Proceedings of APCC2008 copyright (c) 2008 IEICE 08 SB 0083
The MAC protocol of WSN for Supporting Telematics Services in the Road Environment Jae-Han Lim∗ , Jungsook Kim∗ , Changsup Shin∗ , Chong Poh Kit† , ByungTae Jang∗ ∗ Telematics
and USN Research Division, ETRI 161 Gajeong-dong, Yuseong-gu, Daejeon 305-700, Korea † Information and Communications University 103-6 Munji-dong, Yuseong-Gu, Daejeon, 305-714, Korea Email: {ljhar, jungsook96, shincs, jbt}@etri.re.kr∗ ,
[email protected]† Abstract— In this paper, we propose a medium access control(MAC) protocol for supporting telematics service in the road environment, which requires the guarantee of a delay bound on end-to-end transmission and high transmission reliability in a multi-hop network. In order to guarantee a delay bound on end-to-end transmission and high transmission reliability in a multi-hop network, the proposed MAC adopts the access mechanism based on time division multiple access(TDMA) and uses the superframe structure which considers an end-to-end transmission. Moreover, the proposed MAC can extend a network coverage by using special device, T-sink. Through experiments in the real testbed where the proposed MAC is implemented, we can show that the proposed MAC is appropriate for the telematics service in the road environment.
I. I NTRODUCTION Recently, wireless sensor networks(WSN) have been applied in the applications of high level requirements such as reliable transmission and real-time transmission. Among the applications, telematics services are highlighted as a possible application of WSN. However, using WSN for telematics service is challenging since the guarantee of delay bound and high reliability in transmission are necessary. Most of previous works on WSN are concentrated on maximizing network lifetime. [2] is a contention-based random access mechanism with a fixed listen and sleep cycle and uses a coordinated sleeping mechanism. [3] tried to reduce power consumption by turning on the radio as long as one activation event for some time. The activation event includes the reception of data, transmission of data and sensing communication on the channel. However, the access mechanism used in [2] and [3] can cause long transmission delay since packet collision can occur frequently in the high traffic condition. [4] used the access mechanism based on time division multiple access(TDMA) for removing packet collision. However, [4] only considered the distributed method of assigning time slot with adjacent sensor nodes and did not consider the guarantee of delay bound in multi-hop case. Moreover, [4] has problem of inaccurate time synchronization, which can result in packet collision. Therefore, [4] is not appropriate for telematics service in the road environment. [5] proposed MAC which supports power efficiency and delay guarantee. However, [5] has a limitation in the coverage of network since the control message from a central controller, access point(AP), is broadcasted to the entire network in one hop. Therefore, the only telematics
services whose service areas are small are possible by using [5]. In this paper, we introduce a system architecture (topology and device type) for the telematics service in the road environment. Based on the introduced system architecture, we design the MAC protocol which gurantees a delay bound on end-to-end transmission and supports high transmission reliability in a multi-hop network. In order to guarantee the delay bound on end-to-end transmission and high transmission reliability in the multi-hop network, the proposed MAC adopts the access mechanism based on TDMA and uses superframe structure which considers the relay of packets from a source(sensor node) to a destination(central controller). Moreover, the proposed MAC solve the coverage problem of [5] by using special device, T-sink. We implement the proposed MAC and IEEE 802.15.4 MAC in the real-testbed. Through experiment in the testbed, we show that our MAC protocol are appropriate for the telematics service in the road environment and outperforms IEEE 802.15.4. II. P ROPOSED A LGORITHM A. System Requirements Most of telematics services are defined in the road environments, such as routing service, collision warning service and traffic control service. The telematics services in the road environments are generally provided to moving vehicles which do not have enough time to stay in the service area. Therefore, the real-time transmission is necessary for the telematics services in the road environment. Secondly, the failure of the services can engender vehicle accident, especially in case of collision warning service. Therefore, the loss of data should be not be occurred for the success of services. In other words, reliable transmission is necessary. In addition, low power consumption should be supported for each sensor node. This is because sensor nodes are operated based on the battery. Finally, the MAC protocol should be simple since the computational power of sensor nodes is low. B. Network Topology and Devices For the telematics services in road environments, the information about moving vehicles such as detection and velocity, should be gathered. Therefore, the sensor nodes which have vehicle detecting module and transceiver are deployed on the road. In order to prevent the breakdown of the sensor nodes by moving vehicles, the sensor nodes
Proceedings of APCC2008 copyright (c) 2008 IEICE 08 SB 0083 T-SINK
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Fig. 2. T-SINK
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Fig. 1.
