Unit ID or Push-to-talk ID
Many MDC-1200 systems utilize the unit ID option. With each push-to-talk press, the radio sends a data burst identifying the sending radio. Unit IDs are decoded as unique four-digit numbers. Every radio would have a unique four-digit ID, (for example: 0423 or 5990).
Unit ID can be sent as leading or trailing a voice message. In the leading option, the data burst is sent at the moment a user presses the radio's push-to-talk button. An option can be set to make the radio's speaker emit a tone for the length of the unit ID data, (about 1-1.5 seconds). This reminds a user to wait until the data has been sent before talking. The leading unit ID takes slightly more air time (is longer) than a trailing ID because of a header tone and the need to delay the data burst to allow time for CTCSS decoders and voting comparators to open an audio path to the decoder. A default delay is defined with the unit ID option. To adjust for time delay variations in each individual system, radios can be programmed to delay the sending of a radio's unit ID data by up to hundreds of milliseconds within a range. In the trailing option, the data packet is sent at the moment the microphone button is released. This avoids timing issues because the audio path to the base station is already open.
The standard Motorola encoder-decoder has a display which shows the most recent four-digit, push-to-talk ID. A printer can be connected. It would print the unit ID and the time it was received based on the decoder's internal clock.
In computer-aided dispatch (CAD), the four-digit ID is passed to the CAD and may be translated to a local name for the unit. For example, a tow truck with an identifier "Downtown 6" logging on at the beginning of a shift may call the dispatcher and say, "Downtown 6, in service: vehicle radio 0455, hand-held 0771." The CAD computer would translate any push-to-talk ID from either 0771 or 0455 to display "Downtown 6" on the CAD screen.
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