Proposition and validation of an original MAC layer with simultaneous medium accesses for low latency wireless control/command applications

Proposition and validation of an origin al MAC layer with si multaneous accesses for low latency wireless co ntrol/command applications Adrien van den Bossc he, Thierry Val, Eric Campo University of Toulouse, UTM, LATTIS EA 4155 – S CSF group, Blagnac, France {vandenbo, v al, campo}@i ut-blagn ac.fr Abstract: Con trol/comm and processes req uire a transm ission system with som e characteristics like high reliability, low latency and strong guaran tees on messa ges delivery. Concerning wire n etworks, field buses technologies like FIP offer this kind of service (periodi c tasks, real time const raints…). Unfortunately, few wireless technologies can propose a comm unication system which re sp ects such constraints. Indeed, wireless transmissions m ust deal with medium charact eristics which make impossible the direct translation of mechanism s used with wire networ ks. The pur pose of this paper is to present an original Medium Access Control (MAC) laye r for a real time Low Powe r-Wireless Perso nal Area Network (LP- WPAN). The proposed MAC -layer has been validated by seve ral complem entary methods; i n this paper, we focus on th e spec ific Si multan eous G uaran teed Time Slot (SGTS) part. 1. INTRODUCTION Today, wireless net work technologi es are widely used in many applicati ons. Wireless eliminates ex pensive, heavy , not aesthetic cables, which are not easy to install o r to use. Using a wireless network i s sometim es a luxury , but it may be necessary in many cases of moving devices, like car tire sensors, em bedded sensors on robots, et c. All these new needs encourage research and industrial to develop technologies and produ cts in this domain. A typical wir eless sensor net work (Cul ler et al. 2004) technology has to propose stron g and reliable m echanisms for each level of the OSI m odel: Phys ical layer (PHY) must deal with poor Bit Error Rate, Medium Access Control layer (MAC) must avoid collisions and solv e hidden terminal, Network layer (NWK) must enable automatic routing and insure reliability for mobile nodes (Badis et al., 2004), and so on. For a wireless cont rol/comm and applicat ion, a high reliability is required: the techno logy must propose some guarantees dep ending on the applicatio n (temporal bo unding on transmi ssion latency an d packet for warding, m inimal throughput for critical nodes, maximal packet loses…). Adding Quality of Serv ice (QoS) functionalities to t he network is crucial in this typ e of real-time network application ( Simpl ot-Ryl, 200 5). Our research works take pla ce at the second level of the OSI- model ( link-layer ) for time-constrained and comm unicating applications such as robotics (van den Bossche et al. 20 06) (van den Bossche et al. 2007) in not di sturbed e nvironm ents. The MAC sub-layer is in ch arge of the medium access organizati on, i.e. avoidi ng simultaneou s transmissi ons which imply frame collisions and retransmission s, involving transmission l atency. In th e context of t ime-constrai ned wireless netwo rks, we have pr oposed a new MAC method f or the IEEE 802. 15.4 wireless technology (van den B ossche et al. 2006) (van den B ossche et al. 2007); the proposed MAC enables guaranteed and periodic m edium accesses thanks to a centralized scheduling and an exhaustive timeslot rep artition between nodes . In this pape r, we prop ose an im provement of the MAC in order t o prevent the ra refaction of slot s, by allowing nodes to access medium sim ultaneously (concept of SGTS – Sim ultaneous Guara nteed Tim e Slot, define d later). After this introduction, we fi rst present the IEEE 802.15.4 wireless networ k technology and the weaknesses we identified in the context of time-cons trained networks. Then we propose a brief descri ption of the new MAC and t he SGTS improvement. At last, we present the valid ation by hardware prototyping and the obtained results of the SGTS concep t are pres ented. 2. PRE SENTATION OF IEEE 802.15. 