Research Article REALFLOW: Reliable Real-Time Flooding-Based Routing Protocol for Industrial Wireless Sensor Networks

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1 Hinawi Publishing Corporation International Journal of Distribute Sensor Networks Volume 2014, Article ID , 17 pages Research Article REALFLOW: Reliable Real-Time Flooing-Base Routing Protocol for Inustrial Wireless Sensor Networks Kan Yu, 1 Zhibo Pang, 2 Mikael Gilun, 2 Johan Åkerberg, 2 an Mats Björkman 1 1 School of Innovation, Design an Engineering, Mälaralen University, P.O. Box 883, Västerås, Sween 2 ABB AB, Corporate Research, Tegnér, Västerås, Sween Corresponence shoul be aresse to Kan Yu; kan.yu@mh.se Receive 12 December 2013; Accepte 23 May 2014; Publishe 15 July 2014 Acaemic Eitor: Shancang Li Copyright 2014 Kan Yu et al. This is an open access article istribute uner the Creative Commons Attribution License, which permits unrestricte use, istribution, an reprouction in any meium, provie the original work is properly cite. Wireless technologies have been increasingly applie in inustrial automation systems ue to flexible installation, mobility, an cost reuction. Unlike traitional wireless sensor networks (WSNs), inustrial wireless sensor networks (IWSNs), when expaning from wireless monitoring to wireless control, have more stringent requirements on reliability, real-time performance, an robustness in a number of inustrial applications. Successive transmission failures or ealine misses in these applications may severely egrae the control quality an result in serious economic losses an safety problems. Therefore, when eploying IWSNs in harsh inustrial environments, to achieve reliable an eterministic en-to-en transmissions is critically important. In this paper, we explain the primary challenges of esigning appropriate routing protocols an present a reliable real-time flooing-base routing protocol for IWSNs (REALFLOW). Instea of traitional routing tables, relate noe lists are generate in a simple istribute manner, serving for packet forwaring. A controlle flooing mechanism is applie to improve both reliability an real-time performance. A seamless transition in the event of topology change can be achieve by REALFLOW. Performance evaluations via simulations verify that significant improvements of reliability, real-time performance, an network recovery time can be achieve by REALFLOW, compare with traitional routing protocols. 1. Introuction Nowaays, wireless sensor networks (WSNs) have been exhibiting their attractive avantages over traitional wire counterpart for inustrial automation systems, such as the avoiance of cabling an flexible installation. Inustrial wireless sensor networks (IWSNs) can serve a number of purposes, such as monitoring an control. Currently, wireless monitoring via IWSNs for inustrial applications has been wiely applie [1]. Nevertheless, wireless control, as an essential part of automation, is still lacking support in IWSNs [2]. Compare with wireless monitoring, ata transmissions for wireless control in many inustrial applications shoul be sufficiently reliable [3] an eterministic with the latency of the orer of secons, or even millisecons [2]. Transmission failures or ealine misses may result in isturbances to the process, egraation of the overall control performance, an even more serious economic losses or human safety problems. Once both wireless monitoring an control are fullysupporte,theboomingevelopmentofiwsnapplications can be expecte. Besies high reliability an realtime performance, low energy consumption is also an important issue [4 6]. However, compare with the former two requirements, reucing energy consumption shoul not be prioritize as high as reliable an eterministic transmission for ifferent inustrial applications. Compare with environments for traitional WSNs, inustrial environments for IWSNs are harsher an more ynamic ue to a great number of metallic surfaces, extreme temperature, high vibrations, an mobility of noes an other objects [7]. Measurements from [8, 9] have shown that eep faing an shaowing in inustrial environments may result in extremely low receive signal strength inicator (RSSI) values with high variances or even packet losses. Moreover, authors in [7, 10] pointe out coexisting communication systems in the same frequency ban as another major source of isturbances to wireless inustrial applications. Hence, the

2 2 International Journal of Distribute Sensor Networks major challenge is to achieve reliable an eterministic ento-en transmissions in inustrial environments by using IWSNs. Currently, several stanars have been publishe for process measurements an control applications. Among them, Zigbee [11] isshowntobeunsuitableforinustrial automation because there are no frequency iversity, no path iversity, an the MAC unreliability problem [12, 13]. WirelessHART [14] an ISA a[15] are another two stanars esigne for process automation (PA). Although some successful eployment instances exhibit the confience onthesestanars,thereisstillalongwaytogotofulfill the reliability an real-time performance requirements for most of inustrial applications [2]. The improvement can be achieve on ifferent layers. On existing evices, limite efforts can be taken on the physical layer ue to restrictions impose by harware an/or software limitations. On the MAC layer, a number of techniques, such as channel hopping anblacklisting,areusetoimprovereliability,butinharsh inustrial environments, transmission failures still occur quite often. Although automatic repeat-request (ARQ) is applie in most of the stanars, an it is a straightforwar metho to increase reliability, the real-time performance is egrae ue to retransmissions. The previous empirical results [16] exhibit serious consequences such as network congestions, cause by excessive unexpecte retransmissions. On the network layer, the routing protocol plays an extremely important role in achieving high reliability an low latency [1], but the current stanars fail to provie sufficient guiance on how to guarantee reliable an real-time transmission by using appropriate routing protocols. Although a number of research efforts have been taken to esign a reliable real-time routing protocol for IWSNs [17 22], there is still no sufficient evience to show that the problem has been fully solve. There are several challenges to esign an appropriate routing protocol for IWSNs. Firstly, if high reliability cannot be achieve by the lower layers, multipath iversity can be applie by routing protocols on the network layer to increase reliability. Retransmission is an effective metho to increase reliability, but the transmission elay is also prolonge. For many inustrial applications where har ealine is strictly require, outate packets are of limite use for their estinations. Therefore, the secon challenge is that real-time performance shoul be provie by a routing protocol. Thirly, routing protocols shoul be tolerant of suen topology changes, such as link failures