Effcent QoS Provsonng at the MAC Layer n Heterogeneous Wreless Sensor Networks M.Soul a,, A.Bouabdallah a, A.E.Kamal b a UMR CNRS 7253 HeuDaSyC, Unversté de Technologe de Compègne, Compègne Cedex F-625, France b Department of Electrcal and Computer Engneerng, Iowa State Unversty, Ames, Iowa 5, USA. Abstract New emergng applcatons for wreless sensor networks, such as msson-crtcal and multmeda applcatons, requre sensng heterogeneous phenomena, and that the network supports dfferent types of QoS-constraned traffc at varable rates. Desgnng an effcent Medum Access Control protocol allowng these applcatons to work properly whle copng wth the lmted resources of sensor networks s a challengng task. In ths paper, we present a new Adaptve MAC Protocol wth QoS support for Heterogeneous wreless sensor networks whch provdes hgh channel utlzaton wth a hybrd and adaptve behavor, and ntegrates a new effcent prortzaton scheme to provde QoS support and far data delvery of heterogeneous traffc. The protocol desgn s presented along wth performance results obtaned through extensve smulatons. A mathematcal model s provded and appled to perform an analytcal evaluaton of AMPH. Performance modelng, analyss, and smulaton results show the effectveness of our soluton. Keywords: Medum access control (MAC), qualty of servce (QoS), wreless sensor networks (WSN), hybrd, heterogeneous.. Introducton Durng the past ten years, wreless sensor networks (WSN) have drawn the attenton of the research communty, attracted by ths new concept and the sum of challenges ahead. In the early days of wreless sensor networks, they were typcally composed of a large number of dentcal nodes equpped wth scalar sensors such as temperature, humdty and lght sensors, and a rado chpset allowng communcatons between nodes. Ths new nstrument was envsoned for a broad range of montorng applcatons (contnuous sensng, event detecton, etc.) n several areas such as mltary, envronment, ndustry and home []. Recently, the avalablty of low-cost new hardware such as CMOS cameras and mcrophones, accelerometers, gyroscopes, ECG and EMG sensors, coupled wth technologcal mprovements n wreless sensor nodes n terms of processng capabltes and memory, revolutonzed tradtonal montorng and sensng. The release of ths new generaton of nodes led to the emergence of many new promsng applcatons such as multmeda survellance networks, car traffc montorng and advanced health care delvery [2, 3]. Ths new generaton of applcatons has specfc requrements compared wth classc montorng applcatons. Unlke most tradtonal wreless sensor network applcatons, new applcatons are often hgh data rate real-tme applcatons and combne dfferent sensng modaltes, hence producng several types of traffc wth varous characterstcs lke multmeda streams, crtcal physologcal data, Correspondng author. Tel.: +33 344234645; fax: +33 344234477. Emal addresses: marsoul@utc.fr (M.Soul), bouabdal@utc.fr (A.Bouabdallah), kamal@astate.edu (A.E.Kamal) emergency alerts, etc. Gven the lmted resources of wreless sensor networks, especally bandwdth, the QoS requrements of these applcatons pont out the need for new networks whch are capable of transportng large amounts of data, but also to perform real-tme processng, correlaton and aggregaton of data orgnated from heterogeneous sources, and to provde QoS support. The term QoS may have dfferent meanngs, dependng on the context n whch t s used: t may refer to the degree to whch the system performs the functons requred by the applcaton/user, as well as the mechansms mplemented to provde ths performance. Applcatons have QoS requrements and the network must provde QoS support. These two perspectves are nterdependent [4]. Snce dfferent applcatons may have dfferent requrements, no sngle QoS model can support all applcatons. In wreless networks, the MAC layer plays a key role n QoS provsonng. Snce the rado channel s shared and cannot be accessed smultaneously by several nodes, network performance depends drectly on the optmal management of ths resource. MAC protocols can be classfed nto two categores: contenton-based and schedule-based. Contenton-based protocols lke CSMA/CA are scalable wth no strct tme synchronzaton constrant, however, ther performance degrades under heavy traffc because the probablty of collsons ncreases. In schedule-based protocols such as TDMA, the channel s dvded nto tme slots where each node has an exclusve access rght to the medum durng ts tme slot, hence avodng collsons, dle lstenng, and overhearng problems. Nevertheless, tme synchronzaton s requred, and becomes challengng as the network sze ncreases. Also, as nodes can only transmt durng ther own tme slots, schedule-based protocols ntroduce latency and bandwdth under-utlzaton under lght traffc. Preprnt submtted to Computer Communcatons January 2, 24
Few MAC protocols are desgned to cope wth dfferent types of traffc and varyng traffc load condtons. Desgnng an effcent MAC protocol for such networks s challengng. Ths s because such protocols must mplement servce dfferentaton mechansms, ntra-nodal and nter-nodal traffc prortzaton, and an adaptve behavor accordng to traffc condtons. Ths motvates us to develop a new MAC protocol for wreless sensor networks wth heterogeneous sensng capabltes. In ths paper we propose a new Adaptve MAC Protocol for Heterogeneous wreless sensor networks called AMPH. AMPH provdes effcent delvery of heterogeneous traffc usng servce dfferentaton and traffc prortzaton, and maxmzes channel utlzaton by vrtue of ts hybrd adaptve nature. We evaluate the performance of AMPH usng smulaton by consderng two classes of traffc: hgh prorty real-tme traffc and besteffort traffc. Smulaton results show that AMPH enables effcent and far delvery of both real-tme and best-effort traffc accordng to ther respectve QoS requrements. The hybrd behavor of AMPH, whch combnes the strengths of contentonbased and schedule-based protocols, outperforms contentonbased protocols n terms of relablty and channel utlzaton. The rest of ths paper s organzed as follows. The next secton brefly ntroduces related research. A detaled descrpton of AMPH desgn s presented n Secton 3. Smulaton results are dscussed n Secton 4. Secton 5 presents the analytcal model and reports on numercal results. Fnally, Secton 6 concludes the paper. 