Cluster-Based Semi-Asynchronous Power-Saving Protocols for Multi-hop Ad Hoc Networks

Transcription

1 Cluster-Based emi-asynchronous Power-aving Protocols for Multi-hop Ad Hoc Networks Chih-hun Hsu Department of Information Management Nanya Institute of Technology Chung-i, 320, Taiwan Yu-Chee Tseng Department of Computer cience and Information Engineering National Chiao Tung University Hsin-Chu, 300, Taiwan Abstract Existing power saving (P) protocols for mobile adhoc networks (MANETs) can be categorized into synchronous and asynchronous ones. The synchronous P protocol can not be applied to a multi-hop MANET, because it will cause three problems, namely clock synchronization, neighbor discovery, and network partitioning. Although asynchronous solutions are attractive, yet, the cost is high as opposed to synchronous protocols. Hosts in asynchronous protocols need to keep awake for longer time, so as to discover asynchronous P neighbors. Besides, a broadcast message has to be sent multiple times if a sending host s neighbors wake up asynchronously. To conquer the deficiency of asynchronous P protocols, we propose several cluster-based semi-asynchronous P protocols for multi-hop MANETs. The basic idea is to cluster neighboring hosts such that synchronous P protocols can be adopted within individual clusters, and conserving a lot of energy. Two asynchronous schemes are provided in the inter-cluster level, including NR-probabilitybased and location-based schemes. imulation results show that, the proposed semi-asynchronous approaches outperform the asynchronous P protocols when applied to a multi-hop MANET. I. INTRODUCTION ince most mobile devices do not have plug-in power, battery is the only source to support their operations. Battery energy is a limited resource. Therefore, how to conserve battery energy is a critical issue for a MANET, which consists of mobile hosts only. A lot of works have dedicated to the design of wakeup and sleep patterns of mobile hosts [1], [2], [3]. However, these protocols are not power efficient in a sparse network, because almost all the hosts need to keep active all the time. IEEE s P protocol is synchronous in the sense that all hosts must remain time-synchronized. Clock synchronization in a large scale MANET is difficult. To solve the synchronization problem, asynchronous power-saving (P) protocols have been proposed. It is first pointed out in [4] that without harmonization there could exist neighbor discovery and network partitioning [4] problems. Three asynchronous P protocols, namely dominating-awake-interval, periodically-fully-awakeinterval, and quorum-based protocols, are proposed in [4] for multi-hop MANETs. Another asynchronous P protocol for multi-hop MANETs is proposed in [5], which formulates the problem of generating wakeup schedules as a block design problem in combinatorics. To improve the works in [4], several quorum-based P protocols, namely grid quorum, torus quorum, cyclic quorum, and finite projective plane quorum protocols, are proposed in [6]. In this paper, we observe that although asynchronous P protocols can conquer the clock synchronization problem, they may have some performance concerns. In order to discover asynchronous neighbors, hosts need to keep awake for some extra time. Besides, the broadcasting cost is high, because it takes several transmissions for a host to wake up all its asynchronous neighbors. To conquer these deficiencies, we propose several cluster-based semi-asynchronous P protocols for multi-hop MANETs. Although, a cluster-based semiasynchronous P protocol for wireless sensor network (- MAC) has been proposed in [7], yet, -MAC is designed for a static network. When applied to multi-hop MANETs, neighbor discovery and network partitioning problems may occur. The proposed protocol works as follows. irst, hosts are clustered. Inside each cluster, hosts adopt a synchronous P protocol so as to save more energy. Then each cluster is regarded as a super node and among super nodes we will run an asynchronous P protocol. To realize this idea, within each cluster we will request some hosts to serve as watchers in a rotation manner. Only watchers need to run the asynchronous P protocol. We propose two schemes to delegate hosts as watchers: NR-probability-based and location-based schemes. These schemes are adaptive, energy-efficient, and sensitive to the change of neighbors. imulation results show that the proposed semi-asynchronous approaches do outperform asynchronous P protocols when applied to a multi-hop MANET. The rest of this paper is organized as follows. In ection 2, we present our cluster-based semi-asynchronous powersaving protocols. Routing protocols based on our power-saving mechanisms are discussed in ection 3. imulation results are presented in ection 4. ection 5 concludes this paper. II. CUTER-BAED EMI-AYNCHRONOU POWER-AVING PROTOCO The goal of our P protocol is to achieve the power efficiency of the synchronous P protocol, while still being able to be applied to a multi-hop MANET. We partition the network into several clusters and synchronize all the hosts in the same cluster. The host with enough energy and the fastest

