A probabilistic approach for predictive congestion control in wireless sensor networks

Transcription

1 Anne Uthra et al. / J Zhejang Unv-Sc C (Comput & Electron) (3): Journal of Zhejang Unversty-SCIENCE C (Computers & Electroncs) ISSN (Prnt); ISSN X (Onlne) E-mal: [email protected] A probablstc approach for predctve congeston control n wreless sensor networs R. ANNIE UTHRA 1, S. V. KASMIR RAJA 1, A. JEYASEKAR 1, Anthony J. LATTANZE 2 ( 1 Department of Computer Scence and Engneerng, SRM Unversty, Taml Nadu 6323, Inda) ( 2 Department of Software Engneerng, Carnege Mellon Unversty, Pttsburgh, PA , USA) E-mal: {anneuthra, svr, ajeyasear}@yahoo.com; [email protected] Receved June 28, 213; Revson accepted Dec. 6, 213; Crosscheced Feb. 19, 214 Abstract: Any node n a wreless sensor networ s a resource constraned devce n terms of memory, bandwdth, and energy, whch leads to a large number of pacet drops, low throughput, and sgnfcant waste of energy due to retransmsson. Ths paper presents a new approach for predctng congeston usng a probablstc method and controllng congeston usng new rate control methods. The probablstc approach used for predcton of the occurrence of congeston n a node s developed usng data traffc and buffer occupancy. The rate control method uses a bac-off selecton scheme and also rate allocaton schemes, namely rate regulaton (RRG) and splt protocol (SP), to mprove throughput and reduce pacet drop. A bac-off nterval selecton scheme s ntroduced n combnaton wth rate reducton (RR) and RRG. The bac-off nterval selecton scheme consders channel state and collson-free transmsson to prevent congeston. Smulatons were conducted and the results were compared wth those of decentralzed predctve congeston control (DPCC) and adaptve duty-cycle based congeston control (ADCC). The results showed that the proposed method reduces congeston and mproves performance. Key words: Congeston, Rate allocaton, Congeston control, Pacet loss, Bac-off nterval, Rate control do:1.1631/jzus.c13175 Document code: A CLC number: TP393 1 Introducton Zhejang Unversty and Sprnger-Verlag Berln Hedelberg 214 In a wreless sensor networ (WSN), the sensor nodes scattered n the sensng feld sense physcal phenomena such as pressure, temperature, and humdty, and transfer these sensed data to the fnal destnaton called the gateway node. Congeston occurs n such a networ when the offered load of a node exceeds the avalable capacty of that node or the channel bandwdth drops due to channel fadng. Consequently, pacets may be dropped at the buffers and requre retransmsson of those dropped pacets, whch leads to waste of energy. Therefore, both buffer and ln bandwdth must be effcently used to avod congeston and pacet drop among the nodes. Congeston needs to be controlled n msson crtcal applcatons such as mltary, dsaster management, and mnng, as well as n other applcatons such as habtat montorng and an envronment montorng system (EMS) to avod retransmsson of pacets thereby ncreasng the lfetme of the nodes n the networ. Consder an EMS (Fg. 1) consstng of a number of nodes deployed n the montorng area (MA) nsde a mne to montor events le fre, oxygen reducton n ar, ncrease n pressure, and leaage of posonous gases. In ths system each sensor node conssts of a processor, memory, transcever, power source, and one or more sensors. These sensor nodes communcate wth each other and sensor data are transferred to the gateway node n the system. The gateway node, connected wth the Internet, collects data from the sensor nodes and processes the data. Fnally, t sends the data to the data collecton center for further processng and storage of data. The above sad scenaro s equally applcable to other applcatons le the healthcare montorng system.

2 188 Anne Uthra et al. / J Zhejang Unv-Sc C (Comput & Electron) (3): Such wreless sensor networ systems are data centrc; the sensed data are crucal and should reach the destnaton, the gateway, through the ntermedate nodes. Therefore, data transmsson protocols need to mtgate congeston resultng from excess load and a fadng channel to avod pacet drop and waste of energy due to the retransmsson of dropped pacets. Other reasons for congeston n the networ are lsted below. The many-to-one nature of the event nformaton flow causes congeston because a number of event sensng nodes send ther nformaton to any one of ther next hop nodes. Ths node gets congested f the ncomng rate exceeds the outgong rate, whch results n buffer overflow. Moreover, the transmsson occurrng at the same tme causes pacet collson. Therefore, node densty s a ey factor that ncreases the degree of congeston. Each node shares a common rado channel wth all ts neghbors. An nadequate bandwdth reservaton may degrade the networ performance. Hence, to avod congeston n the networ, the data rate must be controlled and bandwdth must be used effcently. In ths paper, we propose a probablstc method to detect congeston and propose congeston control methodologes to avod congeston by effcently usng the buffer and channel bandwdth consderng collson-free transmsson. Sutable bac-off selecton n meda access control (MAC) layer congeston control s consdered to save the energy of nodes. 