III. T-TMAC P ROTOCOL D ESIGN B. Structure of T-TMACA. Assumptions and Network Architecture To organize the data exchange between sensors in the three tier network we propose simple mechanisms that ﬁt the We propose a heterogeneous and centralized WSN archi- application needs. The sensors of each level follow a dynamictecture described in Figure 1. The sensor nodes are organized scheduling (on / off). They wake up when needed and sleepinto groups: Medical (M ), Coordinator (C), Video (V ) and the rest of the time. The scheduling is organized level by levelSink (S). The network architecture is composed of three as follows: in level 1 between Medical nodes (M ) and theirtiers (Each tier has its own characteristics and requirements): associated Coordinator (C), in level 2 between Coordinators(M, C), (C, V ) and (V, S). A sensor node may interact with (C) and Video (V ), and ﬁnally in level 3 between Video nodesother nodes in inter-tiers or intra-tiers to achieve the common (V ) and Sink (S).goal (healthcare monitoring). This type of architecture offers 1) T-TMAC Superframe: We consider an access methodmany advantages in terms of capacity, coverage, and reliability close to the IEEE-802.15.4 protocol with some modiﬁcations.compared to single tier Ad hoc networks as described in . Indeed, we adapt the parameters setting of the SuperframeMedical nodes collect and relay physiological medical data according to the requirement of each tier taking into account(temperature, ECG, etc.), and Video nodes collect and relay the sleep / active scheduling. In the ﬁrst tier, the commu-ambient data (image, humidity, etc.). nications between (C) and (M ) nodes are organized into M M Superframes managed by the coordinator. The Superframes M M are delimited by Beacons sent by the coordinator, within it C provides information about synchronization, GTS (Guaranteed Wireless link V Time Slot) allocation, etc. The ﬁrst Superframe (Superframe 0) Mobility V V may not contain CFP (Contention Free Period). In fact, there M Medical node S will be only CAP (Contention Access Period) for initialization C Coordinator node V V V (cf. section 2) where medical nodes compete to associate to the V Video node Coordinator and reserve some GTSs (cf. section 4). The other S Sink node Superframes (1 to n) contain only CFP period and remove the WBAN 2 CAP. Figure 2 shows the parametrized Superframe. WBAN 1 CFP CAP CFP CAPFigure 1. Global network architecture for healthcare monitoring at home B …. B We summarize below the main characteristics retained for GTS Superframe 1 GTS Superframe nthis architecture: (i) Low density of sensors: the deployment Figure 2. T-TMAC Superframesarea of the global network is relatively small; we are con-ducting this study in the case of a house. (ii) The Wireless The number of reserved GTS depends on the type and theBody Area Network (WBAN) has a cluster / star topology length of data. In the CAP period (called later “Reporting”that consists of (M) and (C) nodes. (iii) Video nodes are period), the coordinator sends the collected data to (V ) nodestationary, while the WBAN nodes are mobile. (vi) Video to which it is associated. This later, reports data to the Sink.nodes act as relay nodes (forwarding data received by (C)). During this period, medical nodes can turn off their radios(vii) The trafﬁc pattern is periodic: "many-to-one" (from the (sleep mode).bottom to the top (M ) → (C) → (V ) → (S), managed 2) Principle phases of the protocol:by the Sink. The data ﬂow is initiated by (M ) node. (v) Initialization: Its principle role is to synchronize the four(C) node is associated with only one video node at a time groups of heterogeneous nodes between each other for mutualfollowing multi-hop transfer to reach the Sink. Thereafter, we recognition, and to organize data transmission that will be usedpropose the appropriate mechanism for mobility management. in the data collection phase. During this phase, the networkTo enhance the lifetime of sensors, three principle assumptions is created level by level according to a top-down messagesare considered. transfer from (S) to (M ). At the end of network creation, • Data aggregation: (C) node aggregates medical data collection phase begins in bottom-up sense from (M ) to (S)of its WBAN, (V ) node can aggregate data from multiple with periodic data transfer.Coordinators within range. Data Collection and topology reconﬁguration: This phase • Power tuning: we propose to adjust the power transmission takes place immediately after the topology creation. It rep-(TPL: Transmission Power Level) of each sensor node in resents the crucial phase of the protocol. In one hand itthe architecture while maintaining hop-by-hop connectivity allows relaying medical data hop by hop to reach the Sink,between: (M ), (C), (V ) and (S), as shown in Figure 1. This according to the schedule deﬁned in the creation phase. Inleads not only to limit the over-consumption of energy but can other hand, it includes mechanisms for topology maintenancealso leads to limit interferences between nodes . and reconﬁguration (cf. section C). • Sleep / active schedule: nodes operate under activity / 3) Organization of Data Exchange: Both phases operateinactivity mode. (M ) and (V ) nodes can turn off their radio in different ways. Figure 3 describes for each data ﬂow,during sensing data. the different messages exchanged between active nodes per
level. Initially, the Sink node initiates the topology creation by • Slots request by (M): the activity periods of (M ) nodessending "S_Beacon" message to Video (V ) node. This later may differ depending on the data size (temperature, fallsends an association request message "ASC_RQ" to the Sink, detection, ECG, etc.). Indeed, if a medical node wants towhich accepts the request by sending "ASC_ACK" message. send more than one packet in an activity period, we proposeThus, the node (V ) is associated with the Sink. Then, the that it sends a request to its associated Coordinator. In fact,association between (V ) and (C) nodes begins. The node during initialization, (M ) nodes request a certain number of(V ) sends "B_Beacon" message to node (C), followed by the slots (subsequently appointed GTS in Section 1) via ASC_RQexchange of "ASC_RQ" and "ASC_ACK" messages whose message. Then the response will be indicated in C_Beaconﬁnalize their association. Then the initialization of the WBAN message (with the number of allocated slots), for use in thenetwork starts between (M ) and (C) nodes with the exchange collection phase. However, in Data collection phase, the re-of "C_Beacon", "ASC_RQ" and "ASC_ACK" messages. After quest could be integrated in DATA message, and the responsesetting up the WBAN network, the data collection begins. will be in the C_Beacon message (in the next cycle). ThisThe node (M ) collects data, builds the ﬁrst data message mechanism responds to the dynamic behavior of network."DATA" and sends it to its associated Coordinator (C). Then • Reliability: to reduce collisions / transmission errors that(C) responds with acknowledgment message "ACK". "DATA" could occur during the two phases, it is necessary to retransmitmessage will be then relayed by (C) to (V ), and ﬁnally all messages (the number of retransmissions is parametrized)relayed by (V ) to the Sink (S), with the exchange of "DATA" including Beacon messages (except acknowledgments). Thisand "ACK" messages. As shown in Figure 3, after receiving leads to increase the reliability of the data exchange.the "ACK" message, nodes can turn to sleep mode to save • Trafﬁc model: we assume that the trafﬁc model made inenergy. the Data collection phase is periodic with the same period throughout the network. Each node (M ) is the origin of one or more data packets in each period (one data ﬂow). (M ) Sink V C M S_Beacon ASC_RQ nodes can also aggregate all medical data in one packet (the ASC_ACK B_Beacon maximum size of PPDU in IEEE 802.15.4 is 127 bytes). To Topology creation phase ASC_RQ meet the energy needs of the sensors, the activity period should ASC_ACK C_Beacon be optimized and reduced as much as possible. ASC_RQ ASC_ACK C_Beacon DATA Start of data collection of (M) C. Complementary Mechanisms for Topology Maintenance ACK B_Beacon We have improved the topology maintenance with new mech- Data collection and reconfiguration phase DATA ACK DATA Reporting anism. Sending Beacon messages periodically offers other ad- S_Beacon vantages that meet the application requirements. Particularly, DATA