The conversion to SIGTRAN is an important stepping stone to full network transition,
since signaling performs the important function of call control. Converting the signaling
facilities to IP provides many economies to the operator and allows it to quickly
realize additional capacity and throughput not present in its legacy signaling networks.
The business case to IP backbone, therefore, becomes much easier to justify when built
around cost savings, and network growth with added density.
The signaling gateway only converts the transport layers to SIGTRAN/IP. Since ISUP
and SCCP/TCAP are not supported in IMS, these layers must also be converted to the
Session Initiation Protocol (SIP). This conversion is provided by the media gateway controller,
which accepts the SIGTRAN/SS7 signaling from the legacy network and converts
this signaling to SIP-based signaling toward the IMS. This is one of the reasons operators
should contemplate VoIP deployments as the next phase toward IP-based IMS.
A VoIP deployment will consist of all IP entities supporting packetized voice and
data across an all-IP backbone. However, the SS7 legacy must still be supported even
in the VoIP domain. Calls originating in the TDM domain will cross boundaries, as will
the SS7 signaling associated with these calls, requiring support of SS7 even in the IP
domain for some time to come.
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