Balancing Energy Saving and QoS in the Mobile Internet: An Application-Independent Approach

Transcription

1 Balancng Energy Savng and QoS n the Moble Internet: An Applcaton-Independent Approach Unv. of Psa, Dept. of Informaton Engneerng Va Dotsalv Psa, Italy {g.anastas, a.passarella}@et.unp.t, G. Anastas, M. Cont *, E. Gregor *, A. Passarella * CNR-IIT Insttute Va G. Moruzz, PISA, Italy {marco.cont, enrco.gregor}@t.cnr.t Abstract The scarcty of energetc resources n moble computers s a very lmtng factor. In ths paper we propose a soluton that tres to balance energy consumpton and QoS requrements. Our soluton follows an applcaton-ndependent approach and, therefore, t can be used concurrently, and wthout modfcatons, by any network applcaton. Furthermore, our soluton s ndependent from the sub-network technology. We mplemented ths soluton and we extensvely tested t. Expermental results have shown that a relevant energy savng (about 7% on average) can be acheved wth respect to the legacy approach based on the TCP/ protocol stack. Furthermore, these savngs are obtaned wthout a sgnfcant degradaton n the QoS perceved by the user. We also compared our applcaton-ndependent approach wth an applcaton-dependent one (.e., a soluton talored to Web browsng) whch performs (slghtly) better. However, the applcaton-ndependent soluton stll guarantees sgnfcant savngs, and fts better a general-purpose moble envronment.. Introducton The Internet exploson n the last years has demonstrated that accessng nformaton of some nterest n the same moment they are needed s a valuable opportunty. In ths context, the concept of moblty adds a new dmenson: the user s no longer bound to hs/her desktop personal computer to access Internet servces. It s not vsonary to foresee that n a near future mllons of users wll access Web pages (or read ther e-mal) by usng a PDA, a pager or a cellular phone. However, ntegratng moble devces n the legacy Internet scenaro s stll a challengng problem for several reasons. Moble devces have less computatonal, storage, and energetc resources wth respect to desktop computers. Furthermore, they usually connects through wreless lnks The work descrbed n ths paper was carred out under the fnancal support of the Italan Mnstry for Educaton and Scentfc Research (MIUR) n the framework of the Projects "Web Systems wth QoS Guarantees" and "Internet: Effcency, Integraton and Securty". that are characterzed by lower bandwdth and greater bt error rates wth respect to wred lnks. Usng the legacy Internet solutons may thus result n a non-optmal usage of the system resources. In partcular, power savng mechansms need to be ntroduced to optmse energetc resources usage. Fnally, the provson of added-value servces (e.g., QoS support, securty, etc.), that are stll an open ssue even n the legacy Internet scenaro, becomes an extremely challengng problem n the Moble Internet where the path between the communcaton s endponts s not statc but changes n tme. Among these problems, the scarcty of energy resources s a very lmtng factor [8,, 6]. Users are not happy f ther moble computer swtches off n the mddle of a network transacton because the battery s exhausted. At the same tme, the user does not lke to recharge the battery too often. Ths paper focuses on strateges to optmse the usage of the moble-computer battery power. In prncple, energy-related problems n moble access to the Internet could be solved by ether ncreasng the battery capacty or reducng the energy consumpton. Projectons on progresses n battery technology show that only small mprovements n the battery capacty are expected n next future [8]. As the battery capacty cannot sgnfcantly be mproved, t s vtal that energy utlzaton s managed effcently by dentfyng ways to use less power preferably wth no mpact on the applcatons. Strateges for energy savng have been nvestgated at several layers ncludng the physcal-layer transmssons [2], the operatng system (technques can be adopted for hard-dsk management [9], CPU schedulng [5], screen blankng [5]), the network protocols and the applcaton level. At the applcaton level some technques proft of remote tasks executon [7] (e.g., the moble computer dscards on a fxed host some energy-consumng task, takng only the task s results from t); another approach conssts n explotng the applcaton semantc [2] (e.g., the applcaton compresses the data before exchangng them, or fnds some tradeoff between performance and power consumpton). In ths paper we nvestgate energy-savng strateges mplemented n software network protocols. We use an applcaton-ndependent approach n the sense that envsaged strateges do not explot knowledge about the Proceedngs of the 36th Hawa Internatonal Conference on System Scences (HICSS 3)

