Sangam - Efficient Cellular-WiFi CDN-P2P Group Framework for File Sharing Service

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1 Sangam - Effcent Cellular-WF CDN-P2P Group Framework for Fle Sharng Servce Anjal Srdhar Unversty of Illnos, Urbana-Champagn Urbana, USA srdhar3@llnos.edu Klara Nahrstedt Unversty of Illnos, Urbana-Champagn Urbana, USA klara@llnos.edu Long Vu ByteMoble Urbana, USA longvu2@llnos.edu ABSTRACT Solutons to the cellular network bandwdth problem have been presented by the communty such as usage of alternatve wreless (e.g., WF) network and employ peerto-peer (P2P) fle sharng over the group of wreless devces. The mult-rado approach for fle sharng has been valdated analytcally or va smulaton n the lterature. However, often the theoretcal and smulaton approaches for fle sharng wthn mult-rado CDN-P2P groups hde the complexty of systems and networks n real scenaros such as heterogenety of phones n a P2P group, ssues wth schedulng polces wthn a CDN-P2P group of devces, CDN-P2P group formaton and others. In ths paper, we present Sangam, an effcent cellular- WF CDN-P2P group framework for fle sharng where we address extensvely system and network challenges n fle sharng for real phone group scenaros. The Sangam framework accounts n ts protocol, polcy and algorthmc desgns for (a) heterogenety of phone group devces n terms of CPU and power levels, (b) dfferent szes and numbers of chunks n P2P part of the group-based content dstrbuton, (c) hybrd schedulng polces for chunk dssemnaton wthn mult-rado CDN-P2P wreless group envronment, and (d) dfferent group szes. Sangam valdaton shows the mpact and dfference to smulatons when consderng real mplementaton of vdeo fle sharng wthn a cellular-wf CDN-P2P group. 1. INTRODUCTION The moble smartphone ndustry has revolutonzed the way that users use the nternet and connect wth other users. One of the most popular servces provded by smartphones s downloadng and playng vdeos.the majorty of the traffc on the Moble Handheld Devces(MHD) s seen to be multmeda content followed by web browsng.csco reports that t expects vdeo traffc to account for 60% of the global IP traffc by 2014 [2]. The moble devces use the cellular nterface to connect to the nternet. The network speed offered by current cellular nterface s only a few megabytes per second [6]. The tme requred to download a short vdeo fle s n the order of a few mnutes.ths severely degrades the qualty of servce provded to the user. Comparson studes n [13] and [7] show effcency of the W-F nterface n comparson to the cellular nterface. The W-F nterface not only provdes a hgher throughput but also consumes much less power than the cellular nterface whle downloadng large fles. Hence ncreased growth of moble phones and ther capabltes to access mult-rado wreless nfrastructure can lead to extended vdeo fle download servces usng these devces. In places where there s no wred network nfrastructure lke open concerts, n stadums or large factores, the nodes use the cellular lnk to download the fle of nterest.examples of such scenaros are vewng the same vdeo fle among a co-located group of fans at a concert, or downloadng the same class materal to co-located group of students n the classroom onto ther moble devces.to enable such group-based scenaros, cellular content dstrbuton networks (CDN) have been used for fle download and mplct sharng of content among groups of wreless devces [14] [6]. However, the cellular network bandwdth s becomng saturated, hence fle download servces are sufferng from long delays, losses and even drop n connectvty n co-located group scenaros. Another very mportant ssue n moble devces today s the lmted battery power avalable. In [22] we see that the transmsson power over cellular networks and W-F access ponts requre the largest percentage of power consumpton n a devce. The effcency as seen n [7] shows us that the W-F nterface consumes consderably less power than the cellular nterface especally when transferrng large fles. In ths paper, we utlze the moble phone capabltes to access mult-rado(cellular-w-f) wreless nfrastructure and explore solutons to assst co-located group of users to download vdeo fles n tmely and energy effcent manner, hence yeld same qualty of servce n moble phones as n laptops. We present the Sangam framework that uses multple rado nterfaces of a moble group based ecosystem to decrease the tme of fle download. All moble devces belongng to a co-located 1

2 3. Reduced power consumpton s attaned usng the hybrd network whle downloadng large fles. The paper s organzed as follows. In Secton 2 we present some of the related work n ths feld. Secton 3 states our assumpton when solvng the problem. We present our problem descrpton n Secton 4. In Secton 5 we descrbe the proposed Sangam archtecture as a vable soluton. In Secton 6 we present our expermental envronment followed by our evaluaton of the Sangam framework n Secton 7. We present our concluson n Secton 8. Fgure 1: Problem Statement group are equpped wth cellular, W-F and Bluetooth nterfaces and connect drectly wth other moble devces n the vcnty.ths connectvty between moble devces leads to the formaton of a wreless peer-topeer(p2p) network. Ths s what we refer to as a group of peers. If the group of peers are nterested n the same fle, duplcate requests are made to the server. The repeated requests for the same fle by the co-located group of peers results n unnecessary usage of cellular bandwdth. The Sangam framework leverages the proxmty of the co-located group of peers and ther nterest n a common fle to mnmze the fle download tme and cellular bandwdth use. Sangam combnes the cellular and W-F wreless nterfaces to create a hybrd CDN-P2P overlay network for group-based fle sharng. Usng the Sangam framework we answer the followng questons: 1. Does the jont cellular/w-f hybrd network provde us wth a lower download tme than wth a purely cellular network? 