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 lines b ...

(MF) telephone signaling system that was in use from the 1970s for International Direct Distance Dialing (

IDDD Direct distance dialing (DDD) is a telecommunication service feature in North America by which a caller may, without operator assistance, call any other user outside the local calling area. Direct dialing by subscribers typically requires extra ...

). 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 ov ...

, 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 artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope ...

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 American Telephone and Telegraph Company (AT&T), that dominated the telephone services industry in North America for over one hundr ...

multi-frequency (MF) signaling system known by

CCITT The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...

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 (phone 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 pa ...

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

2600 Hz A blue box is an 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 voice circuits. ...

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.

References

{{telsigs Telephony signals

See also

References