Operators can have their own implementations of SIM-based authentication. GSM networks are always SS7 based. Unmatching messages To take subscribers' mobility into account, both mobile radio network types add extra message types to the normal kind moving between the nodes. Recall that GSM's separation of the subscriber identity module from the mobile equipment causes a significant difference in databases within both kinds of networks. This dissimilarity yields different MAPs in the two network types, and the differences are a major barrier to roaming between the two.
Barriers are there to be overcome.
Copyright 1999, The Institute of Electrical and Electronics Engineers, Inc.