A portable communications subscriber unit operates as a data transfer
terminal as well as an analog cellular telephone M-ES. Two modes of
operation, an analog cellular communication mode and a Cellular Digital
Packet Data (CDPD) mode, are available in the subscriber unit. The
subscriber unit distinguishes between paging signals indicative of a CDPD
mode of communication and those indicative of an analog cellular mode of
communication. The subscriber unit automatically preempts CDPD
communications in favor of analog cellular communications such as those
carried out in an AMPS system. The present inventive method enables the
subscriber unit to remain registered in the CDPD communication system
using a CDPD "sleep mode" while performing activities on the AMPS system.
Another aspect of the present invention facilitates the automatic
switching of subscriber unit displays to correspond to the communication
system with which the unit is presently communicating.
