An Enhanced Super-Peer System Considering Mobility and Energy in Mobile Environments

Transcription

1 An Enhanced Super-Peer System Considering Mobility and Energy in Mobile Environments Sun-Kyum Kim, Kwang-Jo Lee, Sung-Bong Yang Departement of Computer Science Yonsei University Repubilc of Korea {skyum, kjlee5435, Abstract As mobile technology advances, more services on peer-to peer(p2p) systems in mobile ad-hoc networks are available in various forms. Recently doublelayered P2P systems have been proposed to reduce the network traffic. Peers in a double-layered system are classified into super peers in the upper layer and subpeers in the lower layer. One of the double-layered systems, called the MOB(MOBility) system, considers the mobility of peers and shows higher performance among other systems. But since the battery power of a mobile device is limited, the energy status of each peer should also be considered. In this paper, we propose a super-peer system considering both energy and mobility of peers when selecting super peers. The experiments have been performed on the network simulator NS-2 and the results showed that the proposed system improved the hit ratio about 5.70% over the MOB system, while maintaining more robust networks. 1. Introduction As mobile technologies advance, researches on file search among peers have extended toward the mobile environments. However, due to the nature of mobile networks, there are such limitations as mobile device, bandwidth, battery power, computing power, memory space, and mobility[1]. MANET (Mobile Ad-hoc NETwork)[2] in which no infrastructure could be available was introduced, and many researchers have studied to overcome these limitations. In MANET, the information stored in peers can be utilized in a singlelayered or double-layered P2P systems[3]. For example, a typical single-layered system ORION(Optimized Routing Independent Overlay Network)[1] uses flooding as its broadcast mechanism. Hence, it could not avoid very high network traffic as the number of peers increases. For reducing network traffic, some double-layered P2P systems were developed[4]. A double-layered system classifies the peers in a network into super peers and sub-peers so that most communications can be done among super peers and relay peers. By a relay peer we mean a sub-peer who connects to two adjacent super peers. In a double-layered system, because a super peer should work harder than its sub-peers due to managing its sub-peers, a super peer consumes more energy than a sub-peer. One of the double-layered systems, called the MOB(MOBility) system, considers the mobility of peers for selecting super peers[5]. In the MOB system, a peer who has lower mobility has a higher chance to be selected a super peer repeatedly during the periodic network update. So a super peer s energy gets easily depleted. In this paper, we propose a super peer system considering both energy and mobility of peers when selecting super peers. Through various experiments, we found that the ratio to the energy to the mobility is 60% to 40%; that is, the energy is more important factor than the mobility. The experimental results show that the proposed system outperforms others in terms of the hit ratio while maintaining more robust networks. The rest of this paper is organized as follows. The backgrounds on the mobile P2P systems are provided in Section 2. The proposed system is described in Section 3. The experimental results are given in Section 4, and the conclusions are made in Section Related work 2.1 A Single-layered Mobile P2P System ORION is one of the typical single-layered systems. In ORION, the communications are performed based on the flooding mechanism and each peer has a routing table and a file route table. The main disadvantage of ORION is that network traffic may be increased rapidly when there are many peers within the communication range and a very large number of different files are distributed throughout the network.

