n (TESLA) pro- tocol for authentication. There are already receivers available that support this feature and, at the time of writing, the signals are already available for testing. Geopolitical risk: Some operators do not want to rely on devices that have "geopolitical risk," whether real or imagined; however, in some geographies, bad actors (including nation states) are always a real and substantial threat to a clean GNSS signal. GNSS-based systems are wonderful tools, and their use as sources of time is very valuable, but their drawbacks must be understood and mitigated. The operator needs to design and prepare the network for that day when the space signal is either not available or becomes untrustworthy. 11.3.1 Full On-Path Support with G.8275.1 and SyncE The first network topology to consider is the case where the network provides full on-path support at the protocol level. For this case, the ITU-T G.8275.1 telecom profile combined with SyncE is the best-in-class method to carry frequency, phase, and time across the transport. Most operators glob- ally are using this approach wherever it is possible for them to do SO. Figure 11-6 illustrates what the network looks like in such a deployment. The timing signal is sourced from a PRTC (typically a GNSS receiver) either as a standalone device or embedded in some aggregation router. These signals are used to generate a SyncE signal for the frequency, and the T-GM function generates a PTP message stream for the phase and time. PTP-aware PTP-aware PTP-aware network network network PRTC / T-GM T-TSC Figure 11-6 Transport of PTP and SyncE Across an FTS Network Every network element in the path between the T-GM and radio equipment is configured as a T-BC operating in hybrid mode with SyncE for frequency (it can also be a T-TC). Typically, the radio equipment has the last PTP clock in the chain embedded inside it and functions as a Telecom Time Slave Clock (T-TSC). For a successful deployment, the network design must address the following points: Every node in the path 