2 above applcatons. Our soluton presents to the above layer a standard socket nterface, and thus t does not requre any modfcaton n the applcatons. In addton, t s completely ndependent from the sub-network technology. Ths work s complementary to a prevous one [] n whch we developed an applcaton-dependent soluton to power savng. Specfcally, the applcatondependent soluton was talored to Web browsng. The two approaches (applcaton dependent and applcaton ndependent) share some basc archtectural desgn aspects. Both approaches explot a network archtecture based on the Indrect-TCP model [3]. The moble computer connects to a fxed host (e.g., a Web server) through a thrd entty (the Access Pont) located at the border between the wreless and wred networks (see Fgure ). Wth respect to the tradtonal TCP/ archtecture, the transport connecton between the moble host and the fxed host (e.g., a Web server) s splt nto two parts. The frst one connects the moble host wth the Access Pont, whle the second connects the Access Pont and the fxed host. At the Access Pont a software agent (the Indrect-TCP Daemon) relays data between the two connectons. To acheve power savng the moble host perodcally swtches the network nterface off. Whle dsconnected, data comng from the Internet and destned to the moble host are temporarly stored by the Indrect- TCP Daemon. To decde when and how long the network nterface should be off, both the approaches (.e., the ndependent and the dependent one) dynamcally estmate the traffc behavor (packet nter-arrval tmes, dle perods, etc.). Swtchng off the network nterface actually reduces the energy consumpton but can heavly ncrease the User Response Tme (URT,.e., the elapsed tme between the generaton of a request from the browser, for the retreval of a Web page, and the renderng of that page at that browser s ste), thus negatvely affectng the QoS perceved by the user. Thus, a trade off between these two orthogonal performance fgures must be reached. Moble Host Wreless Lnk Access Pont Wred Lnk Internet Wred Lnk Fgure. Moble Internet scenaro Fxed Host The basc dfference among the two approaches resdes n the algorthms used to decde when, and how long, to keep the network nterface off. In the applcaton dependent approach the semantc of the Web applcaton s exploted. Hence, ths approach requres a specalzed protocol for each applcaton to be supported. For ths reason, hereafter we desgn and evaluate the ndependent approach that s based on algorthms that do not rely upon any a pror applcaton semantc but try to dynamcally ntercept the behavor of the actve applcaton(s). Ths approach s much more flexble, and t s therefore nterestng to compare ts performance wth those of the applcaton dependent approach that consttute a target reference. It can be expected that the dependent approach provdes better power savng characterstcs. We mplemented the applcaton-ndependent soluton n a prototype system and extensvely tested t. The target of our experments was twofold: () to understand how our soluton performs n an actual Internet scenaro, wth respect to the power savngs, and the QoS perceved by the user; and () to compare and contrast t wth the applcaton dependent soluton. Our expermental results ndcated that both the dependent and the ndependent approach guarantee a sgnfcant power savng: the applcaton dependent soluton saved, on average, more than 8% of the legacy TCP/-archtecture energyconsumpton, whle, n the same applcaton scenaro and under smlar condtons, the applcaton ndependent soluton saved, on average, around 7% of the legacy TCP/-archtecture energy-consumpton. Furthermore, these reductons n the power consumpton were obtaned wthout a sgnfcant degradaton of the QoS. Specfcally, we measured the ncrease n the User Response Tme (URT) caused by our power savng archtectures. Wth respect to the legacy TCP/-archtecture, the applcaton dependent approach ncreased the URT of less than 2 seconds, whle n the applcaton ndependent approach the addtonal URT was less than 2.5 seconds. The paper s organzed as follows. Secton 2 presents and dscusses prevous works. Secton 3 s devoted to the defnton of our applcaton ndependent archtecture. Secton 4 reports some expermental results obtaned by usng the prototype mplementaton. Fnally, Secton 5 concludes the paper. 2. Related works Among the hardware components of a moble computer, the network nterface accounts for a sgnfcant part of the total energy consumpton. For devces such as laptops, ths porton s approxmately %, whle n small sze moble computers (PDA, hand-held devces,...) the percentage ncreases up to 5% [3]. Thus, n current moble computers t s vtal to mplement smart powersavng strateges talored to the networkng subsystem. In prncple, there are dfferent approaches to lmt the power consumed by the network nterface. Some researchers focused on the mpact of the transmsson layer errors on the power consumpton. When the bt error rate of the wreless channel s too hgh, a transmtted Recall that small moble computers frequently have no harddsk, and have lmted computatonal resources, whle the network nterface must stll provde the same functonaltes as n a laptop or a PC. Proceedngs of the 36th Hawa Internatonal Conference on System Scences (HICSS 3)

3 message wll be almost certanly corrupted, and ths wll cause a power wastage. In ths case, t s wse to defer the transmsson. Other works suggest to lmt the number of transmssons, as transmttng consumes more energy than recevng. These strateges apply well n a cellular envronment, due to power consumpton characterstcs of moble phones. But they do not seem useful n a WLAN envronment. In ths scenaro, the network nterface requres nearly the same power n the transmt, receve, and dle states [9, 4]. Thus, the only effectve way to reduce the power consumpton s to swtch the network nterface n a sleep status (or, f possble, swtch t off) when t s not needed. Furthermore, n WLANs, the exchange of messages should be done at the maxmum rate allowed by the wreless channel. Ths polcy reduces the tme n whch the network nterface s on, and therefore the power draned from the battery. The effectveness of ths polcy was ponted out n several research works [9, 3, 4]. In the same work t has also been shown that usng the legacy TCP/ archtecture (n a moble