Energy Saving Techniques in Ad hoc Networks

Transcription

1 Energy Savng Technques n Ad hoc Networks 1 Energy Savng Technques n Ad hoc Networks R. Durga Bhavan 1, S. Nagaman 2 and V. Asha 3 1 Asst. Professor, Dept. of CSE, R. K. College of Engneerng, Vjayawada, Inda 2 Asst. Professor, Dept. of IT, LBR College of Engneerng, Vjayawada, Inda 3 Asst. Professor, R. K. College of Engneerng, Vjayawada, Inda Abstract: Packets traversng an ad hoc network can experence dffcultes from power management at every hop, mpactng the routng protocols and the productvty of the network. The major challenge to the desgn of a power management protocol for ad hoc networks s that energy conservaton usually comes at the cost of degraded performance such as lower throughput or longer delay. Essentally, the goal of power management s to let as many nodes use power-save mode as possble whle mantanng effectve communcaton n the network. Communcaton n the network can be mproved by allowng hgher layer decsons about f a devce should ever use power-savng technques. In ths paper, we nvestgate; a node can be n one of two power management modes: actve mode and power-save mode. In actve mode, a node s awake and may receve at any tme. In power-save mode, a node s suspended most of the tme and resumes perodcally checkng for pendng transmssons. A nave soluton that only consders power savngs of ndvdual nodes may turn out to be detrmental to the operaton of the whole network. The role of a power management protocol s to determne when a node should transton between actve mode and power save mode. Keywords: Power management protocol, throughput, power savng, nodes. 1. INTRODUCTION The lmted energy capacty of moble computng devces has brought energy conservaton to the forefront of concerns for enablng moble communcatons. Ths s a partcular concern for moble ad hoc networks where devces are expected to be deployed for long perods of tme wth lmted potental for rechargng batteres. Such expectatons demand the conservaton of energy n all components of the moble devce to support mprovements n devce lfetme [1] [2] [3]. In wreless networks, there s a drect tradeoff between the amount of data an applcaton sends and the amount of energy consumed by sendng that data. Applcaton-level technques can be used to reduce the amount of data to send, and so the amount of energy consumed. However, once the applcaton decdes to send some data, t s up to the network to try to delver t n an energy-effcent manner. To support energy-effcent communcaton n ad hoc networks, t s necessary to consder energy consumpton at multple layers n the network protocol stack. At the network layer, ntellgent routng protocols can mnmze overhead and ensure * Correspondng author: dbhavan@gmal.com the use of mnmum energy routes [4] [5]. At the medum access control (MAC) layer, technques can be used to reduce the energy consumed durng data transmsson and recepton [6] [7]. Addtonally, an ntellgent MAC protocol can turn off the wreless communcaton devce when the node s dle [8] [9]. Communcaton n ad hoc networks necessarly drans the batteres of the partcpatng nodes, and eventually results n the falure of nodes due to lack of energy. Snce the goal of an ad hoc network s to support some desred communcaton, energy conservaton technques must consder the mpact of specfc node falures on effectve communcaton n the network. At a hgh level, achevng the desred communcaton can be assocated wth a defnton of network lfetme. Current defntons of network lfetme nclude: (1) the tme when the frst node falure occurs (2) the fracton of nodes wth non-zero energy as a functon of tme [10] [11] [12], (3) the tme t takes the aggregate delvery rate to drop below a threshold [13], or (4) the tme to a partton n the network. In the context of any of these defntons, t may also be useful to consder node prorty n the defnton of lfetme. For example, the network lfetme could be defned as the tme the frst hgh prorty node fals. In general, one statc defnton of lfetme does not ft

2 2 R. Durga Bhavan, S. Nagaman & V. Asha all networks. In ths paper, we present approaches to energy conservaton that mnmze energy consumpton for communcaton n ad hoc networks. However, these approaches can be tuned to support the desred communcaton and the defnton of network lfetme as needed by the specfc ad hoc network. Energy conservaton can be acheved n one of two ways: Savng energy durng actve communcaton & Savng energy durng dle tmes n the communcaton. The frst targets the technques used to support communcaton n an ad hoc network and s typcally acheved through the use of energy-effcent MAC and routng protocols. The second focuses on reducng the energy consumed when the node s dle and not partcpatng n communcaton by placng the node n a low-power state. In ths paper, we frst defne the costs assocated wth communcaton n ad hoc networks and then dscuss the use of communcaton-tme and dle-tme energy conservaton. 