2 clock among all its neighbors will be chosen as the cluster head and each host will follow the clock of its cluster head. ince the distance between the cluster head and cluster member is only single hop, it would be much easier for the cluster head to synchronize all the hosts in the same cluster. When all hosts in the same cluster are synchronized, each P host can adopt the synchronous P protocol (eg. IEEE s P protocol) as its intra-cluster protocol, so that it can conserve more power. If we regard each cluster as a super node, the clustering MANET can be regarded as a network consists of several asynchronous super nodes. Therefore, how to design the intercluster strategy, so that the asynchronous super nodes (clusters) can detect each other, is an important issue. Each of the super node (cluster) may adopt the asynchronous P protocol to detect its asynchronous neighbors. ince each cluster may consist of several P hosts, it is not necessary for every cluster member to adopt the asynchronous P protocol simultaneously. The members of each cluster may serve as watchers by adopting the asynchronous P protocol in turn. The P host not serving as watchers may adopt the synchronous P protocol. Each P host can adopt either synchronous or asynchronous P protocols according to the schemes proposed in section II-B. Because the paper length is limited and the clustering problem of MANET has been studied extensively [8], we will not discuss how to cluster the MANET in this paper. In the following subsections, we will first show the channel model of our protocol. Then two schemes are proposed to guide the scheduling of P modes. A. Channel Model ince the proposed P protocol is cluster-based and semiasynchronous, its channel model is different from that of IEEE and asynchronous protocols. To maintain the cluster structure and synchronize the hosts in each individual cluster, the structure of beacon interval needs to be redesigned. Each beacon interval may contain four different windows, namely active window, synchronous window, beacon window, and MTIM (Multi-hop TIM) window. The structure of a beacon interval may vary for different protocols (to be elaborated later). The active window is the extra active period that the P host should turn on its receiver to listen to any packet and take proper actions. The synchronous window is for the cluster head to send its beacon, while the beacon window is for the other P hosts to send their beacons. The MTIM window is for other hosts to send their MTIM frames to the P host. MTIM frames serve the similar purpose as ATIM frames in IEEE To avoid collisions among beacons/mtims, a host should wait for a random number of slots before sending out its beacon/mtims. Normally, the host adopts the synchronous P protocol. The structure of the synchronous interval is similar to that of IEEE , each interval starts with a synchronous window followed by an MTIM window. When serving as a watcher, the host may adopt the periodically-fully-awakeinterval or quorum-based protocols [4]. When the periodically- ig. 1. interval ully-awake Interval ynchronous Window ow-power Interval Beacon Window MTIM Window ynchronous Interval (c) ActiveWindow.ully-awake interval,.ow-power interval (c).ynchronous fully-awake-interval protocol is adopted, the beacon intervals are classified as low-power and fully-awake intervals. The fully-awake intervals arrive periodically every p intervals, and the rest of the intervals are low-power intervals. The structures of these beacon intervals are defined as follows. Each low-power interval starts with a synchronous window followed by a beacon window and an MTIM window. In the rest of the time, the host can go to the sleep mode. Each fully-awake interval also starts with a synchronous window followed by a beacon window and an MTIM window. However, the host must remain awake in the rest of the time. When the quorum-based protocol is adopted, the beacon intervals are classified as fully-awake (quorum) and synchronous (non-quorum) intervals. ig. 1,, and (c) show the structures of the fully-awake, low-power, and synchronous intervals, respectively. B. Power Mode cheduling When a host has idled for a certain period of time, it may switch to P mode. The synchronous P protocol is adopted as the intra-cluster P protocol and the asynchronous P protocol is adopted as the inter-cluster P protocol. The P host follows the clock of the cluster head to turn on and turn off its radio periodically. To detect the approaching asynchronous cluster, some hosts need to serve as watchers by adopting the asynchronous P protocol. To conserve energy, the other P hosts would adopt the synchronous P protocol. The designing features of our protocols are shown as follows: adaptive: the protocol should be adaptive to host density, mobility, and location. sensitive to neighbor change: each cluster should be able to detect the change of neighboring clusters. or any two asynchronous watchers, if their duty time are overlapping, they would have chance to detect each other. energy efficient: the proposed protocol should be more efficient than the asynchronous P protocol in terms of power consumption. According to the above features, we propose two schemes for our power mode scheduling protocol. In the beginning of each round, the P host decides its power mode according to the proposed scheduling schemes. There are p beacon intervals in each round, where p is a multiple of the quorum size or period length of the adopted asynchronous P protocol. If the P host serves as a watcher, it will adopt the asynchronous P protocol in this round, otherwise, it will adopt the synchronous P protocol in this round.