Internet scenaro) causes a very bad energy utlzaton at the moble host. The TCP bandwdth decreases as ( RTT p ), where p s the error probablty along the TCP connecton, and RTT s the sum of the wreless and wred lnks round trp tmes [7]. Because the wreless lnk s typcally nosy, many retransmssons may occur, and hence the RTT value can be hgh, thus sgnfcantly decreasng the TCP performance. Consequently, the data transfer can be very long, and ths results n a great power consumpton. Several solutons have been proposed to handle ths problem [3, 4]. A possble way s to use the Indrect-TCP model [3]. Ths model was orgnally proposed to mprove the TCP performance n a moble Internet scenaro [3], and then t has been used to handle power savng problems [3, 4, ]. The approach n [3] mplements the power savng strateges at the transport layer: swtchng the network nterface off after an nactvty tmeout expres (from the last transmsson or recepton), and resumng t after a sleepng tmeout, or when the applcaton runnng on the moble host generates new data to exchange. The man advantage of ths approach s the applcaton ndependence. In the soluton proposed by [3] both nactvty and sleepng tmeouts have a fxed value. As the traffc characterstcs dynamcally vary, usng fxed tmeouts may result n poor performance. Ths can lead both to a weak management of the power consumed by the network subsystem (e.g., f the sleepng tmeouts are too small, the network nterface s almost always on), and/or to sgnfcant degradatons n the QoS perceved by the users (e.g., f the sleepng tmeouts are too long, the power management subsystem ntroduces large URTs). [4] and [] desgn power savng strateges by explotng some knowledge about the behavor of the applcatons. They use a proxy-based approach, and mplement a power savng subsystem talored to fle transfers and to Web servce, respectvely. Ths approach provdes a nearly optmal power management, because the wreless lnk s used only when there s some message to exchange and the data can flow at the maxmum rate. Moreover, the QoS ssue s well addressed. The man drawback s the dependence from the applcaton semantc. Thus, ths approach fts better a dedcated envronment, where the flexblty wth respect to the applcatons s not a crtcal factor. 3. Network archtecture and protocols In a moble envronment, host moblty s typcally handled at the data-lnk layer (e.g., n WLANs) or at the network layer (e.g., usng the Moble ). In both cases, t s transparent to the transport layer. Thus, the TCP protocol and the legacy Internet applcatons can be used even n a moble envronment. Although very smple and costless, ths soluton presents varous drawbacks whch heavly mpact the power requred by the network subsystem of the moble host.. The TCP bandwdth s proportonal to ( RTT p ), where p s the loss probablty on the connecton, and RTT s the sum of the wred and the wreless round trp tmes [7]. Because the wreless lnk s typcally nosy, the RTT can be very hgh 2, and the avalable bandwdth much lower than the wreless-lnk capacty [4]. A low throughput causes long transfer delays and, hence, the moble s network nterface remans unnecessarly on for long perods. 2. Congestons on the Internet reduce the bandwdth, ncreasng both p and RTT. If the overall throughput s less than the rate avalable on the wreless lnk, the effects are smlar to those dscussed n the pont. 3. The typcal Internet applcatons (e.g., Web, e-mal, ftp,...) do not contnuously generate traffc on the network: ther traffc s characterzed by bursts of data (e.g., when the user requests a Web page from the server) followed by dle tmes (e.g., when the user reads the page that he has just downloaded). Durng the dle tmes, the moble s network nterface remans on, and ths greatly ncreases the power consumed wthout any reason. To overcome these drawbacks, we extended the Indrect-TCP model (see Fgure 2) used n prevous power-savng archtectures, see [4] and []. As shown n the fgure, the Access Pont and the fxed host communcate by usng the TCP protocol. On the other hand, the moble host and the Access Pont 2 Note that the wreless lnk layer s typcally relable, e.g., IEEE 82., and ths hdes the losses due to channel nose to the transport protocol. Proceedngs of the 36th Hawa Internatonal Conference on System Scences (HICSS 3)

4 communcate through a hgh-speed low latency WLAN (e.g., IEEE 82.). In ths case t s more effcent to use a Smplfed Transport Protocol (STP) that mplements only the necessary functonaltes. Specfcally, STP provdes a relable connectonless type of servce relevng the moble host from the TCP computatonal burden. applcaton STP I-TCP Daemon STP TCP applcaton TCP Moble Host Access Pont Fxed Host Fgure 2. The Indrect-TCP model MH->FH drecton FH->MH drecton bursts dle phases nter-arrval tmes dle tmes Fgure 3. Typcal traffc profle, as seen at the moble transport layer As noted n pont 3 above, moble hosts do not contnuously exchange data, but data transfer phases are characterzed by bursts nterleaved by dle phases durng whch data are locally processed. Fgure 3 shows a snapshot of a typcal data exchange 3. Our approach s based on a dynamc estmate of the duraton of dle and data transfer phases. Specfcally, we measure at run tme the data nter-arrval tmes and the length of the dle tmes. From these measures, we predct the future traffc behavor, and hence, we decde whether the network nterface should be swtched off (or not), and when to resume t 4. Our target s to let the network nterface on only when t s energetcally convenent, and to transmt data at the maxmum rate allowed by the wreless lnk. Ths strategy approxmates the optmal strategy, and works well f the estmates are accurate. Therefore, the core of our approach are a set of smart algorthms for estmatng the traffc characterstcs (see Secton 3.2 for detals). We ntegrated our power-strategy n the Indrect- TCP archtecture shown n Fgure 2 by ntroducng, on 3 When several applcatons are concurrently runnng the resultng traffc profle s the superposton of the sngle data exchanges. 