Energy-Consumption in Mobile Peer-to-Peer Quantitative Results from File Sharing

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1 Energy-Consumption in Mobile Peer-to-Peer Quantitative Results from File Sharing Jukka K. Nurminen, Janne Nöyränen Nokia Research Center {jukka.k.nurminen, Abstract Battery consumption is an important but not very widely studied aspect of peer-to-peer applications. This paper describes a series of experiments which obtained detailed measurements of BitTorrent energy consumption on handheld devices. The measurements that were carried out with SymTorrent client on three different models of Nokia S60 mobile phones indicate that peer-to-peer content sharing on handhelds is practical from the energy consumption point of view. The results also show that acting as a true peer and uploading content for other users does not cause much extra energy consumption during active downloading. 1. Introduction One challenge of mobile peer-to-peer (P2P) applications is battery consumption. Contributing resources to the P2P network is little extra burden for a home PC which is plugged-in to electricity and broadband connection. However, for a mobile device the situation is different. Energy consumption is thus a potential inhibitor for the serious use of mobile P2P applications. The purpose of this paper is to provide quantitative results for this discussion. As observed by Feeney and Nilsson [1] bandwidth utilization is not synonymous with energy consumption. By measuring power consumption of handheld devices within an active P2P content sharing community we target a better understanding of the battery usage of P2P technology and of the possibilities of this technology on handheld devices. The number of papers that measure the energy consumption of applications on handheld devices is very limited. A possible reason is that exploration of new techniques and concepts are easier to perform in a simulated environment than with real devices. Simulations and theoretical analysis are typically adequate to show relative improvements brought by new algorithms or protocols. However, they fail to evaluate the practical application of these technologies. Real measurements are thus important to complement the theoretical and simulation studies available. The quantitative numbers characterizing the power consumption are important in assessing the feasibility of mobile P2P solutions on today s devices. In particular we focus on file sharing with the BitTorrent technology. Because BitTorrent is widely used it allows measurements of practical content sharing cases with real communities. With the new possibilities to consume and create multimedia in mobile devices BitTorrent is also an increasingly relevant application for mobile phones. In particular we would like to answer questions like 1. What is the power consumption behavior of a typical P2P download activity? 2. How does the power consumption of P2P downloading relate to other uses of the mobile device? 3. How does P2P downloading compare with direct HTTP downloading? How do the different access technologies influence the results? 4. What is the difference when a mobile phone is only a client of a P2P network versus the case when it acts as a true peer and also serves others? The rest of the paper is organized as follows. Section 2 reviews related energy consumption research. Section 3 presents our test setting in detail. Section 4 focuses on our results and on their interpretation. In section 5 we discuss the limitations and shortcomings of our experiments. Finally, section 6 summarizes our main findings. 2. Related Work There are a number of studies of BitTorrent performance [2]-[6]. They measure and model the performance of BitTorrent in the fixed Internet. Moving the scope from fixed Internet to handheld devices adds emphasis to the energy consumption. An /08/$25.00 IEEE 729

2 application that drains the battery of a mobile handset quickly is not likely to get wide acceptance. The common approach to analyze energy consumption of P2P applications is via simulations and theoretical calculations. Joseph et al. [7] propose an energy efficient caching scheme based on a region concept. Leung and Yu-Kwong [8] propose a new metric Energy based data availability and use it to design a P2P file sharing solution that is optimized for wireless environment. Ding and Bhargava [9] compare different P2P file sharing approaches over mobile ad hoc networks qualitatively. The number of measurement studies of P2P power consumption is very limited. Gurun et al. [10] claim to be the first paper to investigate P2P protocol behavior via power measurements. They performed energyconsumption measurements using a P2P chat application called Chimera on a PDA device. Our work complements their measurements in three aspects. First, by using a mainstream BitTorrent application we are able to analyze practical content sharing scenarios. Secondly, measuring the P2P performance on mobile phone creates results that are applicable to a wide range of devices and user. Thirdly, with a mobile phone we are not limited to access but are also able to measure the use on cellular wide-area network. Research on other handheld applications shows that significant gains in energy-efficiency can be achieved. For instance, Yan et al. [11] and Krashinsky and Balakrishnan [12] have investigated energy consumption in web browsing. By taking advantage of typical behavior patters they have developed energy efficient transport level protocols for handhelds. 