Is Clock Synchronization Essential for Power Management in IEEE 802.11-Based Mobile Ad Hoc Networks? Ming Liu Ten H

Home , Lai (surname)

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 performance 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 management. 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 proposed. 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. 2. Interval structures in the Dominating-Awake-Interval protocol

Station A

Rcv ATIM Xmit ACK Rcv Frame Xmit ACK T (=3) Beacon Interval Station B Host A

Host B

Station C Power-Saving State Beacon Window MTIM Window

Fig. 1. Power management in IBSS in IEEE 802.11 Fig. 3. Beacon interval structure of Periodically-Fully-Awake- Interval protocol

• There is only one station sending the beacon packet in each beacon interval. • Clocks are adjusted forward only. ever the difference between two stations’ clocks is, their Because of these characteristics, Huang et al. point out protocols have a bounded number of intervals, N, in the IEEE 802.11 TSF suffers scalability problem[8]. The which there will be at least one exchange of TIMs be- Adaptive Timing Synchronization Procedure is proposed. tween them. N is a relatively small number specific to In this procedure, instead of all stations participating the protocols. beacon transmission in every beacon interval, each station maintains a different frequency of beacon transmissions. The first protocol is called Dominating-Awake-Interval This frequency is related to the relative speed of the sta- protocol. Its basic idea is to let each device stay awake tion’s clock. The faster a station’s clock is, the more fre- more than 50% of a beacon interval to catch others’ TIM quently it will participate the beacon transmission at the packet (Figure 2). Thus TIM and its acknowledge pack- beginning of beacon intervals. This will reduce the con- ets will be exchanged between any two devices in no tention of beacon transmission while increase the conver- more than two beacon intervals. However, stations run- gence speed of clock synchronization. ning Dominating-Awake-Interval protocol can only be in PS for no more than 50% of a beacon interval. So a more efficient protocol, Periodically-Fully-Awake-Interval pro- B. Power Management in IEEE 802.11 tocol, is proposed. In this protocol, a system parameter, Assuming all stations in an IBSS are synchronized by T , is introduced and there is one in every T beacon in- the TSF, IEEE 802.11 [10] defines power management as tervals. The Fully-Awake Interval is defined as the device shown in Figure 1. At each beacon interval, after a suc- stay awake throughout that beacon interval, while in other cessful beacon broadcast, an Ad hoc TIM (ATIM) window beacon intervals the device only wake up for the beacon of a fixed number of slots is used for ATIM transmissions. and MTIM window as shown in Figure 3. In this case, a All stations in the IBSS stay awake from the start of a bea- pair of stations can exchange TIMs at least once in every con interval till the end of the ATIM window. If a station T beacon intervals. The third protocol is based on the con- is not involved with any transmission, it goes to PS until cept of quorum [?]. A system parameter, n, defines a quo- the next beacon interval. For those stations have packets rum of n×n (Figure 4). Each station chooses one row and to exchange, they stay in AM for the rest of the beacon one column randomly. The beacon intervals in the picked interval. The same CSMA/CA access scheme is used for row and column are quorum intervals. Those 2n − 1 quo- all packet transmissions including the beacon and ATIM rum intervals are the same as the Fully-Awake Intervals in packets. Periodically-Fully-Awake-Interval protocol. In other intervals, the station goes to PS right after the MTIM win- C. Power Management Without Clock Synchronization dow. Because of the property of quorum, there will be at Since clock synchronization has problems itself, Tseng least two overlap quorum intervals between any two sta- et al. have proposed power management protocols not us- tions for a period of n × n intervals. Thus TIM exchanges ing any clock information. The authors prove that what- will be guaranteed. 4