4 The network nterface has a transent n gettng on durng whch t drans power from the battery but s not avalable for exchangng data. Therefore, for small nter-arrval or dle tmes t s energetcally convenent to leave the network nterface on. t top of the STP protocol, the Power Savng Packet Transfer Protocol (PS-PT), see Fgure 4. applcaton PS-PT STP I-TCP Daemon PS-PT STP TCP applcaton TCP Moble Host Access Pont Fxed Host Fgure 4. The Power Savng network archtecture; evdence on added protocols When the I-TCP Daemon or the moble s applcatons generates a new packet for the moble host, the PS-PT at the Access Pont logs the nterval elapsed from the arrval of the prevous packet. These logs are used for estmatng the traffc nter-arrval tmes. Specfcally, when the transmsson queues at both sde are empty, the PS-PT estmates when the next packet wll arrve. Ths value s used to decde f the network nterface must be swtched off. When the network nterface s off, the moble host doesn t know f some packet s watng at the Access Pont. Thus, when the estmated nter-arrval (or dle) tme has elapsed, the moble host must always poll the Access Pont for possble new packets, even f ts applcaton level has not generated new data to be sent. We desgned algorthms that acheve a tradeoff between the responsveness of the protocol (.e., f the moble host polls the Access Pont frequently the addtonal delay ntroduced by the system s mnmum), and ts power-savng performance (.e., f the Access Pont s polled rarely, when the moble host reconnects t s very lkely that new data are avalable thus avodng power wastage due to useless polls). 3.. Power Savng Protocols The core of the PS-PT protocol are the algorthms for estmatng when the next packet wll arrve. We used an adaptve approach that records the hstory of the prevous nter-arrval and dle tmes, and reles on ths nformaton, to estmate the next nter-arrval or dle tme (see Secton 3.2 for detals). In our prototype mplementaton we chose to run these algorthms at the Access Pont n order to mnmze the computatonal burden (and hence the energy consumpton) at the moble host. The PS-PT protocol was mplemented as a smple master/slave protocol. When there are no more data to be exchanged, the Access Pont decde whether t s convenent to the moble host to swtch the network nterface off. If so, t sends a shutdown command to the moble host ncludng an ndcaton of the tme nterval durng whch the moble host should reman dsconnected. The moble host uses ths nterval to set a tmer. Upon the tmer expraton, the moble host polls the Access Pont Proceedngs of the 36th Hawa Internatonal Conference on System Scences (HICSS 3)

5 agan. In the followng, we shall descrbe n detal the actons performed at the moble host, and at the Access Pont, respectvely (the pseudo-code here s optmzed for clarty rather than for effcency). PS-PT Actons at the Moble Host OnPacketFromApplcatons(packet) 2 tmestamp(packet) 3 f(card s OFF) 4 stop tmer 5 turn card ON 6 send packet to Access Pont 7 OnPacketFromAccessPont(packet) 8 cmd = extract_command(packet) 9 f(cmd == OFF) turn card OFF t_ = extract_nterval(packet) 2 set_tmer(t_) 3 OnTmerExpred() 4 turn card ON 5 send ON_sgnal to Access Pont Fgure 5. PS-PT protocol: actons performed at the moble host. Upon recepton of a new packet from the above applcaton(s), the moble host bounds a tmestamp to the packet (ths tmestamp wll be used at the Access Pont to mantan the hstory of the arrval tmes, see lne 2). If the network nterface s OFF, then the tmer used to sgnal when the moble host have to reconnect and poll the Access Pont s actve. The moble host stops ths tmer (.e., the last estmate was too large, lnes 3-5) and sends the packet to the Access Pont (lne 6). When a new packet from the Access Pont arrves, the moble host checks whether t contans a shutdown command (lnes 8-9). In ths case the packet also ncludes the tme nterval durng whch the network nterface should reman OFF. The moble host swtches the network nterface OFF, and sets the tmer accordngly (lnes -2). Fnally, upon tmer expraton, the moble host polls the Access Pont (lnes 3-5). PS-PT Actons at the Access Pont 6 OnNewPacket(packet) 7 f(card s OFF) 8 tmestamp(packet) 9 buffer(packet) 2 else 2 stop tmer 22 send/receve data 23 t_ = evaluate_next_nterarrval() 24 f(card must get OFF) 25 send (OFF_CMD, t_ t_pwon) to Moble Host 26 else 27 set_tmer(t_) 28 OnTmerExpred() 29 t_ = update_estmate() 3 f(card must get OFF) 3 send (OFF_CMD, t_ t_pwon) to Moble Host 32 else 33 set_tmer(t_) 34 OnMobleGetsON() 35 f(there s no data to exchange) 36 t_ = update_estmate() 37 f(card must get OFF) 38 send (OFF_CMD, t_ t_pwon) to Moble Host 39 else 4 set_tmer(t_) 4 else 42 send/receve data 43 t_ = evaluate_next_nterarrval() 44 f(card must get OFF) 45 send (OFF_CMD, t_ t_pwon) to Moble Host 46 else 47 set_tmer(t_) Fgure 6. PS-PT protocol: actons performed at the Access Pont. At the Access Pont sde, the system records the state of the moble host s network nterface. Upon recepton of a