2. Related Work Due to the unque resource constrants and applcaton requrements of sensor networks, standard MAC protocols developed for wreless networks could not be used, as dscussed n []. Therefore, several MAC protocols wth dfferent obectves were proposed. Intally, the man desgn goal of MAC protocols for WSN was to maxmze network lfetme. A good survey of popular MAC protocols for wreless sensor networks s provded n [5]. As there s a wde varety of WSN applcatons whose requrements may be very dfferent, t has become evdent that no sngle MAC protocol can ft all applcatons. Indeed, several applcaton-specfc characterstcs such as nteractvty and relablty nfluence the network desgn. Thus, the underlyng network must provde guarantees n terms of latency, bandwdth, and packet loss, ust to name a few. There are many applcaton-specfc MAC protocols n the lterature (e.g., delay-senstve applcatons, bandwdth-hungry, mssoncrtcal, etc.) as shown by the surveys n [6, 7, 8]. However, there are a few QoS-aware MAC protocols,.e., protocols whch am to accommodate dfferent types of QoS-constraned traffc and to adapt to varyng traffc loads [9]. Saxena et al. [] proposed a QoS MAC protocol for wreless multmeda sensor networks (WMSN), as multmeda applcatons commonly carry heterogeneous traffc wth dfferent QoS requrements. Ths protocol s based on a CSMA/CA approach and attempts to fulfll end-to-end delay and bandwdth requrements of three types of traffc (streamng vdeo, real-tme, and best-effort) usng an adaptve contenton wndow (CW) and a 2 dynamc duty cycle for energy conservaton. Servce dfferentaton s acheved usng multple queues and a value of CW related to the traffc prorty. Traffc of utmost mportance wll be assgned a small contenton wndow to have a better chance of accessng the medum. CW sze and duty cycle are adusted accordng to network statstcs such as transmsson falures and domnant traffc type. A smlar dea s pursued n the work of Ygtel et al. [] whch proposed a comparable protocol named Dff-MAC. Dff-MAC uses a dfferent approach for ntra-node packet prortzaton and CW sze adaptaton. The MAC protocol n [] mplements one FIFO queue per class of traffc before the packets are scheduled for sendng, whereas Dff- MAC provdes a far prortzaton of packets wthn the same class based on the hop count metrc of each packet and uses a weghted far queung (WFQ) method to control the relatve throughput of each traffc class. Also, Dff-MAC contnuously adapts the CW sze whle Saxena et al. s MAC wats for the neghborng nodes to adust t. Therefore, CW converges more quckly to ts optmal sze n Dff-MAC. These two protocols use smlar mechansms to the IEEE 82.e standard [2], partcularly wth respect to medum access prortzaton. The hybrd coordnaton functon (HCF) n the standard ncludes a method of channel access called Enhanced Dstrbuted Channel Access (EDCA). EDCA defnes four prorty classes called access categores (AC): Background, Best-Effort, Vdeo, and Voce. The prortes are mplemented usng contenton wndows. Voce and Vdeo have smaller contenton wndows than Background and Best-Effort traffc n order to maxmze the chance to transmt the prorty traffc before delay-tolerant traffc. Saxena et al. s MAC and Dff-MAC provde sgnfcant mprovements over classc CSMA/CA approaches: they exhbt better performance n terms of throughput and latency. Dff-MAC also acheves farness among the dfferent traffc classes. However, although dynamc mechansms enable the network to accommodate tme-varyng traffc loads, they ntroduce a sgnfcant complexty. Besdes, contenton-based protocols may not be effcent under hgh contenton as RTS-CTS exchanges consume extra bandwdth. Ths overhead causes the channel utlzaton to be suboptmal. Contenton-free MAC protocols lke TDMA perform better under heavy traffc loads. Indeed, scheduled transmssons allow avodng collsons. Nevertheless, under low contenton, TDMA leads to low channel utlzaton and hgh latency. Therefore, pure TDMA approaches are not sutable for varable traffc envronment. The lmts of contenton-based and contenton-free MAC protocols have led to the development of hybrd MAC protocols whch attempt to combne the advantages of both approaches. Z-MAC [3] protocol proposed by Rhee et al. s based on ths paradgm: t dynamcally adusts ts behavor between CSMA and TDMA dependng on the level of contenton n the network. Durng the setup phase, the nodes run the followng operatons: neghbor dscovery, slot assgnment, local frame exchange, and global tme synchronzaton. The two-hop neghbor lst s used as an nput to the tme slot assgnment algorthm called DRAND [4]. Ths algorthm computes a schedule where two nodes wthn a two-hop communcaton negh-
borhood cannot be assgned to the same slot. When the setup phase s over, the transmsson phase begns. Nodes can transmt durng ther own tme slot, but they may also contend to use a slot that s not used by ts owner, hence enhancng channel utlzaton. Before transmttng, nodes back off for a random tme wthn a gven contenton wndow. When the backoff tme expres, they run a clear channel assessment (CCA) to know f the channel s clear. The CW sze s set n such a way that owners are always gven a better chance of accessng the channel. Ths mechansm makes Z-MAC robust to synchronzaton errors. In case of clock drft, the performance of Z-MAC s smlar to that of CSMA. To overcome the hgh overhead of RTS-CTS, ths mechansm s not used n Z-MAC. Instead, Z-MAC mplements two modes of operaton: low contenton level (LCL) and hgh contenton level (HCL). When hgh contenton s experenced, an explct contenton notfcaton s sent causng the nodes to swtch to HCL mode where nodes are no longer allowed to steal slots owned by two-hop neghbors. Z-MAC dynamcally adusts ts behavor dependng on the level of contenton n the network, thus achevng hgh channel utlzaton. However, Z- MAC s not suted for heterogeneous applcatons snce t does not mplement any servce dfferentaton mechansm and QoS provsonng. I-MAC [5] adds a prortzaton scheme to Z- MAC and ams to take nto account the traffc load for each sensor node accordng to ts role n the network. Hgher prorty wll be assgned to nodes havng a lot of packets to send, such as cluster heads, thus allowng these nodes to have a better chance to access the medum than ther low-prorty