Method and apparatus for enabling roaming of subscriber among plural mobile radio systems, using mobile equipment accepting removable subscriber identity module

An inter-system roaming method, comprising the steps of:

• allocating a temporary mobile subscriber identity for a mobile subscriber in a first mobile radio system having a first radio interface, including indicating the identity of said mobile radio system in predetermined bit locations in said temporary mobile subscriber identity in a system-independent format,
• sending said temporary mobile subscriber identity to a first mobile station of said mobile subscriber, said mobile station comprising a removable subscriber identification module for storing data related to an identification and authentication of a mobile subscriber, as well as a first mobile equipment capable of operating in said first mobile radio system,
• storing said temporary mobile subscriber identity in a predetermined storage location in said removable subscriber identification module,
• removing said mobile subscriber identification module from said first mobile station,
• inserting said mobile subscriber identification module in a second mobile station comprising a second mobile subscriber equipment capable of operating in a second mobile radio system having a second radio interface,
• copying said temporary mobile subscriber identity from said mobile subscriber identification module to said second mobile subscriber equipment,
• sending said temporary mobile subscriber identity to a fixed network of said second mobile radio system,
• analyzing said predetermined bit locations in said temporary mobile subscriber identity in order to find said identity of said first mobile radio system, and
• requesting an actual identity of said mobile subscriber from said first mobile radio system according to said identity of said first mobile radio system as found by said analyzing, and said temporary mobile subscriber identity.