If the message is to be routed internally (within the same domain as the S-CSCF),
then the S-CSCF routes the message to the MGCF. This may be done directly or through
another CSCF, depending on the operators??™ deployment and implementation.
Packet-Switched Domains (PS)
One would think that within the packet domain, there is no need for gateways or other
entities to route into the IMS. However, the packet domain is not necessarily IMS
compatible. In today??™s networks, operators have been evolving the network from circuitswitched
to packet-switched, which means you have hybrid networks.
A good example of this is the GSM network. GSM started as an IN/SS7 architecture
model, using the SS7 protocol to communicate to internal databases that manage
subscriber location and registration within the network. The switches themselves use
circuit switching when routing calls between networks, but in many cases X.25 packet
switching was implemented for calls within a base station subsystem, and back to the
switch itself.
As GSM matured, it began evolving certain aspects to packet-switched using IP. This
is indeed how IMS came about in the first place, as part of this evolution. But there
are still legacy components that must be supported, until such a time when handsets
are equipped as SIP User Agents (UAs) and can communicate directly with the IMS
infrastructure without interfacing with any other entities.
Pages:
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129