## 3 System Architecture and Design
### Telematics Sensor Network
Since the maximum operation time of PES is less than 6 hours and the accuracy of the Temperature Compensated Crystal Oscillator (TCXO) used is 2.5 parts per million (ppm) between -30 \(\mathrm{\SIUnitSymbolCelsius}\) and 75 \(\mathrm{\SIUnitSymbolCelsius}\), with a single time synchronization packet exchange, the worst possible time offset during a 10 minute period between two T-Sensor nodes in the same lane is [10 \(\times\) 60 \(\times\) (2.5/(10\({}^{6}\)) = 1.5 ms) + 1.5 ms, or 3 ms. Although periodic handshaking is necessary to maintain the required time synchronization accuracy, a single synchronization scheme is designed and implemented for an operation time of 6 hours for this pilot project. Before explaining the time synchronization details, we will briefly outline the timer operation in the T-Sensor and T-sink nodes. In PES, the continuous mode of Timer_A is used among various timer operation modes of MSP430 [31]. In the continuous mode, the Timer_A Register (TAR) is repeatedly increased to 0 xFFFF and restarts from zero, according to the timer clock. Whenever TAR reaches the value of Timer_A Capture/Compare 0 Register (TACCR0), a TACCR0 interrupt is triggered. In the Interrupt Service Routine (ISR) for the interrupt, a global variable, _u32 Tick_ (a 32-bit software time tick), is increased by 1 and 0x0A00 is added to TACCR0 to generate the next periodic timer interrupt with a period of 0x0A00 counts on the timer clock. With this configuration, assuming an 8 MHz timer clock, TAR increases every 0.125 \(\mathrm{\SIUnitSymbolMicro s}\), and 0x0A00 counts generates a 0.125 \(\times\) 0x0A00 = 320 \(\mathrm{\SIUnitSymbolMicro s}\) periodic timer interrupt. The detailed procedure is explained in Figures 4 and 5, and the notations used in the figures are summarized in Table 3. In PES, data on the second, minute, hour, day, month, and year is distributed from the T-Mon Host to the T-Sensor nodes via T-Sink nodes using so-called sender-receiver synchronization, but the time tick synchronization between T-Sensor nodes in the same lane is achieved by a broadcast packet from their immediate parent, the T-Sink node based on mutual \begin{table}
\begin{tabular}{c c} \hline \hline
**Notation** & **Description** \\ \hline T\({}_{\mathrm{R}}\) & Broadcast (Sync. Req.) message receiving time, Reference point \\ T\({}_{\mathrm{S}}\) & Synchronized time to the reference point \\ K & the received software time tick from T-Sink Node. Appended in Sync\_Req packet \\ SN & S1, S2, S3: T-Sensor node’s local software time tick before the synchronization \\ TAR & Timer\_A register \\ TACCR0 & Timer\_A Capture/Compare 0 Register. Timer interrupt is occurred whenever TAR is equal to TACCR0 \\ Lp & Processing Latency, the time interval from T\({}_{\mathrm{B}}\) to the beginning of the packet processing \\ Ls & Processing Latency measured by the local software timer \\  & (incremented in ISR(interrupt service routine) triggered by TAR and TACCR0) \\ Ds & Duration of a software time tick \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Notation used in Figures 4 and 5 for the T-Sensor node time synchronization. receiver-receiver synchronization [32,33]. After receiving the START message from the T-Mon Host, a T-Sink node broadcasts a time-synch command (Sync_Req, 1-hop broadcast packet) to every T-Sensor node in each lane. The Sync_Req packet includes the date and time (_i.e._, year, month, day, hour, minute, and second) of the T-Mon Host and the local time tick value (_u32 Tick_) of the T-Sink node which is broadcasting this Sync_Req packet. Whenever a T-Sensor node receives the broadcast message, it takes a time stamp for the packet with its local TAR value and 32-bit local time tick value (_u32 Tick_). Depending on the local processing state of each T-Sensor node, the processing latency for the received Sync_Req packet varies. Although the processing latency of each T-Sensor node in the same lane is diverse, as  
Figure 4: Timing diagram of the synchronization procedure of T-Sensor nodes to the T-sink node.