Signaling System No. 6 (SS6) was introduced in the 1970s as an early
common channel signalling
A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology.
In ...
method for telecommunication
trunks between international switching centers (ISCs). It is specified in
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 ...
Recommendations Q.251-Q.300.
[ITU-T Recommendation Q.251-Q.300](_blank)
- Specifications of Signalling System No. 6
The predecessor protcol to SS6,
SS5, was carried
In-band
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 ...
and was increasingly being exploited by
phreakers. SS6 was
out of band which gave it some protection from these attacks.
Traffic switching in the ISCs was then typically handled by
crossbar switch
In electronics and telecommunications, a crossbar switch (cross-point switch, matrix switch) is a collection of switches arranged in a Matrix (mathematics), matrix configuration. A crossbar switch has multiple input and output lines that form a ...
es or electronically controlled cross-points such as
reed relay
A reed relay is a type of relay that uses an electromagnet to control one or more reed switches. The contacts are of magnetic material and the electromagnet acts directly on them without requiring an armature to move them. Sealed in a long, narro ...
s. The
transmission media
A transmission medium is a system or substance that can mediate the propagation of signals for the purposes of telecommunication. Signals are typically imposed on a wave of some kind suitable for the chosen medium. For example, data can modula ...
were likely
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 or long terrestrial
submarine cable channels. In some cases these were advantaged or multiplied with
time-assignment speech interpolation (TASI) or
digital circuit multiplication equipment (DCME). These transmission methods were not suited to
line or inter-
register signalling. Taking the signalling function away from the traffic channel could cut costs, and would later lead to Common Channel Signalling. There would be no need for filtering signalling away from speech, and the duty cycle of the costly trunk for each revenue earning call would be better. Signalling
time in call set up and
clear down would be significantly shorter than with C4 and C5. With telephone signalling not transmitted over the line, the opportunities for
fraud
In law, fraud is intent (law), intentional deception to deprive a victim of a legal right or to gain from a victim unlawfully or unfairly. Fraud can violate Civil law (common law), civil law (e.g., a fraud victim may sue the fraud perpetrato ...
were reduced.
The CCITT 6 signalling channel was typically a 2.4
kbit/s
In telecommunications, data transfer rate is the average number of bits ( bitrate), characters or symbols ( baudrate), or data blocks per unit time passing through a communication link in a data-transmission system. Common data rate units are mu ...
data link
A data link is a means of telecommunications link, connecting one location to another for the purpose of transmitting and receiving digital information (data communication). It can also refer to a set of electronics assemblies, consisting of a t ...
. Technology at the time was an M1040 or M1020 analogue
Four-wire circuit In telecommunications, a four-wire circuit is a two-way circuit using two paths so arranged that the respective signals are transmitted in one direction only by one path and in the other direction by the other path. The four-wire circuit gets its ...
presented
private circuit link.
Modem
The Democratic Movement (, ; MoDem ) is a centre to centre-right political party in France, whose main ideological trends are liberalism and Christian democracy, and that is characterised by a strong pro-Europeanist stance. MoDem was establis ...
s were slowly beginning to exceed these data rates over 4w circuits; these data links had the advantage that there was no national tail from the ISC out to a normal end point in the country. It was often the National Section that detracted the most from the quality and reliability of such links.
The data circuit known with a
DP designation, would have been terrestrial with no TASI or DCME, and the C6 signalling on the data link could perform for 48 or 96 traffic channels. It could function for up to 2,048 in the maximum. The question is whether two operators would have that many traffic channels between them, dependent on just one bearer for signalling and subject to occasional failure. It was of course practice to have standby route over a diverse path in case of outage on the original DP data circuit link.
Finally it was considered possible that a mesh of such data channels could be set up between major centres, leading to a system where the link between country A and country B could perform the signalling functions for links to country A from country D for example.
CCITT No 6 was a method developed & implemented by administrations in a limited number of countries for use in correspondent International Switching of
analogue telephone calls. The next development was correspondent and non-correspondent switching of telephone calls by new operators in the countries where more sophisticated methods were required.
References
{{telsigs
ITU-T recommendations
Telephony signals