an noe halting. Once the topology changes, the routing protocol shoul provie an alternative path as soon as possible; otherwise ata transmissions may be terminate uring the path recalculation perio. The last challenge is that the workloa of noes shoul be also consiere. Different from traitional WSNs, the network structure of an IWSN is typically centralize, since operators in the central control room must have the knowlege of the status of the whole network. Thus, the gateway or network manager, as the central evice, is involve in most of network activities. The gateway or network manager shoul not spen too long time on calculating routing information, since partial communication in the network may be halte uring the calculation perio ue to the outate routing information. Therefore, when esigning a routing protocol for IWSNs, these challenges shoul be aresse. In this paper, we propose a reliable real-time flooingbase routing protocol for IWSNs (REALFLOW) to aress all challenges mentione above. REALFLOW consists of a routing establishing an maintenance part an packet forwaring part. The gateway perioically broacasts iscovering messages, name as list-upate messages, to iscover the current network topology. After list-upate messages are receive, all noes shoul sen corresponing responses, name as list-upate messages, back to the gateway. The listresponse message transmission stage has two purposes: one is that it helps the gateway obtain the current network topology information;theotheristhatrelatenoelistsaregenerate in all noes base on receiving list-response messages from other noes. For both uplink an ownlink transmissions, packets are forware accoring to the relate noe lists in all intermeiate noes. In orer to improve reliability, REALFLOWisbaseonthecontrolleflooingmechanism to provie multipath iversity. The real-time performance is also consiere, so outate packets are automatically iscare in intermeiate noes. Due to reunant paths an flooing mechanism, REALFLOW can be tolerant of parts of network topology changes. Since relate noe lists are istributively generate in all noes, the workloas of the gateway are greatly reuce. Due to flooing mechanism, more resources may be require than traitional routing protocols in WSNs. However, as we emphasize previously, in many inustrial applications, especially those from inustrial automation systems, reliability an real-time performance shoul be prioritize for IWSNs, rather than energy consumption. Moreover, it is proven that energy consume by local processing, such as sensoring, ominates the overall energy consumption [23]. Although aitional communication is introuce in the network ue to flooing, the increment of energy consumption cause by aitional communication is not obvious. Therefore, our work briges the gap of current research works to provie reliable an real-time en-to-en transmissions in IWSNs. The rest of this paper is organize as follows: Section 2 presents the previous work on routing protocols in WSNs. In Section 3, we briefly escribe the current IWSN architecture as backgroun knowlege. In Section 4, we outline the propose routing protocol REALFLOW. The simulation settings an scenarios are escribe in Section 5, followe by the simulation result analysis in Section 6. Finally,weconclue the paper in Section Relate Work Designing reliable routing protocols in WSNs, as one of the most challenging research topics, has attracte a great number of research interests. Traitional routing protocols, such as ynamic source routing protocol (DSR) [24] an a hoc on-eman istance vector protocol (AODV) [25], have been successfully applie in traitional WSNs, such as

3 International Journal of Distribute Sensor Networks 3 Zigbee networks. For this set of routing protocols, route iscovery messages are broacast to iscover all available paths. In orer to maintain the upate routing information, all noes have to frequently exchange information for being aware of channel status. If any link is broken or noe halts, all corresponing noes have to recalculate the routing information an try to establish another new path. During this perio, parts of the network may not work properly, which makes the network suffer from higher latency an lower reliability. Several extensions of traitional routing protocols were also propose, for instance, the a hoc oneman multipath istance vector protocol (AOMDV) [26], theahocon-emanistancevectormultipathrouting protocol (AODVM) [27], an split multipath routing (SMR) [28]. These extensions inten to improve reliability by ientifying multiple noes or ege-isjoint paths. However, primary rawbacks, such as excessive control messages an unpreictable network recovery time, still exist. Besies these extensions, multipath routing protocols have been extensively stuie in other research works aiming for high reliability [17 20]. Authors in [19] split each packet into several subpackets by using erasure coes an sen them instea of the whole packet. This scheme might be effective in traitional WSNs, but in IWSNs there exist very short packets with the strict ealine; this scheme may be harly use in IWSNs, since the real-time performance is not fully consiere. A reliable information forwaring using multiple paths (ReInForM) protocol propose in [20] can achieve controlle reliability by sening reunant copies of a packet alongmultiplepaths.however,thisprotocolcanonlybeuse uner a number of assumptions. For instance, every noe has knowlege of the local channel error an is able to compute the information content an importance of sense events, which can harly be obtaine in many practical scenarios. Unlike traitional routing protocols an all their extensions,flooingisaneffectiveapproachtoincreasereliability by multipath iversity. However, previous flooing-base routing protocols fail to satisfy inustrial requirements. Authors in [29] try to avoi the rawbacks of flooing by ranomizing the selection of retransmitters. Their approach may harly be use for a centralize TDMA-base IWSN, since it may bring ifficulties for scheuling. A ranom routingstrategybaseonflooingproposein[30]aimsforlow energy consumption. However, accoring to the evaluation results, even higher en-to-en transmission latency may be introuce, although lower energy consumption is provie by their propose routing algorithm. Except for reliable routing protocols in traitional WSNs, there also exist a number of newly propose routing protocols to aress the challenges from IWSNs. MERLIN protocol propose in [21] utilizes multicast for both uplink an ownlink transmissions to improve low latency an energy consumption, but high reliability is not the main scope of this protocol. Authors in [31] propose EARQ, an energy aware routing protocol also for reliable an real-time communications in IWSNs. Next hop selections are base on the estimations of energy consumption, reliability, an ealines. However, accoring to their evaluation settings, this protocol may be more suitable for WLAN or Zigbee networks, not centralize TDMA