2 Related wors Sensor node Sn node Data collecton center Montorng area Fg. 1 Archtecture of an envronment montorng system Much research has been done to control congeston (Uthra and Raja, 212) n WSNs. The end-to-end congeston control schemes need to propagate the onset of congeston between the end-systems. Ths maes the approach slow. In general, the hop-by-hop congeston control scheme reacts faster to congeston and s preferred to for mnmzng pacet losses n a wreless networ. Therefore, the proposed scheme uses congeston algorthms to predct the onset of congeston of a node and gradually reduces the ncomng rates by means of feedbac messages. One of the earlest congeston control protocols, congeston detecton and avodance (CODA) (Wan et al., 23), uses a combnaton of present and past channel loadng and buffer occupancy for detecton of congeston. Hop-by-hop and end-to-end congeston control schemes smply drop the pacets at the node and use the addtve ncrease multplcatve decrease (AIMD) scheme to control the source rate. Ths results n retransmsson of pacets. Fuson (Hull et al., 24) uses a statc threshold value to detect the onset congeston n the networ. Normally, t s dffcult to fnd a statc threshold value for a dynamc channel envronment. Moreover, CODA and Fuson protocols use the broadcast message to nform ther neghborng nodes about the congeston though ths message s not guaranteed to reach the source. The congeston control and farness (CCF) routng scheme (Cheng and Bajcsy, 24) uses pacet servce tme at the node as an ndcator of congeston. However, usng the servce tme alone to determne the onset of congeston may be msleadng. Interferenceaware far rate control (IFRC) (Rangwala et al., 26) s a rate allocaton technque whch detects congeston based on queue length. When congeston s detected, the rates of the flows are throttled on the nterferng tree. When the average queue length exceeds the upper threshold, rates of the flows are adjusted usng the AIMD scheme. Consequently, IFRC reduces the number of pacets by reducng the throughput. On the other hand, the prorty-based congeston control protocol (PCCP) (Wang et al., 26) uses a rato between pacet nter-arrval tme and pacet servce tme to determne the congeston level of a node. Congeston nformaton s pggybaced n the header of data pacets and broadcasted, and receved by chld nodes. However, both CCF and PCCP gnore queue utlzaton, whch leads to frequent buffer overflows. Ths results n ncreased retransmsson.

3 Anne Uthra et al. / J Zhejang Unv-Sc C (Comput & Electron) (3): Moreover, when the source s at multple hops from the congeston regon, the congeston nformaton s not guaranteed to reach the source n case of CODA and PCCP. Congeston aware routng (Kumar et al., 28) s an applcaton specfc, dfferentated routng protocol whch uses the data rate to dentfy congeston and consders data prorty to overcome congeston. Ths protocol s not sutable for applcatons that have equal prorty data. The mult-event congeston control protocol (Hussan et al., 28), on the other hand, s a networ specfc protocol whch uses pacet delvery tme and buffer sze as ndcators of congeston. Based on the buffer sze, slot length can be ether ncreased or decreased. The reportng rate can also be adjusted through the slot length. However, pacet schedulng and mantanng routng table are overhead n ths protocol. Interference mnmzed multpath routng (I2MR) (Teo et al., 28) evaluates multpath for load-balancng. Long-term congestons are determned by montorng the sze of ther data transmt buffers, by usng exponental weghted movng averages (EWMAs). When a source node s congested, the loadng rate s reduced. However, the number of control pacets transmtted durng path dscovery ncreases. Traffc ntensty s taen as a parameter to measure congeston n cluster based congeston control (Karenos et al., 28). Traffc ntensty s measured n terms of arrval rate and the servce tme of the pacets. Rate self-regulaton s done n the source node. Rate control s done smlar to the AIMD technque. Congeston s detected n decentralzed predctve congeston control (DPCC) (Zawodno and Jagannathan, 27) based on buffer occupances at the nodes, along wth the predcted transmtter power. The current queue level tracs the desred queue level. If the queue level exceeds the desred queue level, the desgned feedbac controller forces the queue level to the target value. The rate of node s calculated based on the outgong rate and buffer occupancy error (excess data that cannot be accumulated n the buffer). The bac-off nterval s selected for both rate adaptaton and preventon of congeston. However, DPCC fxes a statc desred queue level to predct the congeston level. By contrast, the proposed scheme uses an adaptve threshold value and vares the rate based on the predcted congeston level. Congeston control (Lee and Chung, 21) s mplemented usng duty-cycle adjustment n adaptve duty-cycle based congeston control (ADCC). Ths scheme uses both resource control approaches n terms of the actve tme of a node and traffc control approaches accordng to the amount of networ traffc for congeston avodance. The traffc-aware dynamc routng (Ren et al., 211) algorthm routes pacets around the congeston areas and scatters the excessve pacets along multple paths consstng of dle and under-loaded nodes. A hybrd vrtual potental feld usng depth and queue length forces the pacets to elmnate the obstacles created by congeston and move toward the sn. The buffer capacty and data rate are consdered as ndcators of congeston n the probablstc approach for congeston control (PACC) (Uthra and Raja, 211). PACC predcts the onset of congeston n the networ. However, outgong traffc and channel state are not consdered for predctng congeston. The proposed method consders ncomng and outgong traffc of nodes and the channel state to determne the onset of congeston. A predctve control theory was used by Wu et al. (213) to control a networ wth factors le tme delays and pacet dropouts. Bacward and forward channels are consdered to analyze transmsson condtons. Acceptng and applyng newer data, compensatng delayed or lost data, and dscardng older