2. ENERGY SAVING IN AD HOC NETWORKS In general there are three components to energy consumpton n ad hoc networks. Frst, energy s consumed durng the transmsson of ndvdual packets. Second, energy s consumed whle forwardng those packets through the network. And fnally, energy s consumed by nodes that are dle and not transmttng or forwardng packets. To understand how and when energy s consumed n ad hoc networks, t s necessary to consder these costs for data packets forwarded through the network and for control packets used to mantan the network. To lay the groundwork for dscussng energy effcent communcaton protocols n ad hoc networks, we defne these costs for communcaton and ntroduce energy-savng mechansms used by many protocols. 3. ENERGY SAVING APPROACHES Once all of these costs are understood, two mechansms affect energy consumpton: Communcaton Tme Energy Conservaton and Energy Aware Routng. If these mechansms are not used wsely, the overall effect could be an ncrease n energy consumpton or reduced communcaton n the network Communcatons-Tme Energy Conservaton The goal of communcaton-tme energy conservaton s to reduce the amount of energy used by ndvdual nodes as well as by the aggregaton of all nodes to transmt data through the ad hoc network. Two components determne the cost of communcaton n the network. Frst, drect node-to-node transmssons consume energy based on the power level of the node, the amount of data sent and the rate at whch t s sent. The amount of data s determned by the applcaton and the rate s determned by the characterstcs of the communcaton channel. Although the transmsson rate can also be adapted by the sender [26], we do not consder such rate control n ths chapter. However, the power level can be controlled by the node to reduce energy consumpton. Such power control must be performed n a careful manner snce t can drectly affect the qualty and quantty of communcaton n the network. Second, energy s consumed at every node that forwards data through the network. Such costs can be mnmzed usng energy-aware routng protocols. Ths secton frst dscusses the use of power control and ts mpact on communcaton n ad hoc networks. We then present power control protocols and energy-aware routng protocols that am to mnmze energy consumpton for communcaton n the network Energy-Aware Routng Routng protocols for ad hoc networks generally use hop count as the routng metrc, whch does not necessarly mnmze the energy to route a packet [27]. Energy-aware routng addresses ths problem by fndng energy-effcent routes for communcaton. At the network layer, routng algorthms should select routes that mnmze the total power needed to forward packets through the network, so-called mnmum energy routng. However, mnmum energy routng may not be optmal from the pont of vew of network lfetme and long-term connectvty, leadng to energy depleton of nodes along frequently used routes and causng network parttons. Therefore, routng algorthms should evenly dstrbute forwardng dutes among nodes to prevent any one node from beng overused (.e., capacty-aware routng). Hybrd protocols explore the combnaton of mnmum energy routng and capacty-aware routng to acheve energy effcent communcaton whle mantanng network lfetme. 4. MINIMUM ENERGY ROUTING TECHNIQUE The routng metrc used by mnmum energy routng s the per-hop mnmum power level P (, j) needed

3 Energy Savng Technques n Ad hoc Networks 3 for node to reach node j The total power level for route r, P r, s the sum of all power levels P (, j) along the route: D-1 P r = Σ P (n, n + 1), where nodes n O and n D are the source and destnaton, respectvely. Mnmum total transmsson power routng (MTPR) [24] [28] fnds a mnmal power route such that: P s = mnp r, rєa where A s the set of all possble routes. Based on a gven mnmum energy topology that defnes the maxmum power level for all nodes, MTPR fnds the mnmum energy routes optmzng the power level for each hop. In contrast, PARO [5] s a mnmum energy routng protocol ad hoc networks that dscovers mnmum energy routes on demand. PARO assumes that all nodes are located wthn drect transmsson range of each other and that a source node ntally uses the threshold power level to reach the destnaton. Each node capable of recevng the packet determnes f t should ntervene and forward the packet to the destnaton tself to reduce the energy needed to transmt the packet. Although, PARO s desgned for one-hop ad hoc networks, the optmzaton can be used by any par of communcatng nodes, whch allows extendng PARO to mult-hop networks. Gven ths defnton of mnmal power routng, both MTPR and PARO favor routes wth more hops (.e., more shorter hops vs. fewer longer hops). Snce the power level, and so the transmsson energy consumpton, depends on dstance (proportonal to d n ), the energy consumed usng many short hops may be less than the energy consumed usng fewer longer hops [5] [15]. However, the more nodes nvolved n routng, the greater the end-to-end delay. Addtonally, a route consstng of more hops s lkely to be unstable due to the hgher probablty of the movement or falure of ntermedate nodes. Furthermore, both protocols gnore the energy consumed at the relay nodes to receve the