The Signalling System No. 5 was a simple protocol to send telephone signalling via international satellite links, as well as between local carriers inside a country. It is described in ITU Q.140-180 (former CCITT). The signals were transmitted inband, i.e. they were sent as tones in the speech frequency band. The benefit was that there was no common signalling channel required. All channels could be used for phone calls. Another benefit was given to 'phone phreaks'. They sent their own signals to route telephone calls to the destination they wanted to, and get charged for the call they initially dialed - a toll free call.
The system uses two frequencies for signalling, 2400 Hz and 2600 Hz:
Signal | Frequency 1 | Frequency 2
----------------+-------------+--------------
seize | 2400 Hz |
proceed-to-send | | 2600 Hz
----------------+-------------+--------------
answer | 2400 Hz |
acknowledge | 2400 Hz |
----------------+-------------+--------------
busy-flash | | 2600 Hz
acknowledge | 2400 Hz |
----------------+-------------+--------------
clear-back | | 2600 Hz
acknowledge | 2400 Hz |
----------------+-------------+--------------
clear-forward | 2400 Hz | 2600 Hz
acknowledge | 2400 Hz | 2600 Hz
|
On an idle channel, a tone of 2400 Hz is sent by the outgoing exchange to seize a call. The incomming exchange recognizes the tone and acknowledges with a 2600 Hz tone, indicating that the outgoing exchange may proceed-to-send. As the acknowledge is detected by the outgoing exchange, it stops the 2400 Hz tone, and waits until the incomming exchange recognizes it, and also stopps it's 2600 Hz tone.
Now the dialing information is sent using 2 out of 6 frequencies. They are 700, 900, 1100, 1300, 1500, 1700 Hz. Each combination of 2 of these tones represent a key prefix, a digit, or a stop information:
Digit | Frequency 1 | Frequency 2 | Length
--------+-------------+-------------+--------
1 | 700 Hz | 900 Hz | 55 ms
2 | 700 Hz | 1100 Hz | 55 ms
3 | 900 Hz | 1100 Hz | 55 ms
4 | 700 Hz | 1300 Hz | 55 ms
5 | 900 Hz | 1300 Hz | 55 ms
6 | 1100 Hz | 1300 Hz | 55 ms
7 | 700 Hz | 1500 Hz | 55 ms
8 | 900 Hz | 1500 Hz | 55 ms
9 | 1100 Hz | 1500 Hz | 55 ms
0 | 1300 Hz | 1500 Hz | 55 ms
Code 11 | 700 Hz | 1700 Hz | 55 ms
Code 12 | 900 Hz | 1700 Hz | 55 ms
KP1 | 1100 Hz | 1700 Hz | 100 ms
KP2 | 1300 Hz | 1700 Hz | 100 ms
ST | 1500 Hz | 1700 Hz | 55 ms
|
KP1 is used to dial national, KP2 is used to dial transit (international). The next digit, the discriminating digit, is used to discriminate different characteristics of a call, e.g. selecting the language used for operator assistance. The ST is used to signal end of number. Code 11 and 12 are used to call an operator. A national call to area code 555 and number 1212 is dialed via the following sequence:
KP1-0-5551212-ST
The length of a digit is 55 miliseconds, except for the KP digits, they are 100 miliseconds. The pause between digits is 50 miliseconds.
During a call, tones of 2400 Hz and 2600 Hz are used to indicate answer of the call, busy-flash, or release of the call.
To really clear the call, the outgoing exchange must send a clear-forward singal. This signal uses both frequencies 2400+2600 Hz in compound. This tone is acknowledged by the incomming exchange with the same compound tone. The line is now idle again. It can be used for another call in either direction.
