Invention

Part 19 - Multiplexing

The single wire system was only suitable for short range use and the sound quality was poor, so soon every telephone was connected to the exchange by a pair of wires. These "analog local loops" carried a voice signal in both directions so two people could talk at the same time. But long distance calls needed to be amplified and, to make this easier, each direction was typically carried on separate wires.

Wires were expensive, (and ruined the streetscape) so engineers found ways to carry many calls on the same pair of wires. Thus different calls were assigned to a number of different 'carrier' frequencies each of which carried a voice signal. Theses were known as "channel banks" or "carrier telephony" and, more recently, Frequency Division Multiplexing, with each voice channel electronically filtered, modulated and demodulated (hence the expression MODEM).

 With amplifiers, it was possible to carry about 12 voice channels on a pair of wires.

Channel banks were multiplexed (combined) into higher order banks, and sent over coaxial cables (a wire surrounded by a tubular conducting shield to prevent external electrical signals (noise) from corrupting the signal). In 1977, this analog technology permitted 13,200 voice channels on one coaxial cable, with 1 mile repeater (amplifier) spacing.

Because calls were silent about half the time, Time Assigned Speech Interpolation (TASI) technology could squeeze more calls through a channel bank by assigning a channel only when people were talking. This was especially useful for undersea cables and satellite communications where the circuits were expensive. See Wikipedia, L-carrier.

With the development of semiconductors, digitally sampled voice signals permitted Time Division Multiplexing, with 24 voice channels on a single twisted pair of wires running at 1.544 megabits per second. This was the T1 level of the T carrier hierarchy. See Wikipedia, T-carrier.

On the world wide web, some of the cables and links are still used for time division multiplexed voice channels, but most of the infrastructure is used for internet traffic except that the Internet can carry voice signals using Voice Over Internet Protocol (VOIP).

In 1945, the University of Pennsylvania in the USA built the first electronic general-purpose digital computer (ENIAC) using the vacuum tube (valve) technology developed for telephone and radio amplifiers. The computer was so large that its vacuum tubes occupied the entire building, and it consumed a huge amount of electricity and produced a lot of heat.

The point-contact transistor, developed to replace the unreliable, fragile, glass vacuum tube amplifiers, was invented by American physicist John Bardeen with William Shockley and Walter Brattain, at Bell Laboratories in the USA, in 1947.

John Bardeen was the only person to be awarded the Nobel Prize in Physics twice: in 1956 for the invention of the transistor and again in 1972 with Leon N. Cooper and John Robert Schrieffer for a fundamental theory of conventional superconductivity known as the BCS theory (named for Bardeen–Cooper–Schrieffer).

The transistor revolutionized the electronics industry, making possible the development of almost every modern electronic device, from telephones to computers. 

Bardeen's developments in superconductivity are used in nuclear magnetic resonance spectroscopy (NMR) and medical magnetic resonance imaging (MRI).

The MOSFET (metal oxide semiconductor, field-effect transistor) was invented by Mohamed M. Atalla and Dawon Kahng at Bell Telephone Laboratories in 1959. The MOS integrated circuit (MOS IC) chip was proposed soon after, but the technology was initially not considered practical for analog telephone applications. This was resolved with the development of the digital-to-analog converter (DAC) chip and the analog-to-digital converter (ADC) chip by former Bell engineer David Hodges with Paul Gray at UC Berkeley in the 1970s. The wide adoption of MOS technology enabled very high speed, pulse-code modulation (PCM) digital telephony.

The history of mobile phones began with two-way radios in taxicabs, police cruisers and railroad trains. Later versions could be used as mobile two-way radios or as portable phones by being patched into the telephone network.

In 1947, Bell Labs engineers Douglas H. Ring, W. Rae Young and Philip T. Porter proposed hexagonal cell transmission areas for mobile phones with cell towers at the corners of the hexagons having directional antennas transmitting and receiving in 3 directions into adjacent cells. The technology was made possible when Richard Frenkiel and Joel Engel of Bell Labs developed the electronics.

The era of the handheld cellular-mobile phone began on 3 April 1973, when Martin Cooper of Motorola placed a cellular-phone call to Dr. Joel S. Engel, head of research at AT&T's Bell Labs.Meanwhile, the 1956 inauguration of the transatlantic telephone cable (TAT-1) connected the different continental telephone networks into a global network making international direct dialling possible.