Ever heard of a term called trunking system? Most of us would have heard of it. STD calls as we know it is an abbreviation of “Subscribers Trunk Dialling”. They allow connection of different cities through trunk lines. It consists of a simple calculation made by calculating the expected number of users of one city that would call the customers of another city and the total number of lines that may be required for connecting all of these users.
The trunking system provides a medium of communication to users at two different ends through a fixed number of trunk lines. Otherwise you would require one line each for every user who wishes to talk to a person in another city. Instead this system automatically finds an empty line for both the transmitter and the receiver for the conversation to occur.
The airport radio uses the same system. Firstly they calculate the total number of flights that may be flying around and crossing each other. Next they decide on a minimum number of lines that may be required for one tower to connect to a flight that is about to take off or land.
A consideration that is made before installing airport radio is there will be no time wherein all flights will be needed to connect at the same time and hence one will always have some lines empty for making a connection to a flight. As it is the chance for such a thing to happen is very few and one always finds some lines empty for connection to be established.