und On-hook Off-hook Ground Ground Battery Off-hook Off-hook Battery Ground Battery Ground Source: Ref. 8. 7.3.2.2 Address Signaling. Address signaling originates as dialed digits (or activated push-buttons) from a calling subscriber, whose local switch accepts these digits and, using that information, directs the telephone call to the desired distant subscriber. If more than one switch is involved in the call setup, signaling is required between switches (both address and supervisory). Address signaling between switches in conventional systems is called interregister signaling. The paragraphs that follow discuss various more popular standard ac signaling tech- niques such as 2VF and MF tone. Although interregister signaling is stressed where Trunk Signaling Signaling Trunk interface Signaling switching interface switching equipment exchange medium exchange equipment Figure 7.3 E&M signaling. 7.3 SIGNALING TECHNIQUES appropriate, some supervisory techniques are also reviewed. Common-channel signaling is discussed in Chapter 13, where we describe the CCITT No. 7 signaling system. 7.3.2.2.1 Multifrequency Signaling. Multifrequency (MF) signaling has been in wide use around the world for interregister signaling. It is an in-band method using five or six tone frequencies, two tones at a time. It works well over metallic pair, FDM, and TDM systems. MF systems are robust and difficult to cheat. Three typical MF systems are reviewed in the following: MULTIFREQUENCY SIGNALING IN NORTH AMERICA-THE R-1 SYSTEM. The MF signaling system principally employed in the United States and Canada is recognized by the CCITT as the R-1 code (where R stands for "regional"). It is a two-out-of-five frequency pulse system. Additional signals for control functions are provided by frequency combination using a sixth basic frequency. Table 7.2 shows the ten basic digits (0-9) and other com- mand functions with their corresponding two-frequency combinations, as well as a brief explanation of "other applications." We will call this system a "s