North American
Telephone Transmission Technologies
Past & Present


To physically transmit multiple telephone calls over a large area is quite a task. There have been many methods over the years to send phone calls over long distances.

Telephone Transmission Technologies

A brief overview of the history and operation of telephone transmission technologies.

Open Wire Carrier

Open wire carrier was developed to carry multiple calls over a pair of copper wires simultaneously. It uses a method of frequency division multiplexing (FDM) where calls are sent on different frequencies. The frequencies are around 100Khz at about 4 KHz per channel.

Open wire carrier has long ago been removed in all areas. The last open wire carrier system I am aware of was located in rural New Mexico and removed in August of 1997.

Coax Cable

Coaxial cable was used for cross country communications. Coax cable was also used in the transmission of radio and television programming. It was also used for use by government and defense department purposes. Many repeaters were involved and many distribution facilities as well. Coaxial cable is no longer in use, and most of it is abandoned in place.

Microwave (Radio) Towers

Developed in the 1940s and 1950s, microwave transmission became a widespread telephone call transmission medium. Many microwave towers were erected in many countries worldwide. Microwave again uses frequency division multiplexing.

The microwave transmission system broadcasts radio waves via a series of radio towers in the Gigahertz (GHz) frequency range.

There are a few microwave systems in use today, but those that are still in use are converted to digital transmission techniques. Many of the old towers have either been decommissioned or are now used as cellular/PCS towers.

(Off-site Link - "The Latest Word in Communications") In 1947, AT&T inaugurated an experimental microwave radio link, connecting Boston and New York City. This brochure was published by the Long Lines Department to describe the system's technology and facilities. (16 pages)


Most people think of satellites for television transmission. But Bell Labs/AT&T invented communications satellites for long distance communications. The first satellite was Telstar in 1962. The major drawback was the 1/2 second delay because of the distance from the Earth to the geostationary orbit over 22,000 miles from Earth and return. Satellites are still used today for far remote places.

Fiber Optics

Ultra pure glass optical fibers using amplitude modulated infrared light, commonly known as Fiber (or "fibre") Optics, revolutionized telecommunications transmission techniques. Developed in the 1970s and implemented in the 1980s and beyond - fiber optics is the high bandwidth and high quality transmissions medium that is in widespread use in the telephone industry today. Almost all telephone companies in North America use fiber optics, as do many telephone companies in the industrialized world.

Other Telephone Network Systems Pages

Telephone Switching Systems - Main Page
Modern Local Central Office Switching Systems
Vintage Local Central Office Switching Systems
Telephone Network Signaling Technology
Telephone Network Transmission Technology
Telephone Tandem Systems
Old (Historic) Number 4 Crossbar (#4XB) List

Copyright 2008 Telephone World
Page last modified December 20, 2008
Contact us for more information