consider: Failure of the GNSS source of time signals, both local (such as a local jamming event) and sys- temic (the entire constellation has some problem). Failure in the transport carrying a timing signal from a remote primary reference time clock (PRTC) and GM (assuming the use of SyncE and PTP). Redundancy of PRTC and GM sources, spaced widely around the network but not too far away from the cell sites that need to consume the signals. Increasing the duration of holdover; for example, by using a physical source of frequency as a method to enhance holdover (see the section "Holdover" later in the chapter for more information). Designing the network to ensure redundancy in the SyncE signals. Deploying several extremely stable reference clocks (such as atomic clocks) to allow autonom- ous operation of the timing solution (meaning it can run without an external source). Methods that improve the ability of a PTP-unaware network to accurately carry timing signals during a GNSS outage in the local area. Figure 11-3 shows how a device at the end of the net- work, such as a cell-site router, can have a local primary source of time with a PTP slave port recovering clock from a remote T-GM. T-TSC PRTC/T-GM slave clock Local primary timing source PTP-aware PTP-aware network network PTP slave port is PTP & SyncE from PRTC/T-GM secondary source remote T-GM Figure 11-3 GNSS Primary Source with Remote PTP Backup Use of the terms "master" and "slave" is ONLY in association with the official terminology used in industry specifications and standards, and in no way diminishes Pearson's commitment to promoting diversity, equity, and inclusion, and challenging, countering and/or combating bias and stereotyping in the global population of the learners we serve. One commonly proposed method is to also utilize the assisted partial timing support (APTS) tech- nique as outlined in ITU-T G.8273.4, where the connection to the remote T-GM is across an un- aware network. The following sections address each of these considerations in turn. 