There are three key criteria for a radio system that should be understood on a fundamental level before specifying the details of each. These 3 are:
The expectations for each of these will to be specified in a procurement process and will influence different aspects of the system and cost.
What follows is a high-level and simplified description of each criteria, intended to show what influences each criteria and thereby differentiate between the criteria.
The areas of coverage for a radio system is indicated by locations where you have sufficient radio signal from your tower sites and conversely locations from where the radio signal transmitted from your user radio will reach the tower site to permit 2-way communication.
In general, the better coverage you require, the more tower sites you will require.
Availability refers to the percentage of time that the radio system is up and running from end-to-end in a way that enables radio users to communicate over the system. The system will be unavailable if some critical component of the system has failed. Therefore the system availability is improved by adding redundant/back-up components into the system, with automated switching between a failed component and the backup. Obviously the more redundancy built into the system, the greater the cost.
An availability close to 100% would be ideal but in reality is not realizable. Typically availability would be specified as something like 99.9% (“three nines”) or 99.999% (“five nines”). Note that five nines availability corresponds to approximately 5 minutes of system downtime per year.
Grade-of-Service refers to the probability that a user of the radio system would not be able to use the radios system, even if within the coverage area and even if the system was available (i.e. not in a failure mode). This inability to use the system is the result of insufficient system capacity in terms of the number of radio channels. If all of the radio channels are in use, the next user attempting to initiate a transmission will find themselves blocked or at least queued, made to wait for an available radio channel.
A grade-of-service (probability of queueing) will be a small number, something of the order of 1% or 3%. The only way to have a perfect 0% grade-of-service would be to have as many radio channels licensed and available as you have radios that can access the system. This would be unaffordable and not permitted from a licensing perspective.
So in summary, the relationships are:
Coverage <===> number of radio sites
Availability <===> automatic redundancy built into the system
Grade-of-Service <===> number of radio channels