The new system will allow the Fire Dept greater flexibility and better coverage. In addition to allowing us many more resources for our own use, it will enable us to communicate with other city departments using a single radio.
What is trunked radio? It is a group of repeaters controlled by a computer that assigns repeaters to users on an as-needed basis. All radios share all repeater channels in the system. Therefore, to determine which radios can talk to each other, they are divided into “talkgroups” instead of channels.
A talkgroup is an identification code that is programmed into radios. Units that have the need to talk to each other, such as apparatus at the scene of a fire, an engine company and an EMS unit responding to a medical call or firefighters participating in a training exercise, have to be set to the same group code. Radios are programmed with a quantity of talkgroups to suit the needs of communicating within the department and with other departments.
The City trunk system is composed of 11 channels. One is the control channel that handles the assignment of the other ten, which are called working channels. When not assigned to a call, all radios monitor the control channel of the system.
To initiate a call, the user presses the Push-To-Talk (PTT) button as usual. But, instead of keying up the repeater directly, it sends a data signal to the system over the control channel requesting a working channel. The system assigns a working channel, and then sends out a data message to the calling radio and all radios with the same group ID code selected, automatically switching them to the assigned channel to listen to the call. The entire request sequence takes place in less than ¼ of a second. When the assignment has been made, the calling radio emits a high frequency acknowledgement beep from the speaker, which comes from the assigned working channel confirming that the assignment was successful. After channel assignment the call takes place just like it would on our present system. When the calling radio’s PTT is released all of the radios involved in that call return to the control channel and wait for the next call.
When receiving a call from someone within the same group, a radio will be directed to the assigned working channel by the system without requiring any action on the operator’s part. In addition, the ID of the calling radio will be displayed on the radio.
The system will only allow one radio to transmit on an individual group ID code at any given time. During the duration of a call all other radios will be disabled from transmitting on the same group ID code. If an attempt is made to transmit, the radio will emit a low frequency tone from the speaker indicating that the call has been denied.