Carrier telephony is a method for carrying several simultaneous long-distance telephone conversations over the same transmission line. Each conversation is shifted to a different frequency band, allowing the channels to share the line by frequency-division multiplexing (FDM). At the receiving end, filters and demodulators separate the channels and return them to ordinary voice frequencies.
The method was developed to increase the capacity of expensive long-distance lines without adding more wire. American inventor George Owen Squier demonstrated what he called "wired wireless" in 1910, and the Bell System introduced commercial carrier systems in the United States after World War I to meet growing demand for long-distance service without adding a new pair of wires for every conversation. Early systems added a few channels to open-wire routes (copper wires on poles). By the late 1930s, Bell Telephone Laboratories had adopted a standard group of twelve voice channels as a building block for open wire, cable and coaxial cable systems; it was later used in microwave transmission and submarine communications cable systems.
Carrier systems grew steadily in capacity. Early open-wire routes carried on utility pole systems carried only a few additional conversations, while the Bell System's L5 coaxial system, introduced in 1974, could carry 10,800 calls on one coaxial pair and 108,000 calls in a ten-pair cable.
Carrier telephony also changed the practice of electrical engineering. The systems needed filters to separate signals only a few kilohertz apart. Amplifiers for cable and coaxial lines had to add gain that matched the loss of the cable, without distortion. On the longest routes where hundreds or thousands of repeaters were in the path, a repeated small error grew to a failure. Consumers expected telephone service simply to work, but behind that apparent simplicity were thousands of miles of cable, thousands of circuits, and new approaches to accuracy, maintenance, testing, and reliability. Much of the later theory of filters, feedback amplifiers, equalizers, and communication channels grew out of this work. Analog carrier systems were gradually replaced by digital transmission systems using pulse-code modulation (PCM), beginning with systems such as T1 in the early 1960s.
Background
Before carrier telephony, long-distance voice circuits were carried at voice frequencies on open-wire pairs, consisting of two bare copper wires carried on poles (a two-wire circuit). Each conversation normally required one pair of wires. Two pairs, or four wires, could also be connected through loading coils to derive a third phantom circuit, allowing three telephone circuits to be carried on two physical pairs.
Open-wire construction made long-distance service possible, although it required large amounts of transmission line and equipment. In 1911, Bell System service reached from the East Coast to Denver using voice-frequency open-wire circuits. In 1915, vacuum-tube repeaters made transcontinental service possible between New York and San Francisco.
Telephone companies also used multi-pair cable. Toll cable (long-distance line) was commonly built from quads, with each quad containing two pairs of wires. With phantom-circuit operation, one quad could support three voice-frequency telephone circuits.
Long-distance traffic was growing faster than the physical network. In 1930, Bell System engineer W. H. Harrison reported that toll usage increased by 34 percent in three years, while traffic on transcontinental routes more than doubled. The growth required substantial expansion of the long-distance plant. Carrier systems provided additional circuits over existing open-wire lines and cables, reducing the number of new wires needed to meet the demand. Historian John Brooks described carrier as providing a much greater increase in call-carrying capacity than phantom operation.
Carrier telephony was developed to place still more conversations on existing wires by moving additional voice channels to higher frequencies. In normalized dollars, the US line cost for transmission decreased by a factor of 1000 in the period of 1925 to 1975.
History
Early carrier telephony (1910–1920s) In 1910, George Owen Squier demonstrated that more than one voice signal could be sent over a single wire circuit by placing a second signal on a high-frequency carrier. This was similar to the way speech could be carried by radio waves. Squier described the method as "wired wireless".
Bell introduced commercial carrier systems in 1918, after further development of the method for telephone lines. Early systems used open-wire lines and added carrier channels above the ordinary voice-frequency circuit. At the receiving end, filters and demodulators separated the channels and returned them to ordinary voice frequencies.
Early commercial systems (1920s)
In 1915, J. R. Carson showed that a modulated signal could be separated into two sidebands, and that only one sideband was required to recover the original telephone signal at the receiving end. This became known as single-sideband modulation. Ralph Hartley later analyzed the relation between carrier and sideband components, showing that transmitting only one sideband saved power and bandwidth, allowing more signals to share the line. He also described a balanced modulator circuit for suppressing the carrier component.
In 1921, E. H. Colpitts and O. B. Blackwell published a description of several frequency-division multiplexed systems in use for telephony. These systems were used on open-wire transmission circuits. The labels Type A and Type B were applied in later publications. Test systems were operated between South Bend, Indiana, and Toledo, Ohio, and between Chicago and Toledo. Demand for telephone circuits during World War I led to installation of a test system between Pittsburgh and Baltimore.