IWSNs. Both authors in [32, 33]propose a two-hop information-base routing protocol, aiming for enhancing real-time performance with energy efficiency. The routing ecision in [32] isbaseonthetwo-hopvelocity integrate with energy balancing mechanism, whereas the routing ecision in [33]isbaseonthenumberofhopsfrom source to gateway an two-hop information. However, reliability is not fully consiere in these two protocols. Authors from [22] also propose an entire reliable graph routing scheme for broacast, uplink, an ownlink transmissions in IWSNs with promising evaluation results. One rawback of their routing protocol is the too high workloa of the gateway or the network manager to calculate all routing graphs [34]. It is notable that energy efficiency an low power consumptionhavebeenstressebyanumberofprevious research works. However, for many time-critical inustrial applications, the high reliability an real-time performance are prior to all other requirements. If there are many packet losses or packets cannot arrive at their estinations before ealines, optimizing energy efficiency an reucing power consumption become meaningless to the whole system. Reliability, availability, an usability of IWSNs shoul take top priority for those inustrial applications. Therefore, ifferent from previous research works, we mainly focus on improving reliability an real-time performance for IWSNs. 3. Inustrial Wireless Sensor Network Architecture Unlike traitional WSNs, the network structure of an IWSN is generally centralize. Operators in the central control room shoul have the knowlege of the status of the whole network. A comprehensive IWSN usually contains a number of components, such as network manager, access point, security manager, an fiel evices. However, the scope of this work is to esign routing protocol for reliable an realtime transmissions in IWSNs, so simplifie components are consiere in this work. Figure 1 shows a typical topology of an IWSN consiere in this work. Three basic evice types areinvolveintheformationofaniwsn. (i) Gateway: it is responsible for managing the whole network, incluing ientifying routing, istributing resources, an scheuling ecision making. It also connects the control system to fiel evices. (ii) Sensor noes: as one type of fiel evices, the responsibility of a sensor noe is to collect all kins of measurement ata an uploa to the gateway. (iii) Actuator noes: as another type of fiel evices, an actuator noe performs basic functions of actuating after receiving ata or commans from the gateway. As shown in Figure 1, in an inustrial process control system, a programmable logic controller (PLC) connecte to a gateway perioically acquires measurement ata from sensor noes at a certain refresh rate. After executing the control applications, the PLC perioically sens the output values to actuator noes for actuating. Usually sensor an actuatornoesareeployeinascatteremannerina

4 4 International Journal of Distribute Sensor Networks 800xA process automation controller Gateway Actuator information. To avoi this problem, we apply a ifferent mechanism an introuce new routing parameters. In this section, we present the etails of REALFLOW. We first introuce the efinitions an notations for our propose protocol. Then we escribe the route establishing an maintenance metho an packet forwaring metho in Section 4.2 an Section 4.3, respectively, followe by an example of our propose scheme for comprehensive unerstaning. Finally, a theoretical analysis is given in Section xA process automation controller Sensor Figure 1: A typical topology of an IWSN. large area. Accoring to ifferent wireless channel conitions, uplink an ownlink communication may involve one hop or multihop packet transmissions. In orer to provie reliable an real-time communication for inustrial applications, an appropriate routing protocol serving packet forwaring plays an extremely important role in fining available transmission paths. We shoul also notice that in many inustrial applications the ata transmission perio is extremely short, for example, 250 ms or 500 ms [2]. Due to the scheulability of transmission timing slots, the network size an noe hop numbers are severely limite. Therefore, in many inustrial applications an IWSN containing hunres of noes an a noe with many hops away from the gateway can harly be seen in those inustrial applications. 4. Propose Reliable Real-Time Flooing- Base Routing Protocol To achieve higher reliability, flooing metho is use in our propose routing protocol to introuce multipath iversity. However, the inherent rawback of uncontrolle flooing is that the network resources will be rapily exhauste. Thus, our propose routing protocol is base on a controlle flooing mechanism in which flooing behaviors are severely restricte within a certain range, so the usage of network resources can be much more efficient. To assist packets in arriving at their estinations, ifferent routing protocols apply ifferent mechanisms. For instance, routing tables are commonly use in a great number of traitional routing protocols, whereas a graph ID embee in a message is use by graph routing protocols for forwaring messages in the stanarwirelesshart.aswementionepreviously,togenerate all routing or graph information at the gateway or network manager may require extremely long computation time an lea to overwhelming workloa of these evices [34]. Aitionally, once the network topology changes, routing tables an graphs nee to be recalculate again. During the calculation an recalculation perio, partial communication inthenetworkmaybeterminateuetotheoutaterouting 4.1. Definitions an Notations. Provie an IWSAN consists of m noes (sensors an actuators), the noe set N is written as N = {N 1,N 2,...,N m }. In orer to control the flooing behavior efficiently, we introuce a new concept, calle relevant, in our propose routing protocol. This concept is efine as below. Definition 1. If the noe N i (1 i m)in the network is involve in forwaring packets transmitte between the noe N k (1 k m,k=i) an the gateway G, thenoen i is a relevant noe to the noe N k. In our propose routing protocol, a noe is only able to forwar packets sent from or to its relevant noe; otherwise packets from or to irrelevant noes will be roppe by this noe. In orer to istinguish relevant noes from irrelevant noes, each noe in the network shoul maintain a list, nametherelatenoelistl.therelate noe list of the noe N k in the network is efine as L k.thelistl k consists of a set of noes with their source aresses. Provie the length of the list L k is p, L k ={A 1,A 2,...,A p }. Definition 2. The source aress noe N i is A i,anthenoe N k has the list L k.ifa i L k, N i is a relevant noe of the noe N k. Sensor an actuator noes are usually eploye ispersely in a large area. Due to the communication istances, interferences, obstacles, an other aspects, packets may require more than one transmission hop to arrive at the estination. To escribe the network topology, we efine parent, chil, an sibling noes as below. Definition 3. If N i is the relevant noe of N k,ann i has smaller hop number away from the gateway than N k, N i is aparentnoeofn k an N k is a chil noe of N i. Definition 4. If N i istherelevantnoeofn k,ann i has same hop number away from the gateway than N k, N i is a sibling noe of N k. The key point of REALFLOW is to obtain an maintain the relate noe list L in each noe. Several steps are require before regular ata transmissions. The establishment an maintenance of routing information an relate noe lists are escribe in etail in the following subsections Establishing an Maintaining Routes. In orer to obtain thecurrentnetworktopologyforthegatewayangenerate