ACK they can be used for mobility management, re-allocation of Active mode new slots for WBANs, and topology re-conﬁguration: addition Sleep mode Level 3 Level 2 Level 1 of new sensor, and removal of a sensor (depleted battery, sensor breakdown, etc.). Actually, each level has a managerFigure 3. Principle mechanisms of the protocol node that takes decisions while an event occurs: (C) node for tier 1, (V ) node for tier 2 and (S) node for tier 3. • Medium access: to minimize collisions in the three tier The mobility of a person has an impact on communicationsarchitecture we manage the medium access as follows: we of tier 1 and tier 2: (i) In tier 1, when 2 WBANs are situateduse “Data reporting” period to manage “inter-tier” interactions in the same range, to reallocate new slots, the (C) node sendsbetween nodes belonging to different levels (nodes report data C_Beacon message containing the new allocated slots. (ii)(by level) as shown in Figure 3. However, it is necessary, to In tier 2, the link between (C) and (V ) could be interrupted:assign the appropriate access method to minimize collisions node (C) should re-associate with a new (V ) node (we can usebetween nodes belonging to the same level. In fact, we propose the initialization mechanism via the exchange of B_Beacon,a hybrid access: ASC_RQ and ASC_ACK messages). Then the new (V ) node - Slotted access for M and C nodes (WBAN): it provides relays the received data to the Sink (via DATA and ACKa guaranteed access and a reduced delay for medical data. messages). - Contention access for (C, V ) and (V, S): this is appropri-ate for mobility. • Maintaining synchronization: during the data collection IV. D IMENSIONING AND P ERFORMANCE E VALUATIONphase, the network must operate under a regular schedule to A. Analytical Modelensure the transit of the data between levels 1, 2, and 3 inorder to reach correctly the Sink. To this end, we propose that The delay and energy consumption are the most importantCoordinator, Video and Sink nodes send periodically Beacon performance criteria for the application. Below we evaluate themessages. These messages have a crucial role because they performance of the protocol in the two phases: initializationpermit to resynchronize nodes (to prevent the clock drift and data collection. We used three important parameters:phenomenon) while keeping the data exchange hop by hop. network size, transmission interval and data size.
• Initialization phase analysis: The ﬁrst association phase Where Tslots is the slot duration and N (i) is the total numberbetween all medical nodes and the Coordinator is represented of slots allocated to medical node (i). To calculate the energyby the duration D : consumption in data collection we reuse the formula (7): D = TC_Beacon + nM i=1 DASC (i) (1) EDC = erx · (TC_Beacon + TACK ) + etx · (TDAT A ) (9)Where DASC (i) is the association duration of one node i and • Other Superframes analysis: In the same way, we cannM the number of medical nodes. Then, let DASC (i) is the evaluate the upper tiers (C, V ) and (V, S). The principlemean association time of node i with the (C) of its WBAN: parameter that changes is the number of nodes per level (nC ,nV ). However, in data collection analysis, the Tcs duration must be added because (C) and (V ) nodes compete for the DASC (i) = Tcs + TASC_RQ + TASC_ACK (2) medium access. Tcs is a random duration before each node i sends its“ASC_RQ”. To reduce collisions, each node initially senses the B. Hardware Implementationchannel during a random duration Tcs uniformly distributed in • Platforms used: we have implemented T-TMAC in Imote2the interval [0, Tf ], where Tf is the maximum of the duration. hardware platform. The Imote2 transceiver operates at ISM 2.4For the sake of simplicity, we consider in a similar manner to T GHz frequency, 17.4 mA with (0 dBm) power output and it the mean value of Tcs : Tcs = 2f (3) allows data rates of up to 250 Kbps. The micro-controller runs at 13−416MHz. This device requires a supply voltage between • When the other medical nodes hear the ﬁrst “ASC_RQ” 3.2−4.5 Volts, and is powered by three 1.5V (AAA) batteries they must wait for a time equal to (TASC_RQ + in series. The sensing unit includes: temperature, acceleration