R31DS0087EU0101 Rev.1.01

Page 9

Sep.28.21

pcm4l Datasheet

Figure 3. Directory Structure

Once a grandmaster (GM) connection has been established, the pcm4l will assign a reference tracker for that GM.

The reference tracker supervisor will manage all reference trackers and maintain the local clock state machine.

The following sections provide more details about each of these blocks.

2.1 Reference Trackers (RTs)

A reference tracker tracks a remote master clock by analyzing the time stamp pairs [T1, T2] from the forward link

and [T3, T4] from the reverse link. It analyzes the statistics of the time stamps, estimates the frequency and time

offset of the local clock compared to the remote master clock, and requests local oscillator corrections through the

reference tracker supervisor. There could be multiple reference trackers running at the same time, but only one

reference tracker is the active tracker and only the local oscillator correction request from this active tracker will be

granted by the reference tracker supervisor.

As described earlier, there are a number of clock recovery techniques that can be used to recover a packet-based

clock. The pcm4l software focuses on linear and adaptive. Linear will be used for 1588 Profiles that negate the

effects of PDV noise (i.e., by using BCs/TCs throughout the network). Adaptive will be used for 1588 Profiles that

require packet measurement data analysis, where packet selection (i.e., outlier detection) is an important

component to the analysis. In other words, in order to reduce the input PDV noise, only a subset of the received

timing packets are used by the filter. The ITU-T has done a lot of work around determining “suitable PDV” for

adaptive clock recover (both for frequency and time); more information is located in Appendix I of ITU-T G.8260.