n 0 1 2 3 0 1 2 3 list some notations we used in the following analysis in

c1 4 5 6 7 4 5 6 7 Table I. n

c2 8 9 10 11 8 9 10 11

12 13 14 15 12 13 14 15 A. No Clock Synchronization r r1 2 (a) (b) (c) [17] has a thorough study for the power-saving protocols in an environment where no clock synchronization is Fig. 4. Quorum-based protocol: (a) intersection of two hosts’ quo- available. Now we will evaluation the time a station has rum intervals, (b) host A’s quorum intervals, and (c) host B’s quorum to stay awake in each of the schemes. intervals 1) Dominating-Awake-Interval : In this protocol, a TABLE I station has to stay awake for at least half of each beacon COMMON NOTATIONS interval, because even no packet to or from the station, it still needs to listen to catch other stations’ beacon and Notation Representation MTIM packets after beacon and MTIM window. tBI Beacon interval length t Average time a station staying awake dur- Awake tListen ≥ tBI /2 − tTIM (1) ing a beacon interval tTIM TIM window length So that result tBW Beacon window length t = t /2 + t (2) tListen Time a station needs to keep listen other Awake BI BW than beacon window and TIM window (if The maximum time for an TIM packet exchange between there is one) any two stations is 2 × tBI . 2) Periodically-Fully-Awake-Interval : In this protocol, by having a Fully-Awake Interval in every T beacon D. Power Management Based on Local Clock Synchro- intervals, the time a station has to stay awake becomes nization (T − 1) × (tBW + tTIM ) + tBI Between the power management scheme which re- tAwake = (3) quires global synchronization and those not using clock T information at all, comes the power-saving scheme based It takes at most T × tBI for any two stations to exchange on local clock synchronization. In this scheme, multiple TIM packets. power-saving schedules based on different clock values 3) Quorum-Based : As deﬁned in the protocol, for ev- exist in a system. Stations synchronize in a distributive ery n × n beacon intervals, 2n − 1 of them are quorum way like IEEE802.11, forming clusters inside which all intervals, while the rest are non-quorum intervals. Based the stations’ clock and power-saving schedules are syn- on that, the time a station stays awake is chronized. In a multi-hop mobile environment, multi- 2 ple clusters may be established in a network. In order (2n − 1) × tBI + (n − 1) × tTIM tAwake = (4) to communicate among those non-synchronized clusters, n2 stations covered by more than one clusters have to fol- 2 During the period of n × tBI , there will be at least two low multiple power-saving schedules. They receive and exchanges of TIM packets between any two stations. handle multiple SYNC messages (beacon packets) in one beacon interval, keep tracking multiple clocks and power- B. Global Clock Synchronization saving schedules in their neighborhood, and move packets around among the stations in different clusters. For a power-saving protocol with a global synchronization scheme like IEEE 802.11, a station will stay awake for only III.ANALYSIS OF POWER CONSUMPTION tAwake = tBW + tTIM (5) In this section, we analyze the power consumption of And TIM packet can be exchanged between any pair of three categories of power-saving protocols reviewed in the stations in every tBI , if we don’t consider the effects of previous section. Due to the complexity of the analysis of contention. However, this does not include any effort to CSMA/CA scheme, we only analyze them under no trafﬁc reach such a global synchronization and keep every sta- situation. This will give us the best-case scenario of the tion stay in that way. How hard those are depends on the time which a station spends in PS. Before we start, we algorithm used and is not easy to quantify. 5

Awake Periods 1 Awake Periods 2 Overlap TABLE II Awake Period 1 Awake Period 2 SUMMARY OF AWAKE TIME RATIOS Clock synchronization method Awake time ratio

