Analogue Networks

In voicegrade networks, the challenge had been to squeeze as many
analogue signals down limited-size cables as possible. One of the
earlier solutions, still very widely used, is frequency division

multiplexing (FDM): each of the original speech paths is modulated
onto one of a specific series of radio frequency carrier waves; each
such rf wave is then suppressed at the transmitting source and
reinserted close to the receiving position so that only one of the
sidebands (the lower), the part that actually contains the
intelligence of the transmission, is actually sent over the main data
path. This is similar to ssb transmission in radio.
The entire series of suppressed carrier waves are then modulated onto
a further carrier wave, which then becomes the main vehicle for
taking the bundle of channels from one end of a line to the other.

Typically, a small coaxial cable can handle 60 to 120 channels in
this way, but large cables (the type dropped on the beds of oceans
and employing several stages of modulation) can carry 2700 analogue
channels. Changing audio channels (as they leave the telephone
instrument and enter the local exchange) into rf channels, as well as
making frequency division multiplexing possible, also brings benefits
in that over long circuits it is easier to amplify rf signals to
overcome losses in the cable.
Just before World War II, the first theoretical work was carried
out to find further ways of economising on cable usage; what was then
developed is called Pulse Code Modulation (PCM).
There are several stages. In the first, an analogue signal is
sampled at specific intervals to produce a series of pulses; this is
called Pulse Amplitude Modulation, and takes advantage of the
characteristic of the human ear that if such pulses are sent down a
line with only a very small interval between them, the brain smoothes
over the gaps and reconstitutes the entire original signal.
In the second stage, the levels of amplitude are sampled and
translated into a binary code. The process of dividing an analogue
signal into digital form and then reassembling it in analogue form is
called quantization. Most PCM systems use 128 quantizing levels, each
pulse being coded into 7 binary digits, with an eighth added for
supervisory purposes.
OPERATION OF A CHARACTER TDM
+-----+-----+-----+-----+-----+-----+-----+--
<------| SYN | CH1 | CH2 | CH3 | CH4 | SYN | CH1 |
+-----+-----+-----+-----+-----+-----+-----+--
+-----------------+ +-----------------+
1 | | | |1
--+ | +---+ +---+ | +--
2 | | | | | | | |2
--+ MULTIPLEXER |==+ M +--\/\/--+ M +==--+ MULTIPLEXER +--
3 | | | | | | | |3
--+ | +---+ +---+ | +--
4 | | | |4



By interleaving coded characters in a highspeed digital stream it
is possible to send several separate voice channels along one
physical link. This process is called Time Division Multiplexing
(TDM) and together with FDM still forms the basis of most of the
globe's voicegrade communications.