Efficient Opportunistic Broadcasting overDuty-Cycled Wireless Sensor Networks

Shouwen Lai, Binoy RavindranDepartment of Electrical and Computer Engineering

Virginia Tech,Blacksburg, VA 24061Email: {swlai,binoy}@vt.edu

Abstract—We consider the problem of supporting multihopbroadcast over adaptively duty-cycled wireless sensor networks(WSNs). We present hybrid-cast, a broadcast protocol whichadopts opportunistic data delivery in order to shorten broadcastlatency. Hybrid-cast achieves less number of transmission timesor redundant transmissions via delivery deferring and onlineforwarder selection. The preliminary simulation results validatethe effectiveness and efficiency of our design.

I. INTRODUCTION

Multihop Broadcasting [1] is an important network servicein WSNs, especially for applications of code update, remotenetwork re-configuration, network-wide queries etc. However,broadcasting becomes difficult in duty-cycled WSNs whereneighborhood nodes are not awake simultaneously for datareceiving, especially in asynchronous wakeup scenario. Tomake things worse, adaptive duty-cycling [2] are proposedin order to reflect the remained energy at a node, whichbrings heterogenous duty-cycling [2] and makes simultaneousneighbor discoveries more difficult.

There are some works proposed to support multihop broad-casting in duty-cycled WSNs. Wang et al. [3] transformed theproblem into a shortest-path problem with the assumption ofduty-cycle awareness, which is not valid for asynchronouslyWSNs. DIP [4], ADB [1] and opportunistic flooding [5]were designed with a gossiping approach as long as thenetwork is connected. They can achieve reliability for flooding.However, these protocols adopt unicast to replace broadcast forflooding, which may lack efficiency in large scale networksand delivering large chunk of data for flooding.

To overcome the disadvantages of pure replacement viaunicast, we present hybrid-cast, an asynchronous broadcastprotocol for the sake of low-latency broadcasting and lessmessage transmissions. In hybrid-cast, a node intelligentlyforwards message for at least one neighbor in an opportunisticapproach. Hybrid-cast defers broadcasting by one or moretime slot after receiving beacon from the first awake neighborin order to reduce the transmission times. It also adoptsonline forwarder selection in order to reduce the transmissionredundancy. Compared to previous protocols, our protocol canachieve less latency and less number of data delivery times.

II. PROTOCOL DESIGN

A. Overview for Hybrid-Cast

We assume that time axes are arranged as equal time slots(defined as Ts), and a node sends out beacon message at

the beginning of wakeup slots as assumed in [6], [7]. Thewakeup schedule can be once every n slots or based onquorum schedules (i.e, cyclic quorum systems or grid quorumsystems). When a node wants to broadcast messages, it willwait until receiving beacons from its neighbors.

Our goal is to reduce both broadcast latency and deliverytimes for flooding a message to the entire network. To reducelatency, we adopt opportunistic delivery: the sender will for-ward message within δ time after it hear the beacon messagesfrom some neighbors, rather than waiting to get beaconsfrom all neighbors. δ(i.e. δ = Ts) is called the deferringtime. By deferring, the first awake neighbor can still receivebroadcasted message. Meanwhile, more neighbors which areawake during the deferred time will be accommodated. Wealso adopt online forwarder selection to reduce redundanttransmissions. ‘’Online” means that a node selects least relaynodes among its one-hop awake neighbors to cover the twohop neighbors, in order to reduce redundancy and collision.

B. Opportunistic Forwarding with Deferring

Transmitter

Receiver 1

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beacon

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Fig. 1. (a). Opportunistic broadcasting with delivery deferring by 1 slot; (b).Online forwarder (marked by blue filling) selection sequence.

Opportunistic forwarding means that a node delivers dataimmediately to the neighbor which is awake earlier for the sakeof least latency. However, pure opportunistic unicast forward-ing suffers more transmission times. Broadcasting deferringmeans that the sender will not broadcast messages immediatelyafter receiving the beacon from the first awake neighbor.In order to accommodate more awake neighbors, the senderdefers the broadcasting by δ = 1 time slot or δ = q time slots(q is the quorum size in quorum-based schedule [7]. By doingthis, the first awake neighbor can still receive the message, andthe neighbors which turn to wake up before the deferring duetime can also receive the broadcast message. The deferring

This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE INFOCOM 2010 proceedings

978-1-4244-6739-6/10/$26.00 ©2010 IEEE

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Fig. 2. Performance comparison on latency, delivery times and ratio of hybrid delivery.

combines the advantages of opportunistic unicast forwardingand the advantages of broadcasting or wireless radio nature.

As shown in Figure 1, suppose there are three neighbors forthe sender. It only needs to broadcast two times (marked byred arrow) to ensure all neighbors receiving the message.