data are the central law of the strategy. Networ utlty maxmzaton (NUM) descrbed by Morell et al. (211) uses a convex decomposton technque to acheve an optmal soluton. RADAR (Boutss and Kalogera, 212) uses elapsed tmes, latences, and resource loads to dynamcally determne the rate allocaton. The problem s solved by maxmzng the rate to meet the deadlne of every applcaton. Mao et al. (212) consdered data and battery buffers for maxmzng the long-term average sensng rate. The rate control s performed based on the power management framewor. Rate allocaton n queue-based channel-measurement and rateallocaton (Q-CMRA) (Bhargava et al., 212) chooses the maxmum allowed physcal-layer rate based on queue length, and the hghest rate s chosen usng channel measurement. Some of the routng protocols that grant relablty, ncludng the mult-path and mult-speed

4 19 Anne Uthra et al. / J Zhejang Unv-Sc C (Comput & Electron) (3): routng protocol (MMSPEED) (Felemban et al., 26), SPEED (He et al., 23), and RAP (Lu et al., 22), use velocty monotonc schedulng. Certan speed s assgned to the pacets. The speed of the pacet s not clear when the networ s congested. Relablty n MMSPEED s acheved through duplcatng pacets, whch further ncreases congeston. The protocols ntroduced earler (Wan et al., 23; Cheng and Bajcsy, 24; Wang et al., 26; Teo et al., 28) do not consder congeston due to fadng channels n a dynamc envronment. The congeston due to the effect of the fadng channel s taen nto account n the proposed system, whch s explaned n the followng secton. Rate reducton was consdered n Wan et al. (23), Hull et al. (24), Rangwala et al. (26), and Teo et al. (28), and DPCC performs the rate control. However, the proposed system performs rate reducton and rate regulaton based on the adaptve threshold value, whch s ntroduced to trgger the executon of the congeston predcton algorthm, thereby nvong the congeston control algorthm mmedately. Thus, the proposed system not only avods pacet drop but utlzes the buffer effcently. The overall objectve of ths paper s to develop (1) a congeston predcton method for detectng the level of congeston of a node, and (2) a congeston control method for mtgatng congeston n each node. Congeston s mtgated by (1) controllng the flow rates of all nodes ncludng the source nodes to prevent buffer overflowng usng the predcted value, and (2) desgnng sutable bac-off ntervals for each node based on channel state and ts current traffc. We have desgned a mathematcal model for congeston control n a networ by consderng both buffer capacty and ln capacty of a node. on the buffer capacty of that node. The buffer occupancy of node at tme t+1 s gven by q (t+1)=q (t)+u (t) v (t), (1) where u (t) and v (t) are the ncomng and outgong traffc rates of node at tme t, respectvely. The threshold value of node s calculated usng v () t ( t1) (BUFMAX q( t)), u () t u () t, v () t, u () t v (), t (2) where BUFMAX s the maxmum buffer sze of node and α (t+1) s the threshold value of buffer for the gven flow rate consderng the remanng capacty of node. α (t+1) s nversely proportonal to the ncomng traffc, and drectly proportonal to the outgong traffc and the remanng buffer capacty. Therefore, the threshold value decreases as the ncomng traffc ncreases. α (t+1) specfes the desred queue level at t+1 based on the current traffc. Eq. (2) can be appled under the condton that there s data flow n the node or that the ncomng traffc s larger than the outgong traffc. Threshold α (t+1) s set to α max, when outgong traffc exceeds ncomng traffc or u (t) s zero. The adaptve threshold allows nodes n the system to tolerate burstng data flows. Table 1 llustrates the threshold values α (t+1) for certan condtons. Intal condton Buffer empty Table 1 Illustraton of threshold values Incomng traffc, u (t) (pacet/s) Outgong traffc, v (t) (pacet/s) Threshold α (t+1) 2 1.5BUFMAX 3 Probablstc method for congeston detecton Buffer not empty BUFMAX 2 1.5(BUFMAX q (t)) The contrbuton comes from the fact that the possblty of congeston s predcted ahead of ts occurrence and hence the remedal actons can be carred out to elmnate congeston. The estmate of the congeston level s used to reduce the source rate. The level of congeston n each node s detected usng the buffer occupancy and an adaptve threshold value Based on the condtons lsted n Table 1, the followng can be stated. When the buffer s empty and the ncomng traffc s greater than double the outgong traffc, the threshold s set to 5% of the buffer sze. If the buffer s not empty, for example, the buffer contans 1 pacets and BUFMAX s 32 pacets, then we calculate the avalable space as 22 pacets. In

5 Anne Uthra et al. / J Zhejang Unv-Sc C (Comput & Electron) (3): ths example, the threshold s set to 5% of the avalable space, whch s 11. Ths threshold value helps the node trgger the congeston control algorthm when the total number of pacets exceeds the threshold value. The buffer occupancy of a node s compared wth ts threshold value to detect congeston. 1. There s no congeston n the node f q (t)<α (t). 2. If q (t) reaches BUFMAX, the maxmum capacty of the buffer, and u (t)>v (t), then the pacets receved by node wll be dropped. Ths results n buffer overflows, whch n turn causes congeston. 