Time Synchronization. It is very challenging to synchronize the clocks between the video and the single- loop data sequences because of uncertainties with the data collection, compression, and transmission processes. The video signal from a WSDOT surveillance camera is transmitted to a control cabinet via coaxial cable and then sent to a communications hub via fiber optic cable. At the communications hub, it is combined with video signals from many other cameras through the Frequency Division Multiplexing (FDM) technique. The combined video signals are then transmitted to the WSDOT TSMC in Shoreline via glass fiber. At the TSMC, the received video signals are de-multiplexed and connected through a switch to a Digital VAX computer. This entire procedure is illustrated in Figure 8-2 (WSDOT, 2006). Figure 8-2: Schematic of the WSDOT Video Signal Communication System Source: WSDOT (2006). Video signals from the selected camera can be transmitted via live video feed fiber cable from the TSMC to the STAR Lab (details of this cable are discussed in Section 6.1). The latency of video signals received at the STAR Lab is approximately 30 milliseconds. While the latency of the video data is relatively stable, the delay of loop detector data is volatile. A dual-loop’s measurements are processed and integrated into 20-second intervals by the controller to which the dual-loop is connected. The calculated interval mean speed, vehicle length, and bin volume data are then shipped to the TSMC over the WSDOT sensor data communication network. Depending on the level of congestion over the communications network, transmission delay varies. In addition to the transmission delay, system delay induced by the controller makes predicting total delay of the loop detector data more challenging. System delay is defined as the difference between the measurement time of a 20-second interval and the shipment time of the data to the TSMC. Although the controller clock is synchronized four times a day, the system delay for a particular loop cannot be easily estimated. At each WSDOT dual-loop station, the controller has a predefined order for stepping through the detector pairs. However, such an order is station dependent and cannot be identified by looking at the detector list. Controllers at dual-loop stations are configured for 40 detectors, but usually only 10 to 20 of them are defined. Suppose a dual-loop station has 20 inputs. The controller processes the detector pairs in the decreasing order of their inp...
Time Synchronization. Time synchronisation is only to be done if required by the Employer:
Time Synchronization. The system clock accuracy also affects the accuracy of the resulting metrics. It is essential for IP SLA operations like one-way latency (delay) to have precise time synchronization. Some of the other operations, also round-trip and interarrival time, are less sensitive to clock variations.
Time Synchronization. The System will support synchronization to a master clock source(s) using Network Time Protocol (NTP) V3 as defined by IETF XXX 0000. The Contractor shall provide a network master clock source for the System and any ports necessary to connect the master clock source to PSAP CAD systems.