5 International Journal of Distribute Sensor Networks 5 relatenoelistsl in all noes in the network, the gateway perioically broacasts iscovering messages. The iscovering messages are name as list-upate messages. Each listupate message contains three important parameters. (1) r pkt (the absolute accumulate RSSI): it is initialize as zero. When a noe receives a message, the RSSI value obtaine from the raio chip register will be accumulatively ae to the previous value. Thus, this parameter is upate at every hop. (2) h pkt (the packet hop): it is initialize as zero. Every time when a message goes through an intermeiate noe, this parameter is increase by one. (3) A prev (the previous noe aress): every time when a message is forware, the aress of this forwaring noe will be save in this parameter. When a noe receives a list-upate message, it will rebroacast the message as neee. Before the message is forware, all those three parameters will be upate. The noe also nees to recor the information obtaine from the list-upate message. In orer to establish the relate noe list, each noe nees to maintain three important parameters: (1) h noe (the noe hop number): this parameter represents the hop number between the current noe an gateway, (2) C (the number of receive list-upate messages): oncethenoereceivesanewlist-upatemessage, this parameter shoul be increase by one, (3) V (the parent or sibling noe recor ata set): once a noe receives a list-upate message from its parent or sibling noe, the aress of this noe, as well as the r pkt value, will be ae to this ata set. This recor history is extremely important for the next relate noe list generation stage. Once the gateway broacasts a list-upate message, this message shoul be rebroacast by all other noes until it propagatesintheentirenetwork.therebroacastingproceure is summarize in Algorithm 1. The notations mentione in the algorithm are summarize as below (1) T link : a preefine link threshol to filter out packets with weak signal strength, (2) r link : the absolute value of measure RSSI of the current receive message, (3) A current : the aress of the current noe, (4) K max : a preefine maximum allowe parent noe number. In orer to establish appropriate routing paths, the receive signal strength of the list-upate message r link shoul be larger than T link ; otherwise this path is consiere to be unstable an unreliable. T link is preefine accoring to ifferent wireless environments. In orer to prevent listupate messages from enless rebroacasting, h pkt will be checke at every hop. If h pkt is larger than h noe,itmeans (1) Extract r pkt, h pkt,ana prt from the list-upate message (2) Obtain r link from RF part (3) if r link <T link then (4) if h pkt h noe then (5) r pkt =r pkt +r link (6) C=C+1 (7) if C<K max then (8) if h pkt =h noe then (9) V = V {A prt,r pkt, sibling} (10) else if h pkt <h noe then (11) V = V {A prt,r pkt, parent} (12) h noe =h pkt +1 (13) en if (14) else if C K max then (15) if r pkt < max(v(, r i,)) then (16) V = V {,r i,} (17) if h pkt =h noe then (18) V = V {A prt,r pkt, sibling} (19) else if h pkt <h noe then (20) V = V {A prt,r pkt, parent} (21) h noe =h pkt +1 (22) en if (23) else (24) Drop (25) en if (26) en if (27) h pkt =h pkt +1 (28) A prt =A current (29) Forwar (30) else if h pkt >h noe then (31) Drop (32) en if (33) else if r link T link then (34) Drop (35) en if Algorithm 1: List-upate message rebroacasting proceure. that this message comes from a chil noe an shoul be iscare immeiately. Different K max value inicates ifferent multipath transmission properties. If more parent an sibling noes are selecte, more paths will be involve in ata transmissions. Thus, by varying K max, the transmission reliability may also be change. Although enlarging K max will increase the reliability performance, more network resources are also neee ue to incremental transmission paths. Therefore, there is a trae-off between the reliability an network resource consumption when etermining the value K max. After receiving list-upate messages, each noe shall broacastarespontothegateway,namelist-response messages. A list-response message also inclues three necessary parameters: (1) s pkt (the packet sequence number): as a unique inicator to avoi packet uplications,

6 6 International Journal of Distribute Sensor Networks (1) Extract s pkt, A src an N fw from the list-response message (2) if {s pkt,a src } H then (3) H = H {s pkt,a src } (4) if A current N fw then (5) if A current is a parent noe type then (6) N fw = V current (7) else if A current is a sibling noe type then (8) Fin {A i,r i, parent/sibling} where r i is minimum in V current (9) N fw ={A i,r i, parent/sibling} (10) en if (11) L = L A src (12) Forwar (13) else if A current N fw then (14) Drop (15) en if (16) else if {s pkt,a src } H then (17) Drop (18) en if Algorithm 2: List-response message forwaring an relate noe list generation proceure. (2) N src (the accepte parent an sibling noe set): containing the aresses of accepte parent an sibling noe obtaine from previous list-upate messages, (3) N fw (the next hop noe set): containing the next hop noe aresses, as well as noe types. In orer to ientify ifferent list-response messages from asamenoe,thesequencenumbers pkt is introuce an always increase by one after each response. N src shoul be reporte to the gateway to calculate the current network topology, an N fw is use to fin out the next hop. The list-upate message transmission has two purposes. Firstly, the gateway is able to obtain sufficient information to calculate the current network topology accoring to the content of these messages. Seconly, the relate noe list L generation in each noe is base on list-response messages. Both list-response messages forwaring proceure an relate noe list-generation proceure in a noe are summarize in Algorithm 2. The notations that appeare in the algorithm are summarize as below: (1) A src : the source aress of the list-response message sent by the noe N src, (2) V current : the parent an sibling noe set of the current noe obtaine from the previous list-upate message stage, (3) H: a history table to recor all seen messages in orer to prevent uplicate forwaring. Since the combination of s pkt an the source aress of the message can ientify ifferent list-upate messages, if these messages are not foun