Signaling (telecommunications) - Classification

Classification

Note that every signaling system can be characterized along each of the above axes of classification. A few examples:

• DTMF is an in-band, channel-associated register signaling system. It is not compelled.
• SS7 (e.g. TUP or ISUP) is an out-of-band, common-channel signaling system that incorporates both line and register signaling.
• Metering pulses (depending on the country, these are 50 Hz, 12 kHz or 16 kHz pulses sent by the exchange to payphones or metering boxes) are out-of-band (because they do not fall within the frequency range used by the telephony signal, which is 300 through 3400 Hz) and channel-associated. They are generally regarded as line signaling, although this is open to debate.
• E and M signaling (E&M) is an out-of-band channel-associated signaling system. The base system is intended for line signaling, but if decadic pulses are used it can also convey register information. E&M line signaling is however usually paired with DTMF register signaling.
• By contrast, the L1 signaling system (which typically employs a 2280 Hz tone of various durations) is an in-band channel-associated signaling system as was the SF 2600 hertz system formerly used in the Bell System.
• Loop start, ground start, reverse battery and revertive pulse systems are all DC, thus out of band, and all are channel-associated, since the DC currents are on the talking wires.

Whereas common-channel signaling systems are out-of-band by definition, and in-band signaling systems are also necessarily channel-associated, the above metering pulse example demonstrates that there exist channel-associated signaling systems which are out-of-band.