White Paper: Test & Measurement

Precision Time Protocol

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The Precision Time Protocol (PTP), fully defined within IEEE standard 1588, is a highly accurate scalable time source that efficiently services a wide range of networks and devices at each node. High timing accuracy is required to understand the order of events in high-speed environments such as banking transactions or the sequence of circuit breaker trips in power distribution system.

PTP elements are Grand Master Clock (GMC), Master Clock (MC), Boundary Clock (BC), Transparent Clock (TC), or a slave device. The illustration depicts where each element is generally in a network. The GMC is the primary network time device. It is synchronized directly by a time standard (e.g., GPS, atomic clock). Having several MCs on a network is common to provide failsafe for network timing functions. IEEE1588 V2 specifies the messaging protocol and how the data is handled.

LANs are comprised of routers, switches, and hubs to support the many client devices. Without PTP aware devices, latencies (routing, traffic shaping, store and forward) are additive at each node resulting in large time disparities between GMC and time that a node or terminal device receives the GMC time packet. Further, latencies vary, so the total latency will have jitter.

Time is kept by setting a counter to the current time of day. The counter is sized to increment at the smallest unit of time (resolution) desired. It is the time master. Some systems may use an atomic clock as the frequency source and some systems may use only the atomic clock. Disciplining of the oscillator output is used to provide a low drift from the reference time standard (e.g., GPS, atomic clock) when not locked.

This paper drills down on into PTP network architecture, explains terms, and talks about available tools.

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