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. Emergency button The emergency button option designates a button on hand-held or vehicle radios which sends the MDC-1200 unit ID with an emergency flag appended. The decoder notes the unit ID but interprets this data packet as an emergency message rather than a unit ID. Options allow emergency messages to always be sent over a specific channel rather than the channel set by the operator using the channel selector. For example, a system with two channels could be programmed to send all emergency messages on channel 2. This reduces interruption of the primary dispatch channel if an emergency button is pressed. The default setup for emergency buttons is for the sending radio to be completely silent