City of Light: The Story of Fiber Optics

The Quest for Capacity
A VISION OF THE FUTURE 79
Both the switches and the pipes determine the capacity of a telephone system.
Reeves focused on developing a new family of pipes with higher capacity.
Engineers had been heading in the same direction for decades, although they
didn’t always express their goals in those terms. Two trends pushed the demand
for higher capacity. One was the sheer volume of traffic; the other was
the shift toward carrying signals that carried more information and thus
needed more capacity per signal. The voices carried on a telephone line represented
more information than the dots and dashes that represented letters
in the telegrapher’s Morse code. The tiny black-and-white pictures John Logie
Baird and Francis Jenkins sent over their first television transmitters carried
more information than voices. Standard color television channels carry more
information than those first television transmissions.
Even back in the Victorian era, engineers tried to pack as much information
as they could onto a single communications line. The first electrical
telegraphs could carry only one set of dots and dashes along their wires at a
time. As telegraph traffic increased, engineers realized it would be much
cheaper to send multiple signals down the wire than to string more wires. A
host of inventors turned to the task of multiplexing telegraph signals; among
them was Alexander Graham Bell, who extended his ideas to sending voices
along wires and made the telephone. 4 As telephone service spread and forests
of overhead wires threatened to darken cities, engineers invented ways to
send multiple conversations through one set of wires.
Along the way, engineers developed a framework that helped them understand
the process of communication, and how to extend their technology.
A simple telegraph works by letting current flow through a wire for short
(dot) and long (dash) periods. Thus, the telegraph key modulates the current.
Step back, and you see a more general process—a signal that carries information
modulates a steady ‘‘carrier’’ that can be an electrical current, an
audio tone, a radio wave, or a beam of light (figure 7-1). In a telephone, a
voice modulates the electrical current, which varies at the same frequency as
the voice. 5
At first, the carrier was a steady electric current. Then engineers realized
they also could modulate the intensity of a wave. An oscillator generated a
particular frequency, while a separate signal modulated the strength of the
carrier at that frequency. This is the basis of AM (amplitude-modulated) radio
transmission. Later, engineers discovered they could modulate the frequency
of a radio signal, used in FM (frequency-modulated) radio and television
broadcasting.
The transmission capacity or ‘‘bandwidth’’ of a system depends on the
frequency of the carrier; the higher the frequency, the greater its bandwidth.
Since radio transmission began, engineers have moved to higher and higher
frequencies to increase transmission bandwidth. They started at frequencies
of thousands or tens of thousands of hertz (cycles per second), then moved