Calling Line Identification Presentation, also known as A-number presentation, is a service made available by most country's telephone-companies (for a small fee, ofcourse :-)

The protocol is (unfortunately), mostly different among the countries, (ex. USA uses Bell 202 or V23, based on FSK, while Denmark uses DTMF for signalling).

The Danish implementation of CLIP uses simple DTMF-signalling, transmitting the A-subscriber number (from the exchange) to the B-subscriber, just before ringing.

Each DTMF (Dual Tone Multiple Frequency) consists of a pair of tones:

Hz	1209	1336	1477	1633
697	1	2	3	A
770	4	5	6	B
852	7	8	7	C
941	*	0	#	D

The frequencies might seem a little odd, but they are chosen very carefully (based on logarithmic distance), to avoid harmonics of each other, and mains-frequencies (50/60Hz).

The receiver in a CLIP-device must be able to detect frequencies with a (+/-1,5% +5Hz) deviation. Furthermore, it must detect:
DTMF with a difference of less than 8dB between the two single tones.
DTMF with a single-tone level between -4dBm and 28dBm.
DTMF with a single-tone level of less than -36dBm must not be detected.

Tones or breaks with a lenght:
Less than 20ms must not be detected.
Between 20ms and 50ms may be detected.
Higher than 50ms must be detected.

The protocol itself is very simple:

A phonenumber #	'A' means that an A-subscriber number will follow, and '#' ends the phonenumber. The phonenumber is transmitted as digits.
Besides the number, the following 'error-codes' may be transmitted:
D 1 #	The number cannot be transmitted because the A-subscriber has made a restriction.
D 2 #	The number cannot be transmitted because the call is international
D 3 #	The number cannot be transmitted because of technical reasons

Example... The following sequence:
A80808080#
tells you that TeleDanmark is calling :-)

Each digit/space between digits uses (at least) 50ms, so the total transmission time is (10*50+9*50) minimum 0,95s.

The protocol has both advantages:

• It's easy to interface to.

• It's slow.
  
• The time/date is not transmitted, meaning that all decent equipment must have a clock built-in.
  
• You get no notification before the exchange starts transmitting the number (only the DTMF 'A'). In other CLIP-protocols, the polarity is either reversed just before transmitting the number, or the number is transmitted just after the first 'ring'. This gives the receiver the possibility of 'waking up' from a powersaving mode.

For further reading, see TDK-TS 900 216 from "TeleStyrelsen"