4 IEEE 802.15.4 standard (IEEE 2003) (I EEE 2006) proposes a two-layer proto col stack (physi cal-layer and data link-layer) for low power transceivers and l ow baud rate communicat ions between em bedded de vices. Innova tive concepts optimize energy saving . Moreover, IEEE 802.15.4 standard is promoted by the ZigBee Alliance (ZigBee Alliance, 2005) as the physical -layer and data-link-layer of the ZigBee Network specifications. 2.1 Overview IEEE 802.15.4 proposes two PHY layers: PHY868/915 and PHY2450. The first on e operates on both 8 68 MHz and 915 MHz radi o bands. It proposes a ve ry low dat a-rate (20 kbps at 8 68 MHz and 40 kbps at 9 15 MHz) wit h a simple BPSK modulat ion. The PH Y2450 laye r is more inte resting: it allows a greater throughput (250 kbps) thanks to an O-QPSK modulation. M oreover tha nks to its Di rect Sequence Spre ad Spectrum (DS SS) coding, PHY2450 has excellent noi se immunity (IEEE, 2003). The two PHY layers were designed for maxim um energy sa ving: prot ocols are optim ized for short and peri odical data transfe rs. Nodes m ostly stay in a “sleeping” m ode called doze mode. Radio m odem al lows ultra low po wer consu mption ( 40 µA) (Freesca le Semiconduct ors, 2005) and nodes becom e operational in a very short tim e (330 µs). In doze m ode, all radi o functionalities are switched off, removing the ability to receive messages. The wa king time has to be set before goi ng in doze mode (synchronous wake-up) but sleeping devices may also wake-u p if a local event occ urs (asynchro nous wake-up): m otion detectio n for exam ple. 2.2 Medium Ac cess Control ( MAC) and t opologies The standard IEEE 802.15.4 pr oposes two data link-layer topologies: P eer-to-Peer an d Star. Peer- to-peer to pology makes possible direct data transfe rs between devices in rad io range on the sam e radio channel . Access to the m edium uses the CSMA/CA protocol without RTS/CTS mechanism. On the contrary, S tar topol ogy needs a st ar coordi nator: all data transfers go thro ugh the coordinat or and messages are buffered during the dozing period. Th is functionality is called indirect data transfer. Star topology allows high energ y saving thanks t o an optim al distribution of sleeping peri ods between embedde d devices. For synchro nization, the star coordinator sends beac on frames. Inter- beacon perio d is called superfra me. During t he superfram e, the nodes sleep until the next beacon, wake up a nd receive the beacon, ask the star coordinator for pendi ng data, trans mit and receive and then go to doze-m ode agai n. In addition to the classical C SMA/CA-based medium access method, IEEE 802.15.4 pr oposes a Content ion Free met hod for the Star beaconed topolo gy. Nodes can request for Guaran teed T ime Slots ( GTS) to the star coordinator. A GTS consists in one or sev eral time slots dedicated to a p articular node and ca nnot be us ed by ot her nodes. GTSs are ann ounced by the beacon frame, a supe rframe contains up to seven GTS. The number of GTS reservatio ns for a terminal node is directly linke d to its communi cation bandwidt h. This process of medium access reservation provid es Quality of Service properties like b andwidth reservation or latency guarantees (Huang et al . 2006), like 802.11e HCF (IEEE, 2005). Fig. 1: superfr ame struct ure The IEEE 802.15.4 s uperfram e mixed structure ( Fig. 1) combines both m ethods as f ollows: First, a star coordin ator sends a beacon fram e to indic ate the networ k and co ordinator addresses, the nodal data pendin g, the sizes of t he Contention Access Period (CAP) a nd Cont e ntion Free Period (CFP). Then starts the CAP where the nodes and coordi nator send/receive frames using CSMA /CA protocol. This time is also used for r equest from a node to obt ain GTS in t he next superframe. At the end of the CA P, the CFP starts as defined by the coordinator and broad casted by the beacon. Medium access is possible only if the no de has successfully obtained a GTS. At the end of the CFP, a ll nodes go to doze mode if not already and wait until the nex t beacon scheduled broadcast by using an internal wak e up timer. This sleeping period is optional but greatl y advised for ene rgy savi ng. 1 0 2 * 36 . 15 ≤ ≤ = BO with ms BI BO (1) BO SO with ms SFAP SO ≤ ≤ = 0 2 * 36 . 