It is the STP
that provides the call control within the IN model. Without the STP, this network model
would be an absolute mess with every switch in the network interconnecting to one another
(a mesh network). The mesh network is simply not scalable, as has already been
proven many times before in legacy networks.
The STP in the IN is first a router for all signaling messages. All switches and other
network entities connect to an STP, which has responsibility for routing the message
to other entities for processing. When signaling is sent to another network, the STP
provides a gateway function. Part of this gateway function includes gateway screening,
which is used to control the access allowed by other operator networks.
So the STP is both router and firewall for the signaling network. It determines
(through operator provisioning, of course) what signaling messages are allowed into
the network, and how those messages are to be routed to their destination.
The STP also provides a routing function to the various network databases. If implemented
properly, all database addresses are provisioned in the routing tables of the
STP rather than the switches. All database queries are then routed to the STP in the
core network, which then has the responsibility of routing to the most available database
(service control point, or SCP). By using the STP to route these queries, you add
an additional level of security within the network, and eliminate the need to provision
SCP addresses in every node throughout the network.
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