2 2.2 Double-layered Mobile P2P Systems A double-layered P2P system has two layers, the upper layer and the lower layer. Super peers are in the upper layer and manage some neighboring sub-peers, in the lower layer. By the neighborhood of a pair of peers we mean that they are within the communication range. Each super peer maintains a route table for routing paths and a file table whose entry contains the ID, address, and file list of each of its sub-peers. A super peer sends a query message to other adjacent super peers via relay peers when searching a file. For example, when a peer p wants a certain file, it first asks its super peer for the file. The super peer then looks into its file table. If the table contains the file information, the super peer sends the location of the file back to p. Otherwise, it sends the file request query message to each of its adjacent super peers. The search process continues similarly afterwards. In order for a pair of adjacent super peer P 1 and P 2 to communicate each other, one or possibly two relay peers are required as shown in Figure 1. A solid line is a management connection line between a super peer and each of its sub-peers. The rectangles are super peers, the white circles are sub-peers and the gray circles are relay peers. A super peer and its sub-peers form a cluster. A dotted line indicates a relay connection line connecting between two peers in different clusters. In a double-layered P2P system, the entire network should be updated periodically to reflect the current locations of the peers in the network; new super peers and their sub-peers are selected and routing paths among peers are reestablished. We call such a reconfiguration of the network a network update. the lowest mobility among other peers within the communication range is selected as a super peer. If a sub-peer has very high mobility then it may cause the reliability problem only locally. But if a super peer has high mobility, the problem will likely affect much larger area. For super peer selection during a network update, each peer calculates its own mobility as the total distance moved between two network updates. Each peer sends its mobility value to its neighboring peers and receives their mobility values. Then each peer compares the mobility values received with its own. The MOB system improved the reliability by enhancing the stability of clusters. Lower mobility of super peers contributes to the robustness of clusters in the system. Figure 2. The MOB System Figure 2 shows the MOB system selects super peers using the moving distances of peers. The rectangles are super peers and the circles are sub-peers. The number in a node indicates the moving distance, namely the mobility value. 3. The proposed system The MOB system may not be a realistic system since it does not consider the energy factor. We now propose a super peer system considering both mobility and energy of peers. For this, we use the following equation to obtain a value for each peer. These values are compared among the peers during a network update for super peer selection. Figure 1. Routing paths between a pair of super peers via relay peers 2.3 The MOB system The MOB system considers the mobility of peers for selecting super peers. In this system, a peer who has F (A) = p + q, where p+q = 1 In the equation, M max is the maximum mobility of a peer, M A is the mobility of peer A, E A is the energy amount of peer A s device, and E max is the maximum energy capacity of a peer. In the equation, for mobility we normalize the value of (M max -M A ), since the lower

3 mobility A has, the better A would be a super peer. On the other hand, energy is opposite to mobility. Hence E A is simply divided by E max. In the equation, p and q are the weights for considering the mobility and the energy, respectively, where the sum of p and q is equal to 1. Observe that as the F value of A is close to 1, it is more likely to be a super peer. Figure 3. The proposed system Figure 3 shows how some peers can be super peers in the proposed system. Each peer computes its F value, where M max is assumed to be 100, E max is 2,000 in this example. Peers C, G, and K are selected as super peers, because they have higher F values than their neighboring peers. In the MOB system, peers A, C, and K should be selected as super peers since they have lower mobility than their neighboring peers. Some peers in the network show their tendency that they do not move very much, so in the MOB system peers A, C, and K may be selected repeatedly during network updates. In the proposed system, however, we can avoid such cases, since the F value of each peer reflects both peer s mobility and energy consumption between two network updates. Hence the proposed system can maintain the robustness of the network more adaptively. 4. Experimental Results In this paper, we used the network simulator NS-2 v2.34[6] for the experiments. The network area is set to 1,000m 1,000m, the number of peers is 100, and initial energy of each peer is 2,000J. The energy consumptions on sending, receiving a message, and idle, are 1.6mW, 1.5mW, and 1.15mW, respectively[7]. Super peers are newly selected during network reconstruction every 100sec. The movement of a peer is set according to the random way point model, which is used to many network experiments, via the setdest movement generator in NS-2. For a network update, each peer computes its F value every ten seconds, since the mobility value is also calculated every ten seconds in the MOB system. We set two file search patterns, five movement patterns and five speed patterns (1m/sec ~ 5m/sec). We tested ten times with each speed patterns; that is, the overall simulations were performed 50 times. The values of p and q were determined as p = 0.4 and q = 0.6 after trying possible values. These parameters depend on a specific environment. In this paper, we obtained these values through various experiments. Table 1 shows values of the experiment environment and Table 2 lists the parameters in setdest. We compared the MOB system, the Energy system[4], and the proposed system under the same experiment environment. Note that the Energy system selects super peers using only peer s remaining energy. Table 1. Experimental environment Parameter Values number of peers communication range maximum energy network area total experiment time energy consumption Send Receive Idle period of calculating F values period of updating the network Table 2. Values in NS-2 setdest Parameters speed type min speed max speed pause type (m) 2,000(J) 1,000 1,000(m 2 ) 2,000(sec) 1.6(mW) 1.2(mW) 1.15(mW) 10(sec) 100(sec) Values Normal 0.1(m/sec) 1, 2, 3, 4, 5 (m/sec) Uniform