Network topology of our system in road environments
should be mounted in the road. For this reason, the transmission range of the sensor node is short. In general, a multi-hop transmission is used for overcoming a short transmission range. However, if a network is composed of only sensor nodes with short transmission range, the number of hops can be large in order for a packet to reach destination, which causes the increase of delay and cost. For this reason, we add new type of nodes with longer transmission range, T-sink, in order to make up for the short transmission range of sensor nodes. As depicted in fig.1, three types of devices are deployed in the road environments; T-sensor, T-sink and Tbasestation. Among three types, T-sensor and T-sink form cluster which is a basic group for operating the network system. The specific explanation about the devices are explained in the followings. T-sensor As a member of cluster, T-sensor can communicate only with T-sink. To be specific, T-sensor detects vehicles and transmits the detection information to T-sink in a single hop. T-sensor is deployed and mounted in the road. For this reason, the transmission range of T-sensor is short. In addition, T-sensor is operated based on battery. Therefore, the power consumption of T-sensor should be reduced as much as possible. T-sink As a cluster head, T-sink manages its cluster and aggregates detection information from T-sensors and relays the information to T-basestation. Since one of the roles of T-sink is complementing the transmission range of T-sensor, T-sink has longer transmission range than Tsensor. Moreover, T-sink is used for making up for the transmission range of T-basestation. To be specific, control messages from T-basestation are relayed by T-sink to a destination. For this reason, the coverage can be extended by installing additional T-sink, which lacks in [5]. In addition, T-sink is deployed in the roadside and is not mounted on road since T-sink does not detect vehicles.
superframe structure for TeMAC
Due to the location of T-sink, it can be connected to the power grid. T-basestation T-basestation is the central controller which gathers detection information and analyzes the information in realtime fashion. Since T-basestation makes the service data1 by analyzing the detecting information, the computational power of T-basestation is superior to T-sensor and T-sink. Moreover, T-basestation has a role of managing the entire network, such as managing the number and size of cluster. Similar to T-sink, T-basestation has a long transmission range2 and is connected to the power grid. C. Superframe Structure Considering Multihop Transmission For guaranteeing a delay bound on end-to-end transmission as well as supporting high transmission reliability, the proposed MAC is based on TDMA. The operation of the proposed MAC is based on superframe. The superframe consists of active period and inactive period. The active period is divided into four blocks; beacon block, time allocation block, forwarding block, and contention block. Among the four blocks, contention block is the time period of carrier sense multiple access with collision avoidance(CSMA/CA) scheme while beacon block, time allocation block and forwarding block are the time period during which TDMA scheme is used. Beacon block is the time period for the transmission of beacon frame. The beacon is generated by T-basestation and transmitted to the entire network with the help of Tsink. In other words, T-sink receives beacon frame from T-basestation or its parent T-sink and transmits beacon to its child T-sink and T-sensors. The beacon includes the information about the superframe structure and time slot allocation. The time slot allocation information in the beacon is about all T-sinks in the network and T-sensors in the cluster whose cluster head is the transmitter of the beacon. This is because the length of beacon frame can be very large if slot information of all T-sensors is included in one beacon when there are too many T-sensors in the network. Time allocation block is the duration for transmission of data frame by T-sensors. The same time slot can be used by T-sensors whose gap is larger than the interference range of T-sensors. For this reason, the length of time allocation 1 Service data means final information which makes drivers or pedestrian react. 2 The transmission range of T-basestation is same with that of T-sink.
Proceedings of APCC2008 copyright (c) 2008 IEICE 08 SB 0083 chip antenna [8]. For the extension of transmission range of T-sink, an power amplifier is added in T-sink. On the T-sink and T-sensor hardware, the proposed MAC and IEEE 802.15.4 non-beacon enabled MAC is programmed. Fig.3 is the topology we used for our experiment. Eight Tsensors generate data every 100ms and T-sink node relays the packet to the T-basestation. B. Results and Discussion
Fig. 3. Topology of experiments: two-hop network with eight sources(Tsensor) and one T-sink and one T-basestation
block is not large even if many T-sensors are deployed in the network. The data frames which are transmitted in time allocation block can be aggregated by T-sink and transmitted to Tbasestation in forwarding block. Similar to beacon frame, the aggregated frames are transmitted by T-sink in hop-byhop manner. To be specific, the aggregated frames arrive T-basestation via parent T-sinks. Due to the superframe structure where the forwarding block follows the time allocation block, the data packet can reach T-basestation within the length of superframe. In other words, the proposed superframe structure can make it possible to guarantee a delay bound on end-to-end transmission. The contention block is used for association of new devices or slot allocation for devices or transmission of management frame. In network initialization, the length of contention block is very large since associations and slot allocations are frequent in the network initialization. After network initialization is over by the notification in beacon frame, the length of contention block is decreased. III. P ERFORMANCE E VALUATION In this section, we show that the proposed MAC is appropriate for the telematics service by investigating the delivery ratio (a transmission reliability) and endto-end latency(the guarantee of delay bound)using the testbed developed by ETRI. Moreover, we compare the performance of the proposed algorithm with that of the IEEE 802.15.43 A. Implementation and experiment setup For the experiment, we made testbed which consists of T-sensor and T-sink. The micro control unit (MCU) on T-sink and T-sensor is MSP430F1161 [6]. The radio transceiver on the T-sensor and T-sink is CC2420 [7] which supports 250kbps transmission rate in the 2.4GHz band. The antenna used for T-sensor and T-sink is Winizen 3 In this paper, we choose IEEE 802.15.4 for showing the performance superiority of the proposed MAC. In the future, we will compare the performance of the proposed MAC with those of TDMA MAC protocols.
