Information Technology Reference
In-Depth Information
going to happen, whereas using the loop start method of signaling just connects the wires
to receive an incoming call or place an outgoing call.
Tip:
Many other types of signaling exist in the analog world. These include supervisory
signaling (on hook, off hook, ringing), informational signaling (dial tone, busy, ringback,
and so on), and address signaling (dual-tone multifrequency (DTMF) and Pulse). These are
discussed in detail as part of the CVOICE certification series (for more information, see
Analog signaling was a massive improvement over tin cans and string, but still posed
plenty of problems of their own. First, an analog electrical signal experiences degradation
(signal fading) over long distances. To increase the distance the analog signal could travel,
the phone company had to install repeaters (shown in Figure 1-5) to regenerate the signal
as it became weak.
Repeaters
Analog Phone Signal
Source
Destination
Figure 1-5
Analog Signal Repeaters
Unfortunately, as the analog signal was regenerated, the repeater device was unable to
differentiate between the voice traveling over the wire and line noise. Each time the re-
peater regenerated the voice, it also amplified the line noise. Thus, the more times a
phone company regenerated a signal, the more distorted and difficult to understand the
signal became.
The second difficulty encountered with analog connections was the sheer number of
wires the phone company had to run to support a large geographical area or a business
with a large number of phones. Because each phone required two wires, the bundles of
wire became massive and difficult to maintain (imagine the hassle of a single pair of wires
in the bundle breaking). A solution to send multiple calls over a single wire was needed. A
digital connection is that solution.
Moving from Analog to Digital
Simply put, digital signals use numbers to represent levels of voice instead of a combina-
tion of electrical signals. When someone talks about “digitizing voice,” they are speaking
of the process of changing analog voice signals into a series of numbers (shown in Figure
1-6) that you can use to put the voice back together at the other end of the line.
