ach time that the carrier (FDM) equipment demodulates to voice. The major advantage of out-of-band signaling is that continuous supervision is provided, whether tone on or tone off, dur- ing the entire telephone conversation. In-band SF signaling and out-of-band signaling are illustrated in Figure 7.2. An example of out-of-band signaling is the regional sig- naling system R-2, prevalent in Europe and nations under European hegeonomy (see Table 7.1). 2600 Hz Speech Frequency (Hz) 3825 Hz (CCITT) 3700 Hz (USA) Speech Frequency (Hz) Figure 7.2 SF signaling (a) in-band and (b) out-of-band. SIGNALING Table 7.1 R-2 Line Signaling (3825 Hz) Direction Circuit State Forward (Go) Backward (Return) Tone on Tone on Seized Tone off Tone on Answered Tone off Tone off Clear back Tone off Tone on Release Tone on Tone on or off Blocked Tone on Tone off 7.3.2.1.4 E&M Signaling. The most common form of trunk supervision in the analog network was E&M signaling. It derived from the SF or 2VF equipment as shown in Figure 7.1. It only becomes true E&M signaling where the trunk interfaces with the switch (see Figure 7.3). E-lead and M-lead signaling systems are semantically derived from the historical designation of signaling leads on circuit drawings covering these systems. Historically, the E and M interface provides two leads between the switch and what we call the trunk signaling equipment (signaling interface). One lead is called the E- lead which carries signals to the switching equipment. Such signal directions are shown Figure 7.3, where we see that signals from switch A and switch B leave A on the M-lead and are delivered to B on the E-lead. Likewise, from B to A, supervisory information leaves B on the M-lead and is delivered to A on the E-lead. For conventional E&M signaling (referring to electromechanical exchanges), the fol- lowing supervisory conditions are valid: Direction Condition at A Condition at B Signal Signal A to B B to A M-Lead E-Lead M-Lead E-Lead On-hook On-hook Ground Ground Off-hook On-hook Battery Ground Gro