In a switched telephone network, signaling conveys the intelligence needed for one subscriber to interconnect with any other in that network. Signaling tells the switch that a subscriber desires service and then gives the local switch the data necessary to identify the required distant subscriber and hence to route the call
properly. It also provides supervision of the call along its path. Signaling also gives the subscriber certain status information, such as dial tone, busy tone (busy
back), and ringing. Metering pulses for call charging may also be considered a form of signaling.
There are several classifications of signaling:
1. General.
a. Subscriber signaling.
b. Interswitch signaling.
2. Functional.
a. Audible–visual (call progress and alerting).
b. Supervisory.
c. Address signaling.
 It should he appreciated that on many telephone calls, more than one switch is involved in call routing. Therefore switches must interchange information among switches in fully automatic service. Address information is provided between modern switching machines by  interregnum signaling, and the supervisory function is provided by line signaling. The audible–visual category of signaling
functions inform the calling . The alerting function informs the called subscriber of a call waiting or an extended “off-hook” condition of his or her handset. Signaling information can be conveyed by a number of means from subscriber to switch or between (and among) switches. Signaling information can be transmitted by means such as
• Duration of pulses (pulse duration bears a specific meaning)
• Combination of pulses
• Frequency of signal
• Combination of frequencies
• Presence or absence of a signal
• Binary code
• For dc systems, the direction or level of transmitted current
Supervisory signaling provides information on line or circuit condition and indicates whether a circuit is in use or idle. It informs the switch and interconnecting trunk circuits whether a calling party is “off hook” or “on hook” or whether a called party is “off hook” or “on hook.” The meaning and importance of the terms
“on hook” and “off hook” were detailed in Chapter 1, Section 2. The assumption is that a telephone in the network can have one of two states: busy or idle. Idle, of course, is represented by the “on-hook” condition.
The reader must appreciate that supervisory information–status must be maintained end to end on every telephone call. It is necessary to know when a calling subscriber lifts his/her telephone off hook, thereby requesting service. It is equally important that we know when the called subscriber answers (i.e.,
lifts her telephone off hook), because that is when we may start metering the call to establish charges. It is also important to know when the called and calling subscribers return their telephones to the on-hook condition. Charges stop, and the intervening trunks comprising the talk path as well as the switching points
are then rendered idle for use by another pair of subscribers. During the period of occupancy of a talk path end to end, we must know that this particular path is busy (is occupied) so that no other call attempt can seize it. Dialing of a subscriber line is merely interruption of the subscriber loop’s off-hook condition, often called “make and break.” The “make” is a current flow condition (or off hook), and the “break” is the no-current condition (or on hook). How do we know the difference between supervisory and dialing? Primarily by duration—the on-hook interval of a dial pulse is relatively short and is distinguishable from an on-hook disconnect signal (subscriber hangs up), which is transmitted in the same direction for a longer duration. Thus the switch is sensitized to duration to distinguish between supervisory and dialing of a subscriber loop.
2.1 E and M Signaling
Probably the most common form of trunk supervision is E and M signaling, particularly with multiplex equipment (Chapters 5 and 8). Yet it only becomes true E and M signaling where the trunk interfaces with the switch (see Figure 4.3). E-lead and M-lead signaling systems are semantically derived from historical designation of signaling leads on circuit drawings covering these systems. Historically, the E and M signaling interface provides two leads between the switch and what we may call trunk-signaling equipment (signaling interface). One lead is called the “E-lead,” which carries signals to the switching equipment.
3.1 General
Up to this point we have reviewed the most employed means of supervisory trunk signaling (or line signaling). Direct-current signaling, such as reverse-battery signaling, has notable limits on distance because it cannot be applied directly to multiplex systems (Chapters 5 and 8) and is limited on metallic pairs due to the IR drop of the lines involved. Direct-current trunk signaling is addressed in Section 10.
There are many ways to extend these limits, but from a cost-effectiveness standpoint there is a limit that we cannot afford to exceed. On trunks exceeding dc capabilities, some form of ac signaling will be used. Traditionally, ac signaling systems are divided into three categories: low-frequency, in-band, and out-band
(out-of-band) systems. Each of these can derive the four E and M signaling states.
3.2 Low-Frequency AC Signaling Systems
An ac signaling system operating below the limits of the conventional voice channel (i.e., <300 Hz) are termed low frequency. Low-frequency signaling systems are one-frequency systems, typically 50 Hz, 80 Hz, 135 Hz, or 200 Hz. It is impossible to operate such systems over carrier-derived channels because of the excessive distortion and band limitation introduced. Thus low frequency signaling is limited to metallic-pair transmission systems. Even on these systems, cumulative distortion limits circuit length. A maximum of two
repeaters may be used, and, depending on the type of circuit (open wire, aerial cable, or buried cable) and wire gauge, a rough rule of thumb is a distance limit of 80–100 km.
