SS6
Encyclopedia
SS6 or CCITT No 6 Signalling System was introduced in the 1970s as an early common channel
signalling method for telephone
trunks
between International Switching Centres
(ISCs). It had limited applications since at that time C4 and C5
signalling systems were still flourishing but there was a need to experience common channel working on a fast digital basis.
The telephone switching in the ISCs was then typically crossbar
or electronic controlled cross-points such as reed relay
s. The transmission media
was likely to be satellite
or lengthy terrestrial submarine cable
channels. In some cases these were advantaged or multiplied with TASI (Time-Assignment Speech Interpolation
) or DCME (Digital Circuit Multiplication Equipment
). These transmission methods were not suited to Line
or Inter-Register signalling and taking the signalling function away from the traffic channel was a better prospect (leads 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 the 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 data link
. Technology at the time was an M1040 or M1020 analogue 4 wire (4w) presented private circuit
link. Modem
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 link 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 counties where more sophisticated methods were required.
Common Channel Signaling
In telephony, Common Channel Signaling , in the US also Common Channel Interoffice Signaling , is the transmission of signaling information on a separate channel from the data, and, more specifically, where that signaling channel controls multiple data channels.For example, in the public switched...
signalling method for telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
trunks
Trunking
In modern communications, trunking is a concept by which a communications system can provide network access to many clients by sharing a set of lines or frequencies instead of providing them individually. This is analogous to the structure of a tree with one trunk and many branches. Examples of...
between International Switching Centres
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
(ISCs). It had limited applications since at that time C4 and C5
Signaling System No. 5
CCITT5 was a multi-frequency telephone signalling system in use from the 1970s for International Direct Distance Dialing . It was sometimes nicknamed "Atlantic Code" because the first IDD connections between Europe and North America used it....
signalling systems were still flourishing but there was a need to experience common channel working on a fast digital basis.
The telephone switching in the ISCs was then typically crossbar
Crossbar switch
In electronics, a crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner....
or electronic controlled cross-points such as reed relay
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...
s. The transmission media
Transmission medium
A transmission medium is a material substance that can propagate energy waves...
was likely to be satellite
Communications satellite
A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...
or lengthy terrestrial submarine cable
Submarine communications cable
A submarine communications cable is a cable laid on the sea bed between land-based stations to carry telecommunication signals across stretches of ocean....
channels. In some cases these were advantaged or multiplied with TASI (Time-Assignment Speech Interpolation
Time-assignment speech interpolation
In telecommunication, a time-assignment speech interpolation was an analog technique used on certain long transmission links to increase voice-transmission capacity....
) or DCME (Digital Circuit Multiplication Equipment
Digital Circuit Multiplication Equipment
Digital Circuit Multiplication Equipment was a type of voice compression equipment that is installed at either end of a long-distance link . The main characteristics of DCME are defined in ITU-T recommendation G.763...
). These transmission methods were not suited to Line
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 party and vice versa...
or Inter-Register signalling and taking the signalling function away from the traffic channel was a better prospect (leads 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
Call set-up time
In telecommunication, the term call set-up time has the following meanings:#The overall length of time required to establish a circuit-switched call between users....
and clear down would be significantly shorter than with C4 and C5. With the 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 data link
Data link
In telecommunication a data link is the means of connecting one location to another for the purpose of transmitting and receiving information. It can also refer to a set of electronics assemblies, consisting of a transmitter and a receiver and the interconnecting data telecommunication circuit...
. Technology at the time was an M1040 or M1020 analogue 4 wire (4w) presented private circuit
Leased line
A leased line is a service contract between a provider and a customer, whereby the provider agrees to deliver a symmetric telecommunications line connecting two or more locations in exchange for a monthly rent . It is sometimes known as a 'Private Circuit' or 'Data Line' in the UK or as CDN in Italy...
link. Modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
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 link 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 counties where more sophisticated methods were required.