SIMPLE SIP is the first form developed for use in
VoIP networks by the IETF, but it lacks many of the more robust attributes added by
the 3GPP for use within the IMS. These extensions to the SIP protocol are required
before a call can terminate within the IMS domain, because most of the extensions were
added for authorization and authentication.
One other important note about routing calls from the circuit-switched domain into
the IMS: the SS7 protocol is routed using Time Division Multiplexed (TDM) facilities,
while the IMS is all IP-based. Many operators have already begun converting their
backbone networks to all-IP. This includes the facilities traditionally used for routing
of SS7 messages through the network.
Signaling gateways are used to convert the lower layers of the SS7 protocol to IP.
The Message Transfer Part (MTP) is compatible only with TDM facilities and therefore
must be replaced with a set of protocols compatible with IP facilities. SIGTRAN
replaces the SS7 MTP while preserving the upper layers of the SS7 protocol (ISUP and
TCAP). This allows operators to maintain their existing signaling infrastructure and
transition the transport backbone to IP in preparation for an all-IP network.
The upper layers of the SS7 protocol (ISUP and TCAP) are still needed for managing
the resources within the circuit-switched network, but they are not needed within
the IP domain, as we have already discussed.
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