Two way radios - Icom America Inc.


Icom brings a proven approach to digital trunking by providing the benefits of familiarity, efficiency and cost effectiveness with the complete line of IDAS trunking digital radios.

At a high level, there are essentially two types of radio systems – conventional and trunked. In both types a single voice is carried over a transmit/receive (TX/RX) frequency (or TX/RX pair for a repeated system). However, in a conventional system it is up to the user to select the frequency to be used after listening to hear whether the frequency is in use. If the frequency on the selected channel is busy, the user has a choice to either wait until the current conversation is complete or manually change the channel to another frequency. This is not very efficient. Of course it depends on the particular radio activity of the users on the system, but you may find that 100 users are about all a single frequency can support on a conventional system without undue congestion.

Trunking is an advancement in which the radio system listens and changes channels automatically alleviating the person from the task. Because a computer does the work, it happens much more quickly and may carry many simultaneous conversations on one physical channel. The trunked system can be loaded more heavily with users. Trunked systems industry averages suggest up to 75-125 users per frequency can be loaded on a trunked system.

Logic Trunked Radio (LTR™) is the most popular trunking protocols in use today. It is a transmission based protocol, which means it may switch frequencies each time the radio is keyed up. The trunking controller has a designated home channel over which it listens to and sends an in-band, sub-audible message to all the other radios on the system. This sub-audible tone directs or controls the radios to chance from frequency to frequency depending  upon which are available at the time. Use of a sub-audible tone control on the same frequency as used for a voice path opens all licensed channels for voice traffic and does not require a dedicated channel for control, which consumes RF spectrum.

IDAS trunking is similar in approach to LTR. IDAS uses in-band control messages, but instead of sub-audible tones as used in LTR, IDAS uses digital control messages in the packet. There are differences between IDAS trunking and LTR, however. IDAS trunking has more capacity than LTR, and the big advantage of automatic switchover to a secondary home channel in the event the primary fails.

Despite the differences, an analog LTR system can have a smooth migration to IDAS trunking since the subscriber radio (handheld or mobile) can understand both protocols and can scan between the two. Of course, since scanning requires checking numerous channels, care should be taken as the system scales in size so undue decoding delay is not experienced.

Another feature of IDAS trunking is the controller is a module that is inserted into the repeater chassis. This saves rack space in the communications room, and simplifies set-up.