Signaling System No. 5 and earlier systems use in-band signaling, in which the call-setup information is sent by generating special multi-frequency tones transmitted on the telephone line audio channels, also known as "bearer channels". As the bearer channel are directly accessible by users, it can be exploited with devices such as the blue box, which plays the tones required for call control and routing. As a remedy, SS6 and SS7 implements out-of-band signaling, carried in a separate signaling channel, thus keeping the speech path separate. SS6 and SS7 are referred to as common-channel signaling (CCS) protocols, or "Common Channel Interoffice Signaling" (CCIS) systems.