This section is directed to the system owner who is considering converting conventional 450 MHz repeaters to trunked operation. Perhaps you are an operator of shared community repeaters who would like to achieve higher customer loading (and greater profits) from your existing system. Or perhaps you operate a private "campus" system of two or more VHF or UHF conventional channels used for plant security or maintenance. Either way, converting your conventional channels to trunked operation will significantly increase your systems capacity while providing privacy, system security, and advanced features such as selective and emergency calling.

What is Trunking?

The term "trunking" originated decades ago in the telephone industry to describe the process of selecting one clear communications path from many possibilities. It is based on the premise that if 100 users are use the network at any one time. Therefore, it is not necessary to install 100 trunk lines to serve 100 telephone customers; only 10 lines will be sufficient to provide a high level of service.

Trunking in Land Mobile Radio (LMR) Systems

Trunked LMR systems were introduced in the early 1980's on the same premise. By "trunking" together groups of frequencies (channels), a communications network could serve a large number of users with a very high level of service.

The efficiencies and features of trunked radio are well known to experienced 800 MHz system operators.

The primary advantages of any trunked radio system can be summarized as follows:

• Automatic selection of a clear channel: In a conventional radio system, the user may only have access to a single channel. By FCC regulation, if the user wishes to make a call, he must first monitor the channel to make sure it's clear. If the channel is busy, the user must continue to monitor the channel until the co-channel user has terminated the conversation. By contrast, in a trunked system the channel selection is automatic. When the user initiates a call, the trunking system electronically "monitors" each channel and selects one clear (unused) channel from many possible channels.
• Channel Privacy: In a conventional system, other co-channel users can easily eavesdrop on conversations. In fact, channel monitoring is required by the FCC when initiating a call, as described above. In a trunked system, other users cannot listen on other conversations. A radio can only join a conversation when directed by the trunking controller.
• Channel Exclusivity: In a conventional system, discourteous co-channel users can "jump" on a channel in the middle on another conversation, thereby interrupting the call in progress. In a trunked system, once a channel is selected, it becomes exclusive for the duration of the transmission. Other users cannot interrupt or interfere with the call.
• Selective Calling: In most conventional radio systems, selective calling is an expensive add-on option or may not be available at all. On the other hand, some trunked radio systems provide extensive selective calling capabilities as a standard feature. This means that users can selectively call different groups or individuals in the system. Each user is typically assigned a unique individuals ID code and one or more group ID codes. These codes can be dialled by other users in the system to select only the specific groups or individuals with whom they wish to communicate.

The Trunking Advantage

The advantages of trunking can be illustrated by a very simple example. Most of us are familiar with problem of standing in line for a bank teller or at a ticket window, as illustrated in Figure 1 below. Queuing analysis shows that more people (customers) can be handed with the same level of service when all servers (channels) are available to all customers.