r long periods with every- day equipment at reasonable cost. So, it is important to do some expectation setting. The authors frequently answer detailed questionnaires on timing requirements from mobile operators. These questionnaires are part of a process of selecting the network equipment that an operator wants to use in its 5G-ready back-haul and front-haul network (see the section "Writing a Request for Proposal (RFP)" in "Chapter 12. Operating and Verifying Timing Solutions"). It is not unusual to see a holdover requirement for a cell-site router (CSR) that must be able to hold the 1.1-us phase accuracy for TDD radios for wildly unrealistic periods. Many times, operators pur- chasing new equipment require that CSRs should support a holdover of 24-72 hours. Some others ask for multiweek holdover or even up to several months! This is possible, but only in combination with (very expensive) atomic clocks. The reality is that an operator should only expect to keep that level of phase alignment for several hours in an everyday network element. As the name suggests, the CSR is a device sitting in every cell site, and, obviously, because of the large numbers of cell sites, cost is a major factor in deciding on the winning vendor. But the tech- nical specification might specify that this multiweek holdover is a mandatory requirement-mean- ing that the device will be denied any chance of selection unless it complies. And yet, the purchaser expects the CSR with all its Ethernet ports, components, and features to cost less than 20% of the cost of a device that can meet this holdover requirement. The ideal situation would be to put an enhanced PRTC (GNSS receiver plus cesium clock) at every cell site. That would deliver amazing accuracy and very long-term holdover capability. But that would cost something like $50~100K per site and OPEX of around $5~7K per year. A simple and totally robust timing solution is simple to deploy if one has unlimited funds. However, for a normal NE or router with a Stratum 3E oscillator, you s