Once these phones are available, the legacy
wireless network is no longer needed. Handsets will be able to interface via UMTS and
other broadband radio interfaces directly into the IMS via the P-CSCF function.
One thing is certain about IMS: while it is being developed and implemented to
replace the IN and SS7, this replacement does not happen overnight. It will be quite
some time before all of the SS7 network is replaced, and there will be many parts of
this network that are never replaced.
This is evident when one looks at how long it has taken other, similar technologies
to be fully developed and implemented. For example, the first discussions and papers
on Asynchronous Transfer Mode (ATM) date back to 1969 (this author has some of the
early papers from the Bell Labs).
Yet it took us nearly 20 years before the technology was really ready for operators to
begin implementing in their networks. The same is true regarding SS7 and the IN. The
development of SS7 began in the mid-1960s, yet it was not really implemented in the
U.S. until the mid-1980s.
Even then it was only implemented to support 8xx services and not for call control. It
was not until later that operators in the U.S. began implementing SS7 for call control
(using the ISDN User Part, or ISUP, protocol).
The migration to SS7 has been even slower in Europe and Asia. Most of the major
operators in Europe, for example, have implemented SS7 only for call control between
switches, but the role of the STP as a hub within the core of the IN was never fully
appreciated until wireless networks began being deployed.
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