POR: A Packet-Based Opportunistic Routing Protocol for Wireless ...

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networks (WSNs) by taking full advantage of wireless link broad- cast features. However, there is not any valid opportunistic rout- ing for WSN applications.
2013 International Conference on Computer Sciences and Applications

POR: A Packet-based Opportunistic Routing Protocol for Wireless Sensor Networks Ziwei Liu1,2, Chuanbo Wei1, Cheng Qin1, Hui Li2, Xiaoguang Niu1,*, Lina Wang1 1

Computer School, Wuhan University Institute of Seismology, China Earthquake Administration {lzw,lihui}@eqhb.gov.cn,{wchuanbo,chengqin,xgniu,lnwang}@whu.edu.cn 2

Abstract—In * recent years, opportunistic routing protocol (Opportunistic Routing, OR) has aroused wide concern because it can improve the throughput and reliability in wireless sensor networks (WSNs) by taking full advantage of wireless link broadcast features. However, there is not any valid opportunistic routing for WSN applications. This paper proposes a lightweight distributed packet-based opportunistic routing mechanism, namely POR, which can effectively alleviate the inherent local unbalanced traffic load distribution in WSNs. The experimental results show that, POR improve the network aggregate throughput by nearly 15%, and reduce the transportation delay by nearly 75% in comparison with the existing opportunistic routing protocols.

effectiveness and accessibility can increase network throughput and energy efficient. The studies on OR claim that OR achieves better throughput performance than that of traditional routing just through simulations and implementations. However, most of the existing OR protocols mainly focus on multi-hop wireless ad hoc networks, and lack in-depth study for the many-to-one data transfer mode WSNs. They are not suitable for WSNs which have the low data rate, poor link quality and stability, many-toone data aggregation characteristics. By combining the broadcast nature of the radio channel, data aggregation characteristics and data forwarding fairness characteristics, this paper proposes a lightweight distributed packet-based opportunistic routing mechanism for wireless sensor networks, namely POR. The POR is practical for small and medium-sized environmental monitoring applications, the high rate of data reception, the system to survive a long time data transmission routing technology solutions. POR also successfully applied in the underground seismic observation environment monitoring system. The experimental results show that POR improve the network aggregate throughput by nearly 15%, and reduce the transportation delay by nearly 75% in comparison with the existing opportunistic routing protocols.

Keywords- wireless sensor networks, routing protocol, energyefficient, opportunistic routing scheme .

I.

INTRODUCTION

With the development of wireless communication, electronics and sensing technologies, wireless sensor networks (WSNs) have been widely applied in many important areas [1]. The improvement of the energy efficiency in WSNs is the most important and primary issue in both industries and academia [2]. Because the routing mechanism is able to systematically reflecting the characteristics of WSNs (such as the limited resource, many-to-one traffic pattern, broadcast nature and spatial diversity of wireless medium, redundant sensing data, instable wireless link of WSNs), it is the common and appropriate practical technology for energy conservation.

II.

Opportunistic routing protocol exploits the spatial diversity of the wireless medium by involving a set of packet forwarding candidates instead of only one optimal relaying node in traditional routing, and then improves the transmission reliability and efficiency. K. Zeng etc. [4] studied the end-to-end throughput capacity of OR networks, which only adopted the wireless interference model which cannot reflect the characteristics of OR networks and considered the scenarios of single flow and saturated traffic demand. Some opportunistic routing protocols, such as ExOR [5] and UBOR [6], select candidates and prioritize their forwarding-candidate-set by exploiting the whole path cost or global network topology information. In the least-cost opportunistic routing (LCOR) [7], it needs to enumerate all the neighboring node combinations to get the least cost OR paths. OR protocols use the location information of

Routing in multi-hop wireless sensor networks presents a great challenge mainly due to unreliable wireless links and interference among concurrent transmissions. Recently, the idea of Opportunistic Routing protocol (OR) [3] has been widely explored to cope with the unreliable transmissions by exploiting the broadcast nature and spatial diversity of the wireless medium in order to improve the throughput and reliability of multi-hop wireless network. The basic idea of OR protocol is as follows: In the process of packet forwarding, the packet sending node first selects the forwarding candidate node-set, and then forward the packet to the best forwarding candidate node according to the packet in the sending process, the actual *