3. Therefore, the proposed method detects congeston when α (t)<q (t)<bufmax. When the buffer occupancy exceeds the threshold value, the level of congeston ncreases;.e., as the number of pacets ncreases to more than the threshold value, the occurrence of congeston also ncreases. Hence, the probablty of occurrence of congeston at node s P( ( t) ) 1 p( ( t) ) p( ( t) n ( t) ( n)) (). t n1 (3) The tme varyng parameter () t descrbes the probablty for the onset of congeston of node when there are pacets more than the threshold value. The proof of Eq. (3) s gven n the Appendx. All the nodes that are n the communcaton range of node belong to N, whch s the neghborhood of node. The probablty of node recevng a pacet from node j, where jn j, s gven by p j p /, (4) where η= N, p s the probablty of node absorbng the pacet from the neghborng nodes, and 1 p denotes the probablty of node droppng the pacet. Nodes are unformly and ndependently dstrbuted. The probablty that a node s n the neghborhood of node s equal to A, whch s the communcaton area of node and has a value of unty. The probablty of node havng neghborhood sze η s gven by n 1 P p N A A 1 n 1 ( ) (1 ). The expected recevng probablty of node s n p n EP [ ( )] pp j [1 (1 A ) ]. na 1 (5) (6) The level of congeston of a node s estmated usng Eqs. (3) and (6) and propagated to the prevous hop nodes for ndcatng the onset of congeston. A zero value of () t ndcates no congeston of a node, and ( t) 1 provdes the level of congeston. Algorthm 1 gves the algorthm for congeston predcton, executed by every node n the networ. Algorthm 1 Congeston occurrence predcton Calculate threshold value α and determne the possblty of congeston of a node. Intalze: BUFMAX s set to the maxmum sze of the buffer of the node; n s set to the number of neghborng nodes from whch the node receves pacets; A s set to the power level of the node. Predcton() // calculate the threshold value α=(v/u)*(bufmax q); f (α>q) status=; // no congeston else f (q>α && q<bufmax) =q α; S=; for (=1; <; ++) S=S+pow(p/(n*A)*(1 pow((1 A), n)), ); status=s; // possblty of congeston end for else status=1; // congeston end f // send feedbac message to the source f (status!=) set ECB=1; pggybac status nto ACK; else set ECB=; error=(α q)*(1 g); pggybac error nto ACK; end f send ACK to prevous hop nodes

6 192 Anne Uthra et al. / J Zhejang Unv-Sc C (Comput & Electron) (3): Congeston control methodology Networ congeston occurs when ether the ncomng traffc (receved and generated) exceeds the capacty of the node or the ln bandwdth drops due to channel fadng caused by path loss, shadowng, and Raylegh fadng. Therefore, we have proposed two methodologes for congeston control, after predctng the occurrence of congeston. The frst methodology s based on algorthms rate regulaton (RRG) and splt protocol (SP), whch regulate data traffc of the recever node by utlzng the buffer capacty. The second congeston control method determnes the bac-off nterval of each node to adjust the outgong rate of a node based on channel capacty. 4.1 Congeston control usng the rate allocaton scheme The rate allocaton scheme taes nto account buffer occupancy, and ncomng and outgong rates of a node. Every node executes Algorthm 1 to determne the possblty of congeston, () t (called the status value). The non-zero value of () t s communcated as a feedbac message between the upstream nodes. The feedbac message s sgnaled to mnmze the effect of congeston on the hop-by-hop bass by estmatng the ncomng and outgong traffc flows when the adaptve rate allocaton scheme (RRG or SP) s mplemented at each node. Upon recevng the feedbac message, the upstream node reduces ts rate proportonal to the status value gven by the recever node. The feedbac nformaton whch conssts of the status nformaton s pggybaced to the acnowledgement (ACK) frame. Ths ensures that the feedbac s successfully receved by the nodes n the prevous hop and avods the transmsson of extra control messages. The outgong rate of neghborng nodes around the congested node s estmated n two dfferent scenaros, as explaned below. Case 1: rate regulaton (RRG) When the queue level q (t) exceeds the threshold value α (t), the status value () t s propagated to the prevous hop nodes toward the source node. Every prevous hop node that receves the feedbac message reduces ts outgong rate proportonal to the status value. u ( t1) (1 ( t)) u ( t). (7) The desred ncomng traffc rate of the recever node s calculated usng Eq. (7). Ths methodology s named rate reducton (RR). For the optmum use of a buffer, RR s modfed nto RRG. If () t, the outgong rate of the source node s reduced proportonal to the status value as n RR; otherwse, the outgong rate of the source node s ncreased proportonal to e (t), where e (t) s the buffer occupancy error of node, defned as e (t)=q (t) α (t). e (t) specfes the number of pacets that can be accommodated n the buffer to reach the threshold value. The ncomng rate of the recever node s estmated usng u ( t1) (1 ( t)) u ( t) (1 g ) e( t), (8) where g 1. g s set to 1 for the non-zero value of () t and RRG reduces the rate. Otherwse, g s vared between and 1, and RRG ncreases the rate. In Fg. 2, the upstream nodes A and B receve feedbac messages,.e., the status value or error value, from the recever node C and ether reduce or ncrease ther data rate as specfed n the feedbac message. A B Incomng traffc C The status value or the buffer occupancy error s pggybaced nto the ACK pacet. The explct congeston bt (ECB) of ACK s set to 1 for the non-zero value of () t and when () t s. The prevous hop nodes that receve feedbac from the upstream node chec the ECB value. If ECB s set to 1, then the node reduces ts outgong rate proportonal to ts status value. Otherwse, the outgong rate s ncremented proportonal to e (t). The algorthm for RRG s shown n Algorthm 2. Algorthm 2 Rate allocaton Calculate the new outgong rate of a node based on ECB (explct congeston bt), status, error values of the ACK pacet. D Feedbac message for rate reducton/ncrease Fg. 2 Feedbac message propagaton for the prevous hop nodes E