2. What schedulng polces for content dssemnaton need to be appled? What are the consderatons entalng such polces? 3. What s the advantage of accountng for the heterogenety of devces? 4. How s the Sangam framework helpful n the power management of devces? Hence, our paper has the followng contrbutons: 1. To the best of our knowledge, ths s the frst system mplementaton and expermentaton of a hybrd network usng multple rado nterfaces. 2. The Sangam framework ntroduces, evaluates and compares three CDN-P2P coordnatng download and dstrbuton polces whle takng nto consderaton the heterogenety of the peer group of devces. 2. RELATED WORK There have been many hybrd models that have been suggested n order to overcome the challenges faced by a pure clent/server model over a sngle network nterface. The hybrd model takes advantage of the fact that there are multple peers nterested n the same fle content and uses a peer to peer network to augment the clent/server model. Rmac et al [23] develop a mathematcal model to evaluate a hybrd model on the bass of churn rate, object sze and upload bandwdth. The evaluatons show that the hybrd model s senstve to churn rate but performs better than the tradtonal model n terms of resource allocaton and tme convergence. Ishare [26] envsoned a scenaro where there were colocated moble devces requestng the same fle from a server. The fle of nterest s dvded nto segments and downloaded by each of the peers over the cellular network. The moble devce also requests fle segments from other moble devces n the peer to peer network. In Ishare desgn, the download from the server over the cellular nterface and the download from the peers over the W-F nterface takes place smultaneously. Each fle segment s requested at random and each peer tres to download the maxmum number chunks possble n the least amount of tme. There have been many suggestons for usng the base staton to assst n the schedulng of P2P wreless networks. One of most common approaches, as mentoned by Hseh et al [16] s to use the base staton for decdng the schedulng of the P2P networks whch wll offer better throughput n terms of less packet loss. The ICAM (Integrated Cellular and Adhoc Multcast) archtecture proposed by Bhata et al [8] puts forth the concept of moble proxes wth the hghest downlnk rate that are used to forward the data to other moble peers. The problem of farness s addressed by UCAN (Unfed Cellular and Adhoc Networks. UCAN, proposed by Luo et al [19], ntroduces the concept of secure credtng. Whle selfsh hosts are not assumed, there s a credtng system that s used to allow proxes and relays transmttng nformaton to be credted for ther relayng work. Augmentng the exstng cellular network wth peer to peer communcaton s also suggested n Madsen et al 2

3 [20]. The paper proposes cooperatve network of cellular and P2P networks called Cellular controlled P2P communcaton networks. The moble hosts use both the cellular network for communcaton wth the base staton and a Bluetooth connecton for communcatng wth peers. An mplementaton of a fle download process on Noka phones N70 runnng the Symban operatng system posted decreased energy consumpton of about 44% wth the download tme beng half the orgnal tme. Sangam uses multple nterfaces on group-based moble devces n conjuncton wth peer-asssted data dssemnaton. There have been several approaches that have used fle segmentaton at the server sde. Hanano et al [15] and Kang et al [18] propose a hybrd network archtecture consstng of cellular and W-F networks. The papers descrbe a fle exchange algorthm n a peer to peer network wth the help of a central server. Another proposed soluton by Doral et al [11] to deal wth P2P downloads s by usng multcast groups. A sngle node s responsble for dstrbutng a gven range of fle segments to moble nodes. The fle s not strped across all moble nodes n ths proposed approach. Huguenn et al [17] propose an approach for peer to peer vdeo on demand. The fle segments are downloaded n such a way that the less advanced moble devces get the latest fle segments from the server whch s sent to the most advanced peer. In return, the least advanced peer downloads the fle segments from the most advanced peer n order. Ths approach ensures that all moble devces take part n the fle download process and ensure that deadlnes are met n a tmely fashon. The concept of fle segmentaton and P2P download has been mplemented snce the start of Napster, Gnutella and BtTorrent. However, snce we are usng moble devces that brng nto consderaton the challenges of moblty, resource constrants and tme, the mplementaton of BtTorrent drectly on moble devces may not be practcal. Ekler et al [12] present a comparson study by mplementng the BtTorrent applcaton on