neghbors, whch mproves the throughput of the former nodes. Four prorty levels are mplemented usng custom CW szes for each prorty group. Although I-MAC reduces collsons, thus achevng a slghtly better channel utlzaton than Z-MAC, t has not been desgned to support QoS-constraned traffc ether. Indeed, the prortzaton scheme of I-MAC s only based on the amount of traffc of each node, whereas t should have also consdered the traffc type n order to provde dfferentated servces. In addton, snce the prorty levels are fxed, a node cannot dynamcally adapt ts prorty level n case of varable traffc condtons. Fnally, ths protocol may be hard to deploy over a large number of nodes. Nodes are assgned a fxed prorty accordng to ther role n the network, so ths mples that nodes must be manually confgured, unless they are able to nfer ther role n the network. Our goal s to provde an effcent MAC protocol for heterogeneous wreless sensor networks. As more and more applcatons have heterogeneous sensng capabltes and requre network support for dfferent types of QoS-constraned traffc at varable rates, wreless sensor network support becomes a necessty. In ths secton, we present n detal the desgn of AMPH, our new adaptve MAC protocol for heterogeneous wreless sensor networks. The basc dea of our soluton s smlar to that of Z-MAC: we adopt a hybrd behavor whch combnes the strengths of both contenton-based and schedulebased approaches to maxmze the channel utlzaton. Our hybrd channel access method allows slot-stealng, thus achevng hgh channel utlzaton, and provdes adaptablty to varable traffc loads. We also ntroduce a new prortzaton scheme whch s desgned to fulfll the requrements of real-tme traffc. In the followng subsectons, we descrbe n detal the basc prncples of AMPH along wth ts two man operaton phases, setup and transmsson. 3.. AMPH Basc Prncples AMPH s a hybrd channel access method. It s manly based on the tme dvson prncple, but nodes may transmt durng any tme slot n order to maxmze channel utlzaton and mnmze latency. Tme s dvded nto several recurrent tme slots of fxed duraton. Nodes are assgned to tme slots n such a way that no two nodes wthn a two-hop communcaton neghborhood are assgned to the same slot. More detals about slot assgnment are gven n the Setup subsecton. We call nodes assgned to a gven slot owners. Otherwse, nodes are non owners. A cycle of N tme slots consttutes a tme frame, where N s the maxmum number of tme slots,.e., equal to the maxmum number of contenders wthn two hops. As stated before, nodes may transmt durng any tme slot. We propose a new prortzaton scheme whch ensures that nodes wth hgh prorty traffc wll be able to transmt ahead of low prorty nodes n case of competton to access the channel. Our scheme also ncludes an ntra-node arbtraton mechansm so that prorty packets take precedence over other packets as soon as they are created. We frst explan the ntra-node arbtraton mechansm. Inter-nodes arbtraton wll be detaled subsequently. AMPH supports two classes of traffc: real-tme (RT) and best-effort (BE), and RT traffc takes precedence over BE traffc. We assume that the traffc class s statcally set at the applcaton layer. When a packet s submtted to the data lnk layer from the upper layer, a classfer checks whether the packet s real-tme or best-effort and puts t nto the approprate packet queue. AMPH mantans two FIFO queues correspondng to the two classes of traffc, as shown n Fg.. We use a strct prorty scheduler to set the next packet to send, so that RT traffc always has prorty over BE traffc. Our scheduler systematcally selects RT packets as long as the queue s not empty, then t contnues wth BE packets. RT packets queue 3. AMPH Protocol Desgn 3 Incomng packet Classfer BE packets queue Scheduler Fgure : AMPH ntra-node arbtraton scheme Selected packets to be transmtted Ths schedulng mechansm allows to select RT packets for transmsson ahead of delay tolerant BE packets. An addtonal
mechansm s needed to organze channel access between competng nodes n order to guarantee that a node havng RT traffc to send has hgher chance to gan access to the medum than a node havng BE traffc, hence ensurng that RT traffc queung tme s mnmzed. We propose a new arbtraton scheme that provdes low channel access delay for RT packets and farness among nodes wth traffc of the same class. Our arbtraton mechansm uses tmers called backoffs and operates as follows. Competng nodes pck a backoff value and wat for the backoff duraton before tryng to transmt. When the backoff tmer of a node expres, t senses the medum by callng the CCA functon of the PHY. If the PHY returns the channel status as dle, the node may start to send packets, otherwse t has to delay ts transmsson. As a result, the node that obtans the smallest backoff wns the contenton and gans access to the medum. When the backoff of the other contenders expres, the channel wll not be dle anymore, snce the wnner s currently transmttng, and they wll back off agan, usng new samples of backoff duratons. Accordng to our desgn goals, RT traffc takes precedence over BE traffc, so nodes havng RT packets to send should be able to access the channel ahead of nodes havng BE traffc. In order to allow ths behavor, nodes havng RT traffc beneft from smaller backoffs than nodes wth BE traffc whch use longer backoffs. The contenton wndow also depends on the role of the node: owner or non owner. Owners have prorty over non owners. Snce all nodes own a tme slot, ths system acheves a far access to the channel among nodes havng traffc of the same class. In addton, our mechansm allows non owners to steal the slots of owners when they have nothng to send, thus reducng channel access tme and ncreasng channel utlzaton. Nodes havng data to send pck the backoff value β n the approprate contenton wndow, accordng to the type of traffc selected by the scheduler and f they are owner or non owner. The contenton wndows form a non-overlappng nterval set, as depcted n Table. Snce the backoff s chosen randomly, the probablty that contenders wth smlar condtons (non-owners havng traffc of the same class) choose low backoff duratons, and the collson probablty wll be low. Owner + RT traffc Interval A β [A mn, A max [ Non owner + RT traffc Interval B β [B mn, B max [ Owner + BE traffc Interval C β [C mn, C max [ Non owner + BE traffc Interval D β [D mn, D max [ where A < B < C < D. Table : Contenton wndows correspondng to the role of the contender and the type of traffc t has to send In Fg. 2, we depct an example scenaro of two competng nodes u and v, where u and v both have RT traffc to send to the base staton at the begnnng of slot. Node u pcks a backoff β u n the nterval A snce t s the owner of the slot, and v pcks ts backoff β v n the nterval B. Snce β u < β v, the backoff of node u expres frst, so t runs a clear channel assessment (CCA) to determne f the channel s clear,.e., that no nodes are currently transmttng. Node u fnds the channel s dle, so t starts ts transmsson. When the backoff of node v expres, v also runs a 4 CCA but as the channel s not dle anymore (node u s currently transmttng), t cannot transmt and has to wat for the begnnng of the next slot (slot ) to retry. As node v s the owner of slot, t wll beneft from a small backoff. Therefore, t wll be gven the hghest prorty to access the channel. Ths example also llustrates how our backoff system ensures that AMPH s far,.e., that the medum s farly shared among all nodes of the network. We can see that our arbtraton mechansm guarantees that all nodes gan access to the channel at some pont, n the worst case scenaro, durng ther reserved tme slot. Besdes, due to the random nature of our scheme, all nodes of the same prorty level have equal chance of stealng unused slots. The whole transmsson process s descrbed n Secton 3.3. Base staton U V U V A B C D t t o t o +b V Slot Slot Backoff CCA Transmsson Fgure 2: AMPH nter-node arbtraton scheme AMPH ensures that a maxmum number of packets can be sent durng a tme slot n order to maxmze the channel utlzaton. Indeed, transmttng a burst of packet s more effcent than transmttng only one packet per slot. It s not necessary to run the full transmsson process for each packet and the overhead caused by the backoff mechansm s absorbed. The number of packets that can be sent nto one burst depends on the packet sze. In our soluton, we use a strct prorty scheduler and a backoff mechansm. Both mechansms always favor RT traffc. As a consequence, BE traffc may suffer from starvaton. In order to avod ths stuaton, we arrange M frames among N n whch BE traffc has prorty over RT traffc, where N s the number of tme slots n a frame and M s a parameter to adust accordng to the amount of each type of traffc. Durng these partcular tme frames, the backoff values of BE traffc are smaller than those for RT traffc, so nodes havng BE traffc have prorty over node havng RT traffc. Ths mechansm s optonal and may be mplemented only n networks wth hgh data rate contnuous RT traffc sources. 3.2. Setup At startup, nodes enter a setup phase and they perform the followng ntalzaton actons: neghbor dscovery, slot assgnment, framng, and synchronzaton. Each node constructs ts two-hop neghbors lst whch s used as an nput for the slot assgnment algorthm. The slot assgnment problem s analogous to the graph colorng problem. In AMPH, slot assgnment s performed usng DRAND, an effcent dstrbuted slot reuse schedulng algorthm also used n Z-MAC. DRAND ensures that no two nodes wthn a two-hop communcaton neghborhood are assgned to the same slot. For more detals on DRAND operaton, the reader may refer to [4]. The maxmum number of slots defnes the tme frame length, and nodes
synchronze ther schedule at the begnnng of the frame. When the setup phase s done, nodes begn ther normal operaton descrbed n Secton 3.3. 3.3. Transmsson As explaned above, our protocol operates accordng to a specfc tme structure. The tme s dvded nto recurrent tme slots formng frames. The MAC routne occurs at the begnnng of each tme slot. Dependng on whether the node has data to transmt or not, or whether t receves traffc from neghborng nodes, the node performs varous operatons. In the followng, we explan the actons performed by a node durng one tme slot, especally durng the transmsson process. There are bascally four possble scenaros: The node wants to transmt and the channel s dle, The node wants to transmt but the channel s not dle, The node receves data, The node has nothng to transmt and does not receve data. We descrbe the operatons of a node n these dfferent scenaros by followng the state transton dagram of AMPH gven n Fg. 3. Recevng new frame Backoff. At the begnnng of each tme slot, f the node has packets to send, t enters the Backoff state and computes a backoff value β randomly wthn the correspondng wndow, as explaned n Secton 3.. Whle watng for the end of the backoff tme, the node stays n the Backoff state. Durng backoff, the node lstens to the rado channel n the event that t receves data. If so, t swtches to the Recever state. CCA. When the backoff expres, the node swtches to the CCA state and performs a clear channel assessment (CCA) to sense the channel. If the channel s dle, the node s allowed to begn the transmsson and goes nto the Data transmsson state; otherwse t returns to the Wat state and wats for the begnnng of the next slot to retry usng the same process. As nodes lsten to the rado channel durng the backoff perod, CCA s not necessary n ths case. However, n a star topology were all nodes only communcate wth the base staton, the rado could then be turned off to save energy, hence CCA would be useful. Data transmsson. Once a node reaches the state Data transmsson, t s allowed to transmt. The node sends packets untl ether ts queues are empty, or the tme slot has expred. When the transmsson s over, the node returns to the Wat state and awats the begnnng of the next slot. A smlar transton to the Wat state happens when the node s n the Recever state and recepton s completed. Int DATA Recevng new frame Start Recever Wat End of recepton New slot AND Data n queue Channel busy Backoff End backoff CCA Recever. In multhop networks, nodes may act as relay nodes and receve data from other nodes that need to be forwarded. Nodes have to lsten for transmssons ntended for them durng the Wat and the Backoff states. Recepton has prorty over transmsson. As soon as a packet recepton begns, the node swtches to the Recever state. The node leaves ths state when the recepton s over and returns to the Wat state. No other event can nterrupt the recepton. Tx over AND (end of slot OR no more data to send) Data transmsson Data n queue DATA Fgure 3: State machne of AMPH Channel dle Int. Durng the setup phase, the node s n the Int state. After the executon of the setup process, the node swtches from the Int state to the Wat state. 