Contention-Based Beaconless Real-Time Routing Protocol for Wireless Sensor Networks*

Transcription

1 Wreless Sensor Network, 2010, 2, do: /wsn Publshed Onlne July 2010 ( Contenton-Based Beaconless Real-Tme Routng Protocol for Wreless Sensor Networks* Abstract Chao Huang, Guol Wang School of Informaton Scence & Technology, Sun Yat-Sen Unversty, Guangzhou, Chna E-mal: Receved Aprl 12, 2010; revsed May 4, 2010; accepted May 17, 2010 Ths paper presents a novel real-tme routng protocol, called CBRR, wth less energy consumpton for wreless sensor networks (s). End-to-End real-tme requrements are fulflled wth speed or delay constrant at each hop through ntegratng the contenton and neghbor table mechansms. More precsely, CBRR mantans a neghbor table va the contenton mechansm beng dependent on wreless broadcast nstead of beacons. Comprehensve smulatons show that CBRR can not only acheve hgher performance n statc networks, but also work well for dynamc networks. Keywords: Wreless Sensor Network, Real-Tme Routng Protocol, Contenton-Based Scheme, Beaconless 1. Introducton s are comprsed of tny, low power sensor nodes, whch are densely deployed n a montored area and collaborate to forward the sensed data to a base staton through multple hops. s can be wdely used n many applcatons, such as envronmental montorng, mltary survellance, dsaster recovery, and healthcare related applcatons [1]. However, many msson-crtcal applcatons requre that s can guarantee the satsfed qualty of servce (QoS), especally wth real-tme constrants. e.g., n forest fre detecton, the abnormal temperature nformaton should be delvered to a base staton as soon as possble. Otherwse, the outdated data wll be rrelevant and even have negatve effects on the applcatons. s have partcular features dfferent from other wreless networks [2-4]. Frstly, each sensor node has severe resource constrants on bandwdth, memory, processng capablty, and especally energy. Secondly, the number of sensor nodes s very large and the nodes are always densely deployed. Thrdly, t has been recognzed that the network topologes of s may change constantly due to sleep polcy, node moblty, node falure, and so on. The features mentoned above wll pose challenges to fulfll the real-tme requrements. *Ths work s supported by Nature Scence Foundaton of Chna under Grant and Guangdong Nature Scence Foundaton of Chna under Grant ue to the hgh routng overhead and the poor scalablty of global routng decsons, several real-tme routng protocols [5-7] utlze the localzed geographc nformaton to heurstcally fnd a satsfed path to a target. These protocols are mostly governed by the conventonal geographc routng schemes, n whch each node has to perodcally broadcast beacons for obtanng the accurate nformaton of ts neghbors and to store the nformaton n ts neghbor table for routng decsons. Although the neghbors nformaton can be collected by broadcastng beacons, some drawbacks nevtably arse. Frstly, redundant beacons can lead to more energy consumpton and hgher communcaton cost. Secondly, only one neghbor or very small subset of neghbors can take part n routng decsons, but each node should store all of neghbors nformaton n a neghbor table. Therefore, the utlzaton of a neghbor table s much neffcent, and the unused neghbors nformaton stll occupes some of the lmted memory. Thrdly, when a network topology changes constantly, the collected neghbor nformaton wll be outdated quckly, whch n turn leads to hgher packet mss rato, and nodes should broadcast beacons more frequently to update the neghbor tables. It nevtably ncurs not only longer delay, but also consumes sgnfcant energy. Furthermore, frequently broadcastng beacons can result n more serous collsons thus hgher packet mss rato. Contenton-based beaconless scheme [8-12] has been proposed to handle wth the ssues abovementoned. The man dea s that when a node has a ATA packet to be Copyrght 2010 ScRes.