3.3 In-Band Signaling In-band signaling refers to signaling systems using an audio tone, or tones inside
the conventional voice channel, to convey signaling information. In-band signaling is broken down into three categories: (1) one frequency (SF or single frequency), (2) two frequency (2VF), and (3) multi frequency (MF). As the term implies, in-band signaling is where signaling is carried out directly in the voice
channel. As the reader is aware, the conventional voice channel as defined by the CCITT occupies the band of frequencies from 300 Hz to 3400 Hz. Single frequency and two-frequency signaling systems utilize the 2000- to 3000-Hz portion, where less speech energy is concentrated.
3.3.1 Single-Frequency Signaling. Single-frequency signaling is used almost exclusively for supervision. In some locations it is used still for interregister signaling, but the practice is diminishing in favor of more versatile
methods such as MF signaling. The most commonly used frequency is 2600 Hz, particularly in North America. On two-wire trunks, 2600 Hz is used in one direction and 2400 Hz is used in the other.
3.3.2 Two-Frequency Signaling. Two-frequency signaling is used for both supervision (line signaling) and address signaling. We often associate SF and 2VF supervisory signaling systems with carrier (FDM) operation. Of course, when we discuss 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 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-delay circuit or slot filters to bypass signaling tones may be used. Such filters do offer some degradation to speech unless they are
switched out during conversation. The tones must be switched out if the circuit is going to be used for data transmission [7].
It becomes apparent why some administrations and telephone companies have turned to the use of 2VF supervision, or out-of-band signaling for that matter. For example, a typical 2VF line signaling arrangement is the CCITT No. 5 code, where f1 (one of the two VF frequencies) is 2400 Hz and f2 is 2600 Hz. 2VF signaling is also used widely for address signaling (see Section 4.1 of this chapter) .
3.4 Out-of-Band Signaling
With out-of-band signaling, supervisory information is transmitted out of band (i.e., above 3400 Hz). In all cases it is a single-frequency system. Some out-of band systems use “tone on when idle,” indicating the on-hook condition, whereas others use “tone off.” The advantage of out-of-band signaling is that either system, tone on or tone off, may be used when idle. Talk-down cannot occur because all supervisory information is passed out of band, away from the speech-information portion of the channel.
The preferred CCITT out-of-band frequency is 3825 Hz, whereas 3700 Hz is commonly used in the United States. It also must be kept in mind that out-of band signaling is used exclusively on carrier systems, not on wire trunks. On the wire side, inside an exchange, its application is E and M signaling. In other
words, out-of-band signaling is one method of extending E and M signaling over a carrier system.
In the short run, out-of-band signaling is attractive in terms of both economy and design. One drawback is that when channel patching is required, signaling leads have to be patched as well. In the long run, the signaling equipment required may indeed make out-of-band signaling even more costly because of the extra
supervisory signaling equipment and signaling lead extensions required at each end and at each 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, during the entire telephone conversation.
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.

An important factor to be considered in switching system design that directly affects both signaling and customer satisfaction is post dialing delay. This is the amount of time it takes after the calling subscriber completes dialing until ring back is received. Ring-back is a backward signal to the calling subscriber telling
her that her dialed number is ringing. Postdialing delay must be made as short as possible.
Another important consideration is register occupancy time for call setup as the setup proceeds from originating exchange to terminating exchange. Call-setup equipment, that equipment used to establish a speech path through a switch and to select the proper outgoing trunk, is expensive. By reducing register occupancy
per call, we may be able to reduce the number of registers (and markers) per switch, thus saving money.
Link-by-link and end-to-end signaling each affect register occupancy and post dialing delay, each differently. Of course, we are considering calls involving one or more tandem exchanges in a call setup, because this situation usually occurs on long-distance or toll calls. Link-by-link signaling may be defined as a signaling
system where all interregister address information must be transferred to the subsequent exchange in the call-setup routing. Once this information is received at this exchange, the preceding exchange control unit (register) releases. This same operation is carried on from the originating exchange through each tandem
(transit) exchange to the terminating exchange of the call. The R-1 system is an
example of link-by-link signaling.
End-to-end signaling abbreviates the process such that tandem (transit) exchanges receive only the minimum information necessary to route the call. For instance, the last four digits of a seven-digit telephone number need be exchanged only between the originating exchange (e.g., the calling subscriber’s local exchange or the first toll exchange in the call setup) and the terminating exchange in the call setup. With this type of signaling, fewer digits are required to be sent (and acknowledged) for the overall call-setup sequence. Thus the
signaling process may be carried out much more rapidly, decreasing post dialing delay. Intervening exchanges on the call route work much less, handling only the digits necessary to pass the call to the next exchange in the sequence


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