new packet (from the Internet) whle the moble host s dsconnected, the Access Pont buffers the packet and wats for a poll from the moble host (lnes 7-9). On the other hand, f the packet s receved whle the moble host s connected, the Access Pont relays the packet to the moble host (f t was receved from the Internet, lne 22), estmates the next packet arrval tme, and decdes whether ts s convenent to shut down the network nterface (lnes 23-27). It s worthwhle to recall that the network nterface has a transent perod t_pwon n gettng on durng whch t s not able to handle data. Ths mples that the moble host must be ON t_pwon unts of tme before the estmated arrval (lne 25). If the Access Pont estmates that t s convenent for the moble host to dsconnect, t sends a OFF command to the moble host together wth the tme nterval durng whch t must reman dsconnected (lnes 24-25). Otherwse, t sets a tmer wth the estmated arrval tme (lnes 26-27). In the latter case the moble host remans connected. Therefore, f a new packet arrves, the network nterface s ON and, hence, the system must stop the tmer (lne 2). When the moble host polls the Access Pont, there mght be data to exchange or not. In the former case, the Access Pont uses the new data to generate a new estmate and performs the same actons descrbed above (lnes 4-47). In the latter case, the last estmate provded to the moble host was too short. Thus, the Access Pont updates ths estmate and decdes what the moble host must do (lnes 35-4). The same stuaton occurs when the tmer expres: the last estmate was too short, but t ddn t cause the swtchng off of the network nterface. The moble host s stll connected and the Access Pont has to decde whether t s convenent that the moble host dsconnects or not (lnes 28-33). Proceedngs of the 36th Hawa Internatonal Conference on System Scences (HICSS 3)

6 3.2. Algorthm for packet arrvals estmates As clearly appears from the prevous secton, our soluton reles upon the predcton of the traffc behavor. Therefore, we need an algorthm that provdes accurate estmates of packet nter-arrvals and dle tmes, and s able to adapt quckly to changes n the traffc condtons. The Varable-Share Update algorthm [] fts these requrements. Ths algorthm has been proposed as a dynamc algorthm to estmate a generc varable spannng a gven range, and s not bound to a specfc problem. Let I be the range of possble values for a varable y that we want to estmate. To predct the value of y, the Varable-Share Update algorthm reles upon a set of experts. Each expert x provdes a (fxed) value wthn the range I,.e., a value that y can assume. The number of experts to be used, as well as ther dstrbuton among the range I, are nput data for the algorthm. Each expert x s assocated wth a weght w, a real number that measures the dependablty of the expert (.e., how accurately the expert has estmated y n the past). At a gven tme nstant, an estmate of y s acheved as the weghted sum of all experts, usng the current w value as the weght (.e., relablty) for the expert x. Once the actual value of the varable y s known, t s compared wth the estmates provded by the experts, to recalculate and update the weght assocated wth each expert. The algorthm s summarzed n Fgure 7. As shown n the fgure, the core of the algorthm s the weghts updatng algorthm. Updates occur every tme a new actual value of the varable y becomes avalable. Frst, an error functon L s evaluated for each expert: ths functon measures the devaton of the (predcton provded by the) expert from the actual varable s value. Then, the Varable-Share Update s executed, as follows:. each expert loses a porton of ts weght, accordng to the devaton from the actual value; the weght w becomes w (f L= the weght s not changed); 2. each expert shares a porton of ts weght, accordng to ts error functon: a pool s created by usng all the shares (f L= the expert doesn t share anythng); 3. for each expert, the new weght s calculated as the sum of two components: a porton of the weght evaluated n, and a porton of the pool evaluated n 2. Both components depend on the error functon (e.g., f L=, the new weght s the old one, plus a fracton of the pool). Parameters: Varables: Intalzaton: Predcton: η >, α, n (number of experts) x (experts), w (weghts), y (actual varable s value), ŷ (estmated varable s value) w = n =,..., n yˆ = n = w x n = w Loss Update: Varable share: w = w ηl e ( y, ) Fgure 7. Varable-Share Update algorthm The Varable-Share Update algorthm reduces the weghts of those experts that provdes bad predctons, and ncreases the weghts of the experts that provde the more accurate predctons. The speed n ncreasng/decreasng weghts s determned by two algorthm parameters: α and η. Ths algorthm has been proposed to mplement a spndown technque n hard dsks power management [9]. In that context, the update polcy guarantees a quck adaptaton to changes of the varable s values. In our problem we have to estmate two varables nter-arrval tmes and dle tmes that span dfferent ranges. Inter-arrval tmes are tme ntervals between two consecutve packets wthn a burst, whle dle tmes are tme ntervals between two consecutve bursts of packets. Typcally, nter-arrval tmes are smaller than dle tmes. We assumed second as the largest value for nter-arrval tmes: ths assumpton reles on prevous works on Web traffc characterzaton [6, 2]. Furthermore, we used ths value to dscrmnate between nter-arrval tmes, and dle tmes, respectvely. Accordngly, we used two dfferent sets of experts for the two quanttes, and