Message Transfer Part 3

ANSI [1] uses 24-bit Point Codes (PCs) for addressing, while ITU [52] 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. (See Chapter 7, "Message Transfer Part 3 (MTP3)" for more information.)

There are some differences in terms of the Service Indicator (part of the SIO) values. Spare and reserved fields differ slightly, and ANSI [1] uses the SI value 2 (Signaling 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.