3. Test Setting To perform the measurements we have used SymTorrent [13] which is an open source BitTorrent implementation for Symbian S60 platform. It is a complete BitTorrent client supporting both downloading and uploading of content. It thus allows a mobile phone to join the BitTorrent community as a full peer and work in a very similar way than a PC based client. To perform the actual measurements we have used a testing tool called Juice [14] which is a power profiling application for S60 devices. It allows measuring and collecting power consumption data during the use of a mobile phone without any external measurement equipment. For the network connectivity we have used two technologies: third generation cellular network () based on WCDMA technology and wireless LAN () based on the standards. The main body of the measurements was done with Nokia N80 mobile phone. Additionally, we used Nokia E61 and Nokia N95 models to analyze how different hardware affects the results. We used two different setups to connect to the real BitTorrent community: 1) Mobile phone connected via cellular network (Elisa network in Helsinki, Finland) to the Internet. 2) Mobile phone connected via to an ADSL router (D-LinkAir DI-514 Wireless Router b) which is connected via ADSL connection (8/1M Saunalahti) to the Internet. As test material we used the Simpson movie trailer distributed by Wuze ( NJTPLQ3YMZX3JKF.html [Accessed Jun 7, 2007]). The trailer is 9.73 MB in size. The high definition format of the trailer is beyond the capabilities of the player and display resolution of our test phone. However, we used it as a test material since it was freely available and very popular during our test period ( ). The fact that it was available in hundreds of peers made it difficult to use it for our upload tests. For those we used a larger file (37MB) that was not very widely available but under heavy demand. That attracted more peers to request the content from our device which allowed more realistic measurement of the effect of the upload traffic. 4. Measurements and Results W1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 0:00 0:30 1:00 1:30 2:00 2:30 3:00 3:30 4:00 4:30 5:00 5:30 6:00 6:30 7:00 7:30 8:00 8:30 9:00 9:30 10:00 Figure 1. Power consumption during typical content download Figure 1 shows typical power consumption curves when the user downloads the movie trailer with and. In this measurement the uploading feature of SymTorrent has been disabled. The first part of the curve (0-1 min) shows the high level of activity to setup the data connection. The middle part of the curve t 730

3 (1-4:45 min) shows the rather stable phase where active downloading is ongoing with power consumption around 1.2 W. At this phase the downloading speed is limited by the data transmission capacity provided by the operator. In our case it was around 50 kb/s. At 4:45 min the download is completed and power consumption drops to 0.4 W. There are still some power peeks that most likely depend on some delayed communication with the peers. (With an improved end-game algorithm in SymTorrent it could be possible to remove these peaks.) With the shape of the curve is similar. However, the higher speed and lower power consumption create a major difference. The download time is half of the time needed for downloading. While the power consumption during the active download phase is rather close to case there is a major difference in the power consumption after the downloading has been completed. From 5 min onwards power is less than half of the power needed by the. 1.4 W 1.2 With SymTorrent uses slightly less power than a voice call. This means that SymTorrent downloads can last at least as long as voice calls when both are started with full battery. This is a useful measure since technical specifications of mobile phones typically list the talk times of their batteries. With the Nokia N80 phone that we used in our test this would mean around three hours of continuous downloading. Figure 2 also shows that playing video or audio content needs much less power than its downloading. The results also show that the power consumption of the direct HTTP download is at the same level with SymTorrent. It thus seems that the extra complexity of the BitTorrent protocol does not require much additional power. The likely explanation for this behavior is the fact that radio traffic is the dominating component of the power consumption, as also observed in [14]. The above results have focused on the case when SymTorrent is only downloading but not uploading at all. This is clearly an undesirable state since in order to work properly in the community the peers should also contribute their resources to help others. Some clients even enforce this tit-for-tat principle by applying choking measures (see e.g. [15]) towards those peers who are not cooperating ,2 W 1,0 Full peer 0.4 0, ,6 0 SymTorrent Voice call Http Viewing