Noka s Symban platform and the Java Mcro Edton platform. The am of the authors was to examne the realstc possblty of constructng a peer to peer network usng moble phones and nvestgatng the challenges faced. Though Sangam uses the concept of fle segments and peer to peer networks, the dea of chunk assgnments based on resources of a group of devces and multple rado nterfaces s not mentoned by authors.claudno et al [10] mplemented a fle sharng applcaton over W- F nterface on the verson 1.6 of the Androd platform and used an exstng Bluetooth fle sharng applcaton. The authors conclude that W-F s the preferred nterface to transfer fles n a peer to peer network. These evaluatons allow us to compare our results wth [10] over the W-F nterface. Fgure 2: Network Model Tysowsk et al [24] mplement a fle sharng protocol on the Blackberry and Wndows Moble. The moble nodes download a fracton of the fle from the server usng the cellular connecton and then dstrbute fle segments among themselves usng the Bluetooth nterface. The results show decreased download tme and throughput for a large number of peers. Another mplementaton s seen n the Push and Track system by Whtbeck et al [27] for dssemnatng content to a large number of moble node wth a defned tme deadlne. The system makes use of the adhoc connectons n the peer to peer network and reduces the load on the wreless nfrastructure. The push and track system does not deal wth large vdeo fles but s more tuned towards traffc updates and news. 3. MODELS AND ASSUMPTIONS Network Model: The network model as seen n Fgure 2 assumes a group of co-located moble devces that are equpped wth cellular and W-F nterfaces. We call ths a group of peers denoted as V = 1, 2..., N wth group sze V = N. The moble phones are connected va P2P to each other over the W-F nterface, and use the W-F access pont(ap) to communcate wth each other. 1 CDN connectons between all group peers {1, 2,.., N} and the server S wll be over the cellular network. The communcaton connectons {e 1, e 2,..., e p } among the group peer devces and the server S represent edges E = {e 1, e 2,.., e p } of the network group G = (V, E) wth V = 1, 2..., N and E = {e 1, e 2,.., e p }. 1 Usng adhoc mode to communcate among the group of peers wll be carred out as an extenson of our work. 3

4 At ths pont we assume G represents the group of colocated peer devces that are nterested n downloadng and sharng a common fle. The Sangam system s represented by Sangam sys = G {S, {(S, 1),..(S, N)}} Data Model: Let the fle of nterest F be dvded nto m chunks represented by {f 1, f 2..., f m }. Each of these chunks resdes on atleast one peer once the chunk assgnment and download from the CDN s complete. Resource Model: The battery level and CPU speed of the devce quered and stored at the server S to assst wth the chunk assgnment and schedulng. The server assgns chunks based on all resources of the entre group of peers. Moblty and Access Model: We assume that the nodes of the group are statc or are movng very slowly, ensurng that all the peers n the group co-located.the model assumes that group of peers requests a common fle of nterest (F ) from the server S wthn a regstraton tme nterval t a rrval whch s approxmately a few seconds long. It means, wthn our desgn, we currently assume to do chunk assgnment and schedulng to peers for a statc group of peers G G wth V = {1, 2,..., n} V = 1, 2,.., N who regster wth the server S wthn an nterval t a rrval. After ths nterval t a rrval, a new request to jon the group of peers and download a fle F s treated as a new group request where server S groups all new requests from new group of peers G G and derve new chunk assgnments and schedule. The scheduled chunk assgnments are derved and dssemnated to all peers n the group of peers once atleast two peers have regstered wth the server S. 4. PROBLEM DESCRIPTION We know that the maxmum data rate that we can acheve usng a cellular nterface s less than what we can get usng a W-F nterface. When multple moble nodes request the same fle usng multple nterfaces, co-located group of peers can download the same fle n a reduced amount of tme. The total number of chunks m n a gven fle s m = F lesze ChunkSze. The number of chunks assgned by the server S to a peer s m. m s the total number of chunks n the fle of nterest F and s gven by m N. The total number of peers s represented by N. Let T swtch be the tme requred to swtch nterfaces from cellular to W-F. Let t j (f x ) be the tme to download a chunk f x from source to destnaton j. The source can be a server or a moble devce but the destnaton wll always be a moble devce j, j n V. We obtan the best chunk assgnment wth optmal fle download tme subject to constrants such as cellular bandwdth, W-F bandwdth, number of chunks, sze of group of peers and the gan metrc. We defne our gan metrc as follows: Gan = T hybrd T cell 1 (1) We wll valdate the gan metrc through our expermental results shown n Secton 7. The overall goal of the group fle sharng, s to maxmze under constrants such as bandwdth avalablty for group of peers n cellular and W-F networks, fle sze, CPU and power of the devce. We use these constrants to formulate the download tme optmzaton problem. The problem s NP complete because the download tme optmzaton problem ncludes dervaton of the optmal chunk assgnment