1 Is Clock Synchronization Essential for Power Management in IEEE 802.11-Based Mobile Ad Hoc Networks? Ming Liu Ten H. Lai Ming-Tsan Liu Department of Computing and Information Science The Ohio State University Columbus, OH 43210 {mliu, lai, liu}@cis.ohio-state.edu Abstract— Power is always a major concern for mobile unable to work at all, for MANETs. It is not a surprise devices. To save precious energy, mobile devices choose to to see reports on IEEE 802.11 protocols’ mediocre stay in a low power mode by turning off their transceivers peformance when applied to MANETs [13], [19], [20]. when it is not involved with transmission. However, they However, despite its non-optimal performance, IEEE need to wake up to allow packets originated from them sent 802.11 is still considered by many people as the most and those coming to them received. This presents a chal- lenge in the distributed environment as in ad hoc networks. promising technology for MANETs. The reason is sim- In this paper, we discuss the relationship between clock syn- ple: 802.11 already has a world-wide customer base chronization and power-saving in IEEE 802.11-like mobile — millions of devices around the world are “802.11- ad hoc networks. Based on observations and studies of other capable”, and that number is increasing fast in these a cou- power-saving protocols, we propose an integrated protocol ple of years. In order for MANET-capable devices to be for both clock synchronization and power-saving targeted appealing to consumers, it is desired that they be 802.11- for a mobile environment. It handles topology changes sat- compatible. Recently, a few 802.11-based, MANET- isfactorily, and simulation results show it has superior per- formance over other protocols. oriented protocols have been proposed for CSMA/CA [1], Keywords: Mobile ad hoc network, Power-saving, clock clock synchronization [22], and power-saving [17]. synchronization, IEEE 802.11 This paper considers two fundamental problems in wireless networks: time synchronization and power man- agement. Power is arguably the scarcest resource for mo- I. INTRODUCTION bile devices; and power- saving has always been a major IEEE 802.11 [10], [11], [12] is the most widely issue for the designers of mobile devices, wireless com- adopted protocol standard for wireless local area networks munication systems and protocols. Many power conserva- (WLANs). It specifies two different modes: the infras- tion schemes have been implemented or proposed for a va- tructure mode and the ad hoc mode. In the ad hoc mode, riety of mobile devices. For example, Intel’s SpeedStepTM current standards are based upon an environment where and AMD’s PowerNow!TM technologies adjust the power all stations in the WLAN are within one another’s trans- consumption of the CPU by changing the operating fre- mission range, and they communicate in a peer-to-peer quency of its internal clock dynamically and shutting fashion. In other words, the ad hoc mode of 802.11 sup- down unnecessary components according to the demand ports only single-hop ad hoc networks, referred to in the from different applications. Some researchers have pro- specification as Independent Basic Service Sets (IBSSes). posed schemes adjusting transmission power to conserve Recently, mobile multi-hop ad hoc networks energy [5], [14], [8], while others have discussed routing (MANETs) have attracted much of attention from protocols which are power-aware [7], [16], [18]. Power researchers. Unlike IBSS, stations in a MANET are not can be saved during clustering stage [3], [15], and also by necessarily within one another’s transmission range. It reducing contention in MAC layer [2], [6]. Also research may take multiple hops for a message to travel from has been done based on the IEEE 802.11 power man- source station to destination (hence the adjective multi- agement scheme, which puts idle stations (those with no hop). Because of this difference, protocols or schemes traffic coming in or out) into temporary sleep by shutting that work well for IBSSes may perform poorly, if not down their transceivers [4], [17], [21]. In this approach, to 2 prevent stations in power-saving mode from missing data network, and more seriously, the network may be parti- coming to them, the stations wake up from time to time tioned temporarily (but long enough to make clocks out of so that others can page them. It is important that stations synchronization)? We propose an integrated protocol that wake up at the same moments or they might still miss each provides both clock synchronization and power-saving in other. For this reason, time synchronization plays an im- such a general environment. Simulation results support portant role for power management in IEEE 802.11-based our belief of merits of the proposed protocol. ad hoc networks1. The rest of this paper is organized as