t0 t0+t AW t1 t1+t AW t t t +t t +t 0 1 0 AW 1 AW DAI 53% (a) (b) No synch. PFAI 32.5% Fig. 5. Multiple schedules in a beacon interval Quorum-Based 35.4% Global synch. 10% 2 schedules 19% C. Local Clock Synchronization Local synch. 3 schedules 28.4% In local clock synchronization schemes, there can be 4 schedules 37.87% more than one power-saving schedules in a system. For those stations which have only one schedule, their awake time is the same as (5) in the global synchronization t0 and t0 + tAW . Based on this and the definition scheme. Here we only focus on the stations which have of awake period length given above, we can calcu- more than one schedules to follow, therefore have multiple late the expected length of the current awake period awake periods in a beacon interval. Since those schedules with an integral of τ from t0 to t0 + tAW . Because are chosen independently by stations during their initial- of the independence of distributions of those starting ization, when those awake periods start are totally inde- points, we have the following formula to calculate pendent with each other. We describe the starting points the expected length of an arbitrary awake period in of those different awake periods with a uniform distribu- case 2: tion throughout the beacon interval, and all those awake Z t0+tAW E(tAwake in case 2) = (τ − t0)× periods have the same length of tAW , which is the sum of t0 beacon window, tBW , and TIM window, tTIM . But when P {an awake period starts at τ AND there is some overlap between two adjacent awake peri- no other awake period starts ods (Figure 5(b)), we define the length of the first awake period as the time between the starting points of the first between t0 and τ}dτ Z t0+tAW awake period and the second awake period. The length 1 τ − t0 2 = (τ − t0) × × (1 − )dτ of beacon interval is tBI . Here, we try to calculate the av- t0 tBI tBI erage length of the time a station has to stay awake when Combine the two cases, we have the expected length of an there are N(N >= 1) schedules. arbitrary awake period as: Let’s take a look at an arbitrary awake period and the awake period right after it. Here we only concern the tAW E(tAwake) = tAW × (1 − )+ awake period right after it so that we will not calculate tBI Z t0+tAW overlap more than once. There can be two basic cases of 1 τ − t0 (τ − t0) × × (1 − )dτ the relationship between them. t0 tBI tBI • One is when there is no overlap between those two t2 t3 = t − AW − AW (6) awake periods (Figure 5(a)). In this case, the length AW 2 2tBI 3tBI of the awake period we are looking at (not the one where tAW = tBW + tTIM and N >= 3. The above right after it. The length of that awake period is cal- 2 tAW formula reduces to tAW when N = 1, and to tAW − culated with its own follower.) is tAW . And also we 2tBI can easily figure out the probability of this situation when N = 2. From this average length, because of the happens is P 1 = 1 − tAW . independent and identical distribution of all awake peri- tBI • While in the other case (Figure 5(b)), the length of ods, the average total awake time for a station which has the awake period we are looking at is defined as the multiple schedules is just the number of awake periods it time between the starting point of current awake pe- has multiplies the average length of an awake period, i.e. N × (6) riod and the start of the next one (t0 − t1). If we use . a random variable, τ, to describe the starting point of the next awake period, the statistical representa- D. Summary tion of this situation is that an awake period starts We summarize the power performance of three schemes at t0 and another starts at τ, while no other awake based on different clock synchronization methods in Ta- period starts between t0 and τ, where τ is between ble II, using typical values for the parameters: tBI is 6