4. Smulaton experments and results In ths secton, we study the effcency of AMPH through smulaton experments. We descrbe our approach to perform ths evaluaton, then we analyze the relatve performance of AMPH wth Dff-MAC, whch s the best compettor n the lterature. We evaluate the channel utlzaton, the latency, and the relablty acheved by both protocols. Fnally, we dscuss the results and the ablty of AMPH to support the hgh requrements of heterogeneous WSN applcatons. 4.. Scenaro and Smulaton Parameters Wat. The node ends up n Wat at the end of each tme slot and stays n ths state when t has nothng to do at the begnnng of a new slot. The rado may be swtched off f the followng condtons are met: the node has no data to send, and the node s not supposed to receve any data (n a star topology for example, where every node can reach the base staton drectly). 5 The goal of our soluton s to provde hgh channel utlzaton, effcent prortzaton of real-tme traffc, and far data delvery n heterogeneous WSNs. In order to assess the performance of our protocol, we carred out extensve smulatons for two dfferent classes of traffc and we compared the results wth those of Dff-MAC. We selected Dff-MAC as a bass for
comparson snce t s a well-known QoS-aware MAC protocol, and t s the closest protocol n the lterature to our protocol. Lke AMPH, Dff-MAC ams to meet the QoS requrements of heterogeneous traffc by provdng dfferentated servces and fast delvery of the prorty data. By usng effectve QoS mechansms, t acheves hgh performance n terms of throughput and latency []. Accordng to our study of the related work, Dff-MAC s currently the most effcent MAC protocol for heterogeneous wreless sensor networks. Our obectve s to show the benefts of our hybrd channel access technque over a contenton-based approach, as used n Dff-MAC, and to demonstrate the effcency of our prortzaton scheme. In order to evaluate the performance of AMPH, we examne the followng metrcs: throughput, latency, and relablty. We used the MXM framework developed under the OMNeT++ network smulator [6] to smulate AMPH and compare t wth Dff-MAC. Snce our protocol s desgned for heterogeneous WSNs wth varable traffc load, we set up an example scenaro smlar to a multmeda montorng applcaton. We consder a wreless multmeda sensor network composed of nodes equpped wth a vdeo camera producng a contnuous multmeda stream, and also wth envronment sensors whch gather nformaton such as temperature and lumnous ntensty. The applcaton requres that the multmeda content s delvered n real-tme, whereas lght and temperature data are consdered of secondary mportance. In order to smulate ths applcaton scenaro n OMNeT++, we mplemented a custom applcaton layer whch generates two types of packets at dfferent rates, correspondng to scalar data and multmeda content. Mean nterarrval tme Average packet rate. s packets/s.5 s 2 packets/s.2 s 5 packets/s. s packets/s Table 2: NRT/BE traffc loads Frame rate. frames/s.5 frames/s.2 frames/s. frames/s Table 3: RT traffc loads n one slot. Gven that the sze of one vdeo frame s, bts, and assumng that the avalable bandwdth s 256, bps, the duraton of a tme slot must be at least 39.625 ms. We set t to 4.96 ms to correspond to 28 tme unts of.32 ms, whch s the duraton of auntbackoffperod, the basc tme perod used n the IEEE 82.5.4 MAC. The sze of the backoff ntervals A, B, C, and D, expressed n tme unts, are provded n Table 4. Intervals A and C are only one tme unt long, snce there s no contenton durng these perods, unless the nodes are not synchronzed. Addtonal parameters are shown n Table 5. Interval Duraton (tme unts) A B 8 C D 8 Table 4: Backoff ntervals Smulaton of scalar data: to smulate the temperature and lght measurements, our applcaton layer generates small data packets (2 bts) whose packet nter-arrval tmes follow a Posson dstrbuton. Smulaton of multmeda content: we assume that vdeo cameras produce perodc vdeo frames of, bts whch are fragmented nto, bt-long packets. In order to reproduce ths traffc, our applcaton layer perodcally generates packets of, bts each. Dff-MAC mplements three classes of traffc: BE, RT, and non real-tme (NRT), whch s an ntermedary class of traffc for scalar data wth hgher QoS requrements than BE. As a consequence, our applcaton layer tags one scalar data packet out of every two as an NRT packet. Snce AMPH does not support ths class of traffc, NRT packets are processed as BE packets. Data generaton rates are nput parameters whch are vared to evaluate the performance of AMPH and Dff-MAC under varous traffc loads. The dfferent traffc loads offered to the network are presented n Tables 2 and 3. 4.2. MAC Parameters In the smulatons, we set the duraton of a tme slot such that the owner of a tme slot can send a complete vdeo frame 6 Parameter RT packets buffer sze NRT/BE packets buffer sze Avalable bandwdth CCA duraton Value Table 5: Addtonal smulaton parameters 5 Kbts 4 Kbts 256 bps.28 ms We mplemented Dff-MAC accordng to the nformaton provded n [9]. Snce Dff-MAC adopts a CSMA/CA based medum access method, we adapted the mplementaton of CSMA/CA provded n MXM by addng the extra features of Dff-MAC: contenton wndow sze adaptaton, and ntra-node and ntra-queue prortzaton. Dff-MAC uses RTS/CTS and acknowledgments. Just as AMPH, Dff-MAC sends RT packets n a burst. The length of a burst corresponds to the number of fragments of one vdeo frame. 4.3. Smulaton results We evaluated the performance of AMPH through extensve smulatons usng the OMNeT++ smulaton engne and compared t to Dff-MAC. We smulated a network of eght multmeda nodes and a base staton organzed n a star topology where each node s wthn communcaton range of each other and we studed the relatve performance of AMPH and Dff- MAC under varous traffc loads. Each scenaro s smulated
