Precision Time Synchronization using IEEE 1588 for Wireless Sensor Networks

Transcription

1 2009 International Conference on Computational Science and Engineering Precision Time Synchronization using IEEE 1588 for Wireless Sensor Networks Hyuntae Cho, Jeonsu Jung, Bongrae Cho, Youngwoo Jin, Seung-Woo Lee and Yunju Baek Dept. of Computer Engineering, Pusan National University Busan, Republic of Korea BcN Division NoC Technology Team, ETRI Daejeon, Republic of Korea Abstract Wireless sensor networks are evolving from relatively undemanding applications to applications which have stronger requirements. The coordination of distributed entities and events requires time synchronization. Although a number of methods have been studied for WSNs, some applications require high precision time synchronization. Precision time synchronization enables a variety of extensions of applications. The IEEE 1588 precision time protocol (PTP) provides a standard method to synchronize devices in a network with sub-microsecond precision. This paper deals with precision time synchronization using IEEE 1588 over wireless sensor networks. Precision time synchronization using IEEE 1588 provides compatibility between heterogeneous systems in WSNs. This paper also presents experiments and performance evaluation of precision time synchronization in WSNs. Our result established a method for nodes in a network to maintain their clocks to within a 200 nanosecond offset from the reference clock of a master node. Keywords-IEEE 1588; precision time protocol; wireless sensor networks; time synchronization; I. INTRODUCTION Time synchronization of distributed computing elements is a common requirement for a number of distributed applications. Especially, time synchronization in wireless sensor networks (WSNs) is essential to facilitating group operations such as time division multiplexing, sensor localization, data aggregation, distributed sampling, etc. A WSN consists of a number of heterogeneous systems which include distinct network protocols. These heterogeneous systems can easily result in a network partition due to their incompatibility. A number of time synchronization protocols for WSNs have appeared, such as RBS[1], TPSN[2], FTSP[3], the work by Cox[4], etc. These time synchronization protocols do not provide compatibility and they result in a network partition. Therefore, standardization for time synchronization in WSNs, as well as distributed applications, is necessary[5]. The IEEE 1588 precision time protocol (PTP)[6] provides a standard method to synchronize devices in a network with sub-microsecond precision. IEEE 1588 provides not only compatibility between heterogeneous systems but also 0 Correspondence to Prof. Yunju Baek ( yunju@pnu.edu, phone : ) high precision time synchronization. Higher accuracy time synchronization with sub-microsecond precision can enable much sophisticated extensions of applications. The PTP specially addresses spatially localized networks, maintaining microsecond to sub-microsecond accuracy, and it is administration free, and accessible for both high-end and lowend devices between heterogeneous systems. However, most research on IEEE 1588 has been targeted at Ethernet[7-11]. The objective of this paper is to apply IEEE 1588 over Ethernet to sensor nodes in WSNs. In order to achieve convergence of IEEE 1588 with WSNs, we designed a system prototype which is composed of a PTP gateway and a sensor node. Time synchronization using IEEE 1588 for WSN maintains a time difference of within 200 nanoseconds between the master node and slave nodes. The organization of the paper is as follows. We present the traditional IEEE 1588 PTP in the next section. Then, in section 3, we describe precision time synchronization using IEEE 1588 in WSNs. Section 4 includes experiments and the performance evaluation of our system. Finally, in section 5, we conclude this paper with plans for future work. II. RELATED WORK The IEEE 1588 precision time protocol (PTP) defines a protocol enabling precise synchronization of clocks in measurement and control systems implemented with technologies such as network communication[6]. The protocol synchronizes slave clocks with a master clock, ensuring that events and timestamps in all nodes use the same timer values. Because a time difference between the master clock and a slave clock is a combination of the clock offset and message transmission delay, correcting the clock skew involves two phases; offset correction and delay correction. The master node initiates offset correction using a sync and follow up message. When the master node sends a sync message, a slave uses its local clock to timestamp the arrival of the sync message. The slave compares it to the actual sync transmission timestamp in the master clock s follow up message. The difference between the two timestamps represents the sum of the offset of the slave and the message transmission delay. The second set of /09 $ IEEE DOI /CSE

