The SCP is where the service
such as HLR, Number Portability, or Calling Name resides.
This becomes important because as databases grow, a need arises to add additional
elements into the network. If an operator is providing a Number Portability function,
and it has to add another database to support the service, the routing tables would
require changing in every switch accessing the database. STPs and a function known
as Global Title Translation (GTT) eliminate this requirement.
This is a very simple control network, and not all that dissimilar to the IMS. When
you examine the functions within the IMS, think of a wireless network and its entities
(as identified in Figure 1.2) and you will have a clearer idea of how the IMS works.
In the wireless implementation, the messaging between databases and switches increases
significantly. These databases provide an important function to the wireless
network such as location, registration, and authentication/authorization. These concepts
have been extended for use in the IMS, which comes as little surprise given the
architects of IMS are also the architects of the GSM architecture.
The communications in the wireless network pass between the switches and the
base stations themselves. The base stations communicate with the switches using
the SS7 protocol as a transport. The Mobile Application Part (MAP) carries the real
signaling information that is relevant to mobile communications.
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