we consdered two non-overlappng ntervals for experts values: the frst nterval ranges from to second, the second one from to 6 seconds. Choosng 6 seconds as the largest value for an estmated dle tme s a tradeoff between power savng and QoS requrements. When an dle phase occurs, the moble host polls the Access Pont wth a maxmum perod of 6 seconds (see Secton 3, lnes 35-4). Thus, the maxmum addtonal delay ntroduced by the algorthm to the frst packet of the new burst s 6 seconds, whch s an acceptable upper bound for applcatons wthout real-tme requrements. We used 2 experts for each set. The experts values are unformly dstrbuted over the correspondng ntervals (experts of the frst set are placed every 5 msec, whle experts of the second set are spaced by approxmately 3 seconds each other). The update polcy shown n Fgure 7 needs two addtonal parameters, α and η. Accordng to [9] 5, we 5 [9] compares several values for α and η assumng a unform dstrbuton of experts. The expermental results ndcate α=.8 and η=4 as the best choce. x L [ ( ) ( y, x ) α ] pool = w w = pool α L ( ) ( y, x ) α w + n L ( ) ( y, x ) w Proceedngs of the 36th Hawa Internatonal Conference on System Scences (HICSS 3)

7 used α=.8 and η=4. Fnally, the algorthm requres the defnton of a loss functon L. Ths functon provdes a measure of the devaton of each expert from the actual value of the varable, and ts values must le n [,], see []. In our mplementaton we used L x, y = x y max x as the error functon. ( ) y The last steps to buld our system requres () to decde how to manage an estmate that happens to be too short, and () to decde when to use the frst set of experts, and when the second one. Let us consder a too short nter-arrval tme estmate ŷ. When the moble host polls the Access Pont the system must update ths estmate (see lnes of Secton 3). As all the experts below ŷ have clearly provded a wrong predcton, we update the estmate by takng nto consderng only the other experts n the set. Specfcally, the new estmate y ˆ s the weghted sum of all experts that have an assocated value greater than ŷ. In addton, as ŷ seconds have already elapsed snce the prevous arrval, the next arrval s expected n yˆ yˆ seconds. The PS-PT protocol uses ths value ( yˆ yˆ ) to decde whether t s convenent (or not) to the moble host to swtch the network nterface off (lnes 37-4). The system reuses the same procedure every tme t detects that the prevous estmate was too short, untl the nterarrval tme becomes bgger than the maxmum value for that set of experts ( second). At ths pont n tme, t s assumed that an dle phase has begun, and thus, the second set of experts s used to estmate the length of the dle phase. If even the second set of the experts provde a too short estmate, a smlar updatng procedure s used: the estmate s updated by consderng only experts that provded predctons greater than wrong estmate untl the dle phase reaches the maxmum value,.e., 6 seconds. From ths tme nstant onward, the moble host polls the Access Pont perodcally every mnute, untl some data becomes avalable. The arrval of a new packet s nterpreted as the begnnng of a new burst, and thus, the algorthm swtches agan to the frst set of experts. Our updatng polcy guarantees a smooth ncrease towards greater estmates. Thus, t mnmzes the probablty of nterpretng an actual nter-arrval tme as an dle phase, and so t ntroduces small delays to the packet transfers (see the expermental results n Secton 4). 4. Expermental results The objectve of the proposed soluton s twofold: t should allow sgnfcant power savngs wth respect to the legacy TCP/ approach by mnmzng, at the same tme, the degradaton of the QoS perceved by the users. To evaluate our archtecture n an actual network scenaro, we used the Web as the testng applcaton. The Web s today one of the most popular Internet applcaton, and t s the canddate to become the kller applcaton even for moble Internet. Moreover, Web users are senstve to delays. Hence, t s mportant to acheve sgnfcant power savngs, whle mantanng acceptable QoS levels,.e., to mnmze the URT ncrease. To evaluate the performance of our archtecture we defned two ndces: a Power Savng Index and a QoS ndex. The Power Savng Index, I_ps, s defned as: net.nt. consumpton n our archtecture I _ ps = () net.nt. consumpton n TCParchtecture I_ps measures the percentage of power consumed by our archtecture wth respect to the legacy one, and, hence, provdes an ndcaton of the power savng acheved by usng our archtecture. The QoS ndex, named the Page Delay Index (I_pd), s defned as: I_pd = (URT n our arch.) (URT n TCP arch.) (2) I_pd measures the addtonal URT ntroduced by our archtecture wth respect to the legacy archtecture, and hence provdes an ndcaton of the URT ncrease to pay to acheve power savngs. In our experments we used SURGE as the applcaton layer at the clent sde [5]. SURGE s a Web traffc smulator, desgned by Barford and Crovella, that models the statstcal propertes of the traffc generated by a realstc Web user browsng the Internet. Furthermore, at the server sde, we used a real Web server. Specfcally, n our experments the Web server was located at the Unversty of Texas at Arlngton, whle the clent was located at the Department of Informaton Engneerng of the Unversty of Psa (Italy). Hence, our clent-server path crossed (congested) ntercontnental lnks, and ths allowed us to test our archtecture n a congested stuaton. To sgnfcantly evaluate our archtecture, we performed a large set of experments. Each experment ncluded 5 fle-transfer operatons from the Web server to the clent 6 (an experment stopped when the whole page n flght arrved at the clent). In each experment, the same set of fles were requested n parallel both n our archtecture, and n the legacy one. Ths guarantees the same network condtons n both cases. We ran a set of experments, where each experment spanned an entre workng day. To ncrease results relablty, we replcated the experments n several workng days. Fnally, we compared the results obtaned wth those related to an applcaton-dependent soluton developed 6 Transferrng 5 fles allowed a sgnfcant samplng from the Web fle dmensons dstrbuton. Proceedngs of the 36th Hawa Internatonal Conference on System Scences (HICSS 3)

