The problem: loss and distortion¶
An analog voice signal gets weaker the farther it travels down copper. The line's resistance and capacitance also affect high frequencies more than low ones, which muddies speech. Early long-distance calls were faint and hard to understand, and beyond a few hundred kilometres they were impossible.
Loading coils¶
Around 1900, Michael Pupin and George Campbell showed that adding inductors (loading coils) at regular intervals along a line balanced its capacitance and greatly reduced distortion across the voice band. Loading coils extended practical range significantly and stayed on long rural loops for the rest of the century.
That fix later caused a problem. Loading coils block frequencies above the voice band, so they have to be removed from a line before it can carry DSL. Many rural customers could not get DSL for exactly this reason.
Amplifiers and the transcontinental call¶
Range was still limited until the signal could be amplified. The vacuum tube (triode), refined into a practical amplifier by about 1912–1914, made it possible to put repeaters along the route. In January 1915, Alexander Graham Bell in New York called Thomas Watson in San Francisco in the first transcontinental call, repeating his famous line from 1876.
Across the ocean¶
- 1927: commercial transatlantic telephone service began by radio between New York and London. Calls were very expensive, limited in capacity, and at the mercy of atmospheric conditions.
- 1956: TAT-1, the first transatlantic telephone cable, ran from Scotland to Newfoundland. It carried 36 simultaneous calls using amplifiers sealed inside the cable on the sea floor. Undersea cables have carried most intercontinental traffic ever since. Today they are fibre optic and carry most of the world's internet traffic.
Multiplexing¶
Running one pair of wires per call between cities was too expensive. Frequency-division multiplexing shifted many voice channels to different frequency bands so they could share one coaxial cable or microwave radio link. Microwave relay towers spread across the continent from the late 1940s through the 1960s. Canada's Trans-Canada Microwave system was completed in 1958.
Area codes and direct dialing¶
Long-distance calls used to go through operators, who relayed the call from exchange to exchange. Automating them needed a numbering plan that every switch could understand.
- 1947: AT&T and Bell Canada created the North American Numbering Plan (NANP). It divided Canada and the U.S. into numbered areas with three-digit area codes, and set the 3-3-4 format (area code, exchange, line number) still used today. Several of the original 1947 codes are still in use, including 416 (Toronto) and 519 (southwestern Ontario).
- 1951: the first direct distance dialing call went from Englewood, New Jersey, to Alameda, California, with no operator involved.
- Direct dialing spread through the 1950s and 1960s. Operator-assisted long distance became the exception.