nal antenna to synchronize its clock to this level of phase accuracy. Suppose this GPS antenna fails for some reason (say, a lightning strike). In that case, the clock in this cell site equipment will immediately move into holdover. The operation of this cell site can only con- tinue SO long as its clock phase alignment of that cell site radio stays within +1.5 us of accuracy. Should this GPS antenna failure happen during the night, it might take some time for operations staff to discover and locate the failure, and then dispatch a technician to the correct location. If it was a long weekend, it might take quite some time for the technician to reach the location and fix the issue (assuming the technician knows what the problem is and has spares for the GPS antenna readily available). Of course, it would be very convenient if, during this whole duration, the clock was able to stay within a phase alignment of +1.5 us and provide uninterrupted service to that area. If the maximum time to detect and correct this fault is (say) 24 hours, then for uninterrupted service, the clock on this cell site requires a holdover period of 24 hours for +1.5 us. In fact, the generally accepted use case at the ITU-T is that a clock should need holdover performance for up to 72 hours (considering a long weekend). Note that this requirement (24 hours for +1.5 us) is quite a strict holdover specification and just about all currently deployed systems (cell site radios and routers) are not able to achieve this level of holdover. Being able to do it for 72 hours is very difficult and requires a clock with a very high- quality oscillator, such as one based on rubidium. Adding to the difficulty, the weather conditions at a cell site (up some mountaintop) might be ex- tremely challenging with only minimal control of environmental conditions for the equipment. Temperature is a very significant factor contributing to the stability of an oscillator, and that stabil- ity feeds directly into holdover performance. For this reason, the ITU-T G.8273.