06-04-2012, 04:24 PM
In-Band Single-Frequency Signaling
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INTRODUCTION
The signaling requirements for dial telephone operation are naturally
more exacting than those for manual switching methods. This means a
high order of signaling system is needed to satisfy the requirements for
the toll ~elephone plant and for automatic toll switching systems described
in recent papers in this JO~JRNAn), ~ Indeed the advantages in
speed and economy of dial telephone systems depend to a large extent
upon the type of signaling provided for them. The signaling arrangements
for intertoll telephone trunks which are the links between telephone
switching systems, therefore, become most important.
BASIC PLAN
The in-band single-frequency signaling system, although fairly complex
in detail, is very simple in principle. Normally, i.e., when the circuit
is idle, steady tone is transmitted over the line and holds relays operated
at the receiving end. Signaling is accomplished by removing and reapplying
this tone, which in turn releases and reoperates the distant
relays. Independent operation is obtained in each direction with one
signal frequency on four-wire line~, which have separate one-way transmission
pathes from terminM to terminal, and with two signal frequencies,
one for each direction of transmission, on two-wire lines.
GENERAL DESIGN CONSIDERATIONS
The successful use of the voice path ~or signaling, especially for continuous
as contrasted to "spurt" signaling, is feasible only by a compromise
among a number of conflicting factors. These factors or design considerations
are (a) choice of signal frequency, (b) signaling power and
receiver sensitivity, (e) imitation of signal by speech or tones, (d) interferenee
to signal by other tones and noises, and (e) andibility of signaling
tone to operators and subscribers.
(a) Choice qf Signal Frequency
The choice of signal frequency is determined mainly by considerations
of signal imitation by speech. As will be shown later on, signal imitation
decreases rapidly as the signaling frequency is raised with the result that
the highest frequency that can be reliably handled by the transmission
path is used. In the ease of some four-wire type lines, such as EB carrier,
the highest frequency that should be used is 1,600 cycles.
NOISE CONSIDERATIONS
Noise affects the signaling system in a variety of ways depending upon
the nature of the noise and Upon the particular signaling function being
performed. When tone is first applied it is of course desired that the receiver
operate. However at this time the "guard" circuit is hmetioning
because it is also desired that ~he receiver be non responsive to speech.
Noise, which acts on the guard circuit like speech, will therefore tend to
prevent operation of the receive
DESCRIPTION OF 2,600-CYCLE DESIGN
The 2,600-cycle unit, shown in Fig. 10, is just half the size of the 1,600-
cycle unit, costsless than half as much, and is of the "plug in" type so it
can be readily replaced for maintenance action.