The Multicast time synchronization is the Johnson Controls implementation of SNTP w32time with Multicast capabilities and RFC-2030 compliance. This method delivers the same features as the Windows method, but also provides Multicast functionality. The Multicast method provides improved Metasys time synchronization between the Site Director and supervisory devices. A time server provides the master time to the Site Director, and the Site Director in turn multicasts the time to all supervisory devices on the Metasys network.
When a supervisory device first signs up with the Site Director, it polls the Site Director for the current time and matches its time with the Site Director time. By default, every 5 minutes the Site Director broadcasts the current time to all supervisory devices. If a particular device time differs 1.5 seconds or more from the Site Director time, the device adjusts it's time to match. Additionally, if the Site Director time changes by more than 1 to 1.5 seconds, it sends out a Multicast time message to all devices within 2 seconds of the change.
This form of time synchronization requires that all routers on the site support Multicast routing (Internet Group Multicast Protocol [IGMP]) because the Multicast time message crosses routers. The Johnson Controls SNTP time synchronization reduces the time interval in which devices are out of sync with the SNTP master time server.