Time Synchronization

Time synchronization refers to proposed modifications to the existing IEEE P802.1AS standard that was defined for for Audio Video Bridging.  


To understand the proposed changes, first we should describe IEEE 802.1AS.


IEEE802.1AS is the Audio-Visual Bridging (AVB) profile of the IEEE1588 Precision Time Protocol. IEEE1588, and therefore IEEE802.1AS, uses a master-slave protocol to synchronize real-time clocks in the nodes of a distributed system that communicates using a network. In simple terms, PTP ensures that every node on the network knows what time it is. It does not specify what a given node does with its knowledge of time.  The AVB profile (IEEE802.1AS), has features tailored to the “plug and play requirements” of AVB components. PTP messages can be routed at either layer 2 (MAC address) or layer 3 (IP address), effectively making it media independent. It does not use “transparent clocks” to compensate for bridge latency because transparent clocks violate IEEE802 layering conventions. Instead, each node accepts time information from the network master (known as a “Grand Master”) and produces a new clock, in a manner similar to a boundary clock, to compensate for latency.


PIEEE802.1AS-REV introduces new features needed for time-sensitive applications. These features include the ability to support a multiple time domains to allow rapid switchover, should a Grand Master fail, and more precise measurement of time.