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.