100ms, tBW is 3ms, and tTIM is 7ms. For no synchro- it wakes up, its SYNC broadcast will collide with current nization, T in Periodically-Fully-Awake-Interval is 4, and sender’s packet. This will further reduce throughput and n for quorum is 6. And for local synchronization, we cal- increase delay. culate stations with 2, 3, or 4 schedules. For local synchronization based power management, maintaining multiple schedules also has its own issues: IV. SHOULD CLOCK SYNCHRONIZATION BE • A station which has more than one schedules to fol- GLOBAL,LOCAL, OR NONE? low stays awake during all awake periods of those After the review and the analysis of different power schedules. Its sleep time reduces dramatically with management schemes, we come to the question we make the increase of the number of schedules it has to fol- the title of this paper, Should clock synchronization be low. Even though it can be argued that those multi- global, local, or none when considering power manage- schedule stations do not have to follow all those ment? Each of those schemes has its advantages and lim- schedules in every beacon interval, they still have itations. to track other schedules by listening to their bea- The global clock synchronization gives the most simple cons from time to time to maintain synchronization and efficient support to the power management, and has and transmission. All these maintenance procedures the best performance on power-saving. However, it relies present considerable cost on the system performance. on the efficiency of the clock synchronization mechanism • With multiple schedules exist in the system, clus- underlying. In a system where a central station exists and ters of stations with different schedules which are all other stations all synchronize with it, like cellular net- unaware of each other may move into each other’s works, it is very easy to achieve such global synchroniza- coverage area. A discovery scheme is needed to set tion, but in a distributed mobile system like MANET, both up connections among such clusters in order to trans- scalability and mobility bring serious challenges to clock fer packets across them. synchronization across the system. We will further elab- So, a global clock synchronization scheme in a multi-hop orate on the clock synchronization problem in the next environment would be a great benefit, if not essential, for section. an efficient power management protocol. How to achieve The schemes based on no clock synchronization ap- and maintain such a synchronization is the main chal- proaches from the other end. Since those clock syn- lenge, and that becomes the problem we try to solve in chronization algorithms are neither efficient nor accu- the next section. rate enough in MANET, they will just do without it. These schemes eliminate the need of tracking other nodes’ V. PROPOSED PROTOCOL clocks, but each station spends a lot of time on listening to catch beacon or TIM packets. Since they do not keep In the preceding section we argued that global syn- neighbors’ clocks and power-saving schedules, a station chronization is important for power management in an has to use the same procedure every time to catch beacon IEEE 802.11-based MANET. The schemes based on local packets whenever it initiates a transmission session even or no clock synchronization are less efficient in terms of just talked to the same node not long ago. The efficiency power saving. They were proposed mainly because global of power-saving suffers greatly, and it also introduce ex- clock synchronization is difficult to achieve in an IEEE tra delays for traffic. These delays can be quite signifi- 802.11-based MANET. Recently, Lai and Zhou [22] have cant since it occurs for every transmission sessions, and it proposed a global clock synchronization protocol with a also affects the throughput once packets flow gets heav- synchronization accuracy of less than 100 microseconds. ier. In the systems where beacon intervals of different sta- This protocol focuses on the scalability and accuracy of tions are not synchronized, including the scheme based on the clock synchronization in MANET, and it is proposed no clock synchronization and the scheme based on local based on two assumptions: clock synchronization that we will talk about next, there • The MANET is initiated by a single station. Because is a common problem of how to coordinate data packets of the lack of merging groups of unsynchronized sta- and/or signaling packets (like SYNC, and RTS/CTS pack- tions, the protocol handle it one station a time. It re- ets) at the nodes. Since the senders of those packets are sults as if the MANET is “grow” from a single point. not all synchronized, the RTS/CTS in CSMA/CA lost a • The MANET, as a graph, is always connected. If big part of its collision avoidance function. For example, there exist two disconnected subgraphs, they must be another sender within the neighborhood might miss cur- considered as two different MANETs and there is no rent sender’s RTS/CTS exchange because it is in PS. After communication between them. 7