Fig.4 and fig.5 shows the delivery ratio and end-to-end delay with experiment time in order to visualize the stable behavior of the proposed MAC and the unstable behavior of IEEE 802.15.4. In fig.4, the delivery ratio and end-to-end delay of the proposed MAC is stable and the stable points are 100 % and 60ms, respectively. However, we can observe that the delivery ratio and end-to-end delay of IEEE 802.15.4 fluctuate with the average value of 60 % and 50ms, respectively, in fig.5. In these figures, we can say that the proposed MAC is appropriate for the telematics service in the road environment, since it supports high transmission reliability and it can guarantee the delay bound.4 In these figures, the delay of IEEE 802.15.4 is lower than the delay of the proposed MAC since the end-toend delay of the proposed MAC and IEEE 802.15.4 are calculated only from the successfully transmitted packets. However, in IEEE 802.15.4, a transmitter drops a packet if the number of retries which the packet experiences exceeds the retry limit predefined in the IEEE 802.15.4. Therefore, the end-to-end delay of IEEE 802.15.4 is lower than that of the proposed MAC if the delay is calculated from the values of successfully transmitted packets. IV. C ONCLUSION AND FUTURE WORK In this paper, we propose a MAC protocol for the telematics service in the road environment. The proposed MAC is operated on the introduced system architecture (device type and network topology) for telematics service in the road environment. For the telematics service in the road environment, the delay bound on end-to-end transmission should be guaranteed and high transmission reliability should be supported. In order to support the guarantee of delay bound with high transmission reliability, the proposed MAC adopts the access method based on TDMA and uses superframe structure which considers the relay of packets from a source(T-sensor) to a destination(Tbasestation). Moreover, the proposed MAC can extend a network coverage by using special device, T-sink. We implement the proposed MAC and IEEE 802.15.4 MAC in the real-testbed. Through experiment in the testbed, we show that our MAC protocol is appropriate for the telematics service in the road environment. ACKNOWLEDGMENT This work was supported by the IT R&D program of MIC/IITA. [2006-S024-02, Development of Telematics Application Service Technology based on USN Infrastructure] 4 The delay of the proposed MAC can be reduced by adjusting the lengths of contention block and inactive period.
Proceedings of APCC2008 copyright (c) 2008 IEICE 08 SB 0083
(a) Delivery ratio of the proposed MAC
(a) Delivery ratio of 802.15.4 (b) End to end delay of the proposed MAC Fig. 4. Delivery ratio and end-to-end delay in using the proposed MAC
R EFERENCES [1] IEEE Std 802.15.4, Part 15.4: Wireless Medium Access Control (MAC) and physical layer(PHY) specification for Low Rate Wireless Personal Area Networks (LR-WPANs), Dec. 2006. [2] W.Ye, J.Heidemann, and D.Estrin, ”Medium Access Control with Coordinated, Adaptive Sleeping for Wireless Sensor Networks,” in IEEE/ACM Transaction on Networking, June, 2004. [3] T.van Dam and K.Langendoen, ”An Adaptive Energy Efficient MAC Protocol for Wireless Sensor Networks,” in Proc. ACM Sensys, Nov, 2003. [4] V. Rajendran, K. Obraczka, and J.J. Garcia-Luna-Aceves, ”EnergyEfficient, Collision-Free Medium Access Control for Wireless Sensor Networks,” Proc. ACM Conf. Embedded Networked Sensor Systems, Nov. 2003. [5] Sinem Coleri Ergen, and Pravin Varaiya, ”PEDAMACS: Power Efficient and Delay Aware Medium Access Protocol for Sensor Networks,” in IEEE Transactions on Mobile Computing, July, 2006. [6] ”MSP430x15x, MSP430x16x, MSP430x161x MIXED SIGNAL MICROCONTROLLER,” http://focus.ti.com/lit/ds/symlink/msp430f169.pdf [7] ”CC2420,” http://focus.ti.com/lit/ds/symlink/cc2420.pdf. [8] ”http://www.winizen.com.”
(b) End to end delay of 802.15.4 Fig. 5. MAC
Delivery ratio and end-to-end delay in using IEEE 802.15.4