2 C. HUANG ET AL. 529 transmtted, t frst broadcasts a contenton request, and one neghbor wth the shortest wat delay s selected as the most sutable next-hop forwarder. The contentonbased scheme s completely stateless and reactve because each node can on-lne select an avalable next-hop forwarder wthout any pror knowledge of ts neghbors. Ths results n less energy consumpton. Furthermore, t can be found that the current forwarder can obtan the nformaton of the selected next-hop forwarder durng a contenton procedure. Although the nformaton of the selected neghbor s not utlzed n the scheme, t can beneft to future routng decsons. The character ndcates that the approach for selectng the nformaton of neghborhoods s energy effcent. Ths paper presents a Contenton-based Beaconless Real-tme Routng protocol for s, called CBRR. CBRR ams at fulfllng end-to-end real-tme requrements wth less energy consumpton through ntegratng the contenton and neghbor table mechansms. The advantage of our approach s to remove the lmtatons of the beacon-based routng schemes, especally, n dynamc networks. Three remarkable aspects of CBRR are emphaszed as follows. Frstly, the key to CBRR s that the nformaton n a neghbor table s obtaned through a contenton mechansm wthout beacons, and CBRR employs the contenton mechansm to select a new forwardder only when there s no an avalable neghbor, whch meets the real-tme requrements, n the neghbor table. Secondly, the neghbor table allows CBRR to select a next-hop forwarder by drect uncast or contenton-based forwardng. As a consequence, the end-to-end real-tme requrements can be fulflled wth speed or delay constrant at each hop. Thrdly, CBRR can collect the nformaton from the two-hop neghbors durng the wreless broadcastng, and the two-hop neghbor table can be helpful for further meetng the real-tme requrements n the two-hop range. The rest of the paper s organzed as follows. Secton 2 outlnes the related works. Secton 3 presents the proposed real-tme routng protocol. Secton 4 reports the expermental results. 2. Related Works Recently, real-tme routng protocol has aroused much research nterests n s. SAR [13] was the frst QoS protocol for s. It could fnd multple paths between a source and a target wth dfferent prorty n terms of energy effcency and fault tolerance. EQR [14] tred to fnd a least-cost yet energy effcent path for real-tme data, and to maxmze the throughput for non-real-tme data wth a class-based queue model, smultaneously. However, both SAR and EQR were based on global routng decsons, whch result n much hgher routng overheads. Therefore, localzed geographc routng decsons have been popular employed n a varety of realtme routng protocols. SPEE [5] was desgned to provde soft end-to-end real-tme guaranty wth a desred delvery speed, S setpont, whch could support the real-tme communcaton between a source and a snk, through a combnaton of feedback control and non-determnstc geographc forwardng. Only the node, whose progress speed was hgher than S setpont, could be selected as the next-hop forwarder. MMSPEE [6] was an extenson to SPEE wth addtonal servce dfferentatons both n the tmelness and relablty domans. RPAR [7] fulflled the end-to-end real-tme requrements wth low energy consumpton by dynamcally adaptng transmsson power and assgnng one-hop velocty. Unfortunately, these localzed routng protocols had to broadcast beacons to collect the neghbors nformaton and were nevtably suffered from the aforementoned drawbacks. Contenton-based beaconless scheme has been developed to address the ssues aforementoned. IGF [8] only allowed the nodes wthn the 60-degree sector towards the snk to partcpate n the contenton. The wat delay of each avalable node was determned by the combnaton of the progress dstance to a snk, the remanng energy of the node and a random delay. GeRaF [9,10] used the busy tone to avod collsons and replaced the wat delay functon by tme slots, whch were assgned to dfferent regons of forwardng areas. SGF [11] ntroduced a forwardng scheme through ntegratng the contenton mechansm and gradent for unexpected node/lnk falures n hghly dynamc networks. OGF [12] combned the contenton mechansm wth a neghbor table to forward packets n slowly dynamc networks. OGF was smlar to our approach, but t dd not consder the real-tme constrants. 3. Man Results Our desgn goal s to fulfll the end-to-end real-tme requrements wth less energy consumpton for s. Two versons of CBRR protocol are proposed, such as CBRR-OneHop and CBRR-TwoHop, whch are based on one-hop neghbor table and two-hop neghbor table, respectvely. CBRR-OneHop protocol s the foundaton of CBRR, and CBRR-TwoHop protocol s an extenson to CBRR-OneHop. Before descrbng CBRR protocol, we frst ntroduce the relevant defntons and assumptons efntons and Assumptons enote source node and snk node as S and. Let d(, j) j and elay be the Eucldean dstance and the delay between node and node j, respectvely. eadlne() s the requred deadlne of a ATA packet for and s Copyrght 2010 ScRes.