15 (2) Therefore, the superfram e is characterized by two tem poral parameters Beacon Order ( BO ), Superframe Order ( SO ) announced in beacon frames: BO defines the time interval between two beacon messages. Beacon Interval (BI) is calculated as ment ioned in (1). SO defines the Su perFrame “Active Portion” (SFAP = TCAP +TCFP) and is calculated as mentioned i n (2). If BO and SO values are sm all, the network is more reactive (low latency) with lower energ y saving. The greater is the difference be tween BO and SO, the m ore energy is saved. Thus, an appropriate value for these two parameters will be required considering the applications requirements. 3. IDENTIFIED WEAKNESSES OF THE MAC LAYER IN A LOW LATE NCY APPLICATI ON CONTEXT The medium access met hod propose d by the IEEE 802.15. 4 standard is simple and flexible eno ugh for auto-confi gured and spontane ous networ ks. The CSM A/CA prot ocol ena bles automatic adaptation of the mediu m access, even if the number of nodes is important. Howeve r, if an IEEE 802.15.4 network is used in a tim e-constraine d or real-t ime context , the CSMA/CA based MAC does not fit because i t does not propose any guara ntee on messages deliver y, since it is a best effort protocol. As shown in the abov e section, 802.15.4 adopts an interesting mechanism of medium access reservation (GTS) t o free some pri vileged nodes fr om the collision phenome non. However, the medium reservation is conditioned b y two factors: Fi rst, the network m ust be maintained within its capacity an d avoid saturation (Misic et al . 2006). Unfort unately, the standard d oes not grant to a star coordinator the capability t o permanently maintain some bandwidth for a particular node. Th e GTS reservation process works as “first come, first served” a nd is not an acceptable rule of distribution in terms of Qu ality of Service. Second, the primitive call “GTS.request” gen erates a message sent to the star coordinator during the CAP using the CSM A/CA protocol. As t his protoc ol is Best -effort, it can not provide any temporal guarantee. Many contri butions ha ve been proposed in order to in sure QoS functionalities at MAC-level by an optimi zation of beaco n or GTS schedu ling, for exampl e (Koubâa et al. 2007) or (Franco mme et al. 2007). In this paper, the work is focussed on th e proposition of a full determi nistic MAC by using a ne w schedul ing. To achieve a communication bet ween devices in an application with temporal co nstraints, it is essential to insure bandwidth and network latency fo r a number of known critical nodes. Moreover, the differe nt devi ces may have different comm unication re quirement s: strong s poradic fl ows, regular flows with time dependency, etc. In this time-critical context, the standard IEEE 802.15.4 ha s others weaknesse s: - The GTS frequency is based upon t he superfram e frequency, th e star coordi nator BO and SO internal clock parameters. T he nodes m ay only need t o communi cate from tim e to time and not on a regular base. I n othe r words, it is extremely difficu lt for sensors with different data communi cation need to c ohabit on the same star without loss of continuity an d optimal GTS distribution. - If several stars are in the same radio range and on the same channel, there is a high probability for co llisions even during the CFP because the standard doe s not provide com munication p rotocols bet ween star coordinators. According to all these observation s, the mechanism of medium reservation c ould be great ly im proved by: - A fully deterministic access method to insure GTS for selected known nodes at each superframe, - A more flexible GTS allocation to support various access frequencies a nd band width, - The introduction of a new p rotocol betwee n star coordinators in order to avoid GTS collisions. 