in the current history table, they can be accepte for the next step. N fw contains the next hop noe aresses. Thus, if the current noe aress A current is seen in N fw,thismessageisallowetobeforware.the relationship between the current noe an previous noe etermines the next hop behavior. If the current noe is a parent noe of the previous noe, N fw is simply replace by V current. If the current noe is a sibling noe of the previous noe, the current noe nees to fin one of its parent or sibling noes with the minimum accumulative RSSI r i as the next hop noe. The last step before forwaring is to a the source aress of this list-upate message A src to the local relate noe list L.Thearrivalrange of thelist-upate message irectly etermines the flooing range of the noe N src. In orer to limit the flooing range an avoi excessive packet flooing, the arrival range of the list-upate message shoul be severely restricte. If the list-upate message arrivesatasiblingnoe,thismessageisstillthesamehops away from the gateway. Therefore, we only choose the noe with the minimum accumulative RSSI to be the next hop to prevent enlarging the flooing range. After this stage, the gateway is able to get the latest network topology, an a relate noe table is, respectively, generate in each noe. Accoring to the existing IWSN stanars, IWSNs are typically centralize, an TDMA mechanism is applie on the MAC layer. Thus, a TDMA scheuling ecision shall be mae by the gateway, so each noe in the network has the knowlege of its available timeslots for sening an receiving packets. Since esigning a TDMA scheuling schemeisoutofthescopeofthispaper,asimplifietdma scheuling is utilize. Apparently, a parent noe requires more timeslots than its chil noe, since the parent noe is involve in forwaring packets for its chil noe. The number of timeslots require by a parent noe epens on thenumberofitschilnoes,theatarefreshrateofitschil noes, an the ata refresh rate of itself. Therefore, to make an appropriate scheuling ecision, the network topology nees to be generate at the gateway first. As we efine previously, an IWSN consists of m noes, excluing the gateway. A k is the aress of the noe N k. N k istheparentnoesetofthenoen k,obtainefromits

7 International Journal of Distribute Sensor Networks 7 (1) n=m (2) M = M {A gw } (3) while n =0 o (4) for all A i such that A i M o (5) for all N k, k=1,...,m o (6) if A i N k then (7) Set N k as a chil or sibling of N i. (8) N k = N k {A i } (9) if N k =0then (10) n=n 1 (11) en if (12) if A k M then (13) M = M {A k } (14) en if (15) en if (16) en for (17) en for (18) en while Algorithm 3: Network topology tree generation. list-response message. M is the noe set alreay processe by the topology generation algorithm. The generation of the network topology tree is summarize in Algorithm 3. The generation proceure is quite straightforwar, since sufficient information can be extracte from list-response messages to escribe the relationship of two noes in the network. Afterwars the simplifie scheuling scheme can also be calculatebaseonthelatestnetworktopology.itisnotable that exploring the optimal scheuling scheme of the routing protocol to maximize the resource efficiency is out of the scope of this paper. Oncethescheulingecisionismae,thegatewaysens a list-confirme message to each noe as a new routing confirmation. Moreover, the list-confirme message also contains the latest scheuling ecision. The forwaring scheme of listconfirme messages is ientical to the packet forwaring scheme, which is escribe in etail in the next subsection. Once a noe receives its list-confirme message, both new relatenoelistanscheulingecisionwillbeapplie. As a centralize network, before sening out list-confirme messages,thegatewayisabletochangethenetworktopology. If the gateway intens to change the topology, it nees change thecalculatetopologyinformationlocally.thenthegateway encloses a comman in the list-confirme message to inform the corresponing noe to elete a certain noe in its relate noe list L Packet Forwaring Metho. After all noes successfully generate an apply their latest relate noe list an scheuling ecisions, they are able to forwar packets. As we escribe before, the inherent rawback of flooing transmission is excessive packet forwaring. Thus, relate noe lists are use in our propose routing protocol to restrict the flooing range. Now we can formulate packet forwaring criteria of REALFLOW in etail. The first forwaring criterion is written as A L. (1) This criterion inicates that if a noe receives a packet with the source aress or estination aress A, whichcan be foun in its local relate noe list L, thispacketcanbe consiere to be forware. However,sinceflooingisapplieratherthanunicasting, uplicate packets may appear. In orer to filter out uplicate packets an avoi unnecessary packet forwaring, each outgoing ata packet must contain a unique ientifier. AuniquepairP is efine as the ientifier. P consists of the sequence number s pkt an the source aress A src of the current noe, P=(s pkt,a src ). After each ata transmission, the sequence number s pkt is increase by one. Each noe shoul also maintain a history table H. Once a new ata packet is receive, its P is recore in H.IfP appears in H,it inicates that a uplicate packet is receive. Then, the secon forwaring criterion can be expresse as P H. (2) As mentione previously, real-time performance is one of the stringent requirements of IWSNs. Thus, ata packet shoul arrive at its estination before the ealine. In many inustrial applications, outate packets are of limite use for inustrial systems. It is more reasonable to iscar outate packets in the intermeiate noe to save the network resources for other transmissions. The final forwaring criterion of our scheme is T age <T refresh, (3) where T age is the packet age an T refresh is the refresh interval of its originating noe. Theoretically the refresh interval T refresh equals the packet ealine, so a packet shoul arrive at the estination within T refresh. The packet age shoul be checke before being sent out, since packet buffering in the intermeiate noe may introuce aitional elay. Finally,theentirepacketforwaringschemeforboth uplink an ownlink is summarize in Algorithm 4. So far the etails of our propose routing protocol REALFLOW have been presente. In summary, four proceeing steps an four message types are involve in REALFLOW.First,thegatewaybroacastslist-upatemessages to iscover the latest network topology. Then each noe replies with a list-response message to report their parent noes an a itself into the relate noe lists in all intermeiate noes. After receiving all responses from all noes, the gateway is able to obtain the latest network topology an calculate the latest TDMA scheuling ecisions an sen them back to all noes by list-confirm messages. Finally, all noes are able to communicate with the gateway baseoncontrolleflooingaccoringtoourforwaring rules for both uplink an ownlink. A summary of all steps an corresponing messages is shown in Figure 2.

