There are several protocols provided in the SIGTRAN protocol suite:
?– M2UA
?– M2PA
?– M3UA
?– SUA
SIGTRAN is used to support the transport of SS7 level 4 signaling data (ISUP and
TCAP) over an IP facility. The payload of these SIGTRAN protocols is still ISUP and
TCAP, so the entities themselves must still support SS7. There are also specific entities
that interface with the IMS that support SIGTRAN and provide the conversion
between the two signaling types, as we will discuss in later chapters.
One of these functions is the signaling gateway (SG). The signaling gateway is responsible
for interfacing into the SS7 network, and converting the MTP layers to SIGTRAN
32 Chapter 2
back into the packet network. The ISUP and SCCP/TCAP layers remain intact and
unaffected by the changes.
The signaling gateway is another means of evolving the network from a TDM-based
core to an IP-based core. For example, an operator may decide to maintain its existing
TDM facilities in the core network, while at the same time implementing an IP network
as an overlay. As new entities are added to the network, they are added on the IP side
of the network, using a ???cap and grow??? approach rather than a replacement strategy.
Using the signaling gateway at the edge of the TDM networks provides a bridge into
the IP network. The operator can then depend on media gateway controllers (MGCs) to
accept the SS7 signaling, and eventually make the conversion to SIP.
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