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 were used until very recently (2010s) until fiber optics were able to reach very 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 Historical Pages

Landline Telephone History – Main Page
Main page for Telephone History at Telephone World

A Visit to the Connections Museum Seattle
Pictures taken at the Museum of Communications in Seattle, Washington in May 2005.

AT&T Crash of 1990
An excerpt from a magazine with details of the “crash” of the AT&T long distance network in January 1990.

Morris, IL Electronic Switch Experiment
A technical history for the first “electronic” switching system field trial experiment in Morris, IL from 1958 to 1962.

History of the (former) North Pittsburgh Telephone Company
An unofficial technological history of the former North Pittsburgh Telephone Company in Gibsonia, PA (suburban Pittsburgh) from its origins to when it ceased to exist.

Telephone Signaling System Technologies Past & Present
Details on the signaling methods used within the North American telephone network to allow central office and tandem systems to “talk” with each other.

Telephone Transmission Technologies Past & Present
Details on the physical means on how a telephone call is handled through the network between different systems.

Old Bell System Operator Routing Codes
A historical listing of old routing codes that Bell System (AT&T Long Lines) operators used to use that performed special functions within the toll network.

Old Number 4 Crossbar Toll Tandem List
An extensive historical listing of Bell System Number 4 Crossbar (#4XB) toll tandem systems from the 1940s to the 1980s.

Other Telephone History
Technological history on various telephone company systems from the webmaster’s personal perspective over the last 40 years.

Historical Videos
Various videos from YouTube that we’ve found that showcase telephone history.