Another way of providing conversion between the PSTN and IMS domains is through
the use of a signaling gateway. This is useful when there is no VoIP network already
established. The trunking then is managed by the network switches and eventually
must be sent to a packet network, although the packet network could be provided by
another service provider.
The primary purpose of the signaling gateway is to support SS7 over IP networks.
SS7 is a packet protocol, but the transport and network layers of the SS7 protocol (the
message transfer part) were designed for TDM facilities. The Internet Engineering
Task Force (IETF) has developed new transport and network layers for SS7 ISUP and
SCCP/TCAP using the SIGTRAN protocol on IP facilities.
The signaling gateway then receives the SS7 traffic over TDM facilities, next converting
the MTP layers to SIGTRAN for transport over an IP network. The payload of the
SIGTRAN packet remains the SS7 ISUP and SCCP/TCAP protocols, so no conversion is
made from SS7 to SIP at this function. That remains the responsibility of the MGCF.
The MGCF does not replace the P-CSCF; rather, they work in conjunction with one
another as seen in Figure 1.7. The MGCF provides the connection to the SS7 network,
while the P-CSCF then takes over the SIP dialog as the session is forwarded into the
IMS domain. The MGCF only interfaces to the SS7 network within the same service
provider domain.
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