A multimode mobile station, such as a radiotelephone or personal communicator, is capable of operating in more than one system. By example, and for a dual mode mobile station, the modes may be a digital cellular mode (e.g., GSM or DCS1900) and an analog (FM) cellular mode (e.g., IS-41 (AMPS)).
One such modern cellular system is referred to as IS-136, which is described in IS-136.1 and IS-136.2, Rev. A, February 1996, and subsequent updated releases. This system employs both Digital Control Channels (DCCHs) and Analog Control Channels (ACCS) that enable a mobile station to gain access to the system. When a mobile station scans for and subsequently monitors a DCCH or a ACC, it is said to be "camped" on that particular DCCH or ACC. Page messages and other information are received from the DCCH or ACC.
When a currently registered mobile station is performing a Registration on a new system (DCCH to ACC, Public System Identification (SID), Private System Identification (PSID), Residential System Identification (RSID), etc), IS-136 currently has a deficiency that may result in the mobile station being "cutoff" from any system. The nature of this deficiency is best understood with the following example.
A mobile station that is camped on a DCCH transmits a Registration message to the new system, for example a system having an ACC. The new system receives and processes the Registration message, and executes VLR/HLR updates, if necessary. The new system then transmits an Order Confirmation message to the mobile station over the ACC.
However, due to some RF coverage problem (e.g., the user enters an elevator or a tunnel), the mobile station does not successfully receive the Order Confirmation message. In this case it may happen (depending on circumstances) that after the RF reception conditions improve the mobile station may revert back to the original DCCH of the first system (DCCH). It may also occur that the mobile station may again find the same or a different ACC, and would again attempt to register on the ACC since the previous attempt failed. In the most common case, the mobile station will locate and access another control channel (DCCH) of the first system. Reference can be had to, by example, IS-136.2, Section 2.6.3.10, Action on Registration Failure.
However, the mobile station will consider itself to be still registered with the original system (the attempted registration on the second system did not succeed from the mobile stations's perspective, and the last successful registration was with the original system). In this case the mobile station resumes camping on the DCCH of the original (first) system.
The problem that results is that the mobile station does not receive any pages, as they are sent by the new system (ACC) to which the mobile station (unknowingly) successfully registered. As presently specified, corrective action can take anywhere from 1 minute to 8.5 hours (in one minute increments), with a typical time being between 10 minutes and one hour. During this time the mobile station is effectively cut-off from service, and may miss an important incoming call.