follows. Sec- The Timing Synchronization Function (TSF) as de- tion II categorizes power management schemes according scribed in the 802.11 specifications is known to have a to their relationship to clock synchronization. Section III scalability problem (even just for the single-hop IBSS), analyzes the efficiency of different schemes in terms of which, fortunately, can be easily fixed [9]. The improved power saving. The proposed protocol is described in sec- TSF, like the original 802.11 TSF, was intended only for tion V, with performance studies and simulation results the single-hop IBSSs, and was not expected to work well presented in section VI. Concluding remarks and future in a multi-hop environment. In [22], a time synchroniza- work are given in section VII. tion algorithm for multi-hop ad hoc networks was pro- posed. Under the assumptions 1) that the ad hoc network II. BACKGROUND was started by a single node, and 2) that the network re- One of the most common models for power manage- mains connected (but may change its topology) during ment is to divide time into repeated intervals called bea- its lifetime, the proposed protocol can synchronize 500 con intervals. Inside each interval, stations switch be- clocks to within 100 microseconds, as apposed to IEEE tween two modes, Active Mode (AM) and Power Save 802.11 TSF’s 1200 microseconds. Without the above two mode (PS). Most power management schemes, including assumptions, the clock synchronization problem has yet the one in IEEE 802.11 standard, adopt this model. Dur- to be solved for MANETs. In fact, Tseng et al. argued ing AM there are usually dedicated periods for beacon and that since clock synchronization in a multi-hop MANET Traffic Indication Map (TIM) transfer. If a hand-shaking is difficult, if not impossible, it is desirable to have power- succeeds, both stations will stay in AM to transmit/receive saving protocols that do not rely on clock synchronization packets. Otherwise, the station will turn off its transceiver at all. They proposed three such protocols in [17]. Mean- and switch to PS until next scheduled AM. In order to no- while, Ye et al. proposed another compromise – instead tify the receiver of upcoming packets, there must be some of network-wise clock synchronization, they used local overlap between the AM periods of the sender and the re- clock synchronization. ceiver. Thus clock synchronization plays a very important Thus, depending on the role of clock synchronization, role in power management protocols. there are three approaches to the power management in 802.11-based multi-hop MANETs: using global clock A. Clock Synchronization in IEEE 802.11 synchronization, using no clock synchronization, and us- Before introducing the power management in IEEE ing local (or partial) clock synchronization. In this paper, 802.11, we will first take a brief look at its clock syn- we study the relationship between clock synchronization chronization for IBSS, which is achieved by the TSF de- and power-saving in 802.11-based MANETs, attempting fined in the standard. According to the TSF, at the be- to answer the question: Is clock synchronization essential ginning of every beacon interval, each station calculates for power-saving? We analyze the three approaches and a random delay time uniformly distributed between zero show that with a fully synchronized system, the 802.11- and 2×aCW min×aSlotT ime. If a beacon packet from like power management scheme would be the most effec- other station is received during this period, the station can- tive. This result may not be surprising, but useful. It re- cels the pending beacon transmission. Otherwise, at the enforces the importance of clock synchronization as has end of the delay, the station transfers a beacon packet con- been perceived by the designers of 802.11. We then solve taining its own timestamp. Collision of beacon packets the open problem left in [22]: how to synchronize clocks are resolved by the same CSMA/CA method as in other in a general MANET, where multiple stations may initiate MAC transmissions. Upon receiving a beacon packet, a the formation of the network simultaneously, new stations station will compare the timestamp recorded in it with its may join while old stations move around or even leave the own clock. The station will adjust its clock only if the beacon packet shows a faster clock value of the sending 1Time synchronization is also important for networks that employ frequency hopping, where stations need to hop from frequency to fre- station. So the main characteristics of the TSF in IEEE quency simultaneously. 802.11 are: 3 Beacon Interval Beacon Interval Odd Beacon Interval Even Beacon Interval Target Beacon Time Host A ATIM ATIM ATIM Host B Window Window Window Odd Beacon Interval Even Beacon Interval Beacon Xmit ATIM Beacon Window MTIM Window Rcv ACK Xmit Frame Rcv ACK Fig.