This chapter describes signaling system No.6, the first-generation commonchannel signaling system. There are two versions of this system, both of which were first deployed in the mid-1970s. CCITT No.6 [l, 23 is still in use in the international network, on a number of transatlantic and transpacific trunk interofice signaling (CCIS), defined by the Bell groups. Common-channel System, has been used in the U.S. toll network [3,4], but has now been replaced by the North American version of signaling