2 Figure 2. Time Synchronization System using IEEE 1588 Figure 1. IEEE 1588 Precision Time Protocol messages is necessary to account for variations in network delays. The slave then time stamps the instant when a delay request message is sent. The master clock timestamps the arrival of the delay request message. It then sends a delay response message with the delay request arrival timestamp. The difference between the timestamps is the slave-to-master delay. The slave averages the two directional delays and then adjusts the clock by the time of the delay in order to synchronize the two clocks. Because the master and slave clocks drift independently, periodic offset and delay correction is necessary to maintain clock synchronization. Figure 1 illustrates an example of a precision time protocol process using hardware assisted time stamping. III. PRECISION TIME SYNCHRONIZATION USING IEEE 1588 FOR WSN Within Ethernet, a grandmaster clock is a PTP clock that is the ultimate source of time for clock synchronization using the PTP protocol. Slave nodes synchronize their time values with the grandmaster clock via several Ethernet switches. However, conventional devices for IEEE 1588 over Ethernet cannot provide a method to deliver IEEE 1588 messages to other networks. So, IEEE 1588 for Ethernet cannot transfer messages from its network to WSNs. The objective of this paper is to extend IEEE 1588 over Ethernet to WSNs. A. Time Synchronization System using IEEE 1588 In order to forward IEEE 1588 messages to WSNs, a gateway which involves both networks, and nodes which synchronizes their time values with the grandmaster clock are needed, as shown in Figure 2. We designed and implemented a special PTP gateway involving Ethernet and WSNs. The PTP gateway is implemented on FPGA(Xilinx Vertex 4) for high speed communication. Figure 3 shows the architecture of the PTP gateway, which consists of an Ethernet PHY, a fast Ethernet MAC(FMAC), a message translator, a parallel-to-serial converter, a Zigbee controller, a Zigbee PHY and a time-stamping unit. Marvell 88E1145 was chosen as the Ethernet PHY and CC2420(TI) was chosen as the Zigbee PHY. Others are implemented on a FPGA. The FMAC controls the Ethernet PHY and communicates with it. The message translator communicates with the FMAC and converts Ethernet messages into the Zigbee message format. Because the size of a sync and delay req message over Ethernet is 166 bytes, which includes UDP/IP/Ethernet headers, a Zigbee frame where the maximum transmission unit is 128 bytes cannot include IEEE 1588 messages. So, the message translator fragments the IEEE 1588 message over Ethernet into several messages for WSNs. The parallel-to-serial converter changes parallel data into serial data for SPI communication of a Zigbee chip or vice versa. While fast Ethernet provides a data rate of 100Mbps, Zigbee transfers data at 250kbps. To overcome the difference of the data rate between the two media, the parallel-to-serial converter includes a buffer. The Zigbee controller is responsible for controlling the Zigbee PHY and it communicates with the parallel-to-serial converter. Finally, a time-stamping unit time stamps ingoing and outgoing messages. For precise time synchronization in WSNs, we implement a new sensor node[5]. A sensor node consists of a processing unit, a communication unit and a time processing unit, as shown in Figure 4. Freescale s MCF5235 is used for the processing unit. MCF5235 involves a 32-bit RISC processor core. For accurate time stamping, the node adopts an enhanced time processor unit(etpu), which is the coprocessor of MCF5235. The etpu is an event triggered VLIW processor with 32 input capture channels and an embedded 24-bit timer. The main feature of the etpu is that it can trigger an external event via hardware. Thus, the latency during time stamping is remarkably subtle. For the communication unit, TI s CC2420 transceiver is used. 580