7 Anne Uthra et al. / J Zhejang Unv-Sc C (Comput & Electron) (3): RateRegulaton() UnPac(ACK); // Unpac the ACK pacet to obtan ECB, // status, or error f (ECB==1) v=v status*v; else v=v+error; end f RouteDataPacet(); // Route the data pacet wth new rate v Case 2: splt protocol (SP) The earler method RRG uses ACK to pggybac the feedbac messages. To avod the overhead of pggybacng the feedbac messages nto the ACK pacet, the outgong rate of the recever node s ncreased. Hence, the feedbac message s not propagated to the prevous hop nodes n case of SP; nstead, the recever forwards the pacets to more than one upstream neghbor node (Fg. 3). The recever node executes the predcton algorthm and fnds the status value. When the status value s greater than zero, addtonal neghbor nodes are selected for pacet forwardng towards the gateway node, n order to meet the ncomng traffc. Addtonally, the outgong traffc of the recever node s ncreased proportonal to the status value usng Eq. (9). The optmal use of the buffer s acheved through e (t), the buffer occupancy error. v ( t1) (1 ) v ( t) (1 g ) e ( t). (9) Therefore, the retransmsson of pacets s requred. To avod pacet drop due to channel fadng at a gven node, bac-off ntervals are adjusted sutably at the transmtter node. The bac-off nterval selecton scheme plays a major role n decdng whch node gans access to the channel. We utlze channel qualty to assess the onset of congeston. The capacty of the physcal communcaton channel s nown a pror for any networ. However, the actual capacty avalable for transmsson s nfluenced by channel fadng. Hence, an estmate of the channel capacty s modeled by the Raylegh dstrbuton. Thus, the rate s selected by modfyng the bac-off ntervals of the nodes around the congested node and the bac-off nterval of the congested node tself. To acheve successful transmsson n a shared channel, a collson-free rate allocaton s mplemented by satsfyng the suffcent condton (Cheng et al., 29) of bandwdth allocaton. Therefore, we propose a bac-off selecton algorthm such that the ln capacty s utlzed effcently. A fadng channel results n the reducton of channel qualty, whch n turn reduces the channel capacty. The bac-off nterval selecton scheme specfes the tme nterval for data transmsson. When bac-off tme ncreases (or decreases), the transmsson rate also ncreases (or decreases), whch avods the droppng of pacets due to fadng. 5 Bac-off nterval selecton Newly establshed ln for transmsson Let R denote the data generated by node and R j the data transmtted from node to node j. Therefore, the ncomng traffc of node s gven by Incomng traffc Fg. 3 New data transmt path usng the splt protocol (SP) 4.2 Congeston control usng channel qualty When selectng the outgong rate usng the above algorthms RRG and SP, the fadng channels are not consdered. However, a fadng channel s common n wreless networs. Under a fadng channel stuaton, the transmtted pacets from the node are dropped and thus wll not reach the recevng node. R j R. (1) jn Let c(t) denote the ln capacty of node. Node should satsfy the followng condtons for collsonfree transmsson: () When node s sendng data, t should not receve data from ts neghborng nodes. () When node j s recevng data from node or sendng data to node, t should not send or receve data from ts neghborng node, N j and. Condtons () and () can be represented as

8 194 Anne Uthra et al. / J Zhejang Unv-Sc C (Comput & Electron) (3): follows: () ( Rj Rj ). Node should ether transmt jn data to jn j or receve data from one of ts neghbor nodes, j. () ( R R ) ( R R ). j j j j jn jn N j Hence, the condton gven n Eq. (11) should be satsfed for collson-free transmsson as well as congeston-controlled transmsson: the ntermedate nodes wll be consdered as a metrc for the desgned congeston control methods. Fg. 4 shows the networ structure used for smulaton, whch follows a tree topology for pacet forwardng. (11) B ( R R ) ( R R ) c( t). j j j j jn jn N j The requred rate of node s a fracton of channel bandwdth. The bac-off nterval of the neghbor nodes of node, BOFF(t)=1/u (t), s adjusted to acheve the requred rate. The data rate u (t) s calculated as follows: u() t c() t Rj R B. (12) jn When the channel bandwdth drops to zero due to severe fadng, the bac-off ntervals are set to a large value to prevent unnecessary pacet transmsson. 6 Smulaton results In the smulaton the tree based routng protocol s used to transmt the data, whch s typcal for a sensor networ snce the nodes at the leaf transmt data to the sn node at the root of the tree through the ntermedate nodes. The performances of the proposed schemes RRG and SP are analyzed and compared wth DPCC and ADCC. Fnally, the bac-off selecton algorthm s combned wth RR and RRG, and the results are analyzed. Every node s ntalzed wth a data rate of 5 b/s. The buffer maxmum and α max of each node are set to 32 and 27 pacets, respectvely, wth the pacet sze of 512 bts. Parameter A s taen as the power level of the node, set to 5 dbm. Pacets dropped at the ntermedate nodes due to congeston wll cause low networ throughput and decrease energy effcency due to retransmssons. Consequently, the total number of pacets dropped at Fg. 4 Node arrangement n the smulaton topology The networ topology s scalable, and the proposed schemes can be adapted for large networ topology snce the ntermedate node that receves data pacets from three to four neghbor nodes and transmts to a sngle node s consdered for analyzng the per node pacet drop. The outgong rate of the node consdered for analyss s set to ether one thrd or one fourth of the ncomng rate of that node. 6.1 Rate regulaton Fg. 5a