Extended Abstract (ISPCS) Synchronization of Wireless Sensor Networks Using a Modified IEEE 1588 Protocol Yuan Ma/Darold Wobschall Amherst, USA Esensors, Inc. Abstract— A method of precise time synchronization of wireless sensors employing an IEEE 802.15.4 transceiver, and specifically employing the 6LoWPAN protocol, was developed. It uses the IEEE 1588 synchronization standard and the IEEE 1451.5 Smart Transducer Data standard. A Wireless Transducer Interface Module (WTIM) was designed and fabricated. It utilizes the IEEE 802.15.4 transceiver model TI CC2430 which allows access to a hardware sync signal. The difference in timestamps between two WTIMs was measured. The results show that the synchronization precision is better than 10 s for short synchronization intervals but increases to about 100 s for longer synchronization intervals (1 sec for crystal accuracies of 50ppm). The method was tested for 6LoWPAN wireless protocol but would apply to other wireless sensors based on the IEEE 802.15.4 protocols. Keywords-1588; 1451; wireless sensor network; synchronization I. INTRODUCTION The timing of sensor data or actuator control in a wireless network often is critical to the data acquisition or control process. The IEEE 1588 standard describes the time synchronization process for wired network nodes, in particular using Ethernet, but does not explicitly address how this might be extended to wireless networks. Wireless sensors are difficult to synchronize because the total transmission time is comparatively long and quite variable, especially if re- transmission and relaying of messages between nodes is involved. Also energy and bandwidth restriction limit the length and frequency of synchronization messages. The sensor (or actuator) data are formatted using the IEEE 1451.0 and 1451.5 smart transducer standards which provide a standard sensor protocol. These standards do not explicitly specify a method of synchronizing time clocks between the different nodes, or Wireless Transducer Interface Modules (WTIMs), of the network and we have combined methods in this research. Specific features of this research are (1) synchronization pulses are derived from a source close to the physical layer of the transceiver without hardware modifications, (2) implementation with the Internet-compatible 6LoWPAN protocol and (3) a precisely synchronized real time clock module with IEEE 1451/1588 format fabricated from commercial components. II. DETAILED DESCRIPTION A. Wireless Block Diagram A block diagram of a Wireless Transducer Interface Module (WTIM) and an associated gateway or NCAP front end is shown in Fig. 1. The gateway also functions as a wireless router for the 6LoWPAN network. Both consist of a RF transceiver with IEEE 802.15.4 capability, a microcontroller (which is integrated with the receiver for our system) and a clock module (which here is separate). Figure 1. Block Diagram of WTIM (left) and NCAP front end or gateway/router (right) B. IEEE 802.15.4 Transceivers The transceiver selected (Fig. 2) is the TI model CC2430 (2.4 GHz) because it is well suited for the synchronization process. In addition to being based on IEEE 802.15.4 protocol, two key advantages are access to a hardware synchronization signal and the integration of a microcontroller with the transceiver in the same chip. The time synchronization signal selected is SFD (start frame delimiter), as shown in Fig. 2. It goes high when the initial, required preamble of the message is received (or transmitted). The signal is used for an interrupt on the microcontroller which then immediately sends a sync signal to the clock module. Because the software for this function is deterministic (non-branching), and crystal controlled, it has little jitter (under 1 μs). The SFD function is similar to the beacon signal, which is optional. Figure 2. IEEE 802.15.4 Message Timing -- The SFD functions as the time sync pulse