4. NEW FUNCTIONALITIES PROPOS ED BY THE MAC In (van den B ossche et al. 20 06) (van de n Bossche et al. 2007), we ha ve present ed an ori ginal M AC layer for a small time-constrained sens or network, based on the IEEE 802.15.4 Guaranteed Ti me Slot m echanism. The pr oposed m edium access method enables periodical tasks t o access medium at predetermi ned and regular moments. More over, the new GTSs are guaranteed beyond the star , in order t o avoid GTS collisions between stars in th e same radio range and channel. The principal functionalities of the new MAC are presented in the next paragra phs. 4.1 New proposed functionalities The proposed mechanism s ar e intended to ach ieve the following new functionalities: - W ith the present IEEE 802.15.4 standard, only nodes can request for a GTS. We propose to give a s tar coordinator the ability to allo cate GTS at any time to any kn own node, in anti cipation of the re quest, depe nding o n the application needs. Th is functionality makes po ssible deterministic network associations for critical nodes (i.e. in a bounded time). Inde ed, a mobi le node might be capable of ch anging its co ordinator without l oosing its guaranteed medium access. We call this ability PDS, for Previously Dedicat ed Slot . - W ith the prese nt standard, t he GTSs were m anaged by t he coordinator with internal messages within the star. We now propose a mechanism to extend these communicat ions between c oordinato rs to avoid “GTS collisions” (two coordinators g ive a same GTS for two nodes by t wo different stars in t he same radi o range). - W ith the present standard, the GTSs were placed in the CFP, at the end of the supe rframe. We propose that t he GTSs will be laid out anywhere in th e superframe at the coordinator discretion. Th is functionality will enable u s to optimize GTS distribution and with a possible extension to generate a n optim ized global supe rframe com posed of several stars, even if CSMA performance will be sacrificed because CAP pe riods are non-contiguous. - W ith the present standard, a GT S appeared in every single superframe after allocation. We now suggest regulating this GTS inclusion in the superframe at a lower freq uency to fit the node needs. Thu s , a GTS can appear in one superframe out of two, one ou t of four, one out of eight, etc. We introduce the no tion of reservation lev el n , an integer from 0 to n MAX . The GTS of a node wit h a reservation level n will appear in every p = 2 n superframes. It will allow different QoS traffics to cohabit within the same star without need for adjustments of BO and SO parameters. 4.2 Aimed net work topol ogy and role of the PAN-coordin ator The proposed M AC organizati on is managed by the PAN coordinator. This central en tity: - broadcasts su perbeacon frames to synchronize all th e network devices. This b roadcasting is m andatory to provide a glo bal synchroni zation o ver the net work and reduce frame collision (Rowe et al. , 200 6), - receives all GTS request messages, - schedules the medium accesses o f critical nodes, i.e. nodes which request GTS. Thanks to this centralized organisation, the collision GTS phenomenon i s limited. No w, a GTS is a “real Guaranteed Time Slot”, since n o other node is allowed to t ransmit dur ing the timeslot, even if the GTS is dedicated to another star node. Of c ourse, best-effort medium access using CSMA/CA is still possible for non critical nodes. The ai med network topology is illustrated on figure 2. Fig. 2: Net work topology and mini mal radio ra nges between nodes The proposed network is composed of three node ty pes: a unique P AN-coordi nator, one or several st ar coordinat ors (one for each star) an d one or several simple nodes . Each simple node must be associated to a star coordi nator. In a first approach, we have considered th at each star coordinator is in the radio ra nge of the PAN-coordi nator; thi s hypot hesis is not so constraining th anks to the availability of high-power IEEE 802.15.4 devi ces such as MaxStream XBeePRO (MaxStream 2006) which enab le extended rad io ranges up to 1 mile. In return, a star coor dinator m ay not be in the range of all ot hers star coordinators – the PAN c oordinator solves the hidde n star coordinator issue by di stributin g slots for c oordinat or beacons, GBS ( Guaranteed Beacon Slots ). Fig.3: An example of scheduling by PAN -coordi nator The figure 3 illustrates a m edium sched uling with n MAX = 3. The table in the figure 3 shows the timeslot repartition decided by t he PAN-co ordinator for t he next 2 n MAX future superfram es. The PAN-co ordinator br oadcasts i ts superbeacons on each slot #0. Slots #4, #8 and #12 are GBS for the three coo rdinators ( nodes 1, 2, 3 ). Simple no des (11, 21, 22, 31, 32 and 3 3) have di fferent nee ds, so they have requested one or more GTS with different reservation level n (one superfram e out of eight for node 33, one superfram e out of four for node 32, o ne superf rame out of t wo for no des 11 and 21 and at last a GTS in each superframe for nodes 22 and 31). 