The PAD Protocols

The way a PAD operates is defined by a set of protocols, described in three ITU-T recommendations:

• X.3 defines the way the PAD program itself works.

• X.28 defines the way that interaction between the PAD and the terminal is handled.

• X.29 defines the way that interaction between the PAD and the PSDN is handled.

These three standards are often referred to together as triple X. This is sometimes written XXX.