8 8 International Journal of Distribute Sensor Networks Noe 1 relate list table: L 1 Downlink packet forwaring base on controlle flooing Noe n relate list table: L n Uplink packet forwaring base on controlle flooing Step 1: network iscovering List-upate message Step 2: iscovering reply List-response message Step 3: network confirming List-confirm message Step 4: ata transmission Regular ata message GW Figure 2: REALFLOW proceeing summary. (1) Extract A src, A st an s pkt from the receive packet (2) if A src =A gw then (3) if A st L then (4) Construct the unique pair P=(s pkt,a st ) (5) if P H then (6) Upate T age (7) if T age <T refresh then (8) H = H {P} (9) Forwar (10) en if (11) en if (12) en if (13) else if A st =A gw then (14) if A src L then (15) Construct the unique pair P=(s pkt,a src ) (16) if P H then (17) Upate T age (18) if T age <T refresh then (19) H = H {P} (20) Forwar (21) en if (22) en if (23) en if (24) en if (25) Drop Algorithm 4: Packet forwaring proceure Example: Reliable Real-Time Flooing-Base Routing Protocol. Figure 3 illustrates an example of REALFLOW. Transmissions between Noe 7 an the gateway are escribe in etail. In the beginning, the gateway broacasts a listupate message. Propagation routes of the list-upate messagetonoe7areshownassoliarrowsinthefigure. Numbers on the arrows are the absolute values of RSSI from one noe to another. We efine K max as 2, since multipath iversity is achieve an not too much network resource is require. Noe 7 gets five copies of list-upate messages from Noes 1, 2, 3, 6, an 8. Compare with other noes, r pkt from Noes 1 an 2 are minimum. Thus, Noe 7 chooses GW Figure 3: An example of REALFLOW: route establishment an packet forwaring between the gateway an Noe 7. Noes 1 an 2 to be its parent noes. From the figure, Noe 1 consiers the gateway an Noe 2 as its parent noes, an Noe 2 selects the gateway an Noe 3 to be its parent noes. After receiving the list-upate message, Noe 7 shall reply a list-response message to the gateway. When Noes 1 an 2 receive the list-response message, since its aress is inclue in N fw, so they a the aress of Noe 7 to their relate noe lists an forwar it to their parent noes. After all listresponsemessagesaresenttothegateway,relatenoelists are generate in all noes, an the gateway is able to calculate the latest network topology. Then the gateway nees to sen list-confirme messages to all noes with the confirmation of relate noe lists an scheuling ecisions. After the relate noe lists are confirme by the gateway, when Noe 7 sens a packet to the gateway by flooing, only Noes 1 an 2 are able to forwar the packet, an other noes will rop the packet automatically. By this means, packets aretransmittefromnoe7tothegatewayviatwopaths, 5 4

9 International Journal of Distribute Sensor Networks 9 so the multipath iversity is achieve. Moreover, flooing transmission is severely controlle an restricte within a small range to avoi excessive packet forwaring. Usually, many research works reach a consensus that flooing metho is much less efficient at packet transmission than unicasting, since much more network resources may be require for flooing. However, it is not always the truth. In this example, as TDMA is use on the MAC layer, for uplink, only 3 timeslots are require to achieve two-path iversity. For traitional unicast routing protocol, four timeslots are neee to achieve the same iversity. A similar situation happens in the reverse irection for the ownlink. More analyses of require timeslots, as well as complexity an reliability, are iscusse in the following subsection X 1 GW Y 1 Y 4.5. Theoretical Analysis of REALFLOW. In this subsection, the complexity of the algorithm, the network resource efficiency, an reliability performance are analyze by theory Algorithm Complexity. REALFLOW consists of four algorithms as shown above. Algorithms 1, 2, an 4 run istributively in each noe in the network without requiring the gateway to participate, incluing relate noe list generation. The complexity of these three algorithms is T(n) = O(1), where n is the number of noes in the network excluing the gateway. In a centralize IWSN, the gateway is a bottleneck for the whole network. Thus, these algorithms release the gateway from high workloa. Even if the network size increases significantly, the workloa from these three algorithms for each noe incluing the gateway remains the same. Only Algorithm 3 shoul run at the gateway to generate the network topology. The parameter K max, which is the maximum allowe parent noe number, can affect the complexity of the algorithm from Lines 4 an 5. It is a preefine value an can be consiere as a constant. In the worst case, there is only one noe irectly connecte to the gateway, an all other noes are able to fin K max parent noes. Therefore, the worst-case complexity of Algorithm 3 is: T (n) =5K max n K max n+2n+8 K max =O(n 2 ). (4) X Figure 4: An example of a multihop IWSN for analysis Network Resource Efficiency. Regaring a general case, a TDMA IWSN consisting of X Ynoes is constructe, as shown in Figure 4. On each hop, there exist Y noes, an the furthest noes are X hops away from the gateway. For simplicity, we assume that a noe is able to communicate with any parent noe, chil noe, or sibling noe, an each noe perioically communicates with the gateway with the same refresh interval. Here, network resource efficiency refers to the usage of time slots. Flooing-base transmission is eeme to be inefficient ue to excessive transmissions. In orer to explore the network resource efficiency of REALFLOW, we inten to calculate the upper an lower bounofthetotalrequiretimeslotsoftheentirenetwork.to obtain the upper boun, we assume that any noe is able to fin K max parent noes or sibling noes, an we also assume that all noes irectly connecte to the gateway are always involve in forwaring packets from noes more than one hop away. Then the maximum require timeslots R up for the whole network is calculate as R up Compare with other routing protocols for IWSNs, such as [34], where the complexity is O(n 3 ), REALFLOW hugely ecreases the time complexity. In centralize IWSNs, the latest routing an scheuling ecisions are all mae by the gateway or network manager. Thus, low complexity is severely important for centralize IWSNs, since it may take the gateway or network manager excessively long time for the calculations if the routing algorithm is overcomplicate. During the processing perio, the whole network may behave abnormally, since all noes in the network o not have the latest routing information. Although the optimal scheuling algorithm is not specifie in this paper, the complexity of the simplifie scheuling scheme is still no more than that of the topology generation algorithm. Therefore, the overall complexity remains at the same level. { = Y(K max + (1+Y) +(1+Y+K max )+ +(1+Y+K max +K 2 max + +KX 2 max )), Y K X 2 max, X>2 Y(K max + (1+Y) +(1+Y+K max )+ +(1+2Y+K max +K 2 max + +KX 3 max )), K X 3 max Y<KX 2 max, X>3 Y(K max + (1+Y) +(1+Y+K max )+ +(1+3Y+K max +K 2 max + +KX 4 max )),. K X 4 max Y<KX 3 max, X>4 { Y(K max + (1+Y) + +(1+(X 1) Y)), { 1 Y<K max, (5)

