PSTN: Signal Transmission

In the old days, the path an analog voice signal took from your phone to the CO
switch (or switchboard) was simple.With the appropriate cross-connects, each local
loop was half of the analog circuit required for a phone conversation, and the switch
(or operator) simply connected you with a calling or called party that represented
the other half of that circuit. Although loading coils might have been used to reduce
signal attenuation on the circuit, no amplification or signal processing was used.
Since Bell’s original invention, several improvements had been added. Common
battery from the CO with a separate return path instead of the earth eliminated the
need for a battery in each phone and made the phone less noisy. Ringing was
accomplished through magnetos, first added to the phones themselves and later
pulled in to the CO and standardized as 90 Volts of Alternating Current (AC)—all
other phone/PSTN functions on the line use Direct Current (DC). And eventually,
automated electromechanical switching eliminated much of the need for an operator
within the PSTN.
Still, analog transmission and switching had their limits. Until 1915, it wasn’t
possible to go much further than 1,500 miles on an analog long-distance circuit.And
even when that limit was broken thanks to the vacuum-tube amplifier, these longdistance
calls were very noisy. Radio telephony overseas and to ships further
expanded the reach of analog telephony in 1927. And Frequency Division
Multiplexing techniques were developed in the late 1930s that allowed many calls to
pass over a single voice circuit by using frequency shifting techniques equivalent to
those used by FM radio. Each 4 kHz band of voice conversation would be shifted up
or down to a specific slot, allowing many calls to be carried simultaneously over a
single coaxial cable or radio interface. By the 1950s, 79% of the inner-city CO
trunks in the United States were using FDM. But even the microwave systems in use
since the 1950s were analog systems.
T1 Transmission: Digital Time Division Multiplexing
Even though Alec Reeves of Britain had developed Pulse Code Modulation (PCM)
techniques in 1937 for digitizing audio signals, and Bell labs had invented the transistor
in 1948, which was required for the large-scale implementation of digital techniques,
it would take more than a decade to make digital transmission a reality (and
longer still before the advent of digital switching could make the full signal path digital
outside the local loop). 1963 brought the introduction of the T1 or Transmission
One digital carrier using revolutionary signal manipulation techniques that would
forever change telephony.
Unlike all previous carriers, the T1 started in an all-digital format, meaning that
it was structured as a series of bits (193 per frame to be exact, 8 bits per channel, 24
channels, plus the framing bit—moving at the rate of 8,000 frames, or 1,544
Megabits per second) that by design could be completely regenerated again without
data loss over long distances (see Figures 4.3 and 4.4).This provides a 64-kilobit-persecond
digital bitstream for each of the 24 channels, using Time Division
Multiplexing (TDM).