3 1st round 2nd round 1st round 2nd round ig. 2..An example of adopting the periodically-fully-awake-interval protocol as the asynchronous P protocols.an example of adopting the cyclic quorum scheme as the asynchronous P protocols, where represents synchronous, represents fully-awake, and represents low-power intervals. In ig. 2 and, assuming the P host decides (or is dispatched) to serve as a watcher in the first round and become normal host in the second round. The round length is set as 10 beacon intervals. In ig. 2, the periodically-fully-awakeinterval protocol is adopted as the asynchronous P protocol. The fully-awake interval arrives at the first interval and the low-power interval arrives at the second, third,..., and tenth intervals of the first round. In ig. 2, the cyclic quorum scheme [6] is adopted as the asynchronous P protocol. The quorum interval arrives at the first, second, third, and sixth intervals of the first round. The synchronous interval arrives at the other intervals. To detect the change of neighboring clusters, each cluster should have at least one host to serve as the watcher in each round. In a loosely coupled cluster, since the cluster head does not maintain the cluster structure, each host decides its power mode according to its own knowledge. An N R-probabilitybased is proposed for loosely coupled clusters. The probability is set according to the number of detected cluster members, the NR (ignal Noise Ratio) of the beacon frame sent by the cluster head and the mobility of the host. The host can use the detected N R to estimate its distance from the cluster head. The greater the NR is, the shorter the estimative distance becomes. The host that has longer distance from the cluster head will have higher probability to switch to detecting mode, since it is closer to the border of the cluster. or example, if the number of detected cluster members is n, k is a factor set according to the moving speed of the host, the estimative distance between the host and the cluster head is d, and the communication range is r, the probability that the host will switch to detecting mode is (d + r/2)k/nr. This scheme is adaptive to host density, mobility and distance. In a tightly coupled cluster structure, since the cluster head has the knowledge of all its cluster members, it can use these information to dispatch proper hosts to serve as watchers. A location-based is proposed for tightly coupled clusters. In this scheme, the cluster head needs to record the locations of all its cluster members. (The location of the host may be derived from GP or other location estimating methods.) In each round, the cluster head constructs a convex hull according to all its cluster members locations. ince the coverage area of the hosts inside the convex hull may be covered by other hosts, only the hosts belonging to the convex hull need to be dispatched as watchers. The cluster head dispatches k watchers in each round according to a greedy algorithm. Assuming the set of hosts belonging to the convex hull is denoted as CH. The cluster head first dispatches a host a, which is farthest from the cluster head, from CH. And then host a is removed ig. 3. An example of anycasting in a MANET with asynchronous clusters. from CH. The rest k 1 watchers are dispatched according to their extra coverage area. The k 1 hosts those can cover the greatest extra area are dispatched as the watchers. In the following rounds, the set CH would be updated, but the removed hosts will not be added back to set CH until CH =. When CH =, the cluster head will reset CH. This scheme is adaptive to host density, mobility and location. III. ROUTING PROTOCO OR THE PROPOED POWER-AVING PROTOCO When transmitting packets to the P host in the same cluster, we can adopt the unicast protocol of IEEE s P protocol. or inter-cluster communication, the anycast protocol can be adopted. It is similar to the unicast protocol in IEEE s P protocol, except that the destination host s id is replaced by the destination cluster s id. During the destination cluster s MTIM window, the sender contends to send its MTIM packet to the destination cluster. Any of the host belonging to the destination cluster received the MTIM packet would reply an ACK after an I and stay awake in the remaining of the beacon interval. If the sender hears a busy tone after the MTIM packet has been sent, it will adopt the DC procedure to send the buffered packet to the receiver after the MTIM window. With the anycast protocol, each host needs not to be very sensitive to the change of neighbors. As for the broadcast protocol, since all the hosts in the same cluster are synchronized, we can adopt the broadcast protocol of IEEE s P protocol. ig. 3 shows an example, where host A wants to transmit a packet to a neighboring cluster, where hosts B and C belong to the same cluster. During the cluster s MTIM window, an MTIM frame is sent from host A to the cluster. In response, hosts B and C will reply with ACKs. After the cluster s MTIM window finishes, host A can try to send out its data packet. In our routing protocol, the route request packet is scattered along the backbone, which consists of cluster heads and gateways. irst, the source host (if it is not the cluster head) will unicast the route request packet to the cluster head. After received the route request packet, the cluster head will broadcast the route request packet to its cluster members, and then the gateways use the anycast protocol to scatter the route request packet to the neighboring cluster, and so on. When the route request packet has reached the destination, unicast can be used to send the route reply packets. When P hosts in the

