Evolution of Signaling
Signaling and switching are inextricably tied together. Switching automated the network.
But without signaling, switching systems could not function. Thus it would be better said
that switching with signaling automated the network.
Conventional subscriber line signaling has not changed much over the years, with the
exception of the push-button tones that replaced the dial for address signaling. ISDN,
being a full digital service to the subscriber, uses a unique digital signaling system 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.
. 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.
. 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 22.214.171.124.4. It is used with FDM
equipment, and most commonly the tone frequency was 2600 Hz. Of course, this would
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
Line signaling is the supervisory signaling used among switches.
Called out-band by CCITT and in nations outside of North America.