Voice Over IP

TELEPHONE SIGNALING BASICS

To keep matters simple, the telephone system was designed to perform signaling by onhook and offhook operations. The onhook operation means the telephone set is not being used, a term derived in the old days when the telephone handset was placed on a hook (later a cradle) when it was not being used. The offhook is just the opposite; the handset is being used—it is lifted from the telephone hook.

The offhook and onhook operations change the electrical state of the line between the terminal and the CO (or PBX). The signals listed in Table A-1 are onhook or offhook signals of various durations to convey different meanings, as summarized in the far right-hand column of the table.

Figure A-7 builds on the information provided ...