Listening MP3 Idle Figure 2 Comparison of SymTorrent power consumption with other activities Figure 2 can be used to compare P2P downloading with other typical activities on a mobile phone. It presents the average power computed over six measurements both for SymTorrent and for normal HTTP download. For voice call, viewing, listening, and idle cases the average is only over two measurements since the power consumption of these terminal applications is less dependent on changes in external circumstances. Viewing was measured with a video clip captured with a Nokia N93i handset with settings Handset normal MP4 15 frames per second. Listening was measured by playing MP3 audio (192 kbps) with built-in speakers of the phone. Volume setting was at 50%. Figure 2 can be used to draw several conclusions how content sharing activities would work in practice. 0,4 0,2 0,0 0:00 2:00 Client only 4:00 6:00 8:00 10:00 12:00 Figure 3 Power consumption in full peer and client only mode with Figure 3 allows us to compare the case when the mobile peer supports uploads to others ( full peer ) and the case where it is only working as a downloading client ( client only ). These measurements are for only. Supporting upload functionality with networks is complicated because of the NAT and firewall policies of the cellular operators. Obviously the power consumption is higher in the full peer mode since uploading content to others consumes power. However, the additional power needed to serve others is different in the active download phase and in the following passive phase. In 14:00 16:00 18:00 20:00 22:00 24:00 26:00 t 28:00 731

4 the active phase only around 0.2 W extra power is needed for full peer operation. After the download has been completed, working as a full peer requires 0.4 W additional power. The difference comes from the fact that if there is no traffic the radio can be powered down. The need to serve other peers does not allow this to happen. The conclusion from these measurements is thus that acting as a full peer is feasible as long as active downloading is ongoing. With around 25% extra power the phone is able to help others during the active phase while in the passive phase around 200% extra power is needed. To save power the peer should thus serve others only when it is performing a download operation. In order to analyze how sensitive the results are to different phone models we performed measurements with three different Nokia phone models: N80, E61 and N95. These devices represent different HW architectures and different release dates. We also measured direct HTTP download of the same file to compare how P2P technology compares to it. Table 1. Comparison of power consumption in different phone models and connections Connection Model Download time Active (W) Passive (W) Energy (J) N80 4'55" SymTorrent E61 7'20" N95 2'25" N80 2'10" SymTorrent E61 2'16" N95 1'30" HTTP N80 4'10" HTTP N80 0'48" The figures in Table 1 are averages of six download tests with each device model and connection. HTTP downloads were done from a test server where there were no other users. In that respect it fails to represent the practical case of popular content where multiple users are downloading the content simultaneously where the server capacity soon becomes a bottleneck. HTTP download is thus a baseline allowing us to estimate the additional effort needed by P2P to manage the connections to multiple peers. As Table 1 shows there are no major differences between different phone models in the passive state. During the active download N95 was using more power but the download speed was much faster (N95 supports the new HSDPA connectivity). Moreover the shapes of the power consumption curves are very similar indicating that the same findings apply to all three studied devices. The results show that there is 20% more energy consumption when the same file is downloaded with SymTorrent than with HTTP via connection. With the difference is bigger: 150% more energy is needed. We estimate that the difference is caused by three factors. First, reaching full download speed takes time for SymTorrent because it needs to find and negotiate with peers. Secondly, the peer serving speed is likely to be a bottleneck in SymTorrent while the radio connection to the mobile phone is the bottleneck for HTTP download. Thirdly, the performance of the SymTorrent application is not as efficient as the normal HTTP socket connection. In comparison with is over two times faster and around three times more energy-efficient. is thus the preferred choice if available. However, the strength of the solution is its wide coverage which allows downloads to proceed even if the user is genuinely on the move. Energy consumption in case is typically even more important because electricity is often available in places covered by like offices, cafes, or traffic hubs. 5. Discussion A limitation of our experiment, like any measurements with a living network, is that we are not able to control how the network and other peers are operating. While our study allows the analysis of practical possibilities for P2P in handheld devices it should be complemented with further laboratory studies where the behavior of other peers can be controlled. Our