Method and apparatus for providing a predictable upper bound for a channel search time of a portable subscriber unit

A portable subscriber unit operating in a plurality of messaging systems for providing a predictable upper bound for a channel search time, the portable subscriber unit comprising:

• a receiver for receiving messages and control information;
• and
• a processing system coupled to the receiver for controlling the receiver and for processing the messages and control information,
• wherein the processing system is programmed to:
• select a first messaging system to monitor for messages;
• initiate background scanning in accordance with a scan list, comprising a plurality of system parameters ranked by appropriateness for the portable subscriber unit, at a rate not slower than the rate established by a predetermined set of rules known to a controller which sends the messages to the portable subscriber unit, in an attempt to find a system identifier transmitted during a plurality of predetermined time periods, the system identifier identifying a second messaging system more appropriate for the portable subscriber unit than the first messaging system, the background scanning initiated during at least one of a plurality of exempted time periods coinciding with at least one of the plurality of predetermined time periods;
• and
• concurrcntly monitor the first messaging system for receiving messages until identifying the second messaging system.

A controller in a messaging system of a plurality of messaging systems for providing a predictable upper bound for a channel search time of a portable subscriber unit assigned to the controller, the controller comprising:

• a network interface for receiving a message from a message originator and for conducting communications with others of thc plurality of messaging systems;
• a processing system coupled to the network interface for processing the message and the commnunications;
• and
• a base station interface coupled to the processing system for controlling a base station,
• wherein the processing system is programmed to control the base station to transmit, during a plurality of predetermined time periods, a system identifier, and
• wherein the processing system is further programmed with information about a scan list in the portable subscriber unit, comprising a plurality of system parameters ranked by appropriateness for the portable subscriber unit, and with information defining for the portable subscriber unit a predetermined set of rules by which the portable subscriber unit initiates background scanning, and
• wherein the processing system is further programmed to cooperate with others of the plurality of messaging systems, with regard to message delivery to the portable subscriber unit, in accordance with the information about said scan list and said predetermined set of rules, thereby providing said predictable upper bound for the channel search time, and
• wherein the processing system is further programmed with information defining for the portable subscriber unit a plurality of exempted time periods during which the portable subscriber unit is permitted to perform background scanning, in accordance with the scan list, for finding and selecting a system more appropriate for the portable subscriber unit than a currently monitored system, and wherein at least one of the plurality of exempted time periods coincides with at least one of the plurality of predetermined time periods.