AMPH DffMAC AMPH DffMAC Channel utlzaton (%) 8 6 4 2 5 NRT/BE packets / s 2.5 Fgure 4: Comparatve channel utlzaton.5 2 Vdeo frames / s ten tmes wth dfferent seeds and the average was computed. In ths secton, we analyze the smulaton results. We focus on the comparatve channel utlzaton, average latency, and successful packet delvery rato. The channel utlzaton s calculated as the throughput to channel capacty rato. The defnton of the latency s the tme elapsed between the recepton of a packet by the MAC layer and the transmsson of ths packet. The successful packet delvery rato s calculated as the fracton of packets whch were correctly receved by the base staton. Snce hgh throughput s necessary for hgh data rate applcatons such as multmeda applcatons, achevng hgh channel utlzaton s one of the prmary goals of AMPH. In Fg. 4, we plotted the comparatve channel utlzaton of AMPH and Dff- MAC. As shown n ths fgure, AMPH acheves better throughput than Dff-MAC n all scenaros, partcularly when the traffc load ncreases. Ths confrms our hypothess that the hybrd behavor of AMPH allows hgh channel utlzaton under varable traffc loads through the use of an effcent tme dvson schedule whch enhances the contenton resoluton. The ablty to send multple packets n one slot also contrbutes to maxmzng the channel utlzaton, as well as the fact that we do not use control messages such as RTS / CTS or ACK. AMPH also ams to provde fast data delvery for real-tme and msson-crtcal applcatons. In Fg. 5, we show the average latency of RT traffc usng Dff-MAC and AMPH. At low traffc loads, the latency s very small: 33 ms for Dff-MAC, and 45 ms for AMPH. Indeed, at low contenton levels, nodes n Dff-MAC can access the medum almost mmedately whereas n AMPH, the transmsson process starts only at the begnnng of a new slot. Nevertheless, the gap s not sgnfcant. When the traffc load ncreases, contenton gradually ncreases and access to the channel becomes more dffcult. Usng AMPH, the latency of RT packets stays very low ( 7 ms), thus demonstratng the effcency of our arbtraton and QoS mechansms. At 7 Average Latency of RT traffc (s) Average Latency of BE/NRT traffc (s).4.3.2. NRT/BE packets / s NRT/BE packets / s 5 4 3 2 5 2.5.5 Vdeo frames / s Fgure 5: Comparatve average latency of RT traffc AMPH BE traffc DffMAC NRT traffc DffMAC BE traffc 5 2.5.5 Vdeo frames / s Fgure 6: Comparatve average latency of BE/NRT traffc the same tme, the latency of Dff-MAC rses up to 33 ms. In Fg. 6, we plotted the average latency of BE traffc for Dff-MAC and AMPH. Dff-MAC supports two knds of besteffort traffc: non real-tme, NRT, and true best-effort, BE. NRT has hgher prorty than BE traffc. AMPH assmlates NRT to BE traffc. In almost all scenaros, the latency of BE packets usng our protocol s less than one second. We notce that when the BE load s set to packets/s, the latency of BE packets ncreases up to 22 s. However, t should be noted that the mechansm that favors BE traffc over RT traffc when the BE queue flls up was not mplemented. Ths scenaro shows that even under very hgh traffc condtons and wth no specal mechansm to favor BE traffc over RT traffc, BE traffc does not suffer from starvaton. Globally, we notce that AMPH behaves very well, unlke Dff-MAC whose latency rses as soon as the traffc load reaches 5% of the avalable bandwdth. Fgs. 7 and 8 plot the successful packet delvery rato performed by AMPH and Dff-MAC for all types of traffc. Relable data delvery s an mportant requrement, especally for crtcal and real-tme applcatons, where packet loss decreases the nformaton qualty. However, for some hgh-throughput 2 2
Traffc Receved at Snk Node (%) 8 6 4 2 RT traffc BE traffc NRT/BE packets / s 2 5 2.5 Vdeo frames / s.5 Traffc Receved at Snk Node (% NRT/BE packets / s 8 6 4 2 2 5 RT traffc NRT traffc BE traffc 2.5 Vdeo frames / s.5 Fgure 8: Comparatve successful packet delvery rato of Dff-MAC Fgure 7: Comparatve successful packet delvery rato of AMPH traffc such as multmeda streamng, some packet loss can be tolerated up to a certan extent wthout affectng the playback qualty. Addtonally, codng technques can be used to mtgate the effect of packet loss. Our smulaton results show that AMPH acheves hgh relablty, although t does not mplement RTS/CTS exchanges or packet loss recovery technques. For real-tme traffc, n the worst case scenaro the relablty s 89%, and the average relablty s approxmately equal to 94%, thus demonstratng that AMPH s very relable for ths class of traffc. AMPH s not only relable for RT traffc but also for the BE traffc, snce smulaton results show that the average relablty of BE traffc s approxmately equal to 94%. However, we notce that when the RT frame rate s equal to 2 frames/s and the BE traffc load s also set to the maxmum load level, the relablty drops to approxmately 5%. In ths scenaro, the traffc load causes nodes to encounter buffer overflows. Regardng Dff-MAC, the offered relablty for RT traffc s almost equal to %. Dff-MAC outperforms AMPH, but at the cost of poor throughput. As for NRT and BE traffc, packet loss ncreases as the traffc load grows. The two reasons for that are that packets are dropped when ether they have reached the maxmum number of transmsson attempts, or when buffer overflows. Accordng to the preferental treatment of NRT traffc over BE traffc, AMPH suffers lower losses. Globally, we can say that AMPH outperforms Dff-MAC under NRT/BE traffc, snce for about half of the experment, the relablty of Dff-MAC s lower than 5%. 4.4. Conclusons In ths secton, we performed extensve smulatons n order to demonstrate the performance of AMPH and compare the results wth our closest compettor n the lterature named Dff- MAC. The results have shown that AMPH outperforms Dff- MAC n terms of channel utlzaton and latency for both classes of traffc RT and BE. As for relablty, Dff-MAC offers almost a % relable RT packet transmsson, but at the cost of poor throughput, whereas AMPH experences lmted packet loss ( %) whle not wastng bandwdth wth control messages. We 8 had prevously demonstrated n [7] that AMPH outperforms CSMA/CA. These new experments also tend to confrm the superorty of our hybrd behavor over contenton-based solutons. In concluson, our protocol effectve far servce dfferentaton and QoS mechansms mnmze real-tme traffc latency and prevent best-effort traffc starvaton. The tme dvson schedule enhances the contenton resoluton leadng to hgh channel utlzaton and relablty. Hence, AMPH provdes effcent QoS provsonng for heterogeneous traffc for a new generaton of promsng applcatons wth hgh QoS requrements such as multmeda, trackng, and health care applcatons. 