3 530 C. HUANG ET AL. carred n the header of the ATA packet. R c s the rado range. Neghbor Set of Node : NS s the set of nodes wthn the rado range of node. Formally, NS { j d(, j) R c }. Contenton Canddate Set of Node : CCS s the set of nodes whch are n NS and closer to than node. Formally, CCS { j d(, ) d( j, ), j NS }. j Relay Speed: Speed s the velocty between node and j, whch s the rato of the dstance to the delay between node and j. Formally, Speed d(, j)/ elay j j. Requred Speed: Speed s the velocty, whch meets the real-tme requrements. It s the rato of the dstance between node and to the remanng tme of a ATA packet s deadlne. Formally, Speed d(, )/ ( eadlne( ) T now ), where T now s the current tme. Estmated Hop: Hop s the estmated hop number between node and, whch s the rato of the dstance between node and to R c. Formally, Hop d(, )/ R c. Requred elay: elay s the one-hop delay, whch meets the real-tme requrements. It s the rato of the remanng tme of the packet s deadlne to the estmated hop. Formally, elay ( eadlne( ) T )/ Hop. Speed Constrant: Speed constrant s used for each node to fnd an avalable neghbor, whose relay speed s no less than the requred speed, n ts neghbor table. If havng an avalable neghbor, the node can send the ATA packet to the neghbor by drect uncast. Otherwse, the node has to nvoke a contenton procedure to select a new next-hop forwarder. elay Constrant: elay constrant s used for each node to determne whether ts contenton canddates can take part n the contenton or not. If there s an avalable neghbor, whose one-hop delay s no less than the requred delay, n the neghbor table of the contenton canddate, ths canddate can be admtted to partcpate n the contenton. Otherwse, the canddate s forbdden to do that. Assume that each node s aware of ts poston, and t has the same rado range and ntal energy wth a unque I and an out-of-band busy tone. now cton wth wat delay, delay estmaton and one-hop neghbor table, whch s created or updated only durng a contenton procedure. When a source ntends to send ts frst ATA packet, the node has to employ the contenton forwardng polcy to select a next-hop forwarder because the neghbor table of each node has not been created. After the frst ATA packet s successfully forwarded to the snk, each node, whch s on the prevous forwardng path, has at least one neghbor n ts one-hop neghbor table. Therefore, the frst ATA packet can be regarded as a specal packet for the path dscovery. In the future forwardng, the one-hop neghbor table can play an mportant role to help wth selectng an avalable next-hop forwarder, whch meets the speed constrant or delay constrant, by the uncast forwardng polcy or the contenton forwardng polcy. Uncast Forwardng Polcy As shown n Fgure 1, the uncast forwardng polcy can utlze the speed constrant to determne that a ATA packet s forwarded by drect uncast or nvokng the contenton forwardng polcy. If the neghbor table has an avalable neghbor, whch meets the speed constrant, the current forwarder selects the neghbor as the next-hop forwarder and uncasts the ATA packet to the neghbor drectly. Otherwse, the current forwarder has to nvoke the contenton forwardng polcy to select a new next-hop forwarder. Contenton Forwardng Polcy If the uncast forwardng polcy fals to uncast a A- TA packet, the contenton forwardng polcy can employ ERTS-ECTS-ATA-ACK handshake to select a new next-hop forwarder. The delay constrant s used to determne whether a contenton canddate can take part n the contenton or not. ERTS s an extenson to RTS wth addtonal nformaton ncludng the postons of the current forwarder and the snk, and the requred delay CBRR-OneHop Protocol CBRR-OneHop protocol s the foundaton of CBRR, and t conssts of fve components, such as uncast forwardng polcy, contenton forwardng polcy, contenton fun- Fgure 1. Uncast forwardng procedure. Copyrght 2010 ScRes.