packets. Based on these observatons, the routes found by MTPR and PARO may not be effcent. To overcome these problems, the energy consumed when recevng the packet should be ncluded nto the routng metrc [29] [30], whch s lkely to result n the use of shorter routes. An even more accurate metrc should nclude the total energy consumed n relably delverng the message to ts destnaton (e.g., the energy cost of lnklayer retransmssons). In partcular, t s essental to avod lnks wth relatvely hgh error rates to reduce the energy consumed to relably transmt packets Capacty-Aware Routng Assumng all nodes n the network are equally mportant, no node should be used for routng more often than other nodes. However, f many mnmum energy routes all go though a specfc node, the battery of ths node s draned quckly and eventually the node des. Therefore, the remanng battery capacty of a node should be used to defne a routng metrc that captures the expected lfetme of a node, and so, the lfetme of the network. Gven c t, the battery capacty of node at t the functon f (c t ) captures the cost to forward packets for a node Ths cost can be defned as the nverse of the remanng battery capacty and modeled as [25] [31]: t 1 f () c =, t C The battery cost metrc for route r at tme t, R r, can then be determned as: R r = maxf (c t ), Єr Therefore, the desred capacty-aware route s, where A s the set of all possble routes satsfes: R s = mn {R r r Є A}, It must be noted that the choce of t 1 f () c =, t C does not consder the effect of the traffc load on the node battery capacty. To ths end, dran rate s proposed as a metrc to measure the energy dsspaton rate at a gven node [32]. The Mnmum Dran Rate (MDR) algorthm determnes the battery cost metrc of route r, R r, as: t c R r = mn, DranRate and capacty-aware route satsfes: R s = max {R r r Є A}, Incorporatng the battery cost nto the routng protocol prevents a node from beng overused. However, there s no guarantee that mnmum energy routes are found by the routng protocol. Therefore,

4 4 R. Durga Bhavan, S. Nagaman & V. Asha capacty-aware routng may consume more energy to route traffc, whch can reduce the lfetme of the network. 5. POWER MANAGEMENT TECHNIQUES In ad hoc networks, suspendng a node s communcaton devce can mpact communcaton at multple layers of the protocol stack. At the MAC layer, uncoordnated suspenson between two nodes can prevent the nodes from communcatng. At the routng layer, a node that s suspended could be mscategorzed as havng moved away and so cause a route to break, ncurrng unnecessary route recovery overhead. Addtonally, current devce suspenson protocols place lmtaton on the amount of data that can be supported n the network. If the coordnaton of suspend and resume states between communcatng nodes causes too many packets to be dropped or delayed, the suspenson of devces can actually end up consumng more energy [32] [33] [34]. Smlarly, f not enough data can be supported n the network, the suspenson of devces can lmt the effectveness of the network. Communcaton n the network can be mproved by allowng hgher layer decsons about f a devce should ever use powersavng technques. In ths context, a node can be n one of two power management modes: actve mode and power-save mode. In actve mode, a node s awake and may receve at any tme. In power-save mode, a node s suspended most of the tme and resumes perodcally checkng for pendng transmssons, as descrbed n the prevous secton. The role of a power management protocol s to determne when a node should transton between actve mode and power save mode. Packets traversng an ad hoc network can experence dffcultes from power management at every hop, mpactng the routng protocols and the productvty of the network [34]. The major challenge to the desgn of a power management protocol for ad hoc networks s that energy conservaton usually comes at the cost of degraded performance such as lower throughput or longer delay. Essentally, the goal of power management s to let as many nodes use power-save mode as possble whle mantanng effectve communcaton n the network. A nave soluton that only consders power savngs of ndvdual nodes may turn out to be detrmental to the operaton of the whole network Power Management and Routng The partcular decsons about when a node should be n a power-save mode affect the dscovery of routes as well as the end-to-end delay of packets. Smlar to ad hoc routng protocols, power management schemes range from proactve to reactve. The extreme of proactve can be defned as always-on (.e., all nodes are n actve mode all the tme) and the extreme of reactve can be defned as always-off (.e., all nodes are n power save mode all the tme). Gven the dynamc nature of ad hoc networks, there must be a balance between proactve ness, whch generally provdes more effcent communcaton, and reactve ness, whch generally provdes better power savng. In ths space, we dscuss three approaches to usng power management n ad hoc networks: reactve, proactve, and on-demand Reactve Power Management A pure power savng approach (.e., always