Difference Between ITU and ANSI

ITU-T protocols are used on the international plane; every country that wishes to connect to the International C7 network worldwide strictly adheres to these protocols. The comparison presented here is between the North American ANSI protocols (national plane) and the ITU-T recommendations that are to be adapted for use on the national plane. Apart from North America, China and Japan made some modifications outside of the ITU national recommendation framework.

Message Transfer Part 3

ANSI uses 24-bit Point Codes (PCs) for addressing, while ITU uses 14 bits. This is a result of the greater number of nodes needing to be addressed within North America. China also uses 24-bit PCs to ease numbering strain.

ANSI uses an 8-bit SLS (formerly 5-bit—it actually still supports both), while ITU uses 4 bits and its corresponding load-sharing mechanism is different.

There are some differences in terms of the Service Indicator (part of the SIO) values. Spare and reserved fields differ slightly, and ANSI uses the SI value 2 (Signalling network management messages' special messages).

ANSI assigns message priorities to manage congestion, while ITU does not. ANSI network congestion is measured in four levels: 0 (lowest) through 3 (highest). Each network message is assigned a congestion priority code (level). As the congestion level increases, lower priority messages are not allowed to be sent.

ISDN User Part

ANSI ISUP is based on the ITU ISUP recommendations and adheres to the signalling procedures, parameters, and message types without great exceptions. Therefore, it can be considered a nationalized ISUP. As expected, many of the timers have different values; some timers belong in ITU only, and some belong in ANSI only. ANSI does not specify many ITU messages/parameters and many additional messages/parameters that have been added

Signaling Connection User Part

ITU and ANSI have identical message sets.

ITU SCCP has an Importance parameter in the Connection Request, Connection Confirm, Connection Refused, and Released messages, and ANSI does not.

ANSI and ITU state different lengths for the Calling Party Address and the Data parameters that are used inside Unitdata and Unitdata Service messages.

The specified subsystem numbers (SSNs) are the same, except ANSI specifies SSN 11, 13, and 14 as "Reserved," and ITU specifies them as ISDN supplementary services, broadband ISDN edge-to-edge applications, and TC test responder, respectively.

SCCP management differs between ANSI and ITU in terms of the number of messages available. ITU provides six SCCP management messages, while ANSI provides a total of nine

Transaction Capabilities User Part

Variations are much greater at the TCAP level; the variations are so great above TCAP that a comparison could only be made in general terms.

While ITU uses the term "message types," ANSI uses the term "package types." ANSI TCAP has seven messages, as opposed to ITU-T TCAP's five. ITU-T TCAP does not have the concept of permission.

Table J-1 shows the comparable messages used in the two protocols.

Table J-1. ANSI and ITU TCAP Messages

ANSI "Package Types" ITU-T "Message Types"

Unidirectional Unidirectional

Query with Permission Begin

Query without Permission

Response End

Conversation with Permission Continue

Conversation without Permission

Abort Abort