4 C. HUANG ET AL. 531 ECTS s also an extenson to CTS wth addtonal poston of CTS s sender. Busy tone s an out-of-band sgnal whch s used to avod multple canddates takng part n the contenton, smultaneously. The current forwarder and ts contenton canddates work as follows. As shown n Fgure 2, the current forwarder frst broadcasts an ERTS packet to start a contenton, and then wats for recevng an ECTS packet. If overhearng the ECTS packet, the current forwarder should send a busy tone mmedately, and contnues recevng the ECTS packet. After fnshng recevng the ECTS, the current forwarder can uncast the ATA packet to the selected next-hop forwarder drectly. When the current forwarder fnshes recevng the ACK packet sent by the next-hop forwarder, t nserts the nformaton of the new forwarder ncludng the I, poston and one-hop delay, or updates the one-hop delay, n ts neghbor table. If the current forwarder fals to select a next-hop forwarder, t just dscards the ATA packet. The contenton canddate s contenton procedure s shown n Fgure 3. After recevng the ERTS packet, each contenton canddate has to determne whether to partcpate n the contenton or not accordng to ts one-hop neghbor table. If ts neghbor table has an avalable neghbor, whch meets the speed constrant, the canddate can be assgned hgher contenton prorty wth short wat delay, T short. If none of the neghbors n ts one-hop neghbor table can meet the speed constrant, the canddate s forbdden to take part n the contenton. Otherwse, f the neghbor table s empty, whch means that the canddate has never been the forwarder, the canddate s also admtted to take part n the contenton but wth long wat delay, T long. The canddate wth the shortest wat delay can frst respond an ECTS packet thus to wn the qualfcaton of the new forwarder. Other canddates, whch receve the busy tone durng ther wat delay, must qut the contenton mmedately. When fnshng recevng the ATA packet, the new forwarder should respond an ACK packet to the current forwarder. Fgure 2. Current forwarder s workng procedure. Fgure 3. Contenton canddate s contenton procedure. It needs to be ponted out that although the delay constrant never takes the wat delay nto account, t does not affect the real-tme performance because the selected neghbor s the most sutable forwarder, whch has the shortest wat delay. Contenton Functon and Wat elay Contenton functon s the key factor to the contenton-based beaconless scheme because t determnes the wat delay of each contenton canddate. ue to real-tme constrant and energy effcency, our contenton functon takes nto account the combnaton of the progress dstance toward a snk, the remanng energy of the node and the number of packets watng n the output queue. For node, ts contenton prorty s computed as follows. P ( q ( d / Rc E / Et q / Qt r) (1) where P s the contenton prorty; d s the progress dstance towards a snk; E s the remanng energy and E t s the ntal total energy; q s the number of packets watng n the queue and Q t s the total queue sze; r s a random value between 0 and 1; α, β, γ and η are weghts assgned to dstance, energy, queue and random value, respectvely, and meet α + β + γ + η = 1. Usng the contenton functon, we can calculate the wat delay of T short and T long as follows. T SIFS 0.5 SIFS(1 P ) (2) short T SIFS SIFS(1 P ) (3) short where SIFS s Short Inter Frame Spacng and s defned as 10 μs n IEEE standard [15]. From (2) and (3), t can be guaranteed that T short s smaller than T long. elay Estmaton elay estmaton s responsble to calculate the one- Copyrght 2010 ScRes.

5 532 C. HUANG ET AL. hop delay between two neghborhoods. The current onehop delay s evaluated based on two tmestamps. One s the tme T arrvng when the receved ATA packet enters the tal of the output queue. Another s the tme T ACK when the node receves the ACK packet responded by the next-hop forwarder. The tme nterval between these two tmestamps s employed to characterze the one-hope delay here. The new one-hop delay can be calculated by the combnaton of the currently measured delay and the prevous one-hop delay as follows. onehop onehop elay new ( TACK Tarrvng ) (1 ) elay prevous (4) One-Hop Neghbor Table As shown as n Table 1, NeghborI and Neghbor- Poston are obtaned through an ECTS packet; One- Hopelay can be computed from (4). Counter s used to record how many tmes a neghbor has been a next-hop forwarder. One-hop Neghbor table s managed as follows. 