> I had been told - clearly from a not too informed
source - that GPS timing units were used at each base station.
> Does the same apply to 3G (or GSM for that matter) mobile networks as well?
I'd guess not. Why would they bother? They've got links (whether microwave or
landline) back to somewhere more central. The more central places could have a
time source, whether GPS or other. The time signals could be distributed in
some priviledged data frame which doesn't wait in any queue.
Mobile systems are complicated, of course, and there might be an unobvious
reason to use GPS ...