and schedule [25] [9].Hence, we need heurstc algorthms to solve the optmzaton problem. To download the assgned chunks m j from the server the total tme for any peer n a group of peers s gven by: m j t sj (f k ) (2) k=1 The total tme taken by peer to download the remanng chunks from a group of peers s gven by: n m j j=1,j k=1 t j (f k ) (3) The optmzaton problem for mnmzng the download tme of a fle F to a group G of n peers: mn T (F ) {,j,k} = n m t s (f k ) + =1 k=1 n + n T swtch subject to B cell B max cell B wf B max wf m m gan 1 n N E SANGAM ARCHITECTURE n m j =1 j=1,j k=1 t j (f k ) (4) The desgn of Sangam archtecture encompasses regstraton of the group of peers at the server S, chunk assgnment and schedule by the server based on the resources at each peers of the group, and the download protocols carred out at each of the peers. Each peer 4

5 Symbol m m n N E t j (f x ) T cell T hybrd T swtch Bcell max Bwf max B cell B wf Table 1: Notatons Descrpton Total number of chunks n the fle of nterest chunk assgned to peer Sze of regstered Group of Peers, n N Maxmum Sze of Group of Peers Battery level of peer Download Tme of chunk f x from source to destnaton j Download tme of entre fle over cellular nterface Download tme of entre fle over hybrd network Tme taken to swtch between the cellular and W-F nterface Maxmum cellular bandwdth acheved by peer Maxmum W-F bandwdth acheved by peer cellular bandwdth acheved by peer W-F bandwdth acheved by peer has to regster wth the server S that contans the fle of nterest. The regstraton process nvolves the exchange of nformaton between the server and group of peers. The server performs coordnated chunk assgnment on the fle of nterest based on the receved resource nformaton from each peer. Fnally each peer downloads the assgned chunks from the server and group of peers. The overall Sangam flow s as follows: 1. A peer n the G group regsters wth the server S, specfes the fle of nterest F wthn nterval t a rrval. 2. Each peer durng the regstraton phase also declares resource nformaton such as CPU speed, level of battery power and avalable bandwdth to the server S. 3. The server S performs coordnated chunk assgnment for G group of regstered peers on the fle F, based on the receved resource nformaton from group of regstered peers n the nterval t a rrval. 4. The server S schedules the download of assgned chunks from the server over cellular-cdn network to the group of regstered peers. 5. The peers of the group exchange mssng chunks among each other va W-F - P2P network. 5.1 Coordnated Chunk Polces The servers upon completng the regstraton process for group of peers G ng begns assgnng the chunks to the dfferent peers. The common fle of nterest F s dvded nto m chunks. Each peer s assgned a range of chunks based on dfferent polces as a result of the heurstc approaches to solve the optmzaton problem n (4). The peers are then sent the chunk assgnments along wth the G group of peers nformaton peer lst at the end of the regstraton process. The cellular connecton between the server S and the G group of peers s smlar to a control channel. The server s aware of the chunk assgnments and the group of peers are nformed of ths to assst n the download process. We propose three dfferent approaches to chunk assgnment for the regstered group of peers. We present (1)equal chunk assgnment,(2)chunk assgnment based on battery level, and (3)chunk assgnment based on CPU speed. 1. Equal Chunk Assgnment - Homogeneous system: In the smplest chunk allocaton algorthm, all peers are assumed to be equal as shows n Fgure 3. Ths approach serves as the baselne aganst whch we compare other approaches. The server S calculates the total number of chunks and dvdes them equally among the co-located group of peers. In ths chunk assgnment polcy, the server S does not consder the heterogenety of the G group of peers requestng the common fle of nterest F. Peers are assumed to have smlar battery levels and processor speeds. In essence, the server assumes that they are capable of carryng out the download process at the same capacty as one another. Ths chunk assgnment s what we would expect n a homogeneous envronment where all moble phones not only have the same underlyng hardware but also the same verson of the operatng system and rado nterface optmzatons. In the followng equatons, R equal s the fracton of chunks assgned to each peer, P equal s the number of chunks assgned to each peer and m s the total number of chunks. R equal = 1 n (5) 5

6 Symbol m n R equal P equal R battery P battery R cpu P cpu W Table 2: Notatons Descrpton Total number of chunks n the fle of nterest Sze of regstered group of peers Fracton of the fle of nterest assgned to peer Number of chunks of the fle of nterest assgned to peer Fracton of the fle of nterest assgned to peer based on ts battery level Number of chunks of the fle of nterest assgned to peer based on ts battery level Fracton of the fle of nterest assgned to peer based on ts CPU speed Number of chunks of the fle of nterest assgned to peer based on ts CPU speed Weghted Rato of the battery level and CPU speed of peer Fgure 3: Equal Chunk Assgnment - Homogeneous System Fgure 4: Chunk Assgnment based on Battery Level - Heterogeneous system Chunk assgnment: P equal = R m = m n (6) Note that the chunks are scheduled at S n the FCFS regstraton order. 