Anyone who’s known me for a while may be used to my mantra, but it is worth repeating: “Everything works when it works.” It’s an important mantra to keep in mind as it’s our job to keep systems working under failure conditions for as long as it takes for us to mobilise to fix the problem!
Timing Degradation
What do I mean by that? Well, let’s take GPS timing as a prime example. A timing receiver locked to GNSS signals for 24 hours or so will usually give an excellent output fit for purpose, but what happens if someone cuts through your GNSS cable while maintaining another system? The instant this happens the timing performance of your system will start to degrade, usually in line within the physical properties of your holdover technology (e.g., crystal oscillator, oven crystal, Rubidium Atomic Clock, etc). With good design, the locked performance of your clock will be much better than the performance your application requires, but that performance will be moving towards that lower level at a rate determined by this holdover clock.
Optimum Network Holdover
It’s fair to say that the higher holdover performance comes at a higher cost, and devices at the edge of networks tend to be under more cost pressure. As engineers, it’s our job to balance these costs with performance to keep your networks performing, even when problems arise. As a rule of thumb, you should aim to give yourself at least three days to troubleshoot and mobilise to fix whatever problem you have. A four-hour fix means permanent staff at key locations (this may be appropriate in many systems but unfortunately not timing) or parked at motorway junctions like ambulances often are. In summary: ‘just in case’ is not a cost effective and efficient field force solution!
Summary
So, whether it’s GPS timing or any other potential issue facing your network, it’s important to design and maintain a network that factors in these potential problems while considering the time and resources it’ll take to effectively correct them.
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