TASC_ACK ) before starting to draw again a random and humidity measurements. Two kinds of cards  may be duration Tcs . embedded on Imote2 radio card: Sensing card and Video card. • The number of reserved GTS (reserved by (M )) depends • Protocol implementation: Figure 4 shows the implemented on the kind and the length of data. sensor network and Table 1 shows the measured real parameterThe association duration of the other nodes can be written as values of the Imote2. Four important tasks are realized to buildfollows: the network architecture and to test the network operations: power tuning, frames setting (S_Beacons, DATA, ACK, etc.), Tf DASC (i)i=1 = DASC (i − 1) + i+3 WBAN implementation (aggregation functions for (C), slots (4) +TASC_RQ + TASC_ACK management) and Data forwarding. Energy consumption: To calculate the energy consumption PARAMETER VALUE U NITof the ﬁrst node we can reuse the formula (2) by adding S_Beacon, B_Beacon 11, 7 msconsumption corresponding to each mode (reception or trans- CBeacon 12, 8 msmission), then we obtain: TASC_RQ 12, 2 (M ), 11, 7 (C, V ) ms EASC (i)i=1 = erx · (Tcs + TASC_ACK ) TASC_ACK 11, 9 (M ), 11, 7 (C, V ) ms (5) +etx · (TASC_RQ ) TDAT A(M ) , TDAT A(C) , TData(V ) 13, 7 (M ), 14, 9 (C, V ) ms To calculate the total energy consumed we add the amount TACK 11, 9 (M ), 11, 7 (C, V ) ms(erx ·TC_Beacon ) that corresponds to the receiving of the Coor- ST 1083 (M 1), 450 (M 2) msdinator Beacon (“C_Beacon”) and erx , etx are respectively the etx 74, 2 (92 bytes) mAenergy consumed when receiving and transmitting data. The erx 97, 2 (92 bytes) mAenergy consumption of the other nodes during the association el 56, 4 (listening) mAphase can be written as follows: epx 37 mA T epx(sleep) 500 µA EASC (i)i=1 = erx · [DASC (i − 1) + i+3 f (6) esgx 6, 4 (procOn : 43, 4) mA +TASC_ACK ] + etx · (TASC_RQ ) LED 2 (procOn : 39) mA As shown in Figure 3, in the reporting period, (C) sends N umber of retransmissions 3 /Othe collected data to (V ) to which it is associated. During this T x power (M1 , M2 , C1 , V1 , S) −25, −25, −10, −10, −10 dBmperiod, medical nodes may turn off their radios (sleep mode) Table Ito save energy. M EASURED PARAMETER VALUES• Data collection phase analysis: It corresponds to the datasending and differs from the association, because during thistime, medical nodes don’t have to compete for the medium V. R ESULTS AND DISCUSSIONaccess, however they have to send “DATA” messages larger Figure 5 shows the prototyping results for initializationthan “ASC_RQ”. and data collection phases. The graph on the left side shows the current consumption during initialization of each node DDC = TC_Beacon + TDAT A + TACK (7) (M 1), (M 2), (C), (V ) and (S). We see that it increases de- nM DT otal = Tslots . i=1 N (i) (8) pending on the nodes position in the architecture. (M ) and (C)
nodes are the larger consumers of energy. This is due to the of energy saving. Our on-going work is focused on detailedwaiting time that these nodes spent to receive their appropriate modeling analysis and scenarii evaluation and on the compari-Beacon message (B_Beacon for (C) and C_Beacon for (M 1) son between T-TMAC and other MAC protocols such as IEEEand (M 2) (as shown in Figure 3). The graph on right side 802.15.4. Other perspective of this work concerns extendingshows the current consumption of each node during data the protocol with scalable slot allocation mechanism. Thiscollection phase, during 1 hour of operation, with Deep Sleep mechanism could be appropriate for large applications withperiod of 20 seconds. The results show clearly the advantage dense number of nodes such as in the case of hospitals.of the Deep sleep mode implemented on Imote2 platforms, to Reel prototyping Reel prototypingsave the energy of all nodes. Analytical model Analytical model Current consumption during data Current consumption during Exchange of messages initialization (mA) between neighbor nodes collection (mA) 0x09 1) "Radio"card Terminal Serial Dump Sink 1) "Radio card" 1) "Radio" card 2) "Vidéo" card 0x07 0x08 Sniffer PC Xsniffer V1 V2 Type of node Type of node 1) "Radio" card 0x05 0x06 Figure 6. Comparison of results obtained by the two methods: current C1 C2 consumption by each node during initialization and data collection phases 1) "Radio" card R EFERENCES 2) "Sensor" card 0x01 0x02 0x03 0x04  H. Alemdar and C. Ersoy,"Wireless Sensor Networks for Healthcare: A M1 M2 M3 M4 Survey", Elsevier Computer Networks, Volume 54, Issue 15, pp. 2688- 2710, October 2010. WBAN 1 WBAN 2  G.Z.Yang,"Body Sensor Networks", Springer-Verlag London, 2006.  