10 10 International Journal of Distribute Sensor Networks an (5) can be further simplifie as R up { { Y[K max +Y(X 1) + K max (K X 1 max 1) (K max 1) X K max 1 ], { Y[K = max +Y(X 1) (i 1) iy K max (K X i max 1) (K max 1) 2 (i 1) KX i+1 max X+1 + K max 1 Y KX 2 max, X>2 ], K X i max Y<KX i+1 max, X>i, i=3,...,x. (6) Different from upper boun calculation, there are two theoretical lower bouns for REALFLOW. To calculate the first lower boun, we still assume that all noes can fin K max parent noes or sibling noes, but we also assume that reunant paths overlap with each other at maximum extent, which means that noes with the same hops away from the gatewaysharethesameparentnoes.thus,thefirstlower boun of require timeslots R low1 is calculate as R low1 Y(1+(1+K max )+(1+2K max )+ { +(1+(X 1) K = max )) Y { (1+(1+Y) + (1+2Y) Y K max { + +(1+(X 1) Y)), Y < K max, (7) an (7) can be further simplifie as { XY + (X 1) XYK max, Y K R low1 = 2 max (8) { (X 1) XY2 XY +, Y < K { 2 max. In reality, a noe cannot always fin K max parent or sibling noe ue to interferences or network topology. The secon theoretical lower boun can be obtaine if each noe can only fin one available path. Thus, we assume that each noe is only able to fin one parent or sibling noe. The require time slots for the whole network R low2 is (1+X) XY R low2 =Y+2Y+ +X Y=. (9) 2 From (6), since K X 1 max, KX i max,ankx i+1 max etermine the increment of R up, when the hop number of the noe X increases, the maximum require timeslots also grow rapily. It inicates that REALFLOW may not be very resource efficient in an extremely large size of a network, where noes are many hops away from the gateway. However, REALFLOW canbeapplieiniwsnsfortworeasons.thefirstreasonis that in a great number of inustrial applications the refresh rate is extremely fast, at the orer of secons, or even millisecons [2]. Thus, the network size is seriously constraine an cannot be too large. Seconly, since the gateway is allowe to sen commans to shrink the relate noe list in each noe.theflooingrangecanbefurtherrestricte.evenifthe network size is large with hunres of noes, for inustrial applications with the slow refresh rate, REALFLOW can still be resource efficient. Moreover, (6)is calculate base on the worst case in which every noe has ifferent parent noes. However, in reality, many noes may share the same parent noes,sotheworstcasehappensveryrarely. When noes with the same hops away from the gateway share the same parent noes, (8) is obtaine. Accoring to (8), even if the hop number X increases, the total require time slots will not grow extraorinarily. Equation (9) is an extreme case in which all noes can only fin one path to the gateway. The require timeslot number is the same as unicast transmission. Thus, it is unnecessary to iscuss. In reality, the require timeslot number of the whole network is most probably between (6) an(8). Therefore, the network resource consumption when applying REALFLOW is efinitely not overwhelming to the network, although the control-flooing mechanism is applie Reliability Performance. As we emphasize previously, high reliability is one of the most important requirements of IWSNs. REALFLOW is initially esigne to achieve high reliability by high multipath iversity. We use the same network topology (Figure 4) forthe reliability analysis. The packet elivery ratio (PDR) of a noe X hops away from the gateway is calculate as follows. We assume that there are Z i noes, which are i hops away from the gateway an are intermeiate noes to the estination noe. Also for simplicity, we assume that any noe out of these Z i noes is able to irectly fin K max parent noes, if this noe is more than one hop away from the gateway. Furthermore, we assume that all channels are symmetric. This assumption is vali if the banwith, transmission power, an harware for both uplink an ownlink are the same. (j) is the PDR of the jth noe to its kth parent noe if this P (k) noe is more than one hop away from the gateway; otherwise, P (k) s (j) is the PDR of the kth reunant path from the jth noe to the gateway. P prt (i, j) is the overall PDR of the jth noe from ith hop to all its parent noes at (i 1)th hop. P hop (i) is the overall PDR of noes from ith hop to (i 1)th hop. Then we have (j) + (1 P(1) (j)) P(2) (j) + P prt { (i, j) = { { P (1) +( P (1) s K max 1 m=1 (1 P (m) (j))) P (K max) (j), i =1 (j) + (1 P(1) s K max 1 +( m=1 (j)) P(2) s (j) + (1 P (m) s (j))) P (K max) s (j), i = 1, (10)

11 International Journal of Distribute Sensor Networks 11 P hop (i) =P prt (i, 1) +(1 Pprt (i, 1))Pprt (i, 2) + Z i 1 +( m=1 (1 P prt (i, m))) Pprt (i, Z i). (11) FinallytheoverallPDRbetweenthegatewayanthejth noe with l hops away from the gateway P overall (l), l>1,can be calculate as P overall (l) =P prt l 1 (l, j) (m). (12) P hop m=1 It is apparent that from (10) P prt (i, j) will increase if K max increases. In orer to investigate if P hop (i) monotonically increases with P prt (i, j), the partial erivative can be calculate as P hop (i) P prt (i, j) = K max m=1,m =j (1 P prt (i, m)), j [1, K max]. (13) Since 0 P prt (i, m) < 1, itfollowsthat Phop (i)/ P prt (i, j) > 0. From(12), it is also apparent that Poverall (l) will increase if P hop (m) increases. Thus, we can conclue that P overall (l) will efinitely increase if K max increases. Finally, we prove that, by increasing K max,wecanefinitelyimprove the overall reliability performance of REALFLOW, given that there are more available paths to use. However, accoring to (6) an(8), the require number of time slots will also increase at the same time if we increase K max. Therefore, there is a trae-off between the reliability performance an the network resource efficiency. 5. Experimental Setup The reliability an real-time performance of REALFLOW areevaluateancomparewiththoseofthreetraitional routing protocols via simulations in the iscrete event simulator QualNet. In this section, the experimental settings an simulation scenarios are escribe as follows Simulation Settings. We construct a centralize mesh IWSN place within a m area. Two types of evices are involve in our simulations, namely, a gateway an a number of noes, with the complete communication protocol stack. On the network layer, we implemente REALFLOW in QualNet an utilize three other existing transitional routing protocols from QualNet. Besies the network layer, the configurationsfromotherlayers aresummarizein Table 1. The configuration of the physical layer is efine accoring to the stanar IEEE [35], where the communication operates at 2.4 GHz with the bit rate 250 kbps. In orer to provie eterministic communication, TDMA mechanism is applie rather than traitional CSMA/CA mechanism. The timeslot uration is etermine accoring to the stanar WirelessHART. The reasons for turning off Parameter Application layer Network layer MAC layer Timeslot uration Retransmission Physical layer Bit rate Channel frequency Faing moel Shaowing moel Transmit power Receiver sensitivity Table 1: