, setting BImin to a minute or two is enough for these applications. So we just validate the IBOA in a worst case when the BOmin is set to zero. And the rest of the simulation parameters are set to their defaults [1]. To study the impact of IBOA on the network performance under either light or heavy traffic loads, we increase the traffic rate per node gradually from 1 packet/s to 32 packets/s, and compare the simulations of the IBOA-based IEEE 802.15.4 with that of the traditional one. For the comprehensiveness of comparison, we change the fixed BO of the traditional one from zero to six.
Figure 8. Comparison on power consumption of whole network between IBOA-based IEEE 802.15.4 and traditional one.
with IBOA is relative high in this worst case even the traffic load is light. That is because the majority of the coordinator’s power consumption is used for beacon broadcast when the traffic load is light, while the IBOA have to make the coordinator keep broadcasting beacon every BImin symbols and thus consume power considerably. However, the extra power consumption for additional beacon broadcast is the minority of the total power consumption when there are a lot of data frames need to be received, thus the negative impact of frequent beacon broadcast can be minor then. In addition, the cost brought by BO control is also relative low, because the BO of each node is not always changed rapidly and largely if the traffic load is uniformly distributed in the main. To validate the effectiveness of IBOA, the total power consumption of the network is showed in Fig.8, from which we can see that the increase in the power consumption of coordinator for additional operations does not bring obvious performance degradation to the whole network. Moreover, if there are more nodes working in the network, the extra power consumption can be more neglectable.
A. Analysis of Power Consumption Power consumption is one of the most important metrics for the performance evaluations of WSNs, since most of the sensor nodes working in the networks are powered by batteries. From Fig.6, we can see that the IBOA adapts the BOs well. When the traffic per node is relative light, the performance of the IBOA-based one is close to that of the traditional one with large BOs, and then the power consumption is very low for the long sleep time of the nodes and little power consumption for beacon receptions. Contrarily, if the traffic per node is increased to a relative heavy level, the performance of the IBOA-based one is close to that of the traditional one with small BOs. It should be noticed that the low level of power consumption is not the focus of IBOA anymore, if the traffic load is too heavy. To meet the needs of various kinds of applications, the power consumption of coordinator should be considered, since we also assume that the coordinator is powered by battery. It is showed in Fig.7 that the power consumption of coordinator
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Figure 9. Comparison on average end-to-end delay of whole network between IBOA-based IEEE 802.15.4 and traditional one.
Figure 10. Comparison on throughput of whole network between IBOAbased IEEE 802.15.4 and traditional one.
B. Analysis of End-to-End Delay The IBOA is proposed to deal with the performance tradeoff between power consumption and end-to-end delay, so endto-end delay is another important metric. From Fig.9, we can see that the IBOA also adapts the performance of end-to-end delay well. When the traffic per node is light, the end-to-end delay of the IBOA-based one is relative large, since the main focus of IBOA is not end-to-end delay but power consumption then. However, with the growing traffic per node, the end-toend delay keeps going down to a very small level, although the power consumption of sensor nodes keeps rising as the same time. Therefore, the avoidance of the large end-to-end delay brought by large BOs is well worth the extra power consumption.
channel competition between sensor nodes. Therefore, the IBOA can deal with the trade-off between power consumption and end-to-end delay well, and the degradation of throughput with the growing traffic load can also be alleviated. Our future work will focus on the limitations of the IBOA algorithm. Generally, our proposed algorithm is not suitable enough for the applications with a large amount of data to be transmitted at a time, since the individually adaptation brings fairness problem. Therefore, for the applications such as multimedia, the algorithm needs to be specifically designed if the nodes always have large data sequence to transmit. REFERENCES [1]
C. Analysis of Throughput As mentioned above, the IBOA can prevent all of the nodes with pending frames from waking up at one time and competing for channel access in a same active portion. According to Fig.10, we can see that the throughput of the IBOA-based one can keep increasing at a relative high and steady rate with the growing traffic per node, while the considerable dropping frames of the traditional one for heavy traffic are inevitable. IV.
[2]
[3]
[4]
CONCLUSION
[5]
In this paper, the IBOA algorithm for IEEE 802.15.4 networks is proposed, which can effectively adapt the longlasting large end-to-end delay for low-power WSN applications. Simulations have showed that, under each node’s light traffic loads, the IBOA prolongs its beacon interval to reduce power consumption of the node for longer sleep time and less beacon receptions. Under heavy traffic loads, however, the IBOA shortens its beacon interval to improve the performance of end-to-end delay and throughput for less fierce
[6]
[7]
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