What is Signaling System No. 5?

Answer 1

Signaling system number 5 is a take-off on the Bell System R1 multi-frequency encoding scheme adopted as a standard by the CCITT (from the French "Comité Consultatif International Téléphonique et Télégraphique" which translates into the International Telegraph and Telephone Consulatative Committee). Here's the scoop. A group of unique tones is put together to develop a "code" for use over a phone line. The tones are used in pairs, and the frequencies of the tones are selected to minimize the probability of harmonics being "seen" by listening equipment and being confused as another tone. For CCITT5, there were six tones selected and they were used in (15) unique pairs to transmit the digits zero through nine and also some instructions as regards the number string or the connection to be made. Ever use a telephone touch tone keypad? And hear the tones when dialing? Then you're familiar with something like SS#5 already. The keypad is a dual tone multi-frequency (dtmf) device. On touch tone phones, the keypad is three characters wide (the 1, the 2 and the 3) and four characters high (the 1, the 4, the 7 and the *). Draw a line across through each of the rows the characters, and then one down through each column of characters. Each line is a unique tone, and there are seven of them in all. Each of the characters in a given row has a common tone, but each row has a different one. Each of the numbers in a given column has a common tone, but each column has a different one. Each number that is pressed causes a unique pair of tones to be sent down the line. These tones can be interpreted by switchgear and the number (or instruction character) recovered. (Note: there is another vertical column of characters that are not on the keypad. They are the letters A, B, C and D down the right side of the keypad. There is a unique tone for them, too, and that makes our "matrix" four by four for sixteen unique characters. That gives the switchgear some other options for instructions outside the ten characters zero through nine, and the familiar "*" and "#" at the bottom of the first and third columns, respectively.) The particulars are cited in the Wikipedia article on Signaling System Number 5, and a link is provided to that and the DTMF article (the phone touch pad) to make comparing notes easier. Between the two articles and this answer, it is hoped that understanding follows.