Signaling and Control

Control Requirements
To ensure the establishment of end-to-end connections with the required quality
of service, control information must be passed between users and the network,
and between network elements. This control information comes in many different
forms, both simple and complex. The information required in the control information
varies from system to system, and also at different points within the same
system. Control information is required to provide services of many types, not just
end-to-end connections. Control data in telecommunications networks has traditionally
been called signaling.

The Importance of Signaling Systems
Signaling systems are essential to the operation of any telecommunications system.
As switching and transmission systems have evolved, signaling systems have had
to develop continuously to support the services offered in modern telecommunications
networks.

Today, with truly global communications systems in place, signaling plays a
critical role. However, most users are unaware of the signal processing that takes
place even for a simple local telephone call. This is a long way from the early
systems where operators sitting in the local exchange performed most signaling
functions.

Functions of a Signaling System
A signaling system must be able to perform many functions within the network.
One of the most important functions is that of call setup, where the dialed digits
are transmitted across the network and the subsequent routing of the call to its
destination. Should the called party be engaged, this must be notified to the calling
party — who will normally hear this as a tone. When a call is completed, clear
down signals must be transmitted to both parties, and the transmission and switching
systems released.

When a normal telephone number is received by a telephone exchange, it can
use simple look-up tables, held locally, to route the call. However, in the case of
freephone or toll-free numbers, the exchange will have to access a regional or central
database to route the call. This will be achieved over signaling links, able to
interrogate and report back from the databases. This technique forms the basis of
Intelligent Network (IN) technology.

Without the ability to accurately and reliably transmit billing information across
and between networks, few network operators would stay in business for very long.
Another key area for signaling is for operation and maintenance purposes. Should it
be necessary to remove an inter-exchange trunk from service, this must be signaled
to a remote exchange so that it does not attempt to use the out-of-service circuit.

Access Signaling
Loop/Disconnect Signaling —The term “loop/disconnect” refers to a type of
signaling used within the local loop, whereby the telephone, fax machine, modem,
or other device sends signaling information to the local exchange (Figure 2.27). It
is so called because the system relies on connecting and disconnecting a loop across
the line, allowing an electrical current to flow between the two wires when the loop
is made. This current is detectable by circuitry in the telephone exchange.
Although not commonly used anymore for this purpose, loop/disconnect signaling
can be used for the transmission of the dialed digits. Each number is transmitted
as a corresponding number of breaks or disconnections in the local loop.
This form of dialing has a speed of ten pulses per second and consequently it takes
a long time to transmit national or international telephone numbers.
While loop/disconnect dialing is not often used, loop/disconnect signaling is
still used for the basic signaling requirements of the local exchange, where the telephone
signals to the exchange request to make a call (line seizure) and also when
the call is cleared down.

DTMF Signaling — Dual-tone multi-frequency (DTMF) signaling (Figure 2.28)
is now the preferred signaling method for the transmission of dialed digits. This
system works by representing each digit on the keypad with a combination of two
frequencies. These frequencies are audible to the caller. DTMF signaling has the
advantage of being much faster than loop/disconnect dialing. It also has the advantage
that the user, being able to hear the tones, is aware if a button press is missed
or accidentally repeated.