stem called DSS-1 (Digital Subscriber Signaling No. 1). In the ATM world there is digital subscriber signaling system No. 2, Q.2931 and RFC3033. In the 1930s and 1940s, interregister and line signaling evolved into many types of signaling systems, which made international automatic working a virtual nightmare. Nearly every international circuit required special signaling interfaces. The same was true, to a lesser extent, on the national level. In this section we will cover several of the more utilized signaling techniques used on the analog network which operated with frequency division multiplex equipment (Section 4.5.2). Although these signaling systems are obsolete in light of the digital network, the concepts covered here will help in understanding how signaling works. 7.3.2.1 Supervisory/Line Signaling 7.3.2.1.1 Introduction. Line signaling on wire trunks was based essentially on the presence or absence of dc current. Such dc signals are incompatible with FDM equipment where the voice channel does not extend to 0 Hz. Remember the analog voice channel occupies the band from 300 to 3400 Hz. So the presence or absence of a dc current was converted to an ac tone for one of the states and no tone for the other state. There were two ways to approach the problem. One was called in-band signaling and the other was called out-of-band signaling. 2 7.3.2.1.2 In-Band Signaling. In-band signaling refers to signaling systems using an audio tone, or tones, inside the conventional voice channel to convey signaling informa- tion. There are two such systems we will discuss here: (1) one frequency (SF or single frequency) and (2) two frequency (2VF). These signaling systems used one or two tones in the 2000- to 3000-Hz portion of the band, where less speech energy is concentrated. Single-frequency (SF) signaling is used exclusively for supervision, often with its adjunct called E&M signaling, which we cover in Section 7.3.2.1.4. It is used with FDM equipment, and most commonly the tone frequency was 2600 Hz. Of course, this wo