8 []. In [] we performed an dentcal set of experments, evaluatng the same ndces, under smlar network condtons. For the sake of brevty, below we only present the results from a partcular day. However, the results obtaned n dfferent days exhbt the same statstcal behavor. 4.. Power Savng Analyss As mentoned n Secton 2, the energy draned from the battery by the wreless network nterface of a moble host s almost the same rrespectve of ts status (recevng, transmttng or dle). Thus, the energy consumpton s well approxmated by the tme the network nterface remans on. Accordngly, to evaluate the I_ps ndex (see (2)), we measured ths tme both n our archtecture and n the legacy one. card n ON state (s) C_ps C_surge Energy Consumpton servce rate : 3: 6: 9: 2: 5: 8: 2: talan tme Fgure 8. Energy consumpton as a functon of tme. For each experment, Fgure 8 reports the total tme durng whch the network nterface remans on n the legacy archtecture (mddle curve) and n our archtecture (bottom curve), respectvely. Fgure 8 also ncludes the servce rate (top curve) defned as the rato between the number of bytes transferred on the wred network and the tme needed to transfer them. Therefore, the servce rate provdes a measure of the Web servce responsveness to clent s requests. The servce rate allows us to understand how the energy consumpton depends on the envronment condtons. From Fgure 8 t appears that the power consumpton n our archtecture s almost ndependent from the servce rate. Ths s a jont effect of: () the splttng of the TCP connecton n two parts, and () the use of our powersavng polces. Specfcally, our power-savng polces allow the network nterface to reman on only when there are data to exchange on the wreless lnk. Therefore, the energy consumpton does not depend on the tme needed to complete the whole transfer between the remote and the moble host (ths tme hghly vares wth the network condtons). 4 2 servce rate (Kbps) I_ps I_ps Consumpton rato (I_ps) servce avg(i_ps) rate : 3: 6: 9: 2: 5: 8: 2: talan tme Fgure 9. I_ps as a functon of tme Fgure 9 shows the I_ps ndex as a functon of tme (bottom curve), ts average value on the entre day (straght curve), and the servce rate (top curve). These curves confrms that our archtecture ntroduces a sgnfcant power savngs: the energy consumpton s always less than 45% of the one n the legacy archtecture, ts average value s 32%, wth values less than 5%. Ths means that we can save always more than 55%, on average 68%, and wth peaks over 85%. When usng an applcaton-dependent approach [], we obtaned a slghtly better performance: about 8% savng on average, wth peaks over 9%. Ths dfference can be easly explaned as the applcaton-dependent approach explots nformatons about the traffc characterstcs, and the applcaton behavor. On the other hand, the applcaton-ndependent approach cannot rely on such nformatons QoS Analyss Fgure shows the addtonal delay ntroduced n our archtecture to the transfer delay of a sngle fle. Specfcally, for each fle we measured the addtonal delay, then we averaged the values on each experment of a day, and we plotted these average values (bottom curve), together wth the average value on the entre day (straght curve). For convenence, we also plotted the servce rate (top curve). average delay (s) avg_delay Average addtonal servce overall avg rate FILE delay : 3: 6: 9: 2: 5: 8: 2: talan tme Fgure. Average addtonal fle delays as a functon of tme servce rate (Kbps) servce rate (Kbps) Proceedngs of the 36th Hawa Internatonal Conference on System Scences (HICSS 3)

9 9perc (s) From Fgure t appears that the average addtonal delay ntroduced for a sngle fle s no more than second. Furthermore, the average value on all the experments s below.2 seconds. The fle-delay ndex does not characterze the users QoS. A user does not perceve the delays related to sngle fles but hs/her QoS depends on the URT related to the whole Web page (a page usually contans several fles). Fgure allows us to evaluate the system performance from ths pont of vew. In each experment we measured the average addtonal URT ntroduced to the downloadng of Web pages (average I_pd, bottom curve). We also consdered the average addtonal URT over the entre day (straght curve), and the servce rate (top curve). average I_pd (s) avg(i_pd) overall avg Average addtonal PAGE delay : 3: 6: 9: 2: 5: 8: 2: talan tme servce rate Fgure. Average I_pd as a functon of tme It appears that the addtonal URT, averaged over the entre day, s slghtly hgher than.2 seconds,.e., t s only slghtly hgher than the average addtonal delay of sngle fles. Ths means that the delays added to sngle fles of a Web page do not accumulate n the URT of the whole Web page. Moreover, the average I_pd n each experment s no more than second. So, the user doesn t perceve a sgnfcant