The protocol parameters. Description Constant bit rate application REALFLOW/AODV/DSR/DYMO TDMA 10 ms Off O-QPSK 250 kbps 2.4 GHz Rayleigh faing Log-normal 10 Bm 85 Bm retransmissions are ue to the stringent real-time performance requirement an fast refresh rates. Retransmissions bring significantly higher elivery latency, so packets may be alreay outate when arriving at their estinations if the ealines are very short. For many inustrial applications, outate packets are of limite use for estinations. In the stanar WirelessHART an ISA 100a, channel hopping is use on the MAC layer as well. For simplicity, we also isable channel hopping in our simulation. On the network layer, besies REALFLOW, we choose three traitional routing protocols, AODV, DSR, an DYMO provie by Qual- Net. These traitional routing protocols have been alreay applie in traitional WSNs. Constant bit rate application is selecte on the application layer to provie continuous ata transmissions. Besies the protocol stack, the channel parameters an noe settings are also inclue in Table 1.Asmentione before, ue to a great number of obstacles, NLOS communication ominates in many inustrial environments [8, 9]. Therefore, we choose the Rayleigh faing moel. We assume that inustrial environments are relatively static, so log-normal moel is chosen as the shaowing moel. Accoring to the stanar WirelessHART, the maximum allowe raio output power is 10 Bm. To achieve the longest communication istance, we set the transmission power as 10 Bm. This raio output power has been alreay supporte by existing evices, such as CC2591 [36]. We choose the receiver sensitivity as 85 Bm to filter out too weak signals. A great number of existing evices are able to support this sensitivity such as CC2591 an CC2531 [37]. In orer to prove our simulation setting suitable, we place two noes in QualNetwitharoun40metersawayanmeasureRSSIvalue at the receiver sie. Accoring to the measurement result, although the measure RSSI values from our simulation are aroun 10 B less than those from the real environment, the varianceofthesignalstrengthfollowsthesametren.rssi values may iffer from ifferent platforms, so we can consier the channel status to be relatively similar to that in a real inustrial environment.

12 12 International Journal of Distribute Sensor Networks 50 m 50 m (a) Scenario 1 50 m (b) Scenario 2 (c) Scenario 3 Figure 5: Three simulation scenarios. A mesh network is create in each scenario. The arrows between noes an the gateway o not mean the irect links but are the QualNet configurations on the application layer for perioic ata transmissions Simulation Scenarios. We set up three ifferent scenarios to show the results with respect to reliability an realtime performance from REALFLOW. In each scenario, we consier both uplink an ownlink transmissions. Thus, each noe perioically sens packets to the gateway, an the gateway also perioically sens packets to all noes. The refresh intervals from three scenarios are 250 ms, 500 ms, an 1 s, respectively. These refresh rates are commonly seen in existing inustrial applications [2]. Due to the fast refresh rate, the size of the network is strictly constraine. The faster the refresh rate is, the fewer the timing slots an banwith resources are available for each noe. If too many noes exist in the network, the network resources may become unscheulable. It is also important that ue to the fast application refresh rate an stringent reliability an timing requirements, the hop number of each noe in the network cannot be too large. Not only oes larger hop number require more network resources an timing for packet forwaring, but also the en-to-en transmissions are at the risk of higher failure rate. This is one of the major ifferences between time-critical IWSNs an traitional WSNs. Usually the ate refresh rate of traitional WSNs is much lower. Moreover, traitional WSNs have a much higher tolerance to transmission failures. Therefore, the sizes of the network from three scenarios are ranomly selecte as 10, 18, an 30, respectively, excluing the network manager. In each scenario, 50% are sensors an 50% actuators. Three scenarios are shown in Figure 5. It is very important to know that arrows in Figure 5 o not mean that all noes are able to communicate with the gateway irectly. These arrows are the configurations from QualNet to show that all noes are configure to communicate with the gateway perioically. If noes are farther away than one hop istance to the gateway, intermeiate noes will be involve automatically. After mesh networks are establishe, all noes perioically sen packet to the gateway at a certain refresh rate, an the gateway also perioically sens packet to all noes. Then we measure packet elivery ratio (PDR), en-toen transmission latency, an network recovery time. PDR measurements inicate the reliability of ifferent routing protocols. En-to-en latency is calculate from the time when a packet is generate to the time when it arrives at the final estination, which reveals the real-time performance. The network recovery time means the time perio spent by a routing protocol on recalculating a new path. It shows the robustness of routing protocols to be tolerant of network topology changes. These are the most important criteria to valiate a well-qualifie routing protocol for IWSNs. 6. Evaluation Results an Analysis This section escribes the simulation results from all three scenarios, covering the comparison between REALFLOW an three traitional routing protocols. Results of reliability, real-time performance, an network recovery time are analyze separately in three subsections Packet Delivery Ratio. All PDRs from three scenarios are measure in two ways, namely, overall PDR an ealine PDR. Overall PDRs are measure base on all receive packets at the estinations without consiering application ealines, whereas outate packets are iscare when measuring ealine PDRs. Both PDRs from uplink an ownlink are measure separately. Simulation results from three scenarios are shown in Figures 6, 7, an 8, respectively. From the results, we observe that PDRs measure from all scenarios for both uplink an ownlink are much higher than the other three traitional routing protocols. Even in Scenario 3 where noes are place further away from each other, REALFLOW is still able to achieve more than 70% average PDRs for both uplink an ownlink. This observation is mainly because packets are always sening via several paths from the sources to the estinations by our routing protocol, so the probability of packets arriving at the estinations is much higher. Therefore, even if the channel is in a ba status where broken links occur frequently, packets may be still able to fin one path to the estination. It is also notable that overall PDRs an ealine PDRs from REALFLOW are the same, since all outate packets have been iscare at every intermeiate noe at their respective ealines. Compare with REALFLOW, traitional routing protocols perform much worse in the Scenario 3. Not only are all ealine PDRs of traitional routing protocols for both