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History
of inter-exchange signalling
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Before 1970,
only channel-associated signalling
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(CAS) was used.
In CAS systems, signalling always
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occurs in-band (i.e. over voice channels).
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SS6 = CCIS
(common channel interoffice signaling)
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was widely
deployed in North America, but not in
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Europe (=>
concentrating on SS7 instead).
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Starting from
1980 (mainly in Europe), CAS was
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being replaced
by SS7. The use of stored program
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control (SPC)
exchanges made this possible. Like
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CCIS,
signalling messages are transmitted
over
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separate
signalling channels. Unlike
CCIS, SS7
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technology is based on protocol stacks.
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