Distributing PTP over an IP/Ethernet network requires some specialized equipment, both in the form of common core equipment and within the end devices. When used properly, this technology can provide a very accurate and stable time base for all types of signals within modern media operations.
The major system elements and functions include:
Grandmaster Clock – Normally installed in pairs for redundancy, a grandmaster provides a single, common source for PTP clock signals in the network (called a PTP Domain). Only one grandmaster can be active at one time, so all the devices execute a common algorithm to ensure that one and only one device becomes the grandmaster. This process also ensures that if the active grandmaster fails that another device will take over.
Transparent or Boundary Clocks – Delivering the signal from the grandmaster to the end devices (called slaves in PTP specs) is the responsibility of boundary clocks and transparent clocks. These functions are normally built into network equipment such as IP routers and Ethernet switches.
Transparent clocks allow PTP packets to flow through a device while adding a correction factor to the packet that represents the amount of time that the packet spent in transit through the device. Using this value, the end device can precisely calculate the amount of network delay between the device’s input and the Grandmaster’s output, allowing an accurate PTP time to be calculated.
Boundary clocks act as a slave device to an upstream (grand)master, and as a master device to downstream end devices (called slaves). These allow the output of a grandmaster to be shared with multiple downstream devices without overloading the grandmaster. Accuracy levels are close to what can be achieved with transparent clocks.
While it is possible to run a small network with only a few devices using switches that don’t support transparent or boundary clocks, the overall accuracy of the delivered clocks will be degraded. Networks with more than a handful of devices or with multiple switches should use transparent or boundary clocks to ensure signal integrity.
Hardware Timestamping – In order to precisely synchronize devices within a microsecond, precise measurements must be made of the times that packets enter or leave a specific network interface. This requires timestamp measurement hardware to be built into every network interface that can gather the data used by the PTP algorithm.
Software timestamping is not accurate enough to properly implement PTP, due to the timing inaccuracies that can occur within protocol stacks. Accuracy of a few milliseconds or better is certainly possible with software, which is why NTP (Network Time Protocol) can be implemented using interfaces that don’t support hardware timestamping. PTP requires more than a thousand times better accuracy than NTP, so hardware support is essential.
For additional information about precision timing, its benefits, and applications download It’s About (Precision) Time white paper
Related Solutions: Quarra PTP Ethernet switches