2. Chunk Assgnment based on Battery Level - Heterogeneous system: For any moble phone to carry out a fle download protocol usng a cellular or a W-F lnk or both, the battery power requred for transmsson s one of the hghest cost compared to other processes runnng on the moble devce as shown n Fgure 4. For a moble node j whose battery power s relatvely lower than the power of other moble nodes n the group G, t mght be benefcal to conserve the energy and be able to vew the vdeo nstead of completely dranng ts battery power. Another reason for other nodes to support ths unequal chunk management s to ensure that all fle chunks are downloaded and present n the G group of peers. If a moble devce j loses ts battery power n the mddle of the fle download, all peers wll be forced to request all chunks assgned to the peer j from the server S over ther cellular nterface. Ths defeats the purpose of the cellular- W- F CDN-P2P protocol. When each of the peers usng ths modfed chunk assgnment regsters wth the server S, they send the server S ther current battery level. The server proceeds to calculate the rato of total chunks that the peer can download. Here R battery s the fracton of chunks assgned to each peer, E s the battery level of peer and P battery s the number of chunks assgned to peer. Chunk assgnment: R battery E = n k=1 E k P battery (7) = R battery m (8) Note that all chunks are scheduled at the server S n the FCFS regstraton order. 3. Chunk Assgnment based on CPU Speed - Heterogeneous system: Ths algorthm takes nto account a more realstc settng n the current moble ecosystem. Wth the 6

7 n the FCFS regstraton order. Fgure 5: Chunk Assgnment based on CPU Speed - Heterogeneous system ncreased rate of moble development, each moble node can be composed of dfferent processors and hence can work at dfferent speeds as shown n Fgure 5. In order to account for ths heterogenety, each peer sends ts maxmum CPU speed (e.g. 800MHz, 1GHz) to the server durng the regstraton process. The server calculates the rato that each of the peers needs to download based on a smlar calculaton as shown above. A faster processor mght be handed a larger chunk assgnment from the server S as per ths algorthm. Though there s an uneven chunk assgnment among the peers, there s a beneft for faster moble nodes to download a larger number of chunks from the server. When there s equal chunk assgnment we notce that the slower peers become a bottleneck for completng the download of the fle. The faster peers ether wat for the slower peer to complete the fle download of assgned chunks or end up requestng the chunks from the server drectly. The cellular bandwdth may be the bottleneck for performance of faster peers when downloadng chunks from the server. However, peers wth faster CPU speeds perform at a very hgh rate when exchangng chunks wthn the group of peers. Hence, the benefts of faster peers can be utlzed by usng CPU speed for chunk assgnment. Here R cpu s the fracton of chunks assgned to each peer, S s the CPU speed of the peer and P cpu s the number of chunks assgned to peer. Chunk assgnment: R cpu S = n k=1 S k (9) P cpu = R m (10) Note that all chunks are scheduled at the server S 5.2 Sangam Fle Download Protocol The download protocol takes place once the peer regstraton and chunk assgnment are completed. In order to download the entre fle F, the peer has to go through four download phases. In the frst phase, each peer requests the server S for the chunks assgned to t n the FCFS regstraton order. Between the frst and seconds phase the communcaton nterface s changed from cellular to W-F. The moble devces today have ther default nterface set to W-F nterface. As mentoned prevously, we can drect traffc over the cellular nterface for a partcular destnaton. However, the cellular nterface eventually dsconnects leavng only the W-F nterface. We suspect that both nterfaces are not allowed to be swtched on at the same tme due to an underlyng system polcy. However, the recent release of the Androd 4.0 s able to support both cellular and W-F traffc usng the WFdrect package. The smultaneous turnng on of multple nterfaces mght also prove to be too much of a dran on the battery lfe snce these devces have not been optmzed to carry out network connectons on two dfferent nterfaces. In the second phase, each peer requests the G group of peers for the remanng chunks as per peer lst and chunk assgnment handed down from the server at the end of the regstraton process. In order to avod overloadng any one peer n a gven tme nterval by requestng the same set of chunks, we use a round robn schedulng polcy locally at each peer. Each peer begns phase two of the download process by requestng the peer whose ID s one greater than ts own ID. The peer ID s ssued by the server S at the tme of regstraton. The thrd and fourth phases take place only f a peer s unable to download all chunks from the server S and G group of peers n the frst two phases. In the thrd phase, a moble node requests the peer for the chunk that t was unable to download n phase two. There s an nterval of tme durng whch the moble node mght be unreachable due to swtchng between cellular and W-F nterfaces. Snce we assgn chunks based on the capablty of the nodes, the moble nodes wll not complete phase one at the same tme. The nterface swtch tme T swtch vares among devces wth an average value of 12s n our experments. A few chunks of the fle that mght not be obtaned from each of the peers at the begnnng of phase two s acqured by the peer n phase three. The fourth phase takes place f the chunk s not present among the group of peers. The moble node then requests the server for any chunks that mght not have been downloaded by the peers. If the per node chunk 7