4 chosen route receive the route reply packet, it can go to the active mode. In a cluster-based network, except the host close to the border of the network, most of the hosts would become cluster heads or gateways. To reduce redundant transmissions, two strategies can be adopted while scattering the route request packet. In a loosely coupled cluster, each gateway may give up to send its anycast packet, if it has overheard the same packet, as its own anycast packet, being sent to the same destination. In a tightly coupled cluster, the cluster head will assign proper gateways to transmit packets to neighboring clusters and thus reduce redundant transmissions. IV. IMUATION EXPERIMENT To evaluate the performance of the proposed P protocol, we have developed a simulator using C. In the simulations, we assume that there are 100 hosts in the network, the area size is 1000m 1000m, and the transmission radius is 250 meters. Hosts transmission rate is 2M bits/sec, the length of the beacon interval is 100 ms, and the battery power of each mobile host is 100 J. The MAC part basically follows the IEEE standard [9], except the power management part. or the proposed P protocols, the routing protocol proposed in section III is adopted; for asynchronous P protocols, the AODV routing protocol is adopted. When the route is detected to be broken, the data packets would be dropped. The periodically-fully-awake-interval and cyclic quorum protocols are adopted as the asynchronous P protocols. The source and the destination of each route are randomly selected. Two parameters are tunable in our simulations: Traffic load: Routes are generated by a Poisson distribution with rate between 1 5 routs/sec. or each route, 40 packets, each of size 256 bytes, will be sent. Mobility: Host mobility follows the random way-point model. The pause time is set to 30 seconds. When moving, a host will move at a speed between 0 20 m/sec. Basically, each simulation lasts until all the hosts have run out of energies. Each result is obtained from the average of 100 simulation runs. or simplicity, we assume that all hosts are in the P mode. Three performance metrics are used in the simulations: survival ratio: the number of surviving hosts over the total number of hosts. (A host is said to be surviving if its power is not exhausted yet.) Throughput: the average number of MAC-layer data packets successfully received in the network per second. The throughput is evaluated up to the time when any of the host run out of energy. ifetime throughput: the total number of MAC-layer data packets successfully received during the lifetime of the network. The power model in [10] is adopted, which is obtained by real experiments on ucent WaveAN cards. To make comparison, we also simulate the two asynchronous P protocols: periodically-fully-awake interval [4] and cyclic quorum ig. 4. (a ).Throughput vs. mobility..ifetime throughput vs. mobility. scheme [6]. ince the active ratios of the periodically-fullyawake interval with period length 5 and cyclic quorum scheme with quorum size 35 are almost the same ( 1 5 ), the round lengths are set as 5 and 35 beacon intervals when periodicallyfully-awake interval and cyclic quorum scheme are adopted, respectively. or simplicity, the asynchronous P protocols, periodicallyfully-awake-interval protocol with parameter p and the cyclic quorum scheme with parameter n, are denoted as P(p) and C(n), respectively. The NR-probability-based scheme adopting P(p) and C(n) as the asynchronous P protocols are denoted as PNR(p) and CNR(n), respectively. The location-based scheme adopting P(p) and C(n) as the asynchronous P protocols are denoted as P CB(p) and CCB(n), respectively. A. Impact of Mobility To observe the effect of mobility, we fix the traffic load as 1 route/sec and vary hosts moving speed between 0 20 m/sec. ig. 4 shows the impact of mobility on throughput. Mobility has a negative impact on throughput for all schemes because the routes are more likely to be broken as the moving speed increases, and thus the data packets are more likely to be dropped. The throughput of the proposed schemes (e.g. PNR(5), CNR(35), PCB(5), and CCB(35)) is slightly lower than that of the asynchronous schemes (e.g. P(5) and C(35)), because the asynchronous schemes adopt AODV as their routing protocol, which uses flooding to discover route, and thus are less sensitive to mobility. However, as ig. 5 shows, the hosts adopting the proposed schemes survive much longer than the hosts adopting the asynchronous schemes. Hence, the lifetime throughput of the proposed schemes is much better than that of the asynchronous schemes as shown in ig. 4. Overall, the scheme adopting the cyclic quorum protocol performs better than the one adopting the periodically-fully-awake protocol, because the cyclic quorum scheme transmits less beacons than the periodically-fullyawake scheme and thus causes lees interferences and has a better throughput. The NR-probability-based scheme performs slightly better than the location-based scheme in terms of throughput, because the NR-probability-based scheme is based on a loosely coupled cluster structure, which is less sensitive to mobility. ig. 5 shows the impact of mobility on survival ratio. Higher mobility may bring more hosts to act as watchers and thus consume more power. However, the route is more likely to