Corresponding Author

978-0-7695-5125-8/13 $26.00 © 2013 IEEE DOI 10.1109/CSA.2013.43

RELATED WORKS

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nodes to define the candidate set and relay priority. In GeRaF [8], the next-hop neighbors of the current forwarding node are divided into sets of priority regions with nodes closer to the destination having higher relay priorities. MORE [9] delivers opportunistic routing gains by using of intra-flow network coding, which allows it to exploit the spatial reuse available and be easily extensible to multicast traffic. MIXIT [10] is a symbollevel opportunistic routing, and it takes advantage of bit-level wireless spatial diversity to achieve high throughput, which permits partially correct receptions instead of simply accepting fully correct packet. However, there are few practical opportunistic routing protocols in many-to-one WSNs. III.

B. The design and implementation of POR

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POR: A PACKET-BASED OPPORTUNISTIC ROUTING FOR WIRELESS SENSOR NETWORKS

Niu etc. proposed a maximum-concurrent-flow linear planning-based cumulative delivery transmission model for the maximum throughput of opportunistic routing in WSNs and a centralized algorithm to calculate the throughput bounds [11]. With the centralized algorithm, the opportunism data forwarding mechanism that achieves the optimal network performance is designed. However, it is not suitable for the resource limited sensor network. This paper designs a packet-based distributed opportunism data forwarding mechanism POR with the sensor network channel contention overhearing characteristic.

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Fig.1 Choice of channel acknowledgement candidate node

In POR, node calculates the distance d that from itself to the sink node in the network initialization phase. According to the success reception of packets from the neighbors, node can obtain the maximum distance from it to its candidate forwarding nodes dfclongest (the distance from the node to the candidate forwarding node whose link loss rate is the maximum value that POR permits).While a node needs to send packets to the sink, it first uses effective channel reservation mechanism to obtain the minimum effective candidate forwarding nodes set that satisfied the requirements of accumulated data packet successful transmission rate, thus obtains the approximate maximum concurrent data sending nodes set by the distributed method. The channel reservation mechanism of POR could be modified from the existing MAC protocols channel contention RTS/CTS: sending node Sa broadcast a RTS packet for channel contention, the RTS packet contains the channel contention acknowledgement candidate node Fb specified by Sa; Fb broadcasts CTS packet immediately for channel acknowledgement after it received the RTS packet, other nodes in the communication coverage of Sa and Fb must not send packets or acknowledge the channel in the next packet sending period, as shown in Fig 1. With the difference of choosing channel acknowledgement node Fc(red node in Fig 1) that most close to sink, whose distance is dfclongest in existing MAC protocol [12], POR has consideration of successful packets transmission accumulation characteristics of candidate forwarding nodes set in OR protocol, obtains an approximate maximum concurrent data sending node set by reducing channel acknowledgement node influence on other sending nodes. Sa selects a candidate forwarding node Fack(blue node in Fig 1) from the candidate forwarding node set as the channel acknowledgement node, which is between Sa and Fc, and in the cover range of Fc. The distance from Fack to itself is

A. Main Idea of POR With the difference of batch packets opportunism forwarding mechanism based on the same data flow data in wireless ad hoc networks.POR adopted a lightweight node scheduling mechanism based on packet-piggyback-overhearing to achieve multiple data flows in parallel opportunistic forwarding, it support the long path of large-scale networks. POR adopts the method of "Push-Pull" combined to achieve approximate optimal concurrent scheduling node set self-adapting formation and coordination—implement approximate optimal concurrent scheduling node coordination by specifying the candidate node broadcasting reservation mode in the MAC protocol channel reservation phase;"Pull"—after the source node sent the data packet, the candidate nodes receiving the message acknowledgement by piggybacking when it send/forward the data packet, and the acknowledgement message is spread nearby by limited rumor routing in order to ensure the source node and other candidate nodes could receive the message timely, which avoids repetitive messages sending and reduces the transmission delay. In this way, as many as possible nodes could send packets successfully anytime, and the asymmetric link, unstable link can be used in WSNs. POR has the feature of lowoverhead, low-delay and high adaptability of channel status variation.