In this section, we proposed a protocol addressing the MANET. For clarity, the MANETs to be merged, such issues left out in [22], i.e. how achieve synchronization as M1, ..., Mk, are often referred to as sub-MANET. without the above assumptions. B. Design Issues To solve the MANET Merging Problem and allow sys- A. The MANET Merging Problem tem establishes distributively, multiple schedules should Assume that a MANET is initiated by more than one be allowed as in local synchronization based scheme. We station. From each initiator there grows a cluster of syn- also need no clock synchronization based scheme for dis- chronized stations. The main issue now is how to merge cover process. To solve those issues related to multi- these cluster of stations into a single MANET with a glob- ple schedules, we need to merge schedules when stations ally synchronized clock, because although stations inside meet each other, so that a global synchronization will be each cluster are synchronized, stations belong to different achieved in the whole system eventually. So we need to clusters are not. After such a MANET is formed, it may combine all three kinds of schemes reviewed in section II. sometimes be fragmented into sub-MANETs due to node Before we present our algorithm, we will visit some de- mobility, node failure, or other reasons. Again, we face sign issues of the discovery and merge process first. the problem of MANET merger, which is more precisely • When stations running in PS mode, the discovery specified in the following. might take a very long time since the node in sleep Consider a MANET is global synchronized stations won’t be able to hear other’s beacon broadcast. So a based on the IEEE 802.11 IBSS. It use the same frame- station, even when it has no traffic coming in or out, work of time synchronization and power management as besides being awake during the beacon and TIM win- the standard. The main difference between it and the IBSS dows to catch beacon and TIM packets from the syn- defined in the standard is that here we allow multi-hop chronized stations, it has to stay awake throughout network, while all stations in an IBSS are just one-hop beacon interval periodically to increase the possibil- away from each other. This greatly expands the power of ity of catching those unsynchronized beacon pack- MANET, and is the main focus of this paper. ets. This discovery process cannot be run too of- A MANET is characterized by many parameters, such ten. The frequency of it depends on the trade-off as clock value, the lengths of beacon interval and ATIM between power efficiency and the delay of discov- window, the frequency it operates on (for DSSS) or the ery. In our protocol, an adaptive approach based on frequency hopping sequence it adopts (for FHSS), and the change of neighbors is used as the trigger of the also others like multiple access method adopted, fragmen- discovery process. We use neighbor change as an in- tation threshold, retry limit, transmit/receive lifetime, au- dicator of possible topology change around the sta- thentication and encryption parameters. However, most tion. And we think the topology change would be of these parameters are not related with clock synchro- a good time to run the discovery process. However, nization and power management. Among those which do not only neighbor change doesn’t necessarily mean affect synchronization and power-saving, beacon interval new nodes in the neighborhood, but also power man- length and ATIM window length are included in the bea- agement makes it hard to track such changes, so a con packet and can be updated after discovery and syn- station makes the decision with a probability based chronization are achieved. That leaves only the most im- on the changes of nodes it has noticed, and back it up portant ones, clock value and frequency. These parame- with a fixed interval during which discovery must be ters will affect the discovery procedure directly. In this pa- run at least once. per, we leave the frequency difference aside while study- • After found unsynchronized neighbors, in order to ing MANET discovery. To take different frequencies into keep a high power efficiency, it is desirable to syn- account in the discovery process, the station just has to chronize the nodes and merge them into a synchro- run the discovery process on every frequency band. nized MANET. During the merge, both clock value Two MANETs are said to be adjacent if they have at and power-saving parameters such as beacon interval least one node within each other’s transmission range. We and ATIM window length are brought to the same. define the MANET Merging Problem as: Given a con- • To reduce the convergence time of merging multi- nected set of MANETs, M1, ..., Mk, running on the same ple sub-MANETs, the merge can be triggered not frequency and have other physical and MAC configura- only by passive listening to other’s beacon, but by tions the same except clock values, into a single MANET, active sending beacon at different schedule as well. M, with a synchronized clock throughout the merged This happens when a station just adopts a new clock 8