1) If the newly selected forwarder s not n the neghbor table, the current forwarder has to add the nformaton of the new forwarder n ts neghbor table after recevng the ACK packet sent by the new forwarder. Otherwse, the current forwarder needs to update the One- Hopelay of the neghbor n ts neghbor table. 2) If the Counter n the entry of a neghbor s up to MaxCount, the neghbor s forbdden to take part n the current forwardng, and ts Counter should be cleared to zero. MaxCount s the maxmal tmes that a node can be a next-hop forwarder. If the network topology s stable, a neghbor n the neghbor table can always be vald so as to be the next-hop forwarder contnuously. In ths case, the node not only consumes away ts energy quckly, but also deprves other neghbors of beng a next-hop forwarder. Therefore, MaxCount s used to balance the network load thus to prolong the lfetme of s CBRR-TwoHop Protocol CBRR-TwoHop protocol s an extenson to CBRR-One- Hop protocol. It can depend on wreless broadcast to collect the nformaton of the two-hop neghborhoods for buldng two-hop neghbor table. Wth the help of twohop neghbor table, CBRR-TwoHop s able to further meet the real-tme requrements n the two-hop range. Two-Hop elay Estmaton As shown n Fgure 4, node A, B and C are regarded as the last-hop forwarder, the current forwarder and the next-hop forwarder, respectvely. Whle node B uncastng a ATA packet to node C, node A can also overhear the ATA packet, whch carres the poston nformaton of node C. In order to obtan the two-hop delay, each node needs to create a Sent_ATA table for recordng the ATA packets, whch have been sent by the node successfully. As shown n Table 2, the record of a ATA packet ncludes the tems of source s I, sequence number and Receved_Tme, whch s the tmestamp that the ATA packet enters the tal of the output queue. Wth the help of the Sent_ATA table, the two-hop delay estmaton between node A and C can be obtaned as follows. 1) After recevng the ACK packet sent by node B, node A needs to add the relevant nformaton of the ATA packet, whch has been sent to node B prevously, n ts Sent_ATA table. 2) Whle node B uncastng a ATA packet to node C, node A can overhear the packet so as to look up ts Sent_ATA table to see f A has sent ths ATA packet prevously. If true, the current two-hop delay between node A and C s approxmated by the nterval between the Receved_Tme n A s Sent_ATA table and the tme T now when node B fnshes transmttng the ATA packet to node C. After estmatng, node A needs to delete the entry of ths ATA packet from ts Sent_ATA table for reducng the storage overhead. However, f the ATA packet s not n the Sent_ATA table, node A has to gnore the packet and to do nothng. We also compute the new two-hop delay by the combnaton of the currently measured delay and the prevous two-hop delay as follows. elay ( T T ) (1 ) elay twohop twohop new now Re ceved _ Tme prevous (5) Two-Hop elay Estmaton The two-hop neghbor table extends the one-hop neghbor table wth addtonal nformaton of the two-hop neghbor ncludng I, poston and two-hop delay, as Table 1. Structure of one-hop neghbor table. NeghborI NeghborPoston OneHopelay Counter Table 2. Structure of Sent_ATA table. SourceI SequenceNo. Receved_Tme A ATA B ATA Fgure 4. An example of wreless broadcast. C Copyrght 2010 ScRes.

6 C. HUANG ET AL. 533 shown n Table 3. The I and poston of the two-hop neghbor are obtaned through a ATA packet, and Two- Hop elay can be computed from (5). Other tems are the same as those n one-hop neghbor table. It should be noted that f a node fals to select a nexthop forwarder, the node maybe encounters a vod around t. In ths case, the node wll not uncast the receved ATA packet, and ts last-hop forwarder can never overhear the packet. As a result, the nformaton of the two-hop neghbor should be null n the two-hop neghbor table of the last-hop forwarder. For avodng more nvald contentons, f the two-hop table has more than two records, whch have null nformaton of the two-hop neghbor, the node maybe has a serous vod problem n ts two-hop range