C. Online Forwarder Selection

In order to reduce delivery times or redundant transmissionfor multihop broadcasting, it is necessary to select less relaynodes. However, to reduce broadcasting latency, we shouldselect the relay node or forwarder along the directions ofopportunistic forwarding, which leads to online forwarderselection rather than a static decision. Initially, each nodemaintains its one hop awake neighbors (defined as N(x))and the set of two hop neighbors N2(x) based on underlineneighbor discovery protocols.

The sink node or a relay node nx computes the least numberof relay nodes among its one-hop awake neighbors (defined asNawake(x)) to cover the reachable two hop neighbors (definedas N2

reachable(x)).

N2reachable(x) = ∪y∈Nawake(x)N(y) − Nawake(x) (1)

The forwarder selection algorithm in a relay node or thesink node nx is to compute N2

reachable(x) online as shownin Equation 1, and then compute the minimum number ofrelays to cover N2

reachable(x). For the second part, the heuristicsolution in hybrid-cast is similar to the minimum multipointrelays (MPR) algorithm in [8].

D. Collision and Re-transmission

Due to the problem of hidden terminal, it is possible thatone node gets broadcasting message from two nodes simul-taneously, which leads to collision. For reliable broadcasting,we adopt retransmission if a neighbor does not receive thebroadcast message (by checking the beacon message from theneighbor at the sender side). We do not defer broadcastingfor retransmission. The sender could backoff a random period0 ≤ t ≤ Ts in order to avoid collision.

III. PRELIMINARY RESULTS

We simulated the hybrid-cast via OMNET++ simulator andcompared it against ADB [1] and opportunistic broadcast-ing [5] (denoted as OppFlooding). The network contains 200nodes. We set the wireless loss rate as 0.1 and the duration of

one time slot as 100 ms. We randomly generated 10 topologiesand each plot is the average with 10 runs on each topology.

Figure 2(a) shows the broadcast latency before all nodesreceiving the broadcast data. With deferring, hybrid-cast hadslightly higher latency then ADB and OppFlooding. However,for data delivery times, as shown in Figure 2(b), hybrid-cast outperformed the other two solutions because of lessunicasts being involved. Regarding ratio of data delivery,by adopting online forwarder selection, hybrid-cast causedless collisions because of less redundant transmissions, henceachieved higher delivery ratio, as shown in Figure 2(c).

Due to space limitation, we did not show the performancecomparison for heterogenous duty cycle setting.

IV. DISCUSSION AND FUTURE WORKS

We do not assume local synchronization or duty-cycleawareness which is required by [5] and in [3]. The assumptionin hybrid-cast is neighbor-awareness. Such awareness can beachieved by neighbor discovery protocol, or quorum-basedwakeup scheduling [7]. By adopting quorum-based wakeupscheduling, hybrid-cast can be extended to mobile WSNs.Directions for future works include mathematically modelingthe latency bound, and implement the protocol in real platform,like Telosb or Mica.

REFERENCES

[1] Y. Sun, O. Gurewitz, S. Du, L. Tang, and D. B. Johnson, “Adb: anefficient multihop broadcast protocol based on asynchronous duty-cyclingin wireless sensor networks,” in ACM SenSys, 2009, pp. 43–56.

[2] S. Lai and B. Ravindran, “On distributed time-dependent shortestpaths over duty-cycled wireless sensor networks,” in IEEE InternationalConference on Computer Communications (INFOCOM), 2010. [Online].Available: http://www.real-time.ece.vt.edu/infocom10.pdf

[3] F. Wang and J. Liu, “Duty-cycle-aware broadcast in wireless sensornetworks,” in INFOCOM, 2009, pp. 468–476.

[4] F. Stann, J. Heidemann, R. Shroff, and M. Z. Murtaza, “Rbp: robustbroadcast propagation in wireless networks,” in SenSys, 2006, pp. 85–98.

[5] S. Guo, Y. Gu, B. Jiang, and T. He, “Opportunistic flooding in low-duty-cycle wireless sensor networks with unreliable links,” in ACM conferenceon Mobile computing and networking (MobiCom), 2009, pp. 133–144.

[6] Y. Sun, O. Gurewitz, and D. B. Johnson, “Ri-mac: a receiver-initiatedasynchronous duty cycle mac protocol for dynamic traffic loads inwireless sensor networks,” in ACM Sensys, 2008, pp. 1–14.

[7] S. Lai, B. Zhang, B. Ravindran, and H. Cho, “Cqs-pair: Cyclic quorumsystem pair for wakeup scheduling in wireless sensor networks.” inInternational Conference on Principles of Distributed Systems (OPODIS),vol. 5401. Springer, 2008, pp. 295–310.

[8] A. Qayyum, L. Viennot, and A. Laouiti, “Multipoint relaying for floodingbroadcast messages in mobile wireless networks,” in 35th Annual HawaiiInternational Conference on System Science, 2002, pp. 3866–3875.

This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE INFOCOM 2010 proceedings