What is really interesting about the IMS model is how everything is controlled using
one common signaling method. Signaling is what makes connections happen in any
network. It is signaling that allows network entities to communicate with one another
as well as application servers regarding the service to be delivered.
In the Public Switched Telephone Network (PSTN) today, Signaling System #7 (SS7)
is the signaling method used to connect facilities in the telephone network. SS7 connects
different telephone companies networks with one another, and connects all of the
switches within the network with one another.
In the IMS, SS7 is replaced with a new signaling method: SIP. SIP is used to control
everything in the network, allowing all network entities to communicate with one another
regarding service delivery network-wide.
2 Chapter 2
Architecture of the IMS 3
So when we begin talking about the IMS architecture, we will be looking into how all of
the media types and the various entities are controlled through SIP. However, SIP alone
is not enough. There are network functions that must be developed that work with SIP
to provide a universal control mechanism for all forms of subscriber communications. The
IMS defines these functions as well, and we will be looking at these functions in detail.
So don??™t think of the IMS as a means of delivering killer new applications, and don??™t
look for a ???killer app??? in IMS.
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