5. PRESENTATION OF THE SGTS C ONCEPT In section 4.2, we illustrated the scheduling organized by the PAN-coordi nator. To preve nt the rare faction of slot s, we propose a possibility, fo r the PAN-coordinator, to give the same timeslot for two different star nodes if the rad io conditions make it possible, enab ling the possibility for each node to trans mit its m essage at the same tim e without a ny collision. Th is optional fun ctionality of sp atial reusing, as in (Lee et al. 2006), should increase th e performances of the network since two distant nodes can send their message at the same time. Nevertheless, this functionality may be used carefully: the GTSs must keep their “Guaranteed” characteristic. In a first appro ach, we consider that a SGTS will regroup in a single timeslot the tran smission of no more than two transmitters. In order to determine if two nodes can transmit at the same time without making a pert urbation o n the transmissi on (i.e. if two allocate d GTS can be regroupe d in a singl e SGTS), t he PAN-coordi nator m ust know the radi o recepti on conditi ons of the destinati on nodes. Indeed, the S GTS needs to be negotiated by taking into accou nt the receiver opinions. The PAN-coordinat or must ta ke into account only the two destination node s to decide the SGTS attrib ution. If we consider two node pairs (two tr ansmitters, two receivers), the two receivers must not be pert urbed by the other transm itter, as shown on figure 4. On this illu stration, A2 sends a message to A1 and B2 sends a message to B1. The SGTS can be negotiate d, i.e. A2 and B2 can both transm it their me ssage at the same tim e, only if A1 is not pert urbed by B 2 and B1 i s not perturbe d by A2. A non perturbation threshold, in dB, based on the two signal le vels (RSSI, Received Signal Strength Indic ator ) in the t wo receivers, must be ide ntified. This threshol d has been evaluated by a study on real prototype an d result s are presente d in the next section. Fig. 4: Illustration of th e non-perturbation princip le to negotiate a SGTS betw een two node pairs Nevertheless, each SGTS must be negotiated with precaution , particularly if the conce rned nodes are m obile no des. In this case, we recommend disabling this functio nality. 6. SGTS VALIDAT ION BY HARDWARE PROTOTY PING The concept of Sim ultaneous GTS needs to be vali dated and the non p erturbation th reshold has to be identified. Th e real prototyping se ems to be the best way to validate t he SGTS concept. Therefo re we have develo ped a prototy pe of the MAC layer a nd deploy ed a networ k based o n a couple of F REESCALE TM IEEE 802.15.4 devices (Freescale Semiconduct ors, 2005); t his type of 802 .15.4 devices is totally reprogrammable, which allo wed us to implement the proposed MAC layer and th e SGTS negotiation. This prototype als o enables us to eval uate the non perturbation threshold e voked in t he section 5. 6.1 The prototy pe network and its top ology The prototype network is com posed of five nodes: a PAN- coordinator (PC), two star coordi nators (C 1, C2) and two simple nodes (N1, N2). Each si mple node is associated to a different coordinato r. The netw ork topol ogy is shown on figure 5 while the super frame structure is rep resented on figure 6. Fig.5: The prot otype network t opology Fig. 6: Superfr ame struct ure during the tests For this study case, we cons ider that both N1 and N2 have obtained a GT S and can f reely use it to se nd data messages to their star coordinator (N1 to C1 in slot #3 and N2 to C2 in slot #4). Slot # 5 is used by both N1 and N 2. In this study, the objective is to evalu ate the non perturbatio