off) can be consdered as the most reactve approach to power management. However, a network that reles solely on MAC layer power management such as IEEE can be hghly neffcent even though some communcaton s stll possble [34]. In an always-off network, all nodes must be woken up before any communcaton can occur, causng ncreased delay for both control (e.g., route request or route reply) and data packets. Addtonally, all transmssons must be announced (e.g., va an ATIM). If the resources for announcement (e.g., the ATIM wndow sze), cannot support the load n the network, queues fll up and packets get dropped. In a lghtly loaded network, an always-off approach can generally support the traffc wth lttle or no drops, although there s stll an ncreased delay. However, n a heavly loaded network, the announcements become a bottleneck and lttle or no effectve communcaton occurs Proactve Power Management A proactve approach to power management provdes some persstent mantenance of the network to support effectve communcaton. Snce routng protocols operate at the network layer, proactve power management schemes can take advantage of topologcal nformaton to ensure that a specfc set of nodes stays awake to provde complete connectvty for routng n the ad hoc network [35] [36] [37]. We call ths type of approach topology management. Ths dffers from topology control, snce topology control determnes the topology for all nodes whle topology management determnes whch nodes partcpate n routng n the network. One approach to topology management s to create a connected domnatng set (CDS), where all

5 Energy Savng Technques n Ad hoc Networks 5 nodes are ether a member of the CDS or a drect neghbor of one of the members [59] (see Fgure 1). In general CDS-based routng, nodes n the CDS serve as the routng backbone and all packets are routed through the backbone. In a CDS-based power management protocol, all nodes on the CDS reman actve all the tme to mantan global connectvty (e.g., GAF [68] and Span [8]). All other nodes can choose to use power-save mode or even turn off completely. GAF creates a vrtual grd and chooses one node n every grd locaton to be part of the backbone and reman awake (see Fgure 2). All other nodes turn completely off. Span takes a slghtly dfferent approach and uses local message exchanges to allow a node to determne the effect on ts neghbors f t stays awake or uses a low-power mode lke IEEE PSM. Both Span and GAF assume that sources and destnatons are separated from pure forwardng nodes. In the case of mxed source/destnaton/ forwardng nodes scenaros, the specfcaton of both protocols s ncomplete. Nether protocol has a mechansm for sgnalng the data snk for ncomng transmssons. Fgure 1: Example Connected Domnatng Set. The Black Nodes form the CDS. Nodes 1-5 are all Only one Hop Away from a Node n the CDS In Span, t s unclear whether the electon of coordnators should consder the fact that some nodes may be requred to be turned on as data sources or destnatons. By takng advantage of route redundancy n dense ad hoc networks, topology management approaches save energy by turnng off devces that are not requred for global network connectvty. The challenge to topology management comes from the need to mantan the CDS, generally through local broadcast messages that may consume a sgnfcant amount of energy, especally snce broadcast messages wake up all nodes for some amount of tme. Addtonally, the nodes chosen to partcpate n the CDS are perodcally rotated to prevent any one node from havng ts battery depleted. Ths rotaton essentally results n the formaton of a new CDS, resultng n unnecessary overhead f the CDS does not change. The fnal lmtaton to these approaches comes from the fact that regardless of whether or not traffc s present n the network, all the backbone nodes must be actve all the tme. Essentally, even f there s no traffc n the network, some nodes are stll actve and consumng sgnfcant amounts of energy On-Demand Power Management In response to the lmtatons of both reactve and proactve power management, on-demand power management elmnates the need to mantan any nodes n actve mode f there s no traffc n the network by tyng power management decsons to nformaton about whch nodes are used for routng n the ad hoc network [34]. In on-demand power management, all nodes are treated equal, elmnatng the need to know whch nodes are sources and destnatons. All nodes are ntally n power-save mode. Upon recepton of packets, a node starts a keepalve tmer and swtches to actve mode. Upon expraton of the keep-alve tmer, a node swtches from actve mode to power-save mode. The goal s to have nodes that are actvely forwardng packets stay n actve mode, whle nodes that are not nvolved n packet forwardng may go nto power-save mode. The key dea of on-demand power management s that transtons from power-save mode to actve mode are trggered by communcaton events such as routng control packets or data packets and transtons from actve mode to power save mode are determned by a soft-state tmer. In an ad hoc network, f a route s gong to be used, the nodes along that route should be awake to not cause unnecessary delay for packet transmssons. If a route s not gong to be used, the nodes should be allowed to use power-save mode. Durng the lfetme Fgure 2: GAF s Vrtual Grd. One Node n Each Grd Locaton Remans Awake to Create a Connected Domnatng Set

