ISUP and the SS7 Protocol Stack

As shown in Figure 8-2, ISUP resides at Level 4 of the SS7 stack with its predecessor, the Telephone User Part (TUP). TUP is still used in many countries, but ISUP is supplanting it over time. TUP also provides a call setup and release that is similar to ISUP, but it has only a subset of the capabilities. TUP is not used in North America because its capabilities are not sufficient to support the more complex network requirements.

As you can see in Figure 8-2, a connection exists between ISUP and both the SCCP and MTP3 levels. ISUP uses the MTP3 transport services to exchange network messages, such as those used for call setup and clear down. The connection to SCCP is for the transport of end-to-end signaling. While SCCP provides this capability, today ISUP end-to-end signaling is usually transported directly over MTP3. The "Interworking with ISDN" section of this chapter further discusses end-to-end signaling and the two different methods using MTP3 and SCCP for transport.

ISUP Standards and Variants

The ITU-T defines the international ISUP standards in the Q.767 and the national standards in the Q.761–Q.764 series of specifications. The ITU-T standards provide a basis from which countries or geographical regions can define regional or national versions of the protocol, which are often referred to as variants. For the U.S. network, the following standards provide the primary specifications for the ISUP protocol and its use in local and long distance networks:

• ANSI T1.113–ANSI ISUP

• Telcordia GR-246 Telcordia Technologies Specification of Signaling System No. 7, Volume 3. (ISUP)

• Telcordia GR-317 LSSGR— Switching System Generic Requirements for Call Control Using the Integrated Services Digital Network User Part (ISDNUP)

• Telcordia GR-394 LSSGR— Switching System Generic Requirements for Interexchange Carrier Interconnection (ICI) Using the Integrated Services Digital Network User Part (ISDNUP)

In Europe, the following ETSI standards provide the basis for the national ISUP variants:

• ETSI ETS 300-121 Integrated Services Digital Network (ISDN); Application of the ISDN User Part (ISUP) of CCITT Signaling System No. 7 for international ISDN interconnections

• ETSI ETS 300-156-x Integrated Services Digital Network (ISDN); Signaling System No. 7; ISDN User Part (ISUP) for the international interface

The ETS 300-121 is version 1, and the ETS 300-156-x (where x represents an individual document number) is a suite of specifications that covers ETSI ISUP versions 2–4.

A multitude of different country requirements have created many ISUP variants. A few of the several flavors are Swedish ISUP, U.K. ISUP, Japanese ISUP, Turkish ISUP, Korean ISUP. Each variant is tailored to the specific national requirements. Although not certain of the exact number of variants that are in existence today, the author has encountered over a hundred different ISUP variants while developing software for switching platforms.