Signaling System No. 6

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 relays. 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. 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.

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