6 6 R. Durga Bhavan, S. Nagaman & V. Asha of the network, dfferent packets ndcate dfferent levels of commtment to usng a route. Knowledge of the semantcs of such messages can help make better power management decsons. On one end, most control messages (e.g., lnk state n table-drven ad hoc routng protocols, locaton updates n geographcal routng, route request messages n ondemand routng protocols, etc.) are flooded throughout the network and provde poor hnts for the routng of data. Such control messages should not trgger a node to stay n actve mode. On the other end, data packets are usually bound to a route on relatvely large tme scales. Therefore, data packets are a good hnt for gudng power management decsons. For data packets, nodes should stay actve on the order of packet nter-arrval tmes to ensure that no node along the route goes nto power-save mode durng actve communcaton. There are also some control messages, such as route reply messages n on-demand routng protocols and query messages n sensor networks, that provde a strong ndcaton that subsequent packets wll follow ths route. Therefore, such messages should trgger a node to swtch to actve mode. The tme scale for such a transton should be on the order of the end-to-end delay from source to destnaton so the node does not transton back to power-save mode before the frst data packet arrves. The mprovement n energy consumpton comes at an ncrease n the ntal delay of packets n a newly establshed route. Essentally, f all nodes along the route are asleep, they must all be woken up, ncurrng delay on the order of the length of the route tmes the tme to wake up a node. However, n an actve network, many nodes are expected to be awake. Ondemand power management mplctly fnds routes wth more awake nodes, snce those routes have shorter delays. Snce on-demand power management favors awake nodes, t should be coupled wth capacty-aware routng to support load balancng. 6. IDLE-TIME ENERGY SAVING MECHANISM Effectve dle-tme energy conservaton necessarly spans all layers of the communcaton protocol stack. Each layer has access to dfferent types of nformaton about the communcaton n the network, and thus, uses dfferent mechansms to support energy conservaton. MAC layer protocols can save energy by suspendng the communcaton devce durng short-term dle perods n communcaton (.e., operate n a power-save mode). Such fne-graned control requres ntegrated knowledge of transtons between devce suspend and resume n the MAC protocol to nsure the communcatng nodes are both awake. The delay overhead from wakng up a suspended devce can negatvely mpact communcaton n the network and so power-save modes should not always be used. Power management protocols ntegrate global nformaton based on topology or traffc characterstcs to determne transtons between actve mode (.e., never suspend) and power-save mode. 7. CONCLUSION Energy conservaton n ad hoc networks s a relatvely new feld of research. In ths paper, we have presented some of the recent proposals and specfcatons for achevng that goal. It s clear that there s stll room for new approaches that tackle ths extremely complex problem of balancng energy conservaton and power savng wth communcaton qualty n dynamc ad hoc networks. REFERENCES [1] F. Dougls, P. Krshnan, and B. Marsh. Thwartng the Power-hungry Dsk. In USENIX Symposum. [2] F. Dougls, P. Krshnan, and B. N. Bershad. Adaptve Dsk Spndown Polces for Moble Computers. In Second USENIX Symposum on Moble and Locaton Independent Computng. [3] C. H. Hwang and A. C. H. Wu. A Predctve System Shutdown for Energy Savng of Event-drven Computaton. In IEEE/ACM Internatonal Conference on Computer Aded Desgn. [4] J. H. Chang and L. Tassulas. Energy Conservng Routng n Wreless Adhoc Networks. In IEEE INFOCOM, [5] J. Gomez, A. T. Campbell, M. Naghshneh, and C. Bsdkan. PARO: Supportng Dynamc Power Controlled Routng n Wreless Ad hoc Networks. Wreless Netwo s, 9(5): , [6] T. ElBatt and A. Ephremdes. Jont Schedulng and Power Control for Wreless Ad-hoc Networks. In IEEE INFOCOM, [7] E. S. Jung and N. H. Vadya. A Power Control MAC Protocol for Ad hoc Networks. In 8th Annual Internatonal Conference on Moble Computng and Networkng (MobCom), [8] IEEE 802 LAN/MAN Standards Commttee. Wreless LAN Medum Access Control MAC and Physcal Layer (PHY) Specfcatons. IEEE Standard [9] R. Krashnsky and H. Balakrshnan. Mnmzng Energy for Wreless Web Access wth Bounded Slowdown. In 8th Annual Internatonal Conference on Moble Computng and Networkng (MobCom), [10] W. R. Henzelman, A. Chandrakasan, and H. Balakrshnan. Energyeffcent Communcaton Protocol for Wreless Mcrosensor Networks. In Hawa Internatonal Conference on System Scences (HICSS).

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