Common Channel Signaling (CCS) was first introduced in the United States in the 1960s as Common Channel Interoffice Signaling System #6 (SS6). Developed by the International Telecommunications Union - Telecommunications Standards Society (ITU-TS), SS6 used a separate facility for sending signaling information to distant telephone offices.

The first deployment of SS6 in the United States used 2.4-kbps data links. These were later upgraded to 4.8 kbps. Messages were sent in the form of data packets and were used to request connections on voice trunks between two central offices. This became the first use of packet switching in the Public Switched Telephone Network (PSTN). The packets were assembled by placing 12 signaling units of 28 bits each into a data block. This is similar to the method used in SS7 today.

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