4 Figure 4. Hit ratio for file searches Figure 4 shows the hit ratios for file search. The proposed system showed higher hit ratio than the MOB system about 5.70%. The Energy system improved the hit ratio by only 2.00% over the MOB system, because the Energy system does not consider mobility; that is, the network links tends to be broken more easily. Figure. 5 The average update overhead Figure 7. The total overhead Figures 5 and 6 show the average network update overhead and query overhead of each system. By the overhead we mean the the traffic cost for all the network reconstructions plus the traffic cost for all the file searches during the entire simulation. The update network overhead of the proposed system is 6.53% higher and the query overhead is 16.74% higher than the MOB system each. Especially the query overhead of the proposed system has increase noticeably, but such increase came from the fact that there are more live peers in the proposed system than the MOB system. It shows that the proposed system has better energy efficiency. Figure 7 compares the total overhead that is the sum of the update and query overheads. The overall network traffic amounts of the Energy system and the proposed system are 10.27% and 12.32% higher than the MOB system, respectively. Such traffic overhead increases were caused by more live peers, and hence they contribute to higher hit ratios. Figure 6. The average query overhead Figure 8. The average hop counts Figure 8 shows the averagee hop counts of the systems in file searches. The MOB system has about 3.55, and the Energy system and the proposed system have about 3.73.

5 [3] M. Ylianttila, E. Harjula, T. Koskela and J. Sauvola, "Analytical Model for Mobile P2P Data Management Systems", Consumer Communications and Networking Conference, 2008, pp [4] Jung-Suk Han, Jin-Woo Song, Kwang-Jo Lee, and Sung- Bong Yang, "Mobile Peer-to-Peer System using Super Peers for Mobile Environments", International Conference on Information Networking, 2008, pp 1-4. Figure 8. The number of peers with exhausted energy Figure 8 compares the number of dead peers who have no energy as the simulation proceeds. The number of dead peers in the MOB system is greater than other systems. Both the proposed and Energy systems show almost the same behavior. It can be easily seen that more live peers contribute to improving the hit ratio for file searches. The proposed system, therefore, has an overall good performance by selecting super peer considering both mobility and energy. [5] Ji-Hoon Kim, Jin-Woo Song, Taek-Hun Kim, and Sung- P2P System for Bong Yang, "An Enhanced Double-Layered The Reliability in The Dynamic Mobile Environments", Computing and Informatics, 2010, in press. [6] The Network Simulator NS-2http:// Web site: [7] The NS-2.34 source code, wireless-phsy.cc, line Conclusion In realistic mobile network environments, there are several factors that should be considered to build a system. Mobility should be an obvious factor since peers move around the network area. But it found that the energy factor is more important than the mobility with the ratio of 60% to 40%. In this paper, the proposed system enhanced the system performance by maintaining more robust networks, which contributes higher hit ratio for file searches. We will further study more practical P2P systems to improve the system performance in the future. Acknowledgement This work was supported by the Engineering Foundation(KOSEF) ( ). Reference Korea Science and for the research [1] A. Klemm, C. Lindemann, and O. Waldhorst, "A Special Purpose Peer-to-Peer File Sharing System for Mobile Ad Hoc Networks", Vehicular Technology Conference, Vol. 4, Oct. 2003, pp [2] X. Bangnan, S. Hischke, and B. Walke, "The Role of Ad Hoc Networking in Future Wireless Communications", Communication Technology Proceedings, ICCT 2003, Vol. 2, Apr. 2003, pp