2.3.  Analog Signaling

Analog signaling (DTMF, MF tones, or pulses) transmit the telephone number of the called party to the local CO. For each call, whether an inbound or an outbound call, the entity making the call is the "calling party" and the entity receiving the call is the "called party."

For example, a calling party sends the first dialed digits to the local CO. The local CO uses these digits to determine the next CO in the connection chain. The next CO uses these first dialed digits to determine if they are the destination CO or if the call is to be switched to another CO. Eventually, the call reaches the destination CO. At the destination CO, the call is received and acknowledged. The destination CO eventually gets the last dialed digits, which explicitly identify the called party.

The destination CO checks the called party's line to determine if it is idle or busy. If the called party's line is idle, the destination CO applies ringing to the line and sends ringback tones backwards to the calling party. When the called party answers the call, the calling party is switched through to the called party. If the called party's line is busy, the destination CO sends this information backwards to the calling party via tones.

NOTE:

Analog technology does not provide a means to physically block or unblock an analog line.

Pulse dialing (also called rotary dialing) sends digit information to the CO by momentarily opening and closing (or breaking) the electrical loop from the calling party to the CO. This electrical loop is broken once for the digit 1, twice for 2, etc., and 10 times for the digit 0. (Pulse dialing is not available on DM3 boards.)

DTMF and MF signaling use a multifrequency code system wherein each DTMF or MF signal is composed of two frequencies, as listed in Table 1.  Signaling Used to Dial. Although DTMF signaling is designed for operation on international networks with 15 multifrequency combinations in each direction, in national networks it can be used with a reduced number of signaling frequencies (for example, 10 multifrequency combinations).

Some MF digits use approximately the same frequencies as DTMF digits; for example, the digit 4 uses 770 and 1209 Hz for DTMF or 700 and 1300 Hz for MF transmissions. Because of this frequency overlap, MF digits could be mistaken for DTMF digits if the incorrect tone detection is enabled. Digit detection accuracy depends on the digit sent and the type of detection, MF or DTMF, enabled when the digit is detected. See the Voice API Library Reference for details.

Table 1.  Signaling Used to Dial

Code	Pulse (clicks)	DTMF (Hz)	MF (Hz)
1	1	697, 1209	700, 900
2	2	697, 1336	700, 1100
3	3	697, 1477	900, 1100
4	4	770, 1209	700, 1300
5	5	770, 1336	900, 1300
6	6	770, 1477	1100, 1300
7	7	852, 1209	700, 1500
8	8	852, 1336	900, 1500
9	9	852, 1477	1100, 1500
0	10	941, 1336	1300, 1500
*	-	941, 1209	1100, 1700
#	-	941, 1477	1500, 1700

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