Basic Radio Awareness

Communications Systems

How does trunking work?

Trunked networks get their name from the world of telephony. Traditionally two cities would bundle their connections together into a single thick line like the trunk of a tree, which is known as a trunked line. The local household loop lines operated like the branches of a tree, one line for each household.

The reason why trunking has become so important is because it offers some huge advantages over conventional for larger agencies.

A conventional system is a system that has dedicated channels, allocated to a specific user or a group of users. Channel one might be fire, channel two might be fire chiefs, channel three might be animal control. If you want to communicate with a particular group in conventional, then you need to manually select a channel by hand either by moving a selection knob on the radio or by using a drop down menu.

Conventional has some huge advantages and is still popular around the world today. It’s very fast to set up a call, it’s easy to use and it’s pretty inexpensive. However there are some downsides as well. Remember conventional means using dedicated channels that are manually selected. When a channel is being used, it’s used exclusively by one caller, so anybody else who wants to make a call on that channel has to wait until the call is over. It causes an inefficient use of radio channels. Channel one (the fire channel), would probably be frequently busy. On the other hand, channel three (the animal control channel), can go idle as it probably would not be as frequently used.

So while callers may be waiting to use channel one, they cannot make use of channel three even though it’s completely idle. If a user wanted to add call groups, they would need to add a new selectable channel. In this sense, users are limited by how many channels they can select manually. It also adds the headache of reprogramming all the radios anytime a new channel or user group is added.

For small agencies, a conventional system is absolutely perfect. If they know the groups that will communicate together, a conventional system is a good choice. But as soon as the number of groups or the number of users working on a system increases, trunking may be a better option.

Trunked radio might be better called “computer-controlled” or “computer-aided” radio. When a trunked radio user wishes to communicate with another user or group, the computer automatically assigns them the first free available channel to make each call. The underlying principle of trunking is that not all users or groups who need to communicate in a channel will do so at the same time. Therefore there can be many more users or groups than there are channels in the system.

A good way to explain Trunking to use the following analogy: Consider a conventional bank in which specific tellers are dedicated to specific types of customers. For example, one teller focuses on cash withdrawals, another on cash deposits, and another only on business accounts.

So what happens at lunch time? A long queue develops as household users want to make cash withdrawals or put in cash deposits and that teller gets overloaded. The business teller, however, helps one or two business customers spends most of lunch time idle. This is the problem with conventional systems.

By contrast, a Trunking teller can serve any type of customer and do any sort of transaction. So at lunch time when the long queue of household users wanting to make cash withdrawals or cash deposits is mixed in with the business users, a controller at the head of the queue simply assigns each customer to the first available teller.

This means more transactions can occur because the tellers are far more flexible and the trunking controller at the head of the queue is an intelligent intermediary who can direct traffic and maximize efficiency.

So that’s the basic difference between conventional and Trunking networks: In conventional the users manually control the allocation of channels by selecting from a knob, whereas in Trunking systems, a computer at the center of the network is responsible for channel selection.