thus to be forbdden to take part n the latter contentons. Forwardng polcy The forwardng polcy of CBRR-TwoHop s smlar to that of CBRR-OneHop. The dfference s that CBRR- TwoHop needs to employ the speed constrant between the two-hop neghborhoods. If there s a one-hop neghbor, whch the two-hop relay speed between the current forwarder and the two-hop neghbor s no less than the requred two-hop speed, n the two-hop neghbor table, the current forwarder can uncast the ATA packet to the one-hop neghbor drectly. Otherwse, the current forwarder has to nvoke the contenton forwardng polcy wth the two-hop delay constrant to select a next-hop forwarder. 4. Expermental Studes To valdate the CBRR protocol proposed n the paper, CBRR-OneHop and CBRR-TwoHop are compared wth SPEE. Expermental studes are conducted by means of J-Sm smulator [16], whch s an open-source, component-based network smulaton envronment and s developed entrely n Java. OneHop I Table 3. Structure of two-hop neghbor table. OneHop Poston OneHop elay TwoHop I TwoHop Poston TwoHop elay Table 4. Parameters for experments. Counter Parameter Values Parameter Values Network Sze 200m 200m MAC Layer Node Number 200 Intal Energy 100 J Rado Range 40 m Bandwdth 2 Mbps Packet Sze 512 bytes Send/Receve/Idle power(mw) 660/395/ Smulaton Settng We randomly choose four source nodes n the left of network, and two snk nodes n the rght of network. We test the above protocols n two network topologes: Statc network, where the topology s nvarable ncludng packet generaton rate and node densty scenaros. ynamc network, where the topology s changeable due to node moblty or sleep polcy. We choose constant bt rate (CBR) traffc and set CBR at 2 packets/s n all experments except n the packet generaton rate scenaro. Unless specally noted, all parameters for experments are set as shown n Table Performance n Statc Networks In statc networks, the poston of each node s not changeable. Therefore, we set the nterval of beacon broadcast at 10 s n SPEE. Two scenaros are evaluated n the statc networks ncludng packet generaton rate and packet sze. Impact of packet generaton rate The comparatve results between CBRR and SPEE are plotted n Fgure 5. It can be seen that the both CBRR protocols acheve nearly 100% delvery rato (Fgure 5(a)) and stable end-to-end delay around 0.05s (Fgure 5(b)). All these n CBRR contrbute to the Routng/MAC cross-layer desgn, whch can tmely collect the state nformaton of wreless channel thus to avod more collsons durng the forwardng procedures. In contrast, hgher packet generate rate may brng forth more packet collsons thus lead to hgher packet mss rato (Fgure 5(a)) and longer delay (Fgure 5(b)) n SPEE. In Fgure 5(c), SPEE consumes the average energy about two tmes more than those of the two CBRR protocols due to ts beacon broadcastng. It s worth notng that the average energy consumpton of each protocol decreases slowly as the packet generaton rate ncreasng. The reason s that when the packet generaton rate s small, most nodes are always kept n dle state, whose energy consumpton s the man part of the total consumed energy. However, more and more packets have been forwarded thus can lead to lower average energy consumpton as the generate rate ncreasng. Furthermore, Fgure 5 llustrates that the overall performance of CBRR-TwoHop s lttle better than that of CBRR-OneHop because the two-hop neghbor table can be helpful for further meetng the real-tme requrements n the two-hop range. Impact of packet sze Larger ATA packets may lead to hgher probablty of the collsons between a ATA packet and other packets, such as RTS/ERTS, CTS/ECTS, ACK or beacon. Fgure 6 llustrates the comparatve results between Copyrght 2010 ScRes.

7 534 C. HUANG ET AL. CBRR and SPEE n the packet sze scenaro. It can be observed n Fgure 6(a) that the delvery rato of SPEE elvery Rato SPEE CBRR-OneHop CBRR-TwoHop CBR (packets/s) (a) Packet elvery Rato (a) Packet elvery Rato (b) Average End-to-End elay (b) Average End-to-End elay (c) Average Energy Consumpton Fgure 6. Impact of packet sze. (c) Average Energy Consumpton Fgure 5. Impact of packet generaton rate. drops quckly as the packet sze ncreasng, but the packet sze has very lttle mpact on the two CBRR pro- Copyrght 2010 ScRes.

