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to be entered automatically by the components sending control signals
across the interface. The most common form of this is defined in the V.54
standard, and this is covered in more detail in Chapter 7. Because of this,
modems that perform loopback are often referred to as V.54 modems.

The use of V.54 means that the entire testing process can be carried out
by commands given to the component. Thus a PAD connected to a switch
as shown in Fig. 3.15 can test out the complete circuit up to and including
the modem on the switch, by commands given at a local terminal.

If all of these checks work satisfactorily then the rest of the circuit
could be checked using software in the switch and a command terminal
connected to it. See Fig. 3.16. The types of command used to perform
testing are illustrated in Fig. 3.17.

Fig. 3.16 Effective circuits after loopback from switch end

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