5. Modelng and Performance Analyss In ths secton, we provde an analytcal model of our MAC protocol AMPH. The mathematcal model allows the evaluaton of the MAC latency by estmatng the probablty that a node begns a transmsson wthn a gven tme and also estmates the data delvery relablty by dervng the probablty of success of a transmsson attempt. In addton, our model shows how the network sze and the dstrbuton of traffc (proporton of RT and BE traffc) affect the performance of AMPH. We frst ntroduce our approach for developng the model along wth some defntons and desgn assumptons, then we explan n detal the formulaton of our mathematcal model, and fnally we provde the analytcal performance study of AMPH. 5.. Model Assumptons, Reference Scenaro and Notatons The desgn of our model follows a smlar approach to that of Buratt et al. [8], where the authors provde an analytcal model for evaluatng the performance of the non-beacon enabled mode of the IEEE 82.5.4 standard [9]. The model provded by Buratt et al. allows the evaluaton of the probablty that a gven sender node succeeds n accessng the channel, and that the snk receves the transmtted packet. Smlarly, the goal of our model s to estmate the channel access tme and the data delvery rato of AMPH n order to perform an analytcal evaluaton of ts performance n terms of latency and relablty.
Furthermore, we am to analyze the mpact of the network sze and the traffc dstrbuton. In what follows, we present some assumptons made n the model desgn along wth the notatons used n the formulaton of the model, then we provde a short remnder on AMPH operaton. Topology. We consder N nodes organzed n a star topology and a snk whch does not transmt data whch s located at the center of the star. We assume that all nodes are wthn rado range of each other, and therefore the hdden termnal problem does not occur. Nevertheless, collsons may occur f two or more nodes sense the channel at the same tme, fnd the channel dle and start ther transmssons smultaneously. Traffc. Our model s desgned to allow the performance evaluaton of the two types of traffc supported by AMPH: real-tme (RT) and best-effort (BE). Packet sze. Although AMPH may transmt several packets durng one tme slot, we only take nto account the transmsson of one packet, snce t s suffcent to provde the MAC latency. As a consequence, the packet sze does not affect the results. Resoluton tme. In the defnton of our model, the tme s dscrete and the resoluton tme s equal to auntbackoffperod, the base tme unt n the IEEE 82.5.4. We call auntbackoffperod a tme unt, and one tme unt s equal to.32 ms. The notatons and symbols used n the defnton of our model are summarzed n Table 6. Symbol N P{T } P{S p s p b p f p u V S b v β A β B β C β D } Meanng / Defnton Network sze Probablty to begn a transmsson at tme unt of slot Probablty to be sensng at tme unt of slot Probablty of success of a transmsson Probablty to fnd the channel busy at tme unt of slot Probablty to fnd the channel free at tme unt of slot = p b Probablty that the transmsson started n (, ) s unque Vector contanng the probablty of beng n each sensng state n slot Backoff value computed for a gven node at the begnnng of each slot Upper lmt of the contenton wndow A (cf. Fg.) Upper lmt of the contenton wndow B (cf. Fg.) Upper lmt of the contenton wndow C (cf. Fg.) Upper lmt of the contenton wndow D (cf. Fg.) Table 6: Summary of notatons 9 5.2. Formulaton of the Mathematcal Model The obectve of our model s to derve expressons of the followng metrcs: The probablty that a node begns ts transmsson n a gven slot at the tme unt whch s denoted as P{T } The success probablty for a transmsson,.e the probablty that a node succeeds n transmttng a packet and that no collson occurs whch s denoted as p s In order to compute these metrcs, we analyze n detal the transmsson process of a specfc node denoted as the target node. Accordng to the operaton of AMPH, a node achevng the transmsson process can be n one of the four states represented n Fg. 9: Backoff, Sensng (S), Transmttng (T) or Idle. Idle s the default state when a node wats for the tme slot boundary. At the begnnng of a new tme slot, a node havng data to send computes a backoff value, wats for the backoff to expre, and senses the channel. After sensng, f the channel s found dle, the transmsson begns mmedately. Otherwse, the transmsson s delayed and the node has to wat untl the begnnng of the next tme slot before tryng to transmt agan. New slot begns Idle Transmsson s over Backoff Channel s busy T Backoff has expred S Channel s free Fgure 9: Full state-transtons dagram From ths analyss, we notce that the transmsson of a packet s condtoned on the fact that the channel s free or busy. Evaluatng the probablty that the target node starts a transmsson at a gven tme s equvalent to modelng the channel status when the node senses the channel, snce we can deduce both the probablty that a node begns transmsson at an arbtrary tme, t, gven the probablty that t was sensng the channel at t, and the probablty to fnd the channel free at ths moment. In order to better descrbe the transtons between the sensng states over tme and the transmttng states, we provde n Fg the dfferent possble sensng and transmttng states from slot to a generc slot, and the possble transtons from one state to ts successors. A transmsson may begn n slot at the tme unt only f the channel was not busy when the sender node sensed the channel at tme unt. Gven that the probablty of beng n a sensng state n (, ) s denoted as P{S } and the probablty