A. Cerpa, J. Elson, D. Estrin, L. Girod, M. Hamilton and J. Zhao, "HabitatFigure 4. Implementation of the sensor network monitoring: Application driver for wireless communications technology", In Proc. ACM SIGCOMM Workshop on Data Communications, Latin America and the Caribbean, April 2001. Consumption of each node in Sleep mode  J.A. Paradiso and T. Starner, "Energy scavenging for mobile and wireless the network Deep Sleep mode electronics", Pervasive Computing, Volume 4, Issue 1, pp. 18- 27, 2005.  G. Anastasi, M. Conti, M-D Francesco and A. Passarella, "Energy Current consumption during data conservation in wireless sensor networks : A survey", Ad Hoc Networks, Current consumption during 2009. initialization (mA) collection (mA)  I.F. Akyildiz, T. Melodia and K. Chowdhury, "A Survey on Wireless Multimedia Sensor Networks", Elsevier Computer Networks Journal, March 2007.  S-K Chong, S. Krishnaswamy and S-W Loke, "A context-aware ap- proach to conserving energy in wireless sensor networks", Third IEEE International Conference on Pervasive Computing and Communications Workshops, PerCom 2005 Workshops, pp. 401- 405, March 2005 Type of node Type of node  "Wireless Medium Access Control (MAC) and Physical Layer (PHY) Speciﬁcations for Low-Rate Wireless Personal Area Networks (WPANs)", IEEE Std. 802.15.4, 2006.Figure 5. Prototyping results: initialization and data collection phases  J. Polastre, J. Hill and D. Culler, "Versatile low power media access for wireless sensor networks", In Proceedings of the 2nd international We estimated analytically the average energy consumption conference on embedded networked Sensor Systems (SenSys’04), pp. 95-(EASC ) in initialization and Data collection phases. Figure 6 107, New York, 2004.  W. Ye, J. Heidemann and D. Estrin, "Medium access control with co-shows the comparison between analytical and real prototyping ordinated adaptive sleeping for wireless sensor networks", In IEEE/ACMresults, for one hour of operation of the network, with Deep transactions on networking, vol. 12, no. 3, pp. 493-506, June 2004.sleep of 20 seconds (180 cycles). We added in the analytical  A. El-Hoiydi, J.D. Decotignie, C.C. Enz and E. Leroux, "WiseMAC, an ultra low power MAC protocol for the wiseNET wireless sensor network",calculation the values assigned for data sensing ((M 1): tem- SenSys 03, pp. 302-303, 2003.perature and (M 2): all Data). We show that analytical results  V. Rajendran, K. Obraczka and J. J. Garcia-Luna-Aceves, "Energyﬁt with the prototyping results. efﬁcient collision-free medium access control for wireless sensor net- works", In Proceedings of the 1st international conference on embedded networked Sensor Systems, SenSys ’03, pp. 181-192, 2003. VI. C ONCLUSION  I. Rhee, A. Warrier, M. Aia, J. Min and M.L. Sichitiu, "Z-MAC: a hybrid MAC for wireless sensor networks", IEEE/ACM Transactions on In this paper a new MAC protocol for healthcare moni- Networking, pp. 511-524, 2008.toring is presented. Simple mechanisms based on sleep/active  Y. Zatout, E. Campo and J. Llibre, "WSN-HM: Energy-Efﬁcient Wire- less Sensor Network for Home Monitoring", ISSNIP 09, Melbourne,schedule are proposed for energy efﬁciency. We showed the Australia, 2009.advantages of data aggregation and allocation of slots in a  Y. Zatout, R. Kacimi, J-F. Llibre and E. Campo, "Mobility-awaremulti-tiers architecture. The performance evaluation has been Protocol for Wireless Sensor Networks in Health-care Monitoring", Fifth IEEE International Workshop on Personalized Networks, USA, 2011.realized with an analytical model and with real prototyping on  S. Zhao and D. Raychaudhuri, "Multi tier Ad hoc Mesh Networks withImote2 platforms. The results ﬁt very well. From all results, Radio Forwarding Nodes", IEEE Global Telecommunications Conference,it seems that T-TMAC protocol provide a signiﬁcant amount IEEE GLOBECOM 2007, Washington, USA, 2007.