Signaling System Number 6 - An old standard for controlling call setup, breakdown, and ongoing call maintenance published as CCITT Signaling System No 6 Recommendation. SS6 and AT&T's implementation called CCS6 were slow and designed to work on low bit-rate channels. Moreover, these architectures were not layered, which made changing the code a complex and expensive task. Consequently, the CCITT began work in the mid-1970s on a new generation signaling system, which eventually resulted in the publication of SS7 in 1980.