llustrates the queue utlzaton of the ntermedate node that receves pacets from three neghbor nodes. The rato between outgong and ncomng traffc s ntally.33;.e., the outgong traffc of the node s one thrd of the ncomng traffc. Fg. 5a shows that the buffer sze s mantaned approxmately at 16 for DPCC and 2 for ADCC, whereas the buffer sze reaches a maxmum of 3 pacets for RR and s mantaned approxmately at 24 pacets for RRG. The optmal use of the buffer at the recever node s acheved n RRG among the four methods. The non-zero status value ndcates the congeston level n the ntal tme duraton as the ncomng traffc s more than three tmes the outgong traffc (Fg. 6a). Ths results n a reducton n the source rate and an ncrease n the rate rato (Fgs. 5b and 6b). The status value becomes zero when the outgong to ncomng traffc rato s stablzed at 1, as can be seen by comparng Fgs. 5b and 6a. When the buffer reaches the maxmum threshold value, whch s 27 pacets, the status value becomes non-zero, whch

9 Anne Uthra et al. / J Zhejang Unv-Sc C (Comput & Electron) (3): agan causes a reducton n the source rate and an ncrease n the outgong to ncomng traffc rato. The rato between the outgong and ncomng rates s stablzed at 1.1 for RR and around 1 for RRG (Fg. 5b). The pacet drop s completely avoded n RRG, DPCC, and ADCC. Fg. 6b shows the outgong traffc rates of source nodes 1, 2, and 3 whle usng RRG, DPCC, and ADCC, respectvely. The source node decreases ts rate accordng to the status value and ncreases t based on the error value. The control parameter g s set to 1 for the non-zero status value and set to.7,.8, and.9 to ncrease the source rate. ADCC reduces the source rate and stablzes at 1.6 KB of the source rate, whch s the same as RR. The source rate of DPCC ranges between 1.1 and 1.3 KB. Buffer sze (pacet) Rato Smulaton tme (s) (a) (b) 6.2 Splt protocol RR RRG DPCC ADCC Smulaton tme (s) RR RRG Fg. 5 Performance of rate regulaton (RRG) for an ntermedate node that receves data from three neghbor nodes and sends out data to a sngle node (a) Queue utlzaton; (b) Rato between outgong and ncomng traffc rates The performance of SP s evaluated usng two scenaros. The recever node transmts the data to an addtonal upstream node. In addton, the outgong rate of the recever node s calculated and adjusted based on the status value. The dfference between the prevous cases and the SP s that the data rate s ncreased based on the status value. To use the buffer n an optmum way, the data rate s regulated based on the value of the buffer occupancy error (SP-regulate) as n Eq. (9). Probablty Source rate (bt/s) (a) Smulaton tme (s) (b) Fg. 6 Performance of rate regulaton (RRG) for an ntermedate node that receves data from three to four neghbor nodes and sends out data to a sngle node (a) Congeston probablty; (b) Transmsson rate of source nodes These two scenaros are compared n Fg. 7. The source rate n both SP and SP-regulate remans unchanged. The outgong rate of the recever node s adjusted so that the outgong/ncomng rato s stablzed at 1 and 1.1 (Fg. 7b) whle usng SP-regulate and SP, respectvely. The ncrease n the outgong rate corresponds to the ncrease n the status value as seen n Fg. 7c. The buffer sze s mantaned at an average of three n case of SP and mantaned approxmately at around 27 for SP-regulate, whch uses the buffer more effcently than DPCC and SP. 6.3 Bac-off nterval selecton RR RRG RRG source 1 RRG source 2 RRG source 3 DPCC source 1 DPCC source 2 DPCC source 3 ADCC source 1 ADCC source 2 ADCC source 3 1 Smulaton tme (s) Smulatons are conducted usng a 1-bps channel wth Raylegh fadng. The buffer sze, pacet sze, and routng protocol (tree topology) are the same as n the ntal setup. The channel capacty s vared

10 196 Anne Uthra et al. / J Zhejang Unv-Sc C (Comput & Electron) (3): based on Raylegh fadng. The source rate and transmtter rate are controlled by RR, RRG, and the bac-off nterval. Hence, the channel utlzaton algorthm, Mac bac-off selecton, s combned wth RR (Mac-RR) and RRG (Mac-RRG). Fg. 8 shows the buffer occupancy for Mac-RR and Mac-RRG. Buffer sze (pacet) Rato Probablty Smulaton tme (s) (a) SP-regulate SP DPCC ADCC Smulaton tme (s) (b) (c) SP-regulate SP SP-regulate SP bad channel condton. Ths s reflected by the status value as shown n Fg. 9a. Ths causes the rate rato to fluctuate, as shown n Fg. 9b. Buffer sze (pacet) Fg. 8 Queue utlzaton of bac-off nterval selecton combned wth RR and RRG for an ntermedate node that receves data from three neghbor nodes and sends out data to a sngle node Probablty (a) Mac-RRG Mac-RR Smulaton tme (s) Smulaton tme (s) (b) Mac-RRG DPCC Mac-RR Mac-RRG Mac-RR.1 Rato 1. Smulaton tme (s) Fg. 7 Performance of the splt protocol (SP) for the transmtter node that receves data from three neghbor nodes and sends out data to two upstream nodes (a) Queue utlzaton; (b) Rato between outgong and ncomng rates; (c) Probablty of congeston Buffer utlzaton s approxmately the same for Mac-RR, Mac-RRG, and DPCC. Mac-RRG regulates the outgong traffc of the source n order to meet the outgong traffc of the recever node n the presence of channel fadng and the rate rato vares wth respect to tme (Fg. 9b). The smulaton results are tested n the.5 Smulaton tme (s) Fg. 9 Performance of bac-off nterval selecton combned wth RR and RRG for an ntermedate node that receves data from three to four neghbor nodes and sends out data to a sngle node (a) Probablty of congeston; (b) Rato between outgong and ncomng traffc As a whole, the pacet drop s completely avoded n all the proposed methods, namely RRG, SP, Mac-RR, and Mac-RRG. There s no pacet drop n DPCC. However, use of the buffer s not optmal.