8 C. HUANG ET AL. 535 tocols for ther nearly 100% delvery rato. The results may ndcate that the collsons between ATA packets and beacons are much more severe than others between ATA and RTS/ERTS, CTS/ECTS or ACK packets. As a result, wth larger packet sze, more serous packet collsons can lead to longer end-to-end delay (Fgure 6(b)) and more energy consumpton (Fgure 6(c)) than those of the two CBRR protocols. It can be vewed from above expermental results that the two CBRR protocols have much better performance than SPEE n the statc networks. Furthermore, CBRR- TwoHop can outperform than other two protocols due to the help of the two-hop neghbor table, whch can be helpful for further meetng the real-tme requrements n the two-hop range. In addton, t can also be suggested that broadcastng beacons can aggravate the packet collsons thus to degrade the performance of the beaconbased routng protocols Performance n ynamc Networks s are hghly dynamc networks and ther topologes can change constantly due to node moblty, sleep polcy, node falure, and so on. In the followng experments, we compare CBRR wth SPEE n node moblty and sleep polcy scenaros. In addton, we set the frequency of broadcastng beacons at 1 s and 10 s, respectvely, for SPEE to tmely collect fresh nformaton of the neghborhoods. Impact of node moblty We adopt the Random Waypont moblty model wth zero pause tme n the experment. Fgure 7 plots the comparatve results between the two CBRR protocols and SPEE n the node moblty scenaro. Fgure 7(a) llustrates that the delvery rato of SPEE drops much more quckly, whch shows that SPEE can hardly work when the moble speed s hgher than 10 m/s. Ths s because the nformaton of the neghbor table s totally outdated thus to be useless for the forwardng. In contrast, although the two CBRR protocols drop more packets at hgher moble speed, they can stll acheve about 60% delvery rato at 20 m/s n contrbuton to the contenton forwardng polcy, whch can on-lne select a favorte next-hop forwarder. It can be observed n Fgure 7(b) that the average end-to-end delay of SPEE becomes much nstable for ts extremely hgh packet mss rato. However, at hgher node moblty, the both CBRR protocols have lttle longer delay due to the falure of drect uncast. Fgure 7(c) shows that the two CBRR protocols have very close performance, whch consume far less energy than SPEE. It needs to pont out that we set 100 as the maxmum n Fgure 7(c), and the values of SPEE, whch s plotted as 100, are actually more than 100. Furthermore, t can be observed n Fgure 7 that the performance of SPEE-1 s no better than that of SPEE-10, whch suggests that hgher frequency of beacon broadcastng can not mprove the performance of SPEE n the node moblty scenaro. (a) Packet elvery Rato (b) Average End-to-End elay (c) Average Energy Consumpton Fgure 7. Impact of node moblty. Copyrght 2010 ScRes.

9 536 C. HUANG ET AL. Impact of sleep polcy In order to support energy conservaton n s, the most practcal way s to use node sleep polcy. Our sleep polcy s desgned as that the lfetme of each node s dvded nto several same perods, and each perod ncludes an actve sub-perod and a sleep sub-perod. Fgure 8 plots the comparatve results between the two CBRR protocols and SPEE n the dynamc sleep polcy scenaro. Smlar to the node moblty scenaro, the sleep polcy has far more negatve mpact on SPEE than those on the two CBRR protocols. The mpact can be observed n Fgure 8(a) that f the sleep percentage s larger than 50%, the delvery rato of SPEE s approxmate 0. The reason s that the next-hop forwarder, whch s selected by SPEE n the neghbor table, would be usually n sleep state, and SPEE needs to retransmt more packets. However, the both CBRR protocols acheve more than 90% n delvery rato at 50% sleep percentage. Although droppng more packets after 50%, the two CBRR protocols stll outperforms SPEE very much. Other results are smlar to those n the node moblty scenaro as shown n Fgure 8(b) and Fgure 8(c). (c) Average Energy Consumpton Fgure 8. Impact of sleep polcy. It can be concluded from the above expermental results that CBRR s not only partcularly sutable for the dynamc networks, but also has far well performance than SPEE n the statc scenaros wth much less energy consumpton. Furthermore, t can be observed that CBRR-TwoHop outperforms the other two protocols due to the help of the two-hop neghbor table. It also suggests that SPEE s totally not sutable for the dynamc networks, and ncreasng the frequency of beacon broadcastng can not mprove but degrade the performance of the beacon-based routng protocols. 5. Conclusons (a) Packet elvery Rato Ths paper presents a novel real-tme routng protocol, CBRR, for s. The pont dstngushng our approach from the exstng schemes s that CBRR collects the nformaton of neghborhoods by the contenton mechansm nstead of beacons. Ths contrbuton can lead to more energy effcency. In addton, t s notable that the end-to-end real-tme requrements are well fulflled wth speed or delay constrant at each hop, whch attrbutes to the combnaton of the contenton and neghbor table mechansms. The valdty of CBRR s llustrated by means of expermental studes. It has been shown that CBRR can outperform SPEE n terms of delvery rato, end-toend delay and energy consumpton, especally n dynamc networks. Our future work s to conduct the theoretcal analyss on the energy consumpton of CBRR. It s also nterested to nvestgate how to provde relablty n CBRR. 6. References (b) Average End-to-End elay [1] I. F. Akyldz, W. Su, Y. Sankarasubramanam and E. Cayrc, Wreless Sensor Networks: A Survey, Computer Networks, Vol. 38, No. 4, March 2002, pp Copyrght 2010 ScRes.

