m (GNSS) such as the GPS. With the increased number of cells expected in 5G, the cost of providing independent GPS-based timing will likely become prohibitive. Moreover, the accuracy of time recovery from GNSS is limited to +100 ns, which is insufficient for the most demanding of 5G uses. The alternative is for base stations to obtain timing from the communications infrastructure that feeds it, namely from the backhaul transport, and the physical layer of the transport network is a critical element in delivery of high-accuracy timing information. If the transport is carried over a natively synchronous infrastructure, such as PDH, SDH, OTN, or dark fiber, then highly stable frequency is automatically retrieved by the physi- cal infrastructure, as the physical layer requires this frequency synchronization for its own proper functioning, and obtains it using a PLL. The most common asynchronous physical layer is 802.3 Ethernet, but even Ethernet, for rates of interest here, continuously trans- mits bits at a constant rate (sending idle codes when there is nothing to transmit), although that rate is not locked to a frequency reference with high accuracy. SyncE (ITU-T G.8262) remedies this deficiency by applying conventional mechanisms for locking the frequency of physical layers of synchronous networks to Ethernet. The first Ethernet switch in a chain 6.6 Transport Network of switches has its transmit bit rate locked to a PRC; that is, a frequency reference with accuracy of 10-11 or better. Each switch in the chain locks its internal clock onto an input closer to the PRC, and transmits its outputs accordingly. The ITU-T has specified the Eth- ernet Synchronization Messaging Channel (ESMC) to indicate clock quality (closeness to the PRC) and to aid avoiding timing loops (ITU-T G.8262). Distribution of time-of-day information over a packet network requires a packet time pro- tocol, such as the network time protocol (NTP) (RFC 5905), or the PTP (IEEE1588) (IEEE 1588v2). The latter has the potential to be more accurate th