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1 1     d fclongest , here γ denotes a constant coefficient in 2

For the packets not received before, nodes update its own ToBeAckPktQueue and check all packets in SendingPktBuffer according to acknowledgment list in POR header. If there is a candidate forwarding node much more closely to the sink has received the packet, the node won’t forward the packet. In the process of packet forwarding, this paper adopts delay sending strategy based on node routing cost: the more further from candidate forwarding nodes to the sink, the delay time is longer.

[0,1], which is determined by the demands of packets transmission accumulative success rate, specially while γ = 1, channel acknowledgement candidate node Fack is traditional channel acknowledgement node Fc. Data packet header fields of POR are shown in Fig. 2: SRC_ID, PKT_ID fields represent source node identifier of the packet and the packet identifier, which are filled in by the source node; MIN_DIST field represents the minimum distance to candidate forwarding node that receives this packet, by which the candidate forwarding node selects its own forwarding waiting period; PKT_ACK field list represents the packets that have been received, by which candidate forwarding nodes could determine whether the packets received and waiting for forwarding are received by the node closer to the sink, and Hop field presents the hop count the acknowledgement of the packets in the list has spread, which is for limiting spreading area of the acknowledgement of the packets.

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Insert packet A to the Pkt_Buffer and set a forwarding timer

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Fig. 2 Fields of POR header.

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In POR protocol, node maintains sending packet buffer (SendingPktBuffer) to buffer the packets waiting for forwarding, maintains the received packet acknowledgement queue (ToBeAckPktQueue) to keep the packet information that nearby nodes received recently. Fig. 3 (a) shows the flow chart of the source node sending packets, when node is sending packets, it will first fill in POR header fields according to ToBeAckPktQueue; And then put the packets in the SendingPktBuffer, contending required channel by channel reservation mechanism for sending packets; After sending packets, the node will overhear acknowledgement piggybacked in POR header to confirm whether the packet is received by candidate forwarding nodes, node will continue retransmitting until successful receiving or exceed the maximum retransmission times. Fig. 3 (b) shows the process of candidate forwarding nodes forwarding packets. The main task of candidate forwarding nodes is to ensure optimal candidate forwarding nodes can receive and forward packets from the upstream nodes by coordination work of piggybacking acknowledgment. After received the packet, the candidate forwarding nodes will first judge whether the packet was received before.

Whether has sent it more than threshold times

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Update the MAX_DIST and PKT_ACK List fields the POR header of packet A Forward packet A as the transmission process of source node

Yes End

End

(a) Flow of source node sending packets

(b) Flow of candidate forwarding node forwarding packets

Fig. 3 Flow of packets sending in POR

Nodes select appropriate delay based on MIN_DIST field in the received packets to ensure that packets can be forward by the nodes closest to the sink as soon as possible. If nodes overhear other forwarding nodes haven’t forwarded the packet after the delay, it forwards the packet and wait for responding. Piggybacking mechanism in POR protocol adopts regional rumor spreading: when nodes update its own ToBeAckPktQueue based on PKT_ACK table list in POR header after received packets, nodes make HOP field of each table item accumulate 1; And it only select the items that Hop field no more than a specific threshold Hthreshold to fill in PKT_ACK list when forwarding packets,so that could effec-

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tively avoid the widespread diffusion of acknowledgments. Different with the existing OR protocols, POR protocol doesn't specify candidate nodes set, but adopts packets piggybacking acknowledgments to coordinate actual forwarding nodes, so that can effectively use numerous asymmetric link, unstable link in the network to increase the success rate of network transmission. In addition, the local piggybacking acknowledgment mechanism of POR protocol avoid the forwarding waiting delay or redundant packets transmission caused by end-to-end acknowledgment mechanism of other existing OR protocols, and it can obtain higher aggregate throughput of the network especially in the large-scale network. IV.