8 Fgure 6: Sangam Clent Archtecture assgnment s small, there s a chance that the faster nodes are not able to get all the chunks from the other peers n a tmely fashon and end up requestng a larger porton of the remanng chunks from the server n the end. 6. IMPLEMENTATION Sangam has been mplemented on the Androd operatng system verson 2.1 and above. The fle download applcaton on the clent sde s present n the userspace and uses the Java API for socket connectons. The turnng off and on of the cellular and W-F nterface s carred out usng the WfManager class of the Androd API. The phones used n the expermentaton are Drod, Nexus S and Nexus One. In fgure 6 we hghlght the man modules nvolved n the mplementaton of the clent Sangam framework. The server conssts of a smple C program that accepts clent connectons and responds wth the request fle chunk. The server handles the regstraton of the group of peers and the chunk assgnment based on the receved resources from the group of peers. Regstraton: Regstraton of peer at the server S allows the user to specfy the fle of nterest F, IP address and port number.the regstraton process also nvolves the moble node sendng nformaton about the battery level and CPU speed to the server. In order to ensure that we are able to set up TCP connectons over the cellular and W-F nterface, each moble node has to send the IP address of the W-F nterface along wth the IP of the cellular nterface. There could be a number of ways to accomplsh ths. The moble node could have a statc IP address assocated wth ts W-F nterface. In our current mplementaton we turn on the W-F nterface and record the IP address. The W-F nterface s then turned off to allow for the cellular nterface to come up. The androd phone by default chooses the W-F nterface to connect to and hence an explct call usng the WfManager s used to turn off and turn on the nterface. Each peer s also assgned an ID that s used n the download process. As we have prevously stated n our model, we assume that the server wats for the mnmum number of peers to regster n a certan tme nterval t arrval. If the mnmum number of peers does not regster wthn t arrval, then each peer receves the entre fle over the cellular lnk and no sharng polces are mplemented by the server S. Group of Peers Manager: After regstraton wth the server S, the moble node receves the lst of peers nterested n the same fle F along wth ther IP address and port nformaton. Peer management handles the storage of the IP address, port number, and the chunks {f 1, f 2..., f k } assgned to each of peers who are nterested n the same fle F. The peer lst ncludes an array of peers n G and ther chunk assgnment {f 1, f 2..., f k }. The peer lst s used to request the chunks from the group of peers n phase 2 and phase 3 of the download process. Network Connectons Manager: There are three network connectons that the peer handles durng the fle download process. The frst two are used request fle chunks from the server and group of peers. The thrd one s for handlng ncomng fle chunk requests from other peers. The connecton to the server takes place over the cellular network. The other two peer connectons take place smultaneously over the W-F nterface. The current cellular and W-F connecton status s mantaned by the network connectons manager. We use TCP protocol to carry out all fle requests. Fle Transfer Manager: The fle transfer manager s responsble for carryng out the four phases of the download protocol. It s responsble for swtchng nterfaces between cellular and W-F n a tmely fashon. It keeps track of the number of chunks receved from the group of peers and servces request accordng to the chunk assgnment handed down from the server. It also ensures mutual excluson when wrtng and readng chunks from a gven output fle. The vdeo fle s present at the server S and dvded nto chunks of sze 2048 bytes. We carred out fle download evaluatons wth chunk szes varyng from 256bytes to 4096bytes. We found 2048bytes to be the optmal sze whch balances the lmted memory requrements of the moble devce and the IP packet sze. The vdeo fles used are 2.2MB and 3.9MB. 7. EVALUATION The evaluaton of the Sangam framework s carred 8