5 ig. 5. urvival ratio vs. mobility..moving speed = 0 m/sec, and.moving speed = 20 m/sec. ig. 6. urvival ratio vs. traffic load..traffic load = 1 route/sec, and.traffic load = 5 routes/sec. be broken in high mobility environment and thus less data packets are transmitted and less power is consumed. The hosts adopting the proposed schemes survive much longer than the hosts adopting the asynchronous schemes, because the broadcasting cost is much lower and only a few P hosts need to serve as watchers by adopting asynchronous P protocol. In the proposed schemes, since most of the P hosts adopt the synchronous P protocol for most of the lifetime and lots of redundant transmissions are reduced, lots of power can be conserved. B. Impact of Traffic oad To observe the impact of traffic load, we fix the moving speed as 10 meters/sec and vary the traffic load between 1 5 routes/sec. ig. 6 shows how traffic load decreases the survival ratio of hosts. As the traffic load increases, each P host needs to keep awake for a longer period of time to establish routes and transmit data packets, and thus consumes more power. The effect of traffic load on throughput is shown in ig. 7. As the traffic load becomes higher, the throughput increases. When the traffic load is high, the proposed schemes outperform the asynchronous schemes in terms of throughput. ince the proposed schemes spend less cost to establish a route, the proposed scheme can tolerate a higher traffic load and saturate slower than the asynchronous schemes. ig. 7 shows that the proposed schemes outperform the asynchronous schemes in terms of lifetime throughput significantly, because the proposed schemes possess a longer network lifetime and a higher throughput. Overall, the location-based scheme performs slightly better than the NR-probability-based scheme scheme in terms of survival ratio, and the NR-probabilitybased scheme performs slightly better than the location-based scheme in terms of throughput. V. CONCUION In this paper, we have addressed the problems of asynchronous P protocols in a multi-hop MANET. We have pointed out two major drawbacks. irst, the host needs to keep awake for longer time, so as to discover asynchronous P neighbors. Besides, a broadcasting message has to be sent multiple times if a sending host s neighbors wake up asynchronously. To conquer the deficiency of asynchronous P protocols, we have proposed several cluster-based semiasynchronous P protocols for multi-hop MANETs. Two strategies have been provided in our inter-cluster strategy, including NR-probability-based and location-based schemes. imulation results have shown that the proposed schemes outperform the asynchronous schemes in terms of survival ratio and lifetime throughput. Overall, the NR-probabilitybased scheme adopting the cyclic quorum protocol performs the best in terms of lifetime throughput and the location-based scheme adopting the cyclic quorum protocol outperforms the others in terms of survival ratio. REERENCE [1] B. Chen, K. Jamieson, H. Balakrishnan, and R. Morris, pan: An Energy-Efficient Coordination Algorithm for Topology Maintenance in Ad Hoc Wireless Networks, Proc. of the International Conference on Mobile Computing and Networking, pp , [2] Y. Xu, J. Heidemann, and D. Estrin, Geography-informed Energy Conservation for Ad Hoc Routing, Proc. of the International Conference on Mobile Computing and Networking, pp , [3] Y.-C. Tseng and T.-Y. Hsieh, ully Power-Aware and occation-aware Protocols for Wireless Multi-hop Ad Hoc Networks, IEEE ICCCN, [4] Y.-C. Tseng, C.-. Hsu, and T.-Y. Hsieh, Power-aving Protocols for IEEE Based Multi-Hop Ad Hoc Networks, IEEE INOCOM, pp , [5] R. Zheng, J. C. Hou, and. ha, Asynchronous Wakeup for Ad Hoc Networks, ACM MobiHoc, pp , [6] J.-R. Jiang, Y.-C. Tseng, C.-. Hsu, and T.-H. ai, Quorum-based asynchronous power-saving protocols for ieee ad hoc networks, Proceedings of International Conference on Parallel Processing, [7] W. Ye, J. Heidemann, and D. Estrin, An energy-efficient mac protocol for wireless sensor networks, IEEE INOCOM, pp , [8] W. R. Heinzelman, A. Chandrakasan, and H. Balakrishnan, Energyefficient communication protocol for wireless microsensor networks, Proceedings of the Hawaii International Conference on ystem ciences, pp , [9] AN MAN tandards Committee of the IEEE Computer ociety, IEEE td , Wireless AN Medium Access Control (MAC) and Physical ayer (PHY) specifications, IEEE, [10]. M. eeney and M. Nilsson, Investigating the Energy Consumption of Wireless Network Interface in an Ad Hoc Networking Environment, IEEE INOCOM, pp , ig. 7. load..throughput vs. traffic load..ifetime throughput vs. traffic