degradaton n the QoS. avg(i_pd) (s) avg(i_pd) overall avg Average addtonal PAGE delay : 3: 6: 9: 2: 5: 8: talan tme 4 2 servce rate (Kbps) 9perc Fgure 2. I_pd: confdence nterval of the average and 9 th percentle (conf. level 95%) Fnally, we examned the upper bounds of the addtonal URTs. For each experment, we evaluated the confdence nterval of the average and 9 th percentle of I_pd (confdence level 95%). In Fgure 2 we plotted the average I_pd, wth the related confdence nterval (bottom curve), the average addtonal URT over the entre day (straght curve), and the 9 th percentle measured n each experment (top curve). From Fgure 2 t appears that the average I_pd values are small (the values are around.5 seconds). Furthermore, the 9 th percentle s typcally below.5 seconds, and always below 2.5 seconds. Therefore, we can say that our system, wth a probablty of 9%, does not add more than 2.5 seconds, and therefor, the user doesn t perceve almost any degradaton n the QoS. The QoS performances related to the applcatondependent system are slghtly worse (see [ for detals). In that case we obtaned smlar values wth respect to the average I_pd, ts confdence nterval, and ts overall average over an entre day. On the other hand, the applcaton-dependent approach exhbted hgher peak values (up to 5 seconds) wth respect to the 9 th percentle of the I_pd. Ths can be explaned by the aggressve power-savng polcy adopted by the applcatondependent approach: the moble host s network nterface remans off for more tme (and, thus, the energy consumpton s hgher), but ths causes an ncrease n the upper bound of the addtonal delays. However, on the average, both approaches are equvalent n terms of the QoS perceved by the user. Specfcally, wth respect to the Web applcaton taken nto consderaton n both cases, t can be expected that the user does not experence a sgnfcant degradaton n the QoS. 5. Conclusons In ths work we have desgned and evaluated a novel network archtecture for reducng the power consumpton of moble hosts n a moble Internet scenaro, whle mantanng almost the same QoS level. Our archtecture follows an applcaton-ndependent approach as t does not make any assumpton about the traffc profle generated by the applcatons. Therefore, t fts any Internet applcaton, and can be used concurrently by dfferent applcatons. Furthermore, t s totally transparent to the upper layer as t presents a standard socket nterface. Hence, t can be used wthout any modfcatons n the code of legacy Internet applcatons. Our archtecture explots the Indrect-TCP model: t splts the transport connecton between the moble and the fxed host. Ths allows an mprovement n the bandwdth avalable at the transport layer and, consequently, reduces the energy requred to transfer a gven amount of data. The core of our approach, s the Varable-Share Update algorthm that predcts packet nter-arrval tmes and dle perod lengths, and manages the network nterface of the moble host accordngly. Specfcally, the moble host swtches off ts wreless network nterface when there are no data to be exchanged over the wreless lnk. Ths allows to consume the mnmum amount of power for each data communcatons. Proceedngs of the 36th Hawa Internatonal Conference on System Scences (HICSS 3)

10 We have mplemented a prototype of our archtecture and we have extensvely evaluated t. We used n the evaluaton a Web as applcaton as: () the Web s the most popular Internet applcaton, and () Web users are typcally delays senstve. Furthermore, ths choce has allowed us to compare the results obtaned by usng our applcaton-ndependent approach wth those related to the applcaton-dependent approach developed n a prevous paper. The expermental results have shown that the applcaton-ndependent archtecture s able to save, on average, the 68% of the energy consumed by the legacy TCP/ archtecture, wth peaks over 85%. Furthermore, ths energy savng s obtaned wthout a severe degradaton n the QoS perceved by the users. The addtonal URT ntroduced for transferrng a Web page s no more than 2.5 seconds. The comparson between the applcaton-ndependent approach, and the applcatonndependent one, has shown that explotng some knowledge about the applcaton behavor results n a slghtly better power savng, but may ntroduce n the worst case larger addtonal delays. However, n the average case, both approaches have exhbted smlar performance n terms of QoS provded to Web users. By consderng the hgh flexblty of the applcatonndependent archtecture, these results seem to ndcate that ths approach s a very promsng one for power savng. Experments wth dfferent type of applcatons (e.g., e-mal), and several applcatons concurrently actve on the moble host, are however requred to further valdate ts potentaltes. Acknowledgments The authors wsh to express ther grattude to Paul Barford for provdng the SURGE traffc generator used n the experments. Also many thanks to Mohan Kumar for gvng the opportunty to use the Web server at the Unversty of Texas at Arlngton. 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Brodersen, A Portable Multmeda Termnal, IEEE Communcatons Magazne, December 992. [9] M.Stemm e R.H.Katz, Measurng and Reducng Energy Consumpton of Network Interfaces n Hand-Held Devces, Proc. 3 Internatonal Workshop on Moble Multmeda Communcaton, Prnceton, NJ, September 996. [2] M.Zorz e R.R.Rao, Energy Constraned Error Control for Wreless Channels, Proceedng of IEEE GLOBECOM 96, pp.4-46, 996. Proceedngs of the 36th Hawa Internatonal Conference on System Scences (HICSS 3)