MDC (Motorola Data Communications), also known as Stat-Alert, MDC-1200 and MDC-600, is a Motorola two-way radio low-speed data system using audio frequency shift keying, (AFSK). MDC-600 uses a 600 baud data rate. MDC-1200 uses a 1,200 baud data rate. Systems employ either one of the two baud rates. Mark and space tones are 1,200 Hz and 1,800 Hz. The data are sent in bursts over the radio system's voice channel. MDC signaling includes a number of features: unit ID, status buttons, emergency button, selective inhibit, radio check, and selective calling. These features are programmable and could be used in any combination desired by the user. They are typically incorporated in high-end analog FM commercial and public safety radios made by Motorola and other manufacturers. In addition to Motorola, at least two other companies make compatible base station decoders for MDC-1200. Motorola radios with MDC options have an option allowing the radio to filter out data bursts from the receive audio. Instead of hearing the AFSK data, the user hears a short chirp from the radio speaker each time a data burst occurs. (The user must turn on this feature in the radio's option programming settings). A general option setting for all MDC systems is to enable or disable an acknowledgement (ack) data packet. For example, following a selective call, the called radio replies with an ack. This data "handshake" confirms the called radio is powered on, has received and decoded the call. The encoder beeps to confirm the call got through to the target. In computer-aided dispatch, the encoder/decoder may pass the ack to the dispatch computer system, flagging the selective call as having been received by the mobile radio or automatically marking the time of call. The disadvantage of using an ack on busy channels is that more air time is used: roughly double the air time used by the selective call data alone. The ack packet takes roughly the same amount of air time as the selective call itself.
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 hexadecimal 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.
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 when the emergency button is pressed. The radio will silently send the emergency message, with the four-digit unit ID embedded, three times. In hand-held radios, this increases the probability of at least one packet getting through. In a computer-aided dispatch (CAD) environment, the button press may pop a dialog box or activate some other attention-getting device. For example, on a screen showing status summary of all units, the unit with an activated emergency button may change colors or flash.
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