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