uld be in four-wire operation. Thus we would have a 2600-Hz tone in either/both directions. The direction of the tone is important, especially when working with its E&M signaling adjunct. A diagram showing the application of SF signaling on a four-wire trunk is shown in Figure 7.1. Two-frequency (2VF) signaling can be used for both supervision (line signaling) and address signaling. Its application is with FDM equipment. Of course when discussing such types of line signaling (supervision), we know that the term idle refers to the on- hook condition, while busy refers to the off-hook condition. Thus, for such types of line signaling that are governed by audio tones of which SF and 2VF are typical, we have the conditions of "tone on when idle" and "tone on when busy." The discussion holds equally well for in-band and out-of-band signaling methods. However, for in-band 1 Line signaling is the supervisory signaling used among switches. 2Called out-band by CCITT and in nations outside of North America. SIGNALING Single- frequency tone gen. Transmit Narrow Receive filter Trunk equipment AC signaling equipment Signal Operation Condition On hook Transmitting Ground Receiving Off hook Transmitting Battery Receiving Ground Figure 7.1 Functional block diagram of an SF signaling circuit. Note: Wire pairs "receive" and "transmit" derive from the FDM multiplex equipment. Note also the E-lead and M-lead. signaling, supervision is by necessity tone-on idle; otherwise subscribers would have an annoying 2600-Hz tone on throughout the call. A major problem with in-band signaling is the possibility of "talk-down," which refers to the premature activation or deactivation of supervisory equipment by an inadvertent sequence of voice tones through the normal use of the channel. Such tones could simulate the SF tone, forcing a channel dropout (i.e., the supervisory equipment would return the channel to the idle state). Chances of simulating a 2VF tone set are much less likely. To avoid the possibility of talk-down on SF circuits, a time-