10 – 7
Chapter 10 Machine Overview
10.2.2 Telephone Call Connection Mechanism
1) When the handset is lifted, the hook switch is activated and a transmission signal (400Hz/48VDC),
called a dial tone (DT), is sent from the local switch. The dial tone is audible at the handset
speaker, indicating that the calling party can start dialing.
2) Entering the telephone number by rotating the dial or pushing the buttons transmits the number to
the local switch.
There are two types of telephone line corresponding to the two ways of
transmitting the phone number. They are known as dial types. Most recent
telephones can automatically distinguish the dial type.
One type is "Pulse Dialing (PD)", also called "Dial Pulse (DP)". After the rotary
dial on a dial phone is rotated, the dial returns to its original position. While
returning, the electric current is interrupted (dividing the signal into pulses) the
number of times corresponding to the number dialed. The switch derives the
number from the number of pulses. When the pulse repetition rate is ten pulses
per second, it is referred to as 10PPS (Pulses Per Second), when twenty, 20PPS.
The other type is a method known as "Tone Dialing (TD)", formally called "Dual-
Tone Multi-Frequency (DTMF)". Each button on a push-button phone is assigned
a unique pair of frequencies (the "tone"), from which the switch derives the
number.
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