Page 197 - Asterisk™: The Future of Telephony

known as Dual-Tone Multi Frequency (DTMF)—dialing, the dial pad consists of 12
buttons. Each button has two frequencies assigned to it (see Table 7-1).

Table 7-1. DTMF digits
1209 Hz 1336 Hz 1477 Hz 1633 Hz a
697 Hz 1 2 3 A
770 Hz 4 5 6 B
852 Hz 7 8 9 C
941 Hz * 0 # D
a Notice that this column contains letters that are not typically present as keys on a telephone dial pad. They are part of the DTMF standard
nonetheless, and any proper telephone contains the electronics required to create them, even if it doesn’t contain the buttons themselves.
(These buttons actually do exist on some telephones, which are mostly used in military and government applications.)

When you press a button on your dial pad, the two corresponding frequencies are
transmitted down the line. The far end can interpret these frequencies and note which
digit was pressed.

Hybrid (or network)

The hybrid is a type of transformer that handles the need to combine the signals trans-
mitted and received across a single pair of wires in the PSTN and two pairs of wires in
the handset. One of the functions the hybrid performs is regulating sidetone, which is
the amount of your transmitted signal that is returned to your earpiece; its purpose is
to provide a more natural-sounding conversation. Too much sidetone, and your voice
will sound too loud; too little, and you’ll think the line has gone dead.
Hook switch (or switch hook). This device signals the state of the telephone circuit to the CO.
When you pick up your telephone, the hook switch closes the loop between you and
the CO, which is seen as a request for a dial tone. When you hang up, the hook switch
opens the circuit, which indicates that the call has ended. †
The hook switch can also be used for signaling purposes. Some electronic analog
phones have a button labeled Link that causes an event called a flash. You can perform
a flash manually by depressing the hook switch for a duration of between 200 and 1,200
milliseconds. If you leave it down for longer than that, the carrier may assume you’ve
hung up. The purpose of the Link button is to handle this timing for you. If you’ve ever
used call waiting or three-way calling on an analog line, you have performed a hook
switch flash for the purpose of signaling the network.

† When referring to the state of an analog circuit, people often speak in terms of “off-hook” and “on-hook.”
When your line is “off-hook,” your telephone is “on” a call. If your phone is “on-hook,” the telephone is
essentially “off,” or idle.

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