n threshold , i.e. t o measure the num ber of collis ions in slot #5. In orde r to evaluate the perturbation of the oth er transmitter (N2 for C1 and N1 for C 2), each c oordina tor listens the m essages sent by the two nodes and gets the RSSI value duri ng the slots # 3 and #4; the RSSI differe nce is cal culat ed at the end of slot #4 if both me ssages from N1 and N2 whe re received. In slot #5, each coordinator listens to th e medium; if the coordinator receives the message sent by its node, the result is positive . If the coordinator receives t he message of the other node or a collision, the result is n egative . Note that on the F REESCALE TM IEEE 802.15.4 devices used, the transmit power can be adjusted from -16d Bm up to +3.6dBm; this fun ctionality enables us t o impl ement an a utoma tic variati on of node transmitting power to increase the measure range without moving the nodes. All m easures have bee n realized into an anechoic chamber, i.e. without an y noise. 6.2 Obtained resu lts The results obtained on figure 7 are really in teresting: in most cases, two transmissions can be done at the same time without perturbing the other recei ver. In fac t, measures show (a well-known result) that there is only a 1 0 dB window where the SGTS should not be negotiated b ecause of an important risk of collisio n. Fig.7: SGT S validati on by h ardware p rototypin g: obtai ned results Moreover, t he results prese nted on figure 7 show a nother interesting point: the two lin e plots do not cross at 0 dB. It indicates a certain inequality be tween the two nodes: we notice that the nod e N1 has a greater probability of being received by the two receivers, even if its message is received with a smaller RSSI than the message of N2. The same results have b een obtained wit h other measure s. Indeed, a study on sy nchronisat ion has sh own us that our three-le vel synchronisatio n procedur e (PAN-co ordinator, s tar coordinato r, simpl e node) of our p rototype network was not so perfect: one of the two nodes takes a dvantage o n the other by having a little temporal adv ance (few µs). We have made a cross-comparison of these tw o studies and determined that the favourite node in the SGTS study is the one which takes the temporal advance in the synchron isation study. In order t o verify t his hypot hesis, we m ake othe rs measures with the same synchronisatio n for the two no des, as illustrated on figure 8. Of course, this study is unusable for transmitting data, since the destination of the two message sent at the same time is the same coordinator. Fig. 8: Netw ork topology of the seco nd study Fig.9: Obtained results with the second SGTS study The obtained results, on fi gure 9, sh ow that the node e quality is now correct. The line plots cross now at 0 dB. 6. CONCLUS IONS Our works deal with a new MAC-layer for a LP-WPAN IEEE 802.15.4 with real determ inistic capabilities. Thanks to this medium access m ethod, time-constrained nodes can negotiate a periodical and guaranteed medium access, as required in a control/command application. Moreov er, the reservation level parameter ( n ) enables th e cohabitation o f different profiles of traffics in the sam e network wi thout changing BO and SO . In thi s paper, we have present ed an improvement of this MAC-layer which enables simultaneous accesses by the use of specifically described SGT S. SGTSs have been impl emented and t ested on a couple of hardware devices. Measures h ave proved the possibility of simultaneous transmissions without collision wh en propagation cond itions are accepted by the PAN-coord inator. On theses firs t results, we are very optimisti c about the potential improvem ent of th e performa nces of new MA C- layer for a time-const rained wireless net work. The fut ure works concern the optimisation of th e scheduling, including the optional fun ctionality of SGTS. W e are participating in a national proj ect (OCARI 2007) focussed on the d evelopment of a low power and large scale sensor network with time constrained m essages delive ry for industrial applicatio ns. REFERENCES Badis, H. and Al Agha, K (2004). 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