The Evolution of Telephone Cable

When Alexander Graham Bell invented the telephone over 100 years ago the telephone lines were separate lines that connected pairs of telephones. To connect private telephones with the central interconnection facility telephone lines were needed.

The first telephone lines were single grounded wires made of iron or steel, placed on telephone poles or attached to racks on rooftops. Those lines were inherently noisy. Phosphor bronze wires and compound copper steel wires were made in attempts to decrease the noise in the lines. The benefits of using copper conducting wire were not available until Thomas Doolittle developed the process for hard drawn copper wire in 1877. This wire was strong enough for overhead wires and copper took over the telephone wire market.

Early telephone cables with several wires relied upon the technology used in the manufacture of telegraph cables. Gutta percha and various rubber compounds were used for insulating and water proofing the telegraph and early telephone cables. Telephone cables were employed for aerial, underwater and underground. By 1887 all of the newly manufactured cables were metallic circuit cables. Some of the cables contained up to 100 copper wires. They were insulated with cotton, cotton impregnated with paraffin, gutta percha or a rubber compound, and then wrapped in lead.

There was a large demand for telephone cables as they were needed to replace the large number of aerial wires in the cities. In the 1880's the major telephone cable concerns were noise elimination, waterproofing and fitting more wires in each cable. The technique of wrapping the conducting wires in lead was developed to eliminate electromagnetic noise in the lines. Other insulating techniques, such as covering the insulated wires with tin foil and using additional insulating layers, were employed. Techniques for impregnating the lead tubes after the conducting wires were drawn through with melted paraffin, resin, or high test illuminating oil were used to protect the insulation from moisture. By the late 1890's telephone and electric power cables were laid underground in conduits made of creosoted wood. Then the multiple duct vitrified clay conduit became the main type of underground construction used. A separate square hole was provided for each cable, and when more ducts were required sections of conduit could be added.

The general complaints of the poor voice transmission over the telephone cables were still present. There were two major improvements made to telephone cable in the late 1880's. The first was the issuance of a specification for a standard telephone cable, which outlined a metallic circuit or twisted pair cable. The wires were made of copper and covered with at least two layers of cotton and sheathed in a 97% lead, 3% tin alloy pipe. The spaces in the core and between the core and the pipe were filled with an insulating material. A two inch diameter cable could contain up to 52 pairs of wires. The second improvement was the development of paper insulated dry core cable, where the lead tin alloy sheathing now provided adequate water proofing. By 1891 dry core paper insulated cable was the standard. The size of the conductor and the electrostatic capacity requirement were further reduced and greatly decreased the noise in the lines.

Improvements were continually being made to the telephone cable. Some of them were made possible because of the improvements made in other areas of telephony such as transmitters, receivers, coils and switchboard circuits. The next major step was loading of the telephone lines. Loading the telephone cables increased transmission efficiencies. In 1912 a new 1% antimony, 99% lead alloy was developed for cable sheaths. This material was more economical than the lead tin alloy pipes, it had good tensile strength, corrosion resistance, and the mechanical properties necessary for aerial and underground cables. Another improvement was the invention of the repeater, which amplified voice signals. Carrier systems or multiplexing enabled a single pair of wires to be used for multiple calls.

The end of the 19th and the beginning of the 20th centuries were a time of change and rapid expansion in the telephone industry. The telephone went from being a novelty item that only a few could afford, to what most consider a necessity today. Some of the more recent improvements are the use of plastic insulation and the development of coaxial cable which occurred during World War II. Up to 600 conversations could be transmitted over two coaxial cables. Today most of the improvements in telephone cables have centered around sending more information at a faster rate over the same wires. One area of interest is in the development of ADSL or asymmetrical digital subscriber line. Telephone cable has come a long way since 1877, and the technology is still being improved.

СОДЕРЖАНИЕ

Unit 1

Text 1. The Engineering Profession in the 21st century.............................3

Text 2. What Does an Engineer Do?..........................................................6

Unit 2

Text 1. Telecommunications Fundamentals.............................................12

Text 2. The Development of Telecommunications...................................16

Unit 3

Text 1. Types of Signals............................................................................22

Text 2. Signal Transmission.......................................................................27

Unit 4

Text 1. Modulation....................................................................................32

Unit 5 ….......................................................................................................34

Unit 6 …........................................................................................................42

Unit 7 …........................................................................