niversal Time (UTC) received from GPS. But it makes no sense to say that machine A has a +200-ns phase offset (although UTC might be assumed). So, phase is expressed as a relative offset, although when measured to a common time source like UTC, it is often treated as absolute. This difference between relative and absolute is because in some circumstances the radio in one cell site might need to have close phase alignment with its neighboring radios (relative phase offset). But when all the cell sites in one mobile network needs to be aligned with those in another mobile network, then the easiest way to make this happen is to have every radio closely phase aligned with an absolute time source such as UTC. Achieving this alignment to an absolute phase means they are also relatively phase aligned at the same time. This concept will be revisited when talking about phase synchronization in the 5G mo- bile network in Chapter 10 "Mobile Timing Requirements". In the case shown in Figure 3-3, clock signals Ca, Cb, Cc, and Cd all have the same frequency, while Ce and Cf have higher frequencies. The signals are also mixed with a phase pulse, which in the case of Ca, Cb, Cc, Cd, and Cf is generated on the rising edge of every second square wave. You could imagine that the pulse could be the signal to indicate the "tick" of the second, for ex- ample. The signals Ca and Cb are phase aligned with each other, and Cd and Ce are also phase aligned, although Ce has double the frequency of Cd (the pulse is only generated on every fourth rising edge for Ce). Phase pulse Figure 3-3 Frequency and Phase Alignment The signal C has a phase offset from Ca and Cb, while the phase output of Cf is completely un- aligned because the frequency is too high, and the pulse is still being generated every second wave. Note that the devices generating the signal Cd and Ce are frequency aligned, although they have different frequencies; it is just that the signal Ce is oscillating at twice the frequency of the signal in The point of Figure 3-3 is to d