Signaling System No. 6 (SS6) was introduced in the 1970s as an early
common channel signalling
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
method for telecommunication
trunks between international switching centers (ISCs). It is specified in
CCITT Recommendations Q.251-Q.300.
[ITU-T Recommendation Q.251-Q.300](_blank)
- Specifications of Signalling System No. 6
The system had limited applications since at that time the predecessor systems
Signaling System No. 5 (C5) and C4 were widely in use, but there was a need to experience common channel working on a fast digital basis.
Traffic switching in the ISCs was then typically handled by
crossbar switches 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, nar ...
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 modulate ...
were likely
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 radioi ...
links or long terrestrial
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 ...
channels. In some cases these were advantaged or multiplied with
time-assignment speech interpolation In telecommunication, a time-assignment speech interpolation (TASI) was an analog technique used on certain long transmission links to increase voice-transmission capacity.
TASI was invented by Bell Labs in the early 1960s to increase the capacity ...
(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 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 multi ...
data link
A data link is the means of 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 transmitter and a recei ...
. Technology at the time was an M1040 or M1020 analogue
Four-wire circuit
In telecommunication, 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
A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by Modulation#Digital modulati ...
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
{{Telecomm-stub
ITU-T recommendations
Telephony signals