value and it will also send its beacon with its original 3) if Received beacons from stations running under schedule to inform its neighbors who are still follow- different clock and schedule, enter those schedules ing this schedule. into schedule table. Synchronize clock and schedule to the one has largest clock value. 4) if Schedule table isn’t empty, send beacon in beacon C. Procedures windows of those different schedules and decrease Put ideas we’ve discussed so far into pseudo-code, we expire counter for the entries if the corresponding get three procedures in our protocol: One is initialization beacon sending succeed. Clear schedule table for procedure, which is ran once when a station powers up. expired entries. And there is regular procedure which handles incoming 5) Enter Regular Procedure. and outgoing traffic as well as beacon and ATIM transfer. This is the main procedure which usually a station is exe- cuting, and it is mainly based on power-saving procedure VI.SIMULATION in the IEEE 802.11 standard. The last one is discovery and TABLE III merge procedure, which is ran when the station decide to SIMULATION CONFIGURATION search for unsynchronized stations in the neighborhood. Initialization Procedure Parameter Value Execute this at start up. Beacon interval length 2000 slots 1) Listen for beacons for at least the length of one bea- Beacon window length 64 slots con interval. ATIM window length 200 slots 2) if Beacons received, synchronize with the one with Beacon length 2 slots largest clock value and store other schedules in ATIM packet length 2 slots schedule table. Data packet length 40 slots 3) else Use its own clock and schedule. Min. cont. win. size, CWmin 31 slots 4) Enter Regular Procedure. Max. cont. win. size, CWmax 255 slots Regular Procedure Simulation time 1000 beacon intervals For each beacon interval do following unless specified. To compare results on power-saving, we have im- 1) The same action for beacon, ATIM and incom- plemented the Periodically-Fully-Awake-Interval proto- ing/outgoing data as in a beacon interval in IEEE col from [17], the power-saving scheme described in [21], 802.11 and our protocol in a simulation environment. In the sim- 2) if Neighbor change ratio is above the threshold, cal- ulation, data traffic is presented as a single packet with a culate detection probability based on this ratio. En- fixed length. We describe it using a Bernoulli process with ter Discovery & Merge Procedure in the next bea- access probability pT raffic. We don’t make any particu- con interval with this probability. lar assumption on the network topology. Nodes connect 3) if Received beacons from stations running under with each other randomly, with 20% to 30% of total nodes different clock and schedule, enter those schedules as neighbors. We adjust this ratio to see the effects of into schedule table. Synchronize clock and sched- the density of the network on different algorithms. Node ule to the one has largest clock value. movement is described as another Bernoulli process with 4) if Schedule table isn’t empty, send beacon in bea- a moving probability pMove. However, since we didn’t con windows of those schedules and decrease expire implement any routing procedure, traffic only happens be- counter for the entries if the corresponding beacon tween directly connected neighbors and the node won’t sending succeed. Clear schedule table for expired move when it has packets to send. entries. The clock synchronization algorithm we have used in Discovery & Merge Procedure our simulation is mainly based on the simple scheme us- Do this when specified. ing beacon packet defined in IEEE 802.11. Our algorithm 1) The same action for beacon, ATIM and incom- can always easily adopts any other synchronization mech- ing/outgoing data as in a beacon interval in IEEE anism which is more efficient even with a large number of 802.11. nodes. The effects of a better synchronization algorithm 2) Stay awake for the whole beacon interval even no can be seen as reducing node mobility, since the fact that traffic to listen for beacons. stations loose synchronization some time after it has got 9

