ile infrastructure. These operators typically relied upon GNSS at every cell site to supply phase and frequency synchronization but were becoming concerned about the lack of a backup in case of a local GNSS outage. The solution that the ITU-T came up with was APTS. The APTS design takes advantage of the fact that the cell site already houses GNSS equipment. Figure 11-11 illustrates the concept. During normal operations, the GNSS is the primary source of phase/time and frequency for the radio equipment. The APTS clock, in this case a slave clock called the Telecom Time Slave Clock with assisted partial support (T-TSC-A), has a slave port that is re- ceiving a timing signal from a remote T-GM across an unaware third-party network. If the GNSS signal becomes unavailable and the primary source has lost its traceability to a refer- ence, this slave port will be selected as a backup by the best master clock algorithm (BMCA). Use GNSS to correct network asymmetry PTP-unaware PTP-unaware PTP-unaware network network network PRTC / T-GM T-TSC-A G.8275.2 G.8275.2 GNSS assisted boundary clock boundary clock Figure 11-11 Assisted Partial Timing Support The extra component that makes this work is that during normal operations, while the GNSS is the primary source, the slave clock also recovers the timing signal from the remote T-GM and measures any difference in the recovered clock against the accurate GNSS time signal. Any offset between the two time signals is most likely to come from asymmetry in the third-party network links between the slave and the T-GM. This measurement is taken constantly and used to determine a cor- rection value for the PTP slave port. If the primary GNSS source ever disappears, and the slave port is selected as a backup source, then the last measured value of the asymmetry is used to correct the timing signal from the remote PTP T-GM. It is the use of the local GNSS source (or any other source of timing signal) to assist the PTP slave to recover accurate time that gives this solution its name. In th