3 Figure 5. Time Synchronization between Ethernet and WSN Figure 3. Figure 4. Architecture of the PTP gateway Architecture of the sensor node CC2420 is a 2.4GHz RF transceiver which supports the PHY of IEEE /ZigBee. The clock of a processor is controlled by the crystal oscillator. Under ideal circumstances, physical clocks oscillate at a constant frequency. In the real world, manufacturing variations and exposure to out-of-tolerance conditions (e.g. mechanical shock) result in permanent frequency errors in crystals. An oscillator with a frequency tolerance of one parts per million(ppm) has a drift rate of one microsecond per second. In general, a cheap oscillator has a frequency tolerance ranging from PPM, where the maximum drift rate ranges from microseconds per second. This value is inadequate for a precision time protocol. Thus, a sensor node uses a temperature compensated crystal oscillator (TCXO), which has a 1.5PPM frequency tolerance, in order to reduce the drift rate. Figure 5 shows hardware prototypes for precision time synchronization using IEEE 1588 for WSNs. B. Time Synchronization between Ethernet and WSN A master(or grandmaster) node initiates a time synchronization procedure. Figure 6 illustrates the time synchronization procedure between Ethernet and WSNs. The master node, which provides a reference time over Ethernet, records its local clock time, and encapsulates this within a packet. Then, it broadcasts messages to the slave nodes, switches, and PTP gateways via Ethernet. PTP gateways, which receive time synchronization messages from the master node or switches, analyze the message. If the checksum is correct, the PTP gateway fragments the message to the Zigbee format according to the message type, and then broadcasts the fragmented messages. A sensor node which receives a time synchronization message from the PTP gateway synchronizes its local clock with the clock time obtained from the message. The detailed behavior of nodes participating in a given communication is as follows. The time stamps of the precision time protocol are numbered. These are exchanged between the master node in Ethernet and slave sensor nodes in WSNs. A sync message is sent by the master node. After exchanging a sync message at t 1, the time-stamping unit reads and stores the local clock time of the master node. After a slave sensor node receives a sync message, the slave sensor node records the value of the local clock counter at t 2. If the slave sensor node is aware of t 1 and t 2, it can calculate the offset between the master node and the slave sensor node. However, the slave sensor node does not have information about t 1. The master node inserts the time stamp of t 1 into the consequent follow up message. The master node can also send the follow up message. The follow up message is always associated with a specific sync message and it contains a more precise estimate of the time of the sync message. The slave sensor node uses the information contained in the follow up message to correct its local clock in order to synchronize the local clock with the clock of the master at time t o. Equation (1) presents an offset calculation. offset = clock slave clock master = T 2 T 1 (1) This offset calculation includes the propagation delay[12-13]. So, an enhanced correction can be achieved by taking into account the propagation delay of messages sent from the slave sensor node to the master node. This additional correction can be estimated by reversing the offset calculation procedure That is, the slave sensor sends a message termed a delay req message to the master node, and t 3 is inserted 581

4 Figure 7. Clock drift in time advance Figure 6. Time synchronization between Ethernet and WSN into the local variable by the time-stamping unit. In this case, because the slave sensor node communicates with the master node over Ethernet, the slave node generates a full PTP message including UDP/IP/Ethernet headers. The full PTP message fragmented by the slave node is assembled at the PTP gateway and delivered to the master node. The master then reports the reception time of t 4 to the slave sensor node in a delay resp message. Later, the slave sensor node can recalculate the offset and propagation delay in order to achieve precise time synchronization with the master node. delay = (t2 t1) + (t4 t3) 2 offset = T 2 T 1 delay (2) From this point of view, there are two methods to synchronize clocks; logical and physical. The logical time synchronization methods use a counter to adjust the local clock, whereas the physical methods adjust the rate of the local oscillator to achieve time synchronization. This paper uses the logical method to synchronize a node s clock with the master clock. The clocks in the sensor nodes are generated by crystal oscillators. Such crystal oscillators are susceptible to drifts from the ideal clock or between two nodes, as shown in Figure 7. The error depending on the rate of relative drift between the two nodes affects the accuracy of time synchronization. Because this clock drift increases over time, periodic correction is needed. In this paper, we correct the relative drift between the two nodes. The period for the correction of the clock drift between two nodes is defined according to equation (3). Sync interval = err tolerance 10 6 f drift (3) where f drift is the drift rate of the crystal oscillator including its stability and err tolerance is the tolerance of the time error between the two nodes. Although periodic time synchronization reduces the frequency skew caused by the difference of the clock drift of two nodes, the difference of the clock drift between sync intervals still remains. To eliminate the difference of the frequency skew between two nodes, a slave sensor node calculates both the drift rates of the master node and the slave sensor node, as shown in equation (4). The slave sensor node corrects the drift rate by using the drift rate of the master node. Δ m = t k+1 m t k m Δ s = t k+1 s t k s drift = Δ s Δ m (4) Δ m where, Δ m, the clock drift of the master node, applies to clocks between the sync interval t k m and t k+1 m of the master node, and Δ s, the clock drift of the slave node, applies to clocks between the arrival sync interval t k s and t k+1 s of the slave node. Figure 8 illustrates the time instant of sync messages for drift calculation. The time stamping point is critical, because it affects the accuracy of time synchronization. The time stamping point can be any point within the network layers. However, time stamping in an upper layer such as the application layer has the disadvantage that the protocol stack can cause non-deterministic delays. The delay between the time stamping point and the transmission point can vary between minimal and maximal, depending on the network and protocol states. The transmission can be delayed if it causes a collision. Time stamping of a receiver is performed at the start of an interrupt, after receiving the frame. The delay of a message reception varies according to the protocol stack and kernel activity. To reduce the delay and jitter, time stamps should be taken as near to the wires as possible[7]. In this paper, time stamps for WSNs can be taken at the media independent interface (MII) between the media access 582