10 C. HUANG ET AL. 537 [2]. Z. Cheng and P. K. Varshney, QoS Support n Wreless Sensor Networks: A Survey, Internatonal Conference on Wreless Networks (ICWN 04), LasVegas, Vol. 1, June 2004, pp [3] Y. J. L, C. S. Chen, Y. Q. Song and Z. Wang, Realtme QoS Support In Wreless Sensor Networks: A Survey, Proceedngs of Seventh IFAC Internatonal Conference on Feldbuses and Networks n Industral and Embedded Systems, Toulouse, November 2007, pp [4] M. Youns, K. Akayya and A. Wadaa, On Handlng QoS Traffc n Wreless Sensor Networks, Proceedngs of the 37th Annual Hawa Internatonal Conference on System Scences, Hawa, January 2004, pp [5] T. He, J. A. Stankovc, C. Y. Lu and T. F. Abdelzaher, A Spatotemporal Communcaton Protocol for Wreless Sensor Networks, IEEE Transacton on Parallel strbuted Systems, Vol. 16, No. 10, October 2005, pp ,. [6] E. Felemban, C. G. Lee and E. Ekc, MMSPEE: Multpath Mult-SPEE Protocol for QoS Guarantee of Relablty and Tmelness n Wreless Sensor Networks, IEEE Transactons on Moble Computng, Vol. 5, No. 6, June 2006, pp [7] O. Z. Chpara, Z. M. He, G. L. Xng, Q. Chen and X. R. Wang, Real-Tme Power-Aware Routng n Sensor Networks, Proceedngs of 14th IEEE Internatonal Workshop on Qualty of Servce (IWQoS 2006), New Haven, June 2006, pp [8] T. He, B. M. Blum, Q. Cao, J. A. Stankovc, S. H. Son and T. F. Abdelzaher, Robust and Tmely Communcaton over Hghly ynamc Sensor Networks, Real-Tme Systems, Vol. 37, No. 3, ecember 2007, pp [9] M. Zorz and R. R. Rao, Geographc Random Forwardng (GeRaF) for Ad Hoc and Sensor Networks: Energy and elay Performance, IEEE Transactons on Moble Computng, Vol. 2, No. 4, October-ecember 2003, pp [10] M. Zorz and R. R. Rao, Geographc Random Forwardng (GeRaF) for Ad Hoc and Sensor Networks: Multhop Performance, IEEE Transactons on Moble Computng, Vol. 2, No. 4, October-ecember 2003, pp [11] P. Huang, H. Y. Chen, G. L. Xng and Y.. Tan, SGF: A State-Free Gradent-Based Forwardng Protocol for Wreless Sensor Networks, ACM Transacton on Sensor Networks, Vol. 5, No. 2, March 2009, pp [12]. Z. Chen and P. K. Varshney, On-emand Geographc forwardng for ata elvery n Wreless Sensor Networks, Computer Communcatons, Vol. 30, No , October 2007, pp [13] K. Sohrab, J. Gao, V. Alawadh and G. J. Potte, Protocols for Self-Organzaton of a Wreless Sensor Network, IEEE Personal Communcatons, Vol. 7, No. 5, October 2000, pp [14] K. Akkaya and M. Youns, Energy-Aware QoS Routng n Wreless Sensor Networks, Cluster Computng, Vol. 8, No. 2-3, July 2005, pp [15] IEEE Standard for Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) Specfcatons, ISO/IEC : 1999(E), IEEE, August [16] RCL J-Sm, Copyrght 2010 ScRes.