The Signaling System No. 5 (SS5) is a

multi-frequency In telephony, multi-frequency signaling (MF) is a type of signaling that was introduced by the Bell System after World War II. It uses a combination of audible tones for address ( telephone number) transport and supervision signaling on trunk ...

(MF) telephone signaling system that was in use from the 1970s for International Direct Distance Dialing ( IDDD). Internationally it became known as CCITT5 or CC5.ITU-T Recommendation Q.140-Q.180
- Specifications of Signalling System No. 5 It was also nicknamed ''Atlantic Code'' because it was used for the first IDDD connections between Europe and North America. Signaling systems in use at the time were designed for

in-band signaling In telecommunications, in-band signaling is the sending of control information within the same band or channel used for data such as voice or video. This is in contrast to out-of-band signaling which is sent over a different channel, or even o ...

, meaning they used the same channel as the media that they controlled. SS5 was designed for inter-continental traffic for which many transmission paths were long terrestrial, often

submarine cable Submarine cable is any electrical cable that is laid on the seabed, although the term is often extended to encompass cables laid on the bottom of large freshwater bodies of water. Examples include: *Submarine communications cable *Submarine power ...

, and geostationary

satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

links. Trunks using satellite links also had

echo suppressor Echo suppression and echo cancellation are methods used in telephony to improve voice quality by preventing echo from being created or removing it after it is already present. In addition to improving subjective audio quality, echo suppression i ...

s connected at their end points. SS5 was specifically designed to work within those links. Based on the

Bell System The Bell System was a system of telecommunication companies, led by the Bell Telephone Company and later by the AT&T Corporation, American Telephone and Telegraph Company (AT&T), that dominated the telephone services industry in North America fo ...

multi-frequency (MF) signaling system known by

CCITT The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is one of the three Sectors (branches) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunicat ...

as Regional System R1, it used six signaling frequencies: 700 Hz (''A''), 900 Hz (''B''), 1100 Hz (''C''), 1300 Hz (''D''), 1500 Hz (''E'') and 1700 Hz (''F''). The first five frequencies were used in a

two-out-of-five code A two-out-of-five code is a constant-weight code that provides exactly ten possible combinations of two bits, and is thus used for representing the decimal digits using five bits. Each bit is assigned a weight, such that the set bits sum to the ...

to represent decimal numbers (telephone numbers), and the last frequency in combination with one of the others represented the beginning or end of a sequence of digits. These frequencies were combined to encode the following signals: The ''2 out of 6'' frequency code was used to pass digits forward between Registers in traditional International Switching Centres. Each digit took 55 ms with a 55 ms Inter-digital pause (IDP) and the sequence was sent 'en-bloc' to ensure that Echo Suppressors would not switch out the forward path as the links tended often to be satellite channels. The first digit was a keying Prefix (a KP) to indicate Terminal or Transit working and the last digit was the digit 15 or Keying Finish (KF). In addition, for

line signaling Line signaling is a class of telecommunications signaling protocols. Line signaling is responsible for off-hook, ringing signal, answer, ground start, on-hook unidirectional supervision messaging in each direction from calling party to called ...

a 2 frequency (2VF) code using compelled sequences of 2400 Hz and

2600 Hz 2600 hertz (2600 Hz) is a frequency in hertz (cycles per second) that was used in telecommunication signaling in mid-20th century long-distance telephone networks using carrier systems. Tone signaling carrier systems operated in the sta ...

rather than continuous SF tone was used to seize the line at the beginning of a call over that trunk and clear it at the end of the call - long after the valuable register had sent 15 or KF and dissociated from the call. This Line signalling element was 2VF to introduce the possibility of more meanings and tone-off idle in order that hundreds of channels in transmission media would not be transmitting standing tones simultaneously.

Abuse and fraud

The deficiencies of SS5 are widely known as the root cause for "

Blue box A blue box is an Electronics, electronic device that produces tones used to generate the in-band signaling tones formerly used within the North American long-distance telephone network to send line status and called number information over voi ...

fraud" that enabled phone phreaks, such as Captain Crunch (

John Draper John Thomas Draper (born March 11, 1943), also known as Captain Crunch, Crunch, or Crunchman after a toy boatswain's call whistle once given away in boxes of Cap'n Crunch breakfast cereal that for some years could be used to make free long d ...

), to abuse the telephone system and place telephone calls for free, among exploiting other capabilities. SS5 was abused by sending a "seize" signal to simulate and pose as a telephone switching system from a subscriber telephone line. This enabled the perpetrator to control the remote switch that had no means for distinguishing the signaling from that of another legitimate switch. This fraud was a factor in the widespread adoption of

Signalling System No. 7 Signalling System No. 7 (SS7) is a set of telephony signaling protocols developed in the 1970s that is used to setup and teardown telephone calls on most parts of the global public switched telephone network (PSTN). The protocol also performs ...

(SS7) in replacement of SS5.

References

{{telsigs Telephony signals

Abuse and fraud

See also

References