9 (a) (b) (c) Fgure 8: Scenaro 1: Fle Download Tme wth Equal Chunk Assgnment Approach Fgure 7: Expermental Setup out on Androd phones havng an operatng system verson of 2.1 or greater. We use fve Drod phones whch use the Verzon network for cellular connectvty. To demonstrate the heterogenety of devces we use the Nexus One devce wth a T-moble cellular connecton. The Verzon network uses CDMA technology and offers an average speed of 848kbps [4]. T-moble uses GSM technology wth an average speed of 7.2Mbps [1]. A maxmum of fve phones were used durng ths evaluaton and fles of sze 2.2MB and 3.9MB were requested. We use x for our W-F connectvty. The network uses WPA2 protocol for authentcaton. In our evaluaton, the fle download tme T (F ) s the most mportant metrc. Our goal s to mnmze the tme requred by all peers to download the complete fle. In some cases, when we assgn chunks based on battery level, our goal s also to have as low battery consumpton of certan peers as possble. Each resultng plot has the baselne cellular download tme, server (CDN) download tme over the cellular network and peer(p2p) download tme over the W-F nterface. The Baselne Download - Cellular s the tme taken to download the entre fle from the server over the cellular network(cdn download). The Server Download - Cellular s the tme taken by a peer to download the assgned chunks from the server over the cellular lnk and Peer Download W- F s the tme taken to download the remanng chunks from the group of peers over the W-F nterface (P2P download). We have conducted Baselne measuments where 5 peers smultaneously request the entre fle of sze 2.2MB from the server S over the cellular network. The average download tme was found to be seconds wth a standard devaton of 38s over three trals. The download tme per fle chunk of sze 2048 bytes was seconds. Our orgnal am was to buld the system wth smultaneous download over both cellular and W-F nterface of the moble devce. However n the real system framework we ran nto many problems. For example, usng the requestroutetohost functon n the Connectvty Manager class of the Androd SDK [3] we were able to gan margnal success by specfyng the nterface to be used for a partcular IP. However, t was dffcult to sustan the connectvty of the cellular connecton snce the W-F nterface s the default network to be used. The ncreased dran of the battery power and possble nterference between the two transmssons was one of the reasons for us to swtch to a sequental download process as opposed to a concurrent one. We envson a change n the hardware of moble devces whch wll allow us to use smultaneous network nterfaces smlar to the desgn mentoned t [21]. The addton of the WfP2pManager class to the Androd SDK startng from Androd 4.0 wll enable us to use the W-F nterface for peer to peer ad hoc connectons whle smultaneously mantanng our cellular connectvty [5]. 7.1 Scenaro 1 - Equal Chunk Assgnment In our frst set of experments we demonstrate the ad- 9

10 vantage of the hybrd network usng the Sangam framework. We assgn fle chunks equally to all the peers n the group G. The group of peers G request a fle F of sze 3.9MB from the server S. We carry out ths request for dfferent group szes 3, 4, and 5 regstered peers. In Fgure 8, the download graphs (8(a), 8(b), 8(c)) show that as the number of peers ncreases from 3 to 5, the download tme of one peer decreases by 60s. Each peer s downloadng a smaller number of fle segments over the cellular nterface. The majorty of the chunks are now downloaded from the group of peers over the W-F nterface. There s a decrease n the gan rato due to the decrease n download tme over the cellular nterface. In Fgure 9 we observe that the download tme may not always gve us the best gan rato. In the Fgure 9 plots, we consder a peer-to-peer network of sze 3 peers who request 2.2MB from the server S. In both plots we see that the tme to complete the fle download s only 10s to 50s lesser than the baselne case. We attrbute ths to two factors: Interface Swtch Tme: After the completon of phase 1 of the download process over the cellular network, all peers swtch ther nterface to W-F. The average tme overhead for the swtchng of nterfaces from cellular to W-F s 10s to 15s on average. However, some peers mght not be able to scan and assocate wth the W-F access pont wthn ths nterval of tme. The longer the moble devce takes to assocate and connect to the W-F access pont, the greater the tme s that the node s unreachable by all other peers. Hence peers end up watng for any devce that s unreachable. Contenton: Multple devces n the peer to peer network contend wth one another n order to be scheduled by the W-F access pont. Ths takes place when the number of access ponts are few n number. A contenton graph s formed and results n severe degradaton n the performance of the hybrd network. Multple peers end up watng to download from ther neghborng peers. We observe the fle download process takng place n a subset of peers n a gven nterval of tme. The gan that can be acheved n ths stuaton s very low. 7.2 Scenaro 2 - Chunk Assgnment based on Battery Level In our next expermental scenaro we consder chunk assgnment based on the battery level. The peer wth the greater battery level s assgned a larger share of the fle segments. In ths case the metrc that we choose to mnmze the fle download tme under the constrants of the battery consumpton of the peers. We consder a hybrd network wth 3 and 5 peers requestng a fle of sze 2.2MB. In Fgure 10, we show the result of the hybrd network of 3 and 5 peers, and the download tme T (F ) Peer ID Intal Battery Level Fnal Battery Level Download tme(s) Table 3: Scenaro 2: Download Tme for 5 Peers s only slghtly greater than the scenaro of equal chunk assgnment. Table 3 also shows that the peers wth a lower battery level seem to suffer a greater decrease n battery power than the peers wth a hgher battery level. Hence, havng such peers take on a greater a share of the cellular download would completely exhaust the remanng power. When the number of peers s 3, we see n Fgure 10(a) that the peer wth a lower battery level ends up watng for the cellular download to be completed. We also notce n Table 3 that the tme to download the chunks from the group of peers for a peer wth battery level 20 s larger than t s wth battery level 50 and 70. Due to the lmted tme spent downloadng the chunks over the cellular nterface, the peer does not suffer from a large drop n power level. 