in the 1960s, led to the abuse by phreaking, a subculture that exploited the technology to explore national and international telephone networks and place cost-free long-distance telephone calls. Combating abuse, and improving communications, the telephone industry transitioned to out-of-band signaling systems, such as Signalling System 7 (SS7), by the 1980s. This separated the voice and signaling channels, making it impossible to generate control signals in the voice bearer channel. The development of the T-carrier system in the 1960s helped obsoleting single-frequency (SF) signaling. SF signaling was replaced by advanced methods of common channel signaling, a technology in development since the 1920s. In the 1970s, multi-frequency signaling systems came into use for international direct distance dialing (IDDD) which used the frequency of in line signaling in two-frequency mode with , for line seizure during call setup, and for tear-down. signaling was a standard for many international signaling systems, such as the Regional System R1. The prominent application of the frequency in telecommunications world-wide inspired the name of many hacker communities and publications, such as 2600: The Hacker Quarterly, and the late 20th-century counterculture 2600. See also Blue box Signaling System No. 5 References Phreaking Telecommunication protocols Telephony signals