Grade of Service (for Public Safety Communications) – Part 1

The concept of Grade-of-Service for a Public Safety radio system must be kept separate from two other system parameters: coverage and availability. Grade of Service refers to the probability that a user will be denied immediate access to the operational system. Grade-of-Service is a measure of the system’s capacity, that is how many radio channels the system has and how many radios are trying to access the system at a point in time.

Grade-of-Service applies in many places other than radio systems. For example, at your local supermarket checkout, the Grade-of-Service translates to the probability of you having to wait in line (referred to as queueing) because all of the available cashiers are busy with other customers.

It might be obvious that you wouldn’t have to wait at the grocery store if they would just have more cashiers working. Adding more cashiers, or adding more radio channels, would indeed improve the Grade of Service – so why don’t they? It all comes down to cost – it’s going to cost more, to add more cashiers or more radio channels – the Grade-of-Service is largely about economics.

If Grade of Service is about the probability of waiting , how much of a wait, and how frequently, is tolerable? If you had to wait 3 seconds for the next available cashier at the grocery store you’d probably not even realize that you had waited. However, if you frequently had to wait 3 seconds to get a channel on a radio system you might exclaim that the system is unusable. Grade-of-Service breaks down to two parameters:

  1. the probability that you will be queued (made to wait in line) and
  2. the average time of that wait.

What probability and queueing wait should you expect on a Public Safety radio system? National regulator are likely to give you an answer. In Canada, Industry Canada (a federal department similar to the FCC in the US) has channel loading guidelinesi for Public Safety radio systems. They essentially say you can license enough radio channels so that your Grade-of-Service is 3%. More specifically these guidelines say that during the average busy period, 97% of calls would not be delayed by more than the average “holding” time of one transmission (which might be something like 3 or 4 seconds).

Note that even if you had a virtually unlimited budget to be able to install enough radio channels to have no chance of queueing (i.e. a GoS of 0.0%), the frequency spectrum is a shared and finite resource, so government regulations will limit what Grade of Service is ultimately possible.

[continued in Part 2]

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