5 Figure 8. Determination of drift between two nodes Figure 10. Basic time synchronization verification Figure 9. Time stamping points control (MAC) layer and the Zigbee physical (PHY) layer, using hardware assisted time stamping (etpu). When the Zigbee device receives MAC protocol data from the upper layer, it generates a four byte preamble, a one byte start of frame delimiter (SFD), and a one byte frame length. Then, it transfers the data to the MAC protocol data unit (MPDU) and performs a cyclic redundancy check (CRC). At this point, after transferring the last bit of the SFD, the communication unit causes the SFD pin to become high. The time-stamping unit of the sensor node detects the rising edge of the SFD pin. Then, it stores the value of the local clock counter in an internal register. Figure 9 depicts the time stamping points of a SFD from the time processing unit. IV. PERFORMANCE EVALUATION This section includes the performance evaluation of time synchronization using IEEE 1588 for wireless sensor networks. The purpose of the first evaluation was to verify time synchronization using IEEE 1588 over WSNs. This evaluation is the basis of the IEEE 1588 extension. The system setup used for the evaluation is presented in Figure 10. The system consists of a master sensor node and slave sensor node, a reference pulse generator, and the connecting LAN technology. The master sensor node which provides the global clock periodically sends synchronization messages. Messages for time synchronization are based on IEEE A slave sensor node which receives a message from the master node synchronizes its clock with the master clock. In order to compare two clocks, a reference clock generator periodically sends an external event to the master node and a slave node simultaneously. The two nodes record the time that the event is detected. The sensor nodes are operated by a 37.5MHz clock where the clock tick is approximately nanoseconds. The sync interval is set to 10 per one second, and the external event interval due to the reference pulse generator is one per two seconds. The experiment was repeated 500 times. Figure 11 depicts the values of the clock offset of the slave node from the master node. This graph implies that the closer the offset is to zero, the more precise the time synchronization, and a zero value indicates perfect time synchronization with the master node. A positive value implies that the slave node runs slower than the master node, whereas a negative value implies that the slave node runs faster than the master node. In the graph, the maximum offset is five clock units, where one clock unit means a difference of about nanoseconds, and the minimum offset is -6 clock units. Also, the result shows an average of approximately clock units (about 19.3 nanoseconds) and a standard deviation of approximately 2.05 clock units (about nanoseconds). Figure 12 shows the distribution of the time synchronization result. This result is the basis for the comparison when IEEE 1588 is applied to WSNs via Ethernet. The purpose of the second experiment was to evaluate the time synchronization performance over WSNs via Ethernet. The system setup used for the evaluation is presented in Figure 13. The system consists of a master node, a slave node, two PTP gateways, a fast Ethernet switch(netgear FS608), a reference pulse generator, and an analyzer PC. 583

6 Figure 13. Time synchronization in WSN via Ethernet Figure 11. Result of basic time synchronization Figure 14. Result of time synchronization in WSN via Ethernet Figure 12. Histogram of time synchronization using (unit : clock (1 clock = nanosecond) Two PTP gateways and a fast Ethernet switch are setup between the master node and the slave node. The time synchronization procedure is the same as the one used in the first experiment. Figure 14 depicts the values of the clock offset of the slave from the master node. In the graph, the maximum offset is 40 clock units and the minimum offset is -19 clock units. Also, the result shows an average of approximately 3.4 clock units (about nanoseconds) and a standard deviation of approximately 7.25 clock units (about nanoseconds). Figure 15 shows the distribution of the time synchronization result. Figure 16 illustrates verification of IEEE 1588 messages exchanged via the fast Ethernet switch shown in Figure 13. To verify IEEE 1588 messages, the master node used a broadcast address instead of a multicast address, and a packet sniffer directly connects to the switch s other port. The result shows that messages exchanged between the master node and the slave node correctly identify IEEE 1588 messages at the middle point of both. The results showed that the time synchronization result of the second experiment had a standard deviation of approximately 7.25 clock units (about nanoseconds) while that of the first experiment had a standard deviation of approximately 2.05 clock units (about nanoseconds). This difference is derived from the jitter, which is the delay difference that depends on the network and switch devices. Jitter correction is difficult, while the delay is easily calculated by using the delay req and delay resp messages. A simple method of delay jitter correction is periodic transmission of a message such as a sync message. However, IEEE 1588 uses non-periodic and random pattern delay calculation. Nevertheless, the proposed time synchronization using IEEE 1588 for WSNs can provide an accuracy of within 200 nanoseconds. This result is superior to other time synchronization protocols for WSNs. 584