7.3 Scenaro 3 - Chunk Assgnment based on CPU Speed In order to account for the heterogenety of nodes we assgn chunks to the group of peers based on the CPU speed level of the moble devces. We use two Drods that use the Verzon cellular network and one Nexus one that uses the T-Moble cellular network. The CPU level of the Drod phones are Hz and that of the Nexus One s Hz. All 3 peers request a fle of sze 1MB from the server. The Nexus One s assgned a greater share of the fle segments due to the hgher CPU speed. In Fgure 11(a), the ntal download from one of the Drods carred out by Nexus One s slghtly slower n the begnnng. Ths may be because of the nterface swtch that s takng place at the correspondng Drod. The plot of the correspondng Drod shows a certan delay n swtchng nterfaces and startng the download from the group of peers. We notce the speed of download of the remanng chunks from the group of peers by the Nexus One devce s extremely fast (see Fgure10(a)). The Drod s able to leverage the faster Nexus One devce n order to complete ts download. However, the faster CPU power of the Nexus one was not able to overcome the bottleneck of the throughput of the cellular network. As we can see on the plot n Fgure10(b) of the Nexus 10

11 (a) (b) Fgure 9: Scenaro 1 : Impact of Contenton and Interface Swtch Tme on Fle Download Tme (a) (b) Fgure 10: Scenaro 2 : Fle Download Tme for Chunk Assgnment Approach based on Battery Level Devce/Model Network Network Network Number Fle Download Evaluaton Framework Interface Model Protocol of Peers Sze(MB) Tme(s) Blackberry [24] Bluetooth P2P OBEX Implementaton Wndows Moble Bluetooth P2P UDP Implementaton [24] T Noka N70 [20] Cellular, Hybrd TCP 2 - cellular 2 Implementaton Bluetooth Ishare [26] Cellular, Hybrd TCP Smulaton W-F Sangam Cellular, W-F Hybrd TCP Implementaton Table 4: Comparson of Fle Sharng Technques 11

12 (a) (b) Fgure 11: Scenaro 3 : Fle Download Tme for Chunk Assgnment Approach based on CPU Speed: (a)drod,(b)nexusone One moble devce, the cellular download lne mmcs the baselne plot nspte of havng a greater processng power. 7.4 Comparson wth Ishare As prevously mentoned n Secton 3, Ishare protocol [26] carres out fle download from the server and peers smultaneously and t s valdated only va smulaton. We compare our mplementaton to the smulaton results presented n Vu et al [26]. The smulaton was carred out usng Network Smulator 2 (NS2) wth segment sze 4KB. The smulator uses IEEE b adhoc lnk and 1xEV-DO cellular lnk wth a peak rate of 2.4Mbps. We only compare Scenaro 1- equal chunk assgnment wth the plot dsplayng average download tme. The Ishare evaluaton uses the Random way pont moblty model as opposed to statc or slow movng nodes n the Sangam protocol. We notce that the download tme n the Ishare smulaton s 500s whch s hgher than our mplementaton. Ths mght be because the group sze of Ishare s 15 whch s much larger than 5. Futhermore the cellular rates assumed and moblty models affect the fle download tme. Note that we do not assume moblty n the Sangam mplementaton due already major mplementaton challenges such as heterogenety, varety n battery power levels and overhead n swtchng between multple rado nterfaces, of fle sharng n statc topology scenaros. We have dentfed these practcal challenges and developed feasble solutons. 8. CONCLUSION The ncreased use of moble devces have led to the congeston of cellular networks. Recognzng that most moble devces are equpped wth multple nterfaces, we propose to develop a soluton that would nvolve downloadng a fle over both the cellular and W-F lnks. In scenaros where there s no nfrastructure and co-located peers are nterested n fast download of large fles, the proposed Sangam framework mnmzes the fle download tme. Usng effcent chunk management polces based on resource nformaton from the group of peers, the fle chunks are assgned and dstrbuted accordngly among the group of peers. We mplement the Sangam framework on Androd phones n order to compare and contrast the three chunk management polces - 1) equal chunk assgnment 2)chunk assgnment based on battery level and 3) chunk assgnment based on CPU speed. We obtan a decrease n download tme of 50s -60s by mplementng equal chunk assgnment and CPU speed approach. Though the download tme s slghtly hgher when we mplement chunk assgnment based on battery level, we are able to conserve the power level of certan moble devces ensurng the completon of the download process. 9. ACKNOWLEDGMENTS Ths work was funded by the Boeng - ITI(Informaton Trust Insttute) center. 10. REFERENCES [1] Cdma and gsm network speed. cdma-vs-gsm-examned-whch-3g-network-s-superor [2] Csco newsroom. press-release-content?type=webcontent. [3] Connectvty manager androd ap. androd/net/connectvtymanager.html. [4] Speedtest data. gadgetlab/2011/02/phone-speedtest/. [5] W-f drect ap. 12

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