measurements are snapshots of the P2P network operation in a given time period with a given content. Multiple measurements allow us to count average results. However, different content with varying popularity and availability of peers is likely to change the absolute values but is not likely to change the power consumption characteristics. Our measurements were carried out with high quality radio connections: in both and measurements the phone was stationary and close to the base station. Movement and less reliable connections near the border of connectivity range would also change the results. Another potential source of measurement error is the maturity of the client software. Although SymTorrent is a complete BitTorrent client it does not support all features of the most sophisticated clients. Our results can also be used to point out some parts of SymTorrent that would benefit from further optimization. Terminating all operations to enter idle 732

5 state after downloading has been completed in one such improvement. 6. Conclusions Power consumption measurements add a new dimension to the analysis of P2P content sharing. This series of measurements has shown that P2P content sharing on widely available handheld devices is feasible from the power consumption point of view. The results show that power consumption of BitTorrent is on the same level as voice calls. Battery consumption does not seem to be a problem when a moderate amount of video downloading followed by viewing is done on the move. The results further indicate that the mobile client is able to act as a true peer: only little extra power is needed to support uploading during an on-going download. However, after downloading is finished also uploading should be terminated to allow the radio to return to power saving state. The results give rise to a level of optimism. The performance of P2P content sharing is reasonable already today. Further development with special focus on energy-efficiency is likely to improve the situation further. Better implementations, protocol extensions focusing on energy consumption, new techniques for handling fairness and incentives in a handheld friendly way, and hardware improvements are some of the alternatives for further development. References [1] L. M. Feeney and M. Nilsson, Investigating the energy consumption of a wireless network interface in an ad hoc networking environment, in Proc. INFOCOM 2001, pp [2] D. Qiu and R. Srikant, Modeling and performance analysis of BitTorrent-like peer-to-peer networks, in Proc Conference on Applications, Technologies, Architectures, and Protocols For Computer Communications, Portland, Oregon, USA, 2004 pp [3] L. Ziqian and C. Changjia, Modeling bittorrent-like peer-to-peer systems, IEEE Communications Letters, Vol.10, Iss.7, July 2006, pp [4] G. Lei, C. Songqing, X. Zhen, T. Enhua, D. Xiaoning, Z. Xiaodong, A performance study of BitTorrent-like peerto-peer systems, IEEE Journal on Selected Areas in Communications, Vol.25, Iss.1, Jan. 2007, pp [5] D. Erman, D. Ilie, and A. Popescu, BitTorrent Traffic Characteristics, in Proc. International Multi-Conference on Computing in the Global Information Technology, 2006, pp [6] M. Izal, G. Urvoy-Keller, E.W. Biersack, P.A. Felber, A. Al Hamra, and L. Garcés-Erice, Dissecting BitTorrent: Five Months in a Torrent's Lifetime, in Proc. 5th International Workshop on Passive and Active Network Measurement, 2004, pp [7] M. S. Joseph, M. Kumar, Huaping Shen, and S. Das, Energy efficient data retrieval and caching in mobile peerto-peer networks, in Proc. Third IEEE International Conference on Pervasive Computing and Communications Workshops, 2005, pp [8] A.K.-H. Leung and K. Yu-Kwong, On topology control of wireless peer-to-peer file sharing networks: energy efficiency, fairness and incentive, in Proc. 6th IEEE International Symposium on a World of Wireless Mobile and Multimedia Networks, 2005, pp [9] G. Ding and B. Bharat, Peer-to-peer file-sharing over mobile ad hoc networks, in Proc. Second IEEE Annu. Conf. on Pervasive Computing and Communications Workshops, 2004, pp [10] S. Gurun, P. Nagpurkar, and B. Y. Zhao, Energy consumption and conservation in mobile peer-to-peer systems, in Proc. First International Workshop on Decentralized Resource Sharing in Mobile Computing and Networking, 2006, pp [11] H. Yan, D. K. Lowenthal, and K. Li, ACE: an active, client-directed method for reducing energy during web browsing, in Proc. International Workshop on Network and Operating Systems Support For Digital Audio and Video, 2005, pp [12] R. Krashinsky and H. Balakrishnan, Minimizing energy for wireless web access with bounded slowdown, in Proc. 8th Annual international Conference on Mobile Computing and Networking, 2002, pp [13] I. Kelényi, P. Ekler, and Z. Pszota, SymTorrent 1.27, Budapest University of Technology and Economics. Apr. 11, [Online]. Available: [14] G. Bosch Creus and M. Kuulusa, Optimizing Mobile Software with Built-in Power Profiling, in Mobile Phone Programming and its Application to Wireless Networking, F. Fitzek and F. Reichert, Eds. Springer, [15] B. Cohen, Incentives build robustness in BitTorrent, in Proc. First Workshop on the Economics of Peer-to-Peer Systems, Berkeley, CA,