The Sigtran Protocol Suite

Figure 8-16 shows the Sigtran protocol suite and the relationship between
the Sigtran protocols and standard SS7 protocols. Above IP, we find a protocol
known as the Stream Control Transmission Protocol (SCTP). The primary
motivation behind the development of SCTP is the fact that neither
UDP nor TCP offer both the speed and reliability required of a transport
protocol used to carry signaling. The design of SCTP is an attempt to make
such reliability and speed available to the users of SCTP.
In the SCTP specification, such a user is known as an Upper Layer Protocol
(ULP). A ULP can be any of the protocols directly above the SCTP
layer, as illustrated in Figure 8-16. Each of the protocols above SCTP is an
adaptation layer. For example, M3UA is the MTP3 User Adaptation Layer.
Thus, we could have ISUP over M3UA over SCTP. Each of the adaptation
layers uses the same primitives to and from the layer above, as are used by
the equivalent SS7 layer. Thus, the layer above does not see any difference
between the adaptation layer and its SS7 equivalent. Thus, if we have ISUP
over M3UA, the ISUP layer believes the M3UA to be standard MTP3 and
does not know that the transport is IP-based.

The following applications layers are defined:
■ SS7 MTP2-User Adaptation Layer (M2UA) provides adaptation
between MTP3 and SCTP. It provides an interface between MTP3 and
SCTP such that standard MTP3 may be used in the IP network,
without the MTP3 application software realizing that messages are
being transported over SCTP and IP, instead of MTP2. For example, a
standard MTP3 application implemented at an MGC could exchange
MTP3 signaling network management messages with the external SS7
network. In the same manner that MTP2 provides services to MTP3 in
the SS7 network, M2UA provides services to MTP3 in the IP network.
■ SS7 MTP3-User Adaptation Layer (M3UA) provides an interface
between SCTP and those applications that typically use the services of
MTP3, such as ISUP and SCCP. M3UA and SCTP enable seamless
peer-to-peer communication between MTP3 user applications in the IP
network and identical applications in the SS7 network. The application
in the IP network does not realize that SCTP over IP transport is used
instead of typical SS7. In the same manner that MTP3 provides
services to applications such as ISUP in the SS7 network, M3UA offers
equivalent services to applications in the IP network.
■ SS7 SCCP-User Adaptation Layer (SUA) provides an interface
between SCCP user applications and SCTP. Applications such as
TCAP use the services of SUA in the same way that they use the
services of SCCP in the SS7 network. In fact, those applications do not
know that the underlying transport is different in any way. Hence,
transparent peer-to-peer communication can take place between
applications in the SS7 network and applications in the IP network.
■ ISDN Q.921-User Adaptation Layer (IUA) is the Sigtran equivalent of
the Q.921 Data-link layer which is used to carry Q.931 ISDN signaling.
Thus, Q.931 messages may be passed from the ISDN to the IP network,
with identical Q.931 implementations in each network, and neither of
them recognize any difference in the underlying transport.