source sends to data flow in batches is 32. Candidate forwarding nodes set 5 packet sending period for each waiting time of priority [5]. In MORE, the number of packets that source sends to the same flow in batches is 8[9].The time of dealing with receiving packets, network coding etc. is one packet sending period. Due to the adoption of the piggybacking acknowledgement mechanism based on regional rumor spreading, POR only need to utilize a few information list of local area sending packets acknowledgement in the header of packets, and the additional control packets for coordination of candidate nodes is unnecessary while data transmission. However, MORE and ExOR need contain the entire router of all candidate node information list in packet header. The length of header is proportional to the length of the longest router in WSNs, the payload ratio of packets decreased with the network increased. Moreover, the sink node need broadcast additional acknowledgement packets to all candidate nodes in transmission router for terminating the packets sending. From the above analysis, the communication overhead of POR is minimal, and the protocol field overhead of data packets and the overhead of protocol maintenance control packets are light than other OR protocols.

SIMULATION EXPERIMENT AND RESULTS ANALYSIS

In this section, we discuss simulation experiment and performance analysis of POR. Compared with the simulation performance of representative ExOR [5], MORE [9] and TinyRT [13], we study the performance of POR in terms of throughput rate and transmission delay. S11

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Fig. 4 Illustration of a 7×7 grid WSN

Fig. 5 Network aggregate throughput rate with different network load

The simulation experiment network topology is shown in Fig. 4.The network scale is 49 static nodes; they are uniformly distributed in a 900×900 m2 square area. In the network,7 nodes in the first column are the source nodes set, they send packets to the sink node located in the middle of 7th column periodically. In the simulation experiment, the packets received rate (PRR) threshold of multi-path data forwarding mechanism and opportunistic data forwarding mechanism while select link are 0.8 and 0.2 respectively. The packets received rate (PRR) of link is inverse to the distance between two nodes, and there is a random Gauss offset that expected value is 0.1, the average link packet reception rate of two nodes 250 meters apart is 0.8.Packets sending period is 0.023s in simulation experiment.In ExOR,the number of packets that

Fig. 5 gives the network aggregate throughput rate result of each data forwarding mechanism with different network load. When the packets producing rate increases to 4pp, the aggregate throughput rate of POR is 46% higher than that of TinyRT, and about 15% higher than that of ExOR and MORE . Fig. 6 shows the average end-to-end transmission delay in an unsaturated network with different load where there is only a source node S31 sending packets. Due to packet as a unit in POR, its transmission delay is gradually close to TinyRT as the network load increases. When the network load increases to 8pkt/sec, because the piggybacking acknowledgement mechanism can make better use of a large number of network

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Basic Research Project of China Earthquake Administration (Grand No. 201101014), Director Foundation of the Institute of Seismology, China Earthquake Administration (Grand No. IS200726019, IS200956041).

data packages and the OR protocol reduces forwarding/retransmission times, the transmission delay of POR is less than TinyRT. As the network load increases, network packets loss and retransmission occurs frequently due to much more forwarding times and only one forwarding node in TinyRT, which thus makes end-to-end transmission delay increases. From the above analysis we can see that, POR is suitable to the WSNs with low node data generation rate and high network load.

REFERENCES [1]

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Fig. 6 Transmission delay with different network load. [8]

V. CONCLUSION About the existing research results on opportunistic data forwarding mechanism in WSNs cannot resolve the problem in terms of high packet loss rate, poor stability perfectly, this paper proposes a lightweight distributed packet-based opportunistic forwarding mechanism. Simulation results show that, compared with the existing opportunistic data forwarding mechanism, POR can reduce packets loss rate greatly of a high load WSNs with a little control overhead while forwarding data, and the network throughput is increased by about 15%, the transmission delay is reduced by nearly 75%.

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[10]

[11]

[12]

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ACKNOWLEDGEMENTS This work was partially supported by National Key Basic Research Program of China “973 Project” (grant No. 2011CB707100), Program for Changjiang Scholars and Innovative Research Team in University (grant No. IRT1278),

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