In one carrier arrangement, an ordinary telephone signal occupied the 300 Hz to 3,000 Hz voice band. Three additional two-way circuits were assigned to carrier channels, with one direction occupying the 10 kHz to 20 kHz band and the other direction occupying the 20 kHz to 30 kHz band. Eight two-way telegraph signals were also multiplexed into the 3,333 Hz to 10 kHz band. One commercial system used on the Detroit–Harrisburg circuit transmitted the carrier along with each modulated voice signal. It carried three carrier signals in addition to one ordinary telephone circuit over 596 miles, quadrupling the previous capacity. Arthur A. Oswald later described carrier telephony as the first commercial use of single-sideband modulation.
Single-sideband transmission was used in the Harrisburg–Chicago carrier system, which suppressed the carrier and transmitted a single sideband to improve spectral and power efficiency. That system carried one ordinary telephone circuit and four carrier circuits on a single line. In 1936, a two-channel radiotelephone system was placed in service using the two sidebands independently. Later systems carried as many as four voice channels on a single radio carrier. Early systems also added multiple telegraph signals to lines used for telephony.
Long-distance open-wire systems (late 1920s–1930s)
In a 1928 paper, the four-carrier-channel system described in 1921 was identified as Type A, and the three-carrier-channel system without carrier suppression as Type B. The same paper described a later open-wire system, Type C, which added three bidirectional carrier circuits to a bidirectional voice-band circuit on the same open-wire pair.
The Type C system was intended for long open-wire routes, from about 150 miles to more than 1,000 miles. It used single-sideband transmission with suppressed carrier, as in Type A, and used alternate frequency plans to reduce objectionable crosstalk (unwanted coupling of signals between nearby wire pairs) on the same pole route. From 1926 to 1928, Bell System use of carrier systems increased from about 130,000 channel-miles to 230,000 channel-miles, a measure equal to one telephone channel carried for one mile.The Type D system was developed for shorter toll circuits of about 50 to 200 miles. It added one carrier circuit to a plain voice-frequency circuit, doubling the capacity of a pair at lower cost than the long-distance Type C equipment. For circuits longer than about 125 miles, an added amplifier produced the D-A version. Type C and Type D systems could be used together on the same pole routes. One Bell System example showed a four-crossarm pole line carrying six long-distance Type C circuits, fourteen shorter Type D circuits, thirty voice-band circuits, and eighty telegraph circuits.
Engineering improvements and cost reductions widened the economic use of carrier telephony. By 1930, Bell described three-channel systems as suitable for long-haul routes of about 125 miles or more, while simpler one-channel systems made carrier economical on routes of roughly 75 to 125 miles. The shorter-haul equipment could sometimes be extended to about 200 miles by adding terminal amplification.
The last Type C circuit in the United States was taken out of service around 1980.
Different open-wire carrier technologies emerged outside the Bell System. In 1929, H. Sterky described a Swedish carrier-current system built by Svenska Radioaktiebolaget, Ericsson's subsidiary, for Mexico's telephone network. He described carrier telephony as suitable, for technical reasons, on aerial lines. The lines used copper or aluminium conductors; aluminium was preferred because copper was a theft target. The system lacked repeaters; the number of channels was limited by city-to-city distances. The first Mexican stage was installed on routes linking Mexico City, Vera Cruz, Celaya, Guadalajara, and San Luis Potosí. Individual spans carried up to three carrier channels; the Mexico City–Celaya route, for example, used three carrier-frequency pairs over 359 km. Each channel used one carrier frequency in each direction, spaced 5 kHz apart, with frequencies derived from a 5 kHz master frequency generated in Mexico City. Unlike suppressed-carrier single-sideband systems, the Ericsson system removed the lower sideband but transmitted the carrier together with the upper sideband, eliminating the need for a locally regenerated carrier at the receiver.
Morristown cable carrier trial (1933) In 1933, Bell Telephone Laboratories conducted an experimental cable carrier installation at Morristown, New Jersey. The trial used a 25-mile length of underground cable with repeaters spaced at 25-mile intervals, arranged to simulate an 850-mile circuit with 33 intermediate repeaters. Nine carrier telephone channels were transmitted using frequencies from 4 kHz to 40 kHz. The trial demonstrated "very excellent" transmission quality and manageable crosstalk.
For test purposes, the nine carrier links were connected in tandem to represent circuits as long as 7,650 miles. The simulated circuit had approximately 1,300 dB of total attenuation, which was compensated by the repeaters. Stable operation depended on negative-feedback amplifiers, developed from work by H. S. Black. Bell Labs later described negative feedback as a key prerequisite for developing long-distance cable carrier systems.
Although not adopted immediately for commercial service because of economic conditions during the Great Depression, the Morristown trial established the viability of cable carrier systems. It bridged earlier open-wire carrier systems and the standardized multi-channel cable systems introduced later in the 1930s.
The 12-channel group systems (late 1930s–early 1940s): Types J, K, and L