11 Anne Uthra et al. / J Zhejang Unv-Sc C (Comput & Electron) (3): Only 5% of the buffer s used effcently n DPCC, whereas the buffer capacty s effcently used by RRG, SP-regulate, Mac-RR, and Mac-RRG. Buffer occupancy s more than 8% n case of RRG and SPregulate. Mac-RR and Mac-RRG use the maxmum channel capacty. Data loss due to the sudden ncrease n the data rate can be avoded by the varyng nature of the threshold value. The fxed threshold n DPCC results n pacet drop when there s burstng data traffc. The source rates of RRG and SP are compared wth those of the exstng systems DPCC and ADCC n Table 2. Though RRG ncreases the data rate when there s no congeston, the source rates are reduced (Fg. 6b). As the congeston control method RRG reduces the source rate, t may not be useful for msson crtcal applcatons where the data must be generated wthn the gven tme nterval and are consdered vtal. Ths method can be used by nonmsson-crtcal applcatons. Thrty-three percent of the ntal source rate s preserved n RRG, equal to the ncomng traffc, whereas DPCC preserves only 25% of the ntal source rate. ADCC also preserves 33% of the source rate, but t does not consder channel capacty, whch leads to pacet drop. Table 2 Source rate comparson between congeston control methodologes Congeston control Fnal rate method Source 1 Source 2 Source 3 RRG SP DPCC ADDC The ntal rate s 512 for all the three sources and four methods On the other hand, SP conserves the source rate by ncreasng the recever node transmsson rates when there s congeston. Hence, 1% of the data rate s mantaned. Thus, ths method s sutable for msson crtcal applcatons such as mnng and battle feld montorng. Bac-off nterval selecton can be combned wth both RRG and SP to use the channel capacty effcently. To analyze the accuracy of the congeston predcton method, the smulatons are performed wth 1 source nodes and one recever node. Smulatons are run 15 tmes to collect suffcent statstcs for calculatng the accuracy of the congeston predcton model. The number of actve neghbor nodes sendng pacets to the recever node s vared randomly between 1 and 1. The predcton model controls the source rate n accordance to the change n ncomng pacets. In RRG, the source rates are ncreased to use the buffer occupancy. When the buffer occupancy of the recever node s mantaned at the maxmum threshold value and a sudden ncrease of ncomng pacets from more neghbor nodes, congeston occurs n the recever node and the accuracy of the predcton model becomes 98.6%. SP wors wth 1% effcency as t ncreases the outgong rate of the recever node. The outgong rate of the recever node can be ncreased n SP, tll t reaches the maxmum channel capacty. 7 Conclusons Ths paper presents a congeston predcton method and congeston control schemes where congeston s mtgated by controllng the outgong traffc of the source nodes and selectng the bac-off nterval of all nodes based on the channel condton. The ncomng rate of the recever node s mtgated by consderng buffer occupancy usng RRG, whereas SP regulates the node transmsson rate. Addtonally, the bac-off nterval selecton scheme mtgates the outgong flow of each node. Smulaton results show that the proposed schemes ncrease the per-node throughput and reduce energy n the networ by avodng pacet drops (Energy effcency s not shown explctly n ths paper. Snce there s no retransmsson of pacets, energy consumpton of every node s reduced). The source data rate s preserved n case of RRG and SP. The performance n terms of node throughput ncreases when the bac-off nterval selecton algorthm s combned wth RR and RRG because the ncomng and outgong traffc s balanced. As the buffer occupancy s mnmal n case of RR and SP, burst of data traffc can be handled well. Acnowledgements We than the faculty of Department of Computer Scence and Engneerng, SRM Unversty, for provdng constructve comments whch greatly mproved the qualty of the paper.