7 time difference of within 200 nanoseconds between the master node and slave nodes. This result is superior to other time synchronization protocols, which showed an accuracy of tens of microseconds for WSNs. In addition, our approach enables a variety of extensions of applications for WSNs. Our future work will include much more sophisticated time synchronization using IEEE 1588 v2. ACKNOWLEDGMENT This work was supported by the Grant of the Korean Ministry of Education, Science and Technology (The Regional Core Research Program/Institute of Logistics Information Technology) Figure 15. Histogram of time synchronization in WSN via Ethernet (unit : clock (1 clock = nanosecond) REFERENCES [1] Jeremy Elson, Lewis Girod and Deborah Estrin, Fine-Grained Network Time Synchronization using Reference Broadcasts, The Fifth Symposium on Operating Systems Design and Implementation (OSDI), pp , Dec [2] Ganeriwal, S. Kumar, R., Srivastava, M.B., Timing-sync protocol for sensor networks, Proceedings of the ACM International Conference on Embedded Networked Sensor Systems, pp , Figure 16. IEEE 1588 verification by a packet sniffer [3] Maroti, M., Kusy, B., Simon, G., Ledeczi, A., The flooding time synchronization protocol, Proceedings of the ACM International Conference on Embedded Networked Sensor Systems, pp , [4] D. Cox, E. Jovanov, and A.Milenkovic, Time synchronization for Zigbee networks, in Proceedings of the 37th Annual Southeastern Symposium on System Theory (SSST 05), pp , Mar [5] Hyuntae Cho, Sanghyun Son and Yunju Baek, Implementation of a Precision Time Protocol over Low Rate Wireless Personal Area Networks, The Thirteenth IEEE Asia-Pacific Computer Systems Architecture Conference (ACSAC 2008), Aug V. CONCLUSION AND FUTURE WORK Time synchronization of distributed computing elements is a common requirement for a number of distributed applications. Especially, time synchronization in WSNs can be essential to facilitating group operations. A WSN consists of a number of heterogeneous systems which include distinct network protocols. These heterogeneous systems can easily result in a network partition due to their incompatibility. The IEEE 1588 PTP provides a standard method to synchronize devices in a network. This paper applied IEEE 1588 over Ethernet to sensor nodes in WSNs. In order to achieve convergence of IEEE 1588 with WSNs, we designed a system prototype which is composed of a PTP gateway and a sensor node. Our system for time synchronization provides a precise global time value not only for wireless sensor networks but also for Zigbee networks. This paper also included experiments for performance evaluation. Time synchronization using IEEE 1588 for WSNs maintains a [6] IEEE , IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems, IEEE Instrumentation and Measurement Society, Nov [7] Weibel, Hans, High Precision Clock Synchronization according to IEEE Implementation and Performance Issues, Embedded World, pp , [8] Juha Kannisto, Timo Vanhatupa, Marko Hannikainen, and Timo D. Hamalainen, Precision Time Protocol Prototype on Wireless LAN, Lecture Notes in Computer Science, vol. 3124, pp , [9] Kannisto, J.,Vanhatupa, T. Hannikainen, M. Hamalainen, T.D. Software and hardware prototypes of the IEEE 1588 precision time protocol on wireless LAN, 14th IEEE Workshop on Local and Metropolitan Area Networks 2005, Sep [10] Hans Weibel, Dominic Bechaz, IEEE 1588 Implementation and Performance of Time Stamping Techniques, 2004 Conference on IEEE 1588, Sep. 28,

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