70 70

65 65 eters and various trafﬁc loads on the performance of the

60 60 55 power-saving scheme based on local clock synchroniza- 55 50 50 p_move=0.2, overlap=40% 45 tion. In graph (a), there are n = 100 nodes, s = 20 differ- 45 p_move=0.5, overlap=40% p_move=0, overlap=40% 40 Awake ration (%) Awake ration (%) 40 p_move=0.2, overlap=15% s=20 p_move=0.5, overlap=15% 35 s=10 ent schedules in the system, and each node scans for new 35 p_move=0, overlap=15% s=4 30 s=2 30 25 neighbors every T = 10 beacon intervals. We can see 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 p_traffic p_traffic the algorithm is not sensitive to node mobility because (a) (b) multiple schedules are observed. When some neighbors 70 80 65 70 with schedule A are replaced by neighbors with sched- 60 60 55 ule B, as long as the total number of schedules a node 50 50 45 40 follows isn’t changing, it doesn’t put much more works 40 T=4 Awake ration (%) T=4, s=4 Awake ration (%) 30 n=20 35 T=10 n=50 on a node. The percentage of nodes which have multiple T=20 20 n=100 30 25 10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 schedules, Overlap, has an apparent effect. Those nodes p_traffic p_traffic stay awake much longer than nodes with only one sched- (c) (d) ule. Graph (b) shows that the average number of schedules Fig. 7. Simulation results of power-saving scheme based on local of those multi-schedule nodes following, s, has the most synchronization. significant effect on the awake ratio. Other parameters are: n = 100, T = 10, pMove = 0.5, Overlap = 40%. With s increasing from 2 to 20, when there is no traffic, synchronized due to the inefficiency of the clock synchro- the average awake time of each node climbs from 25% to nization algorithm can also be translated as those stations over 50% of a beacon interval, although less than half of leaving the system while the same number of new stations the nodes are multi-schedule nodes. The authors of [21] joining the system (with a unsynchronized clock). So in argue that there would be only a few different schedules the simulation results we present next, the effects of clock- in the system. However, since no schedule change is pro- drifting can be evaluated from node mobility. posed in that scheme, and consider the complex situations First, we show the performance of each protocol. The in MANET, we think it is important to make the protocol results of Periodical-Fully-Awake-Interval protocol are less sensitive to the number of different schedules. The presented in Figure 6. In (a), where 100 nodes are in the effect of the frequency that a node has to stay awake for system, we can see the distance between adjacent Fully- the whole beacon interval to scan for different schedules Awake-Intervals, T, has a major effect on the awake ra- is shown in (c) with n = 100, s = 20 if not specified, tio, and the effect of the node mobility on PFAI protocol p = 0.5 and Overlap = 15%. The effect is clear, is almost negligent. That is because in this protocol, a Move but not as significant as the number of schedules. And in node’s wake-sleep schedule is totally unrelated with other (d) (s = 4, T = 20, p = 0.2 and Overlap = 15%), nodes. In (b) and (c) (p = 0.2 and T = 4 for both), Move Move the number of nodes in the system changes the behavior of PFAI’s behavior changes with the node density under dif- the protocol when the traffic is heavy, but it doesn’t have ferent traffic loads. When network node density is high, much effect when the traffic is light. We had the same i.e., more nodes are inside a node’s communication range, observation for PFAI protocol. the power consumption starts to level off even the traffic load keeps increasing. However, when there are less Figure 8 shows our protocol’s performance and the ef- nodes in the system, the average percentage of time when fect of parameters. First in graph (a) at n = 100, s = 20 a node stays awake increases almost linearly with traffic and T = 10, the protocol is more sensitive to the node load. The reason of this is contention. When node density movement, pMove, than to Overlap. This is because our is high, heavy traffic load causes a lot of contentions dur- protocol tries to unify the schedules in the system once ing the TIM transferring. Those stations which couldn’t detects them. Those nodes in the overlap area (have mul- send TIM successfully will go to PS until next beacon in- tiple schedules) eventually reduce to one schedule only. terval. So more beacon intervals are spent in PS for a However, when the node mobility is high, nodes with dif- station even when it has outstanding data packets. This ferent schedules have a greater chance to discover each can be verified from the delay graph in (c). A dense sys- other due to node movements. It takes more effort to get tem, n=100, has a much higher delay than a system that synchronized. This merge effect can also be seen clearly is sparse, n=20. And the delay keeps increasing with traf- in graph (b) at n = 100, T = 10, pMove = 0.2, and fic load. When n=20, the average packet delay doesn’t Overlap = 15%, which shows the number of different change much with traffic load. schedules has a much less impact on the results. In (c) In Figure 7, we can see the effects of different param- (n = 100, s = 20 if not specified, pMove = 0.2 and 10

75 70 9000 100 nodes 70 65 8000 50 nodes 65 60 7000 20 nodes 60 55 6000 55 50 5000 50 T=2, p_move=0.2 45 4000 45 20 nodes T=3, p_move=0.2 40 3000 Awake ration (%) 40 T=4, p_move=0 Awake ration (%) 50 nodes 35 T=4, p_move=0.5 35 100 nodes 2000 30 30 Average delay (per packet) 1000 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 p_traffic p_traffic p_traffic (a) (b) (c) Fig. 6. Simulation results of Periodical-Fully-Awake-Interval protocol

