This means that these networks must be able to interwork
with the IMS architecture. The procedures already described apply to the IMS network,
not the legacy network. The only thing that changes will be the events and procedures
implemented at the access layers of the network prior to reaching the IMS. Once the
IMS is reached, all of these procedures become valid.
The traditional wireline network typically relies on the control protocol Signaling
System #7 (SS7). Like the IMS and SIP, the SS7 protocol is used to establish and
Establishing a Session in the IMS 119
manage voice sessions. However, SS7 does not support other media types; hence the
need to move to a new architecture and network model such as IMS.
SS7 relies on a separate network for call control, just like the IMS. This network is
often referred to as the Intelligent Network (IN). The idea behind the IN was to create
a control network that all network switches would connect to, and move the call intelligence
and control into the core of the network. This in theory would reduce operations
cost and allow operators to implement new services more quickly, without having to
enable every switch in the network to support these new services.
There were many obstacles to full implementation of the IN concept, not the least of
them cost. But many of the tier 1 network operators have implemented this model and
have been relying on the IN architecture for decades.
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