T -p b p b S T -p b p b S T -p b S p b state S B, etc. We depct a state-transton dagram of the meta sensng states n Fg 2. A node n the sensng state can become, at the next tme unt, ether transmttng f the channel s free, or dle f the channel s found busy (cf. Fg ). If the node fals to access the channel, the node wll retry to access the channel at the next tme slot and compute a new backoff value accordng to ts new role and type of traffc. The dagram represents the feasble transtons from all the possble sensng states n slot to the possble sensng states n slot +. Fgure : Representaton of the transtons between Sensng and Transmttng states S A that the channel s found busy n (, ) s denoted as p b, the probablty to begn a transmsson n (, ) denoted as P{T } s: P{T } = P{S } ( p b ) () Snce P{T } only depends on the probablty to be n the sensng state and to fnd the channel free, our model ams to determne all the possble sensng states and the assocated probabltes to fnd the channel free. In the followng, the sensng states are denoted as S, where represents the slot number and the tme unt at whch the node carres out the CCA functon. As the CCA duraton s less that one tme unt, we assume that t s performed durng the last.28ms of the backoff b v, so = b v. The backoff s modeled as follows. The backoff tme value b v s unformly dstrbuted n contenton wndows whch depends on the type of traffc that the target node wants to send and f t s the owner of the current slot. The contenton wndows are non-overlappng ntervals set as shown n Fg.. Slot Slot 2 A B C D b A b B b C b D A = [, β A ) B = [ β A, β B ) C = [ β B, β C ) D = [ β C, β D ) Owner and RT Non Owner and RT Owner and BE Non Owner and BE Fgure : Backoff contenton wndows The value of b v can be any number between and β D, thus enablng the followng sensng states: S, S,..., S β D. However, the behavor of the protocol s unchanged for values of b v belongng to the same contenton wndow. Therefore, t s possble to group the sensng states accordng to the values of : the sensng states S where A are grouped n the meta state S A, the sensng states S where B are grouped n the meta S D S C Fgure 2: State-transton dagram of a generc node The transton from state S to state S parameters : S B + depends on three The probablty to fnd the channel busy n (, ) p b The role of the node n slot + The type of traffc the node has to transmt at the begnnng of tme slot + The transton probablty from state S to state S + depends only on the frst parameter p b, as explaned below. The other two parameters determne whch meta state the transton leads to. Indeed, the role of the nodes evolves and n addton, they can receve RT packets from upper layers anytme. As we want to strctly favor RT traffc over BE, f a node fals to access the channel to transmt a BE packet n slot and receves a RT packet n the meantme, n slot + the node wll be n the sensng state S A or S B, whle t was n S C or S D n. The sendng process of the BE packet s nterrupted. However, n the model, we consder the process of sendng a gven packet from begnnng to end. As a consequence, all transtons from states S C and S D to states S A and S B are mpossble. We represent the remanng possble transtons n Fg. 3 and we further provde the assocated transton probabltes, accordng to the role and type of traffc of the node n slot +. We denote by P{S A S B } the transton probablty from state S where B to S + where A. In Table 7, we gve the transton probabltes of all possble transtons accordng to the type of traffc that the target node wants to transmt and ts role at slot +.
S A S C S B S D Trans s used when the target node has not been owner yet and s not the owner of the next slot Trans 2 s used when the target node s the owner of the next slot Trans 3 s used when the target node has already been owner n the current frame Fgure 3: Smplfed state-transton dagram Accordng to Table 7, these matrces can be wrtten as: Node parameters Node wth RT traffc, owner at slot + Transton probablty P{S A S B } = p b B Node wth RT traffc, P{S B S A } = non owner at slot + P{S B S B B } = p b Node wth BE traffc, owner at slot + P{S C S D D }= p b Node wth BE traffc, non owner at slot + P{S D S C C }= p b P{S D S D D }= p b Table 7: Transton probabltes 5.3. Calculaton In the prevous secton, we have formulated the bass of the mathematcal model. In what follows, we explan n detal the calculaton of the varous elements provded durng the model defnton: the probablty that the target node s sensng, the probablty to fnd the channel busy, and the probablty that the transmsson starts and s successful. 5.3.. Calculaton of the probablty of sensng at the next slot Let V S be a vector formed of the probablty that the target node s n one of the four meta sensng states at tme slot. } V S = {P{S A }, P{S B }, P{S C }, P{S D } (2) The probablty V S + that the target node ends up n the four sensng states at tme slot + s: V S + = V S Trans (3) where Trans s a state-transton matrx. The process s a chan, however, t s not a Markov chan snce our process s not memoryless. Indeed, Trans depends on the hstory of the node, as we explan heren after. The possble transtons from S to S + are determned by the role of the node n slot + (owner or non owner), but f the node has already been owner n the current frame, t cannot be owner anymore n ths frame, and therefore, states S A and S C are no longer accessble. In order to reflect ths evoluton of the role of the node, we represent the transton probabltes as three dstnct transton matrces: Trans, Trans 2, and Trans 3. The computaton of V S + through Equaton 3 uses one of these three transton matrces dependng on the followng scenaros: B p Trans = b D p b B p Trans 2 = b D p b A p b B p Trans 3 = b C p b D p b The probablty P{S + } that the target node fals to access the channel n (, ) and ends up n the sensng state n ( +, ) s expressed as: (4) (5) (6) P{S + } = V S + ( ) (7) In order to ntalze the computaton process, an ntalzaton vector whch descrbes the role and the type of traffc that the target node has to send s necessary. Let V S be the vector whch represents the state of the target node at slot. } V S = {P{S A }, P{S B }, P{S C }, P{S D } (8) The possble values of V S are represented n Table 8. Target node parameters Value of V S Owner wth RT traffc {,,, } Non owner wth RT traffc {,,, } Owner wth BE traffc {,,, } Non owner wth BE traffc {,,, } Table 8: Possble values of the ntalzaton vector V S 5.3.2. Calculaton of the probablty to fnd the channel busy The status of the channel when a node senses the channel determnes f t may start to transmt or not. If the channel s found busy, ths means that another node s already transmttng. Therefore, the node must delay ts transmsson, otherwse a collson wll ensue. In AMPH, once a node gans access to the channel, t transmts as many packets as t can before the