| 摘要 |
To help prevent call bridging and increase the reliability of channel de-assignment in a trunked radio repeater system, this working channel sends a drop channel "pre-alert" message to on-channel mobile units just before sending the drop channel signal. This "pre-alert" message instructs the mobile units to continue to receive but to not attempt to access this channel by transmitting. The pre-alert messages thus makes the overall trunked communications system more forgiving o~ coincident mobile transmissions during drop channel signalling. Under good signalling conditions there are two possibilities: (1) all mobile units detect the pre-alert, nothing is transmitted to the repeater during the pre-alert message, and a drop channel message is transmitted by the repeater following the pre-alert signal to successfully de-assign the channel; or (2) a mobile unit transmits during the pre-alert to thereby prevent the repeater from ever transmitting the drop channel message but to instead permit all mobile units to stay on the channel and continue communications. Thug, no mobile will attempt channel access during the drop channel message, and either: (a) all mobile units leave the working channel and return to the control channel, or (b) all mobile units remain on the working channel and continue to communicate.
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