1.2.1. Understanding the Different PTP Clocks
In addition to the clock nodes shown in the figure, there is also a management node communicating management messages to query and update the PTP datasets maintained by the clock nodes for the purposes of initialization and fault management. This node typically resides in a CPU.
- Ordinary Clock:
An ordinary clock (OC) device can be either a master clock or a slave clock and it is a single port in a 1588 clock domain network.
- Transparent Clock:
A transparent clock (TC) device updates the PTP messages with the time taken by them to traverse through the network device from an ingress Ethernet port to an egress Ethernet port. In other words, the TC device updates the PTP event messages with their residence delay in the devices before the messages are transmitted out. The TC has two modes; end-to-end mode and peer-to-peer mode. The end-to-end TC uses the end-to-end delay measurement mechanism between slave clocks and the master clock. The peer-to-peer TC involves link delay correction using peer-to-peer delay measurement mechanism, in addition to the residence delay correction.
- Boundary Clock:
A boundary clock (BC) device has multiple ports in a 1588 clock domain with one slave clock port and possibly more than one master clock port. The BC maintains the same timescale for the slave clock port and possibly multiple master clock ports. A BC helps to avoid the long chain of transparent clocks between the network 1588 grandmaster and slave, leading to higher inaccuracy in the slave’s ToD synchronization. The BC also helps to divide a larger 1588 network, thus reducing the traffic going all the way back to the original 1588 master. Typical application BC devices include routers, gateways and bridges.
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