12 198 Anne Uthra et al. / J Zhejang Unv-Sc C (Comput & Electron) (3): References Bhargava, V., Jose, J., Srnvasan, K., et al., 212. Q-CMRA: queue-based channel-measurement and rate-allocaton. IEEE Trans. Wrel. Commun., 11(11): [do:1. 119/TWC ] Boutss, I., Kalogera, V., 212. RADAR: adaptve rate allocaton n dstrbuted stream processng systems under bursty worloads. Proc. 31st Symp. on Relable Dstrbuted Systems, p [do:1.119/srds ] Cheng, M., Gong, X., Ca, L., 29. Jont routng and ln rate allocaton under bandwdth and energy constrants n sensor networs. IEEE Trans. Wrel. Commun., 8(7): [do:1.119/twc ] Cheng, T.E., Bajcsy, R., 24. Congeston control and farness for many-to-one routng n sensor networs. Proc. 2nd Int. Conf. on Embedded Networed Sensor Systems, p [do:1.1145/ ] Felemban, E., Lee, C., Ec, E., 26. MMSPEED: multpath mult-speed protocol for QoS guarantee of relablty and tmelness n wreless sensor networs. IEEE Trans. Mob. Comput., 5(6): [do:1.119/tmc.26.79] He, T., Stanovc, J.A., Lu, C., et al., 23. SPEED: a stateless protocol for real-tme communcaton n sensor networs. Proc. 23rd Int. Conf. on Dstrbuted Computng Systems, p [do:1.119/icdcs ] Hull, B., Jameson, K., Balarshnan, H., 24. Mtgatng congeston n wreless sensor networs. Proc. 2nd Int. Conf. on Embedded Networed Sensor Systems, p [do:1.1145/ ] Hussan, F.B., Ceb, Y., Shah, G.A., 28. A multevent congeston control protocol for wreless sensor networs. EURASIP J. Wrel. Commun. Netw., 28: [do: /28/83271] Karenos, K., Kalogera, V., Krshnamurthy, S.V., 28. Cluster-based congeston control for sensor networs. ACM Trans. Sens. Netw., 4(1):5:1-5:39. [do:1.1145/ ] Kumar, R., Crepald, R., Rowahy, H., et al., 28. Mtgatng performance degradaton n congested sensor networs. IEEE Trans. Mob. Comput., 7(6): [do:1.119/ TMC.28.2] Lee, D., Chung, K., 21. Adaptve duty-cycle based congeston control for home automaton networs. IEEE Trans. Consum. Electron., 56(1): [do:1.119/tce ] Lu, C., Blum, B.M., Abdelzaher, T.F., et al., 22. RAP: a real-tme communcaton archtecture for large-scale wreless sensor networs. Proc. 8th IEEE Real-Tme and Embedded Technology and Applcatons Symp., p [do:1.119/rttas ] Mao, Z., Kosal, C.E., Shroff, N.B., 212. Near optmal power and rate control of mult-hop sensor networs wth energy replenshment: basc lmtatons wth fnte energy and data storage. IEEE Trans. Automat. Contr., 57(4): [do:1.119/tac ] Morell, A., Vcaro, J.L., Vlajosana, X., et al., 211. Optmal rate allocaton n cluster-tree WSNs. Sensors, 11(4): [do:1.339/s ] Rangwala, S., Gummad, R., Govndan, R., et al., 26. Interference-aware far rate control n wreless sensor networs. Proc. Conf. on Applcatons, Technologes, Archtectures, and Protocols for Computer Communcatons, p [do:1.1145/ ] Ren, F., He, T., Das, S., et al., 211. Traffc-aware dynamc routng to allevate congeston n wreless sensor networs. IEEE Trans. Parall. Dstr. Syst., 22(9): [do:1.119/tpds ] Teo, J.Y., Ha, Y., Tham, C.K., 28. Interference-mnmzed multpath routng wth congeston control n wreless sensor networ for hgh-rate streamng. IEEE Trans. Mob. Comput., 7(9): [do:1.119/tmc.28.24] Uthra, R.A., Raja, S.V.K., 211. PACC: probablstc approach for congeston control n wreless sensor networ. CT Int. J. Wrel. Commun., 3: Uthra, R.A., Raja, S.V.K., 212. QoS routng n wreless sensor networs a survey. ACM Comput. Surv., 45(1): [do:1.1145/ ] Wan, C.Y., Esenman, S.B., Campbell, A.T., 23. CODA: congeston detecton and avodance n sensor networs. Proc. 1st Int. Conf. on Embedded Networed Sensor Systems, p [do:1.1145/ ] Wang, C., Sohraby, K., Lawrence, V., et al., 26. Prortybased congeston control n wreless sensor networs. Proc. IEEE Int. Conf. on Sensor Networs, Ubqutous, and Trustworthy Computng, p [do:1.119/ SUTC ] Wu, Y., Yuan, Z., Wu, Y., 213. A predctve control strategy for networed control system wth destablzng transmsson factors. Adv. Sc. Eng. Med., 5(1):83-9. [do: /asem ] Zawodno, M., Jagannathan, S., 27. Predctve congeston control protocol for wreless sensor networs. IEEE Trans. Wrel. Commun., 6(11): [do:1.119/twc ] Appendx: Proof of Eq. (3) Let P(α(t)+1) be the probablty of a node havng one pacet greater than the threshold value at tme t. P(α(t)+1)=p(α(t)+1), where p(α(t)+1) s the probablty of one pacet arrval, calculated usng Eq. (3). Smlarly, P(α(t)+2)=p(α(t)+2)+p(α(t)+1 α(t)+1). The probablty that the node contans more than two pacets above the threshold value depends on two new arrvals of pacets or one new arrval provded that there exsts one pacet more than the threshold value. When generatng such functons, we have (for

13 Anne Uthra et al. / J Zhejang Unv-Sc C (Comput & Electron) (3): smplcty, α(t) s replaced as α) P(α+1)=p(α+1), P(α+2)=p(α+2)+p(α+1 α+1), P(α+3)=p(α+3)+p(α+1 α+2)+p(α+2 α+1), P(α+)=p(α+)+p(α+1 α+( 1))+p(α+2 α+( 2)) + +p(α+( 1) α+1). In general, 1 P( ) p( ) p n ( n), n1 where =2, 3,, BUFMAX α wth =1 as the ntal condton.