55 44 80 16000 Local synch. s=20, T=20, Overlap=15% 42 Local synch. s=4, T=20, Overlap=40% 50 14000 No synch. T=3 40 70 No synch. T=4 45 38 12000 Global synch. s=20, T=20, Overlap=40% 60 40 36 10000 50 34 8000 35 32 40 p_move=0.5, overlap=40% 6000 p_move=0.5, overlap=15% No synch. T=3 Awake ration (%) 30 Awake ration (%) 30 s=20 Awake ration (%) p_move=0.2, overlap=40% 30 Local synch. s=20, T=20, Overlap=15% s=10 Local synch. s=4, T=20, Overlap=40% 4000 p_move=0.2, overlap=15% 28 s=4 No synch. T=4 Average delay (per packet) 25 p_move=0, overlap=40% s=2 20 2000 p_move=0, overlap=15% 26 Global synch. s=20, T=20, Overlap=40% 20 24 10 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 p_traffic p_traffic p_traffic p_traffic (a) (b) (a) (b)

55 75 80 16000 No synch. T=3 70 No synch. T=4 50 14000 Local synch. s=20, T=20, Overlap=15% 65 70 Local synch. s=4, T=20, Overlap=40% 60 12000 Global synch. s=20, T=20, Overlap=40% 45 60 55 10000 50 40 50 8000 45 40 35 T=4 No synch. T=3 6000

Awake ration (%) Awake ration (%) 40 Awake ration (%) T=4, s=4 30 No synch. T=4 4000 T=10 35 n=20 Global synch. s=20, T=20, Overlap=40%

30 Average delay (per packet) T=20 n=50 20 Local synch. s=20, T=20, Overlap=15% 30 n=100 Local synch. s=4, T=20, Overlap=40% 2000 25 25 10 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 p_traffic p_traffic p_traffic p_traffic (c) (d) (c) (d) Fig. 8. Simulation results of our protocol. Fig. 9. Comparison of protocols at high mobility, pMove = 0.5. (a) and (b) show a system with 100 nodes; (c) and (d) show a system with 20 nodes. Overlap = 40%), T becomes one of the main parameters that will define the power performance in this protocol. Although awake ratio changes a lot from T = 4 keeps increasing. When there are only 20 nodes in (c) to T = 10, but it does not change much as the distance and (d), the system hasn’t reached its saturation point. of detecting for new neighbors increases from every 10 The awake ratio becomes higher and higher to accommo- beacon intervals to every 20 beacon intervals. This is ex- date the increasing traffic, while delay does not change pected since periodically detection is not the only way our much. Among three protocols, Periodically-Fully-Awake- protocol decides when to execute the detection procedure. Interval protocol shows some stability. It responds to most The protocol also monitors the change of the neighbors of parameters slowly except the distance between Fully- and runs detection procedure based on it. The simula- Awake-Intervals, T . It has the highest awake ratio when tion results show that after a certain point, the estimation the traffic is very light, because of the short period be- used in the protocol for neighbors discovery is so effec- tween Fully-Awake-Intervals. With the increase of traffic tive that most of the discovery procedure is started based load, both awake ratio and packet delay increase, but at a on the neighbor change rather than the schedule, T . In (d), relatively low rate. Although the scheme based on local it’s shown the effect of the number of nodes with s = 4, clock synchronization has a lower awake ratio and delay T = 10, pMove = 0.2 and Overlap = 40%. value at light traffic situation, i.e. pT raffic < 0.2, it sur- When we put all three protocols together for compar- passes PFAI with T = 4 when traffic picks up, and the de- ison, some interesting results unveiled. First, we com- lay increment is more dramatic. These can be contributed pare them in a highly mobile system where about half to the high contentions coming from issues with multiple of nodes change their positions (and also loose synchro- schedules we pointed out in previous sections. Our pro- nizations) in Figure 9. In graph (a) and (b), we can see tocol shows the best result overall. Because of clock syn- a saturated system in which awake ratio goes up as traf- chronization and merged schedule, its performance close fic load increasing and then goes back down, and delay to the underlying CSMA/CA protocol used by all packet 11 transfer. After the system reaches the saturation point, first ACM international symposium on Mobile and ad hoc net- the power-saving scheme saves energy by preventing data working and computing (MobiHoc’00), pp. 99–106, Aug. 2000. transmissions from keeping increasing, since that can only [2] L. Bononi, M. Conti, and L. 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