Certainly it is the IN that supports
services such as Freephone, Calling Name Display, and Number Portability, all
of these being implemented worldwide. Of course, wireless networks could not support
roaming today without the use of SS7 and its architecture.
The IN architecture is really quite simple. Each switch in the network is equipped
with a service switching point (SSP) function, which allows the switch to generate SS7
messages and interact with the rest of the network via SS7. The switch in turn can be
connected to other switches, or connected through a hub and router referred to as the
signal transfer point (STP). The STP provides many services outside the scope of this
book (my Signaling System #7 book goes into much greater detail of SS7 and the IN),
but for the purposes of this topic the STP becomes a focal point in interworking with
the IMS.
Applications and services are deployed on servers referred to as service control points
(SCPs). These servers are large, dedicated servers that are SS7 specific but will also
have to interwork with the IMS if operators want to utilize their legacy services in an
IMS environment. This is discussed in more detail in Chapters 1 and 2.
The gateway into the PSTN from the IMS network is the Media Gateway Control
Function (MGCF). It is up to the MGCF to convert SS7 messages into SIP messages
for forwarding into the IMS network.
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