This is generally how the switches in the PSTN intercommunicate. User-to-Network Signaling
Generally, when using twisted copper pair as the transport, a user connects to the PSTN through analog, Integrated Services Digital Network (ISDN), or through a T1 carrier.
The most common signaling method for user-to-network analog communication is Dual Tone Multi-Frequency (DTMF) . DTMF is known as in-band signaling because the tones are carried through the voice path. Figure 1-9 shows how DTMF tones are derived.
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