In M a y of 1976, a new Common Channel Interoffice Signaling (CCIS) system linked together t h e last new N o . 4A toll crossbar office in M a d ison, Wisconsin a n d the first No. 4 ESS toll office in Chicago, Illinois. This was a major milestone in a long-range program t o achieve a nationwide Stored-Program Controlled (SPC) network of stored-program controlled switching offices interconnected by a new high-speed, high-capacity interoffice signaling system, CCIS. T h e SPC network will provide faster, m o r e reliable communications a n d will m a k e possible a myriad of new communication services. An evolutionary transition to t h e SPC network is in progress. CCIS is now being implemented in t h e toll or long-distance network. I t will s u b s e q u e n t l y be e x t e n d e d to Traffic Service Position S y s t e m (TSPS) u n i t s a n d to Electronic Switching System (ESS) local switching offices t o provide customer-to-customer CCIS service. T h i s issue of t h e Bell System Technical J o u r n a l is devoted to CCIS. T h i s overview a n d t h e following articles cover t h e inception a n d goals of t h e p r o g r a m a n d t h e i m p l e m e n t a t i o n of CCIS in t h e toll network.In telephony, interoffice signaling has two functionally different c o m p o n e n t s : supervisory signaling, used t o initiate a n d t e r m i n a t e con nections, a n d t o indicate call status; a n d address or control signaling, used t o c o m m u n i c a t e t h e destination of a call. Supervisory signaling 221

This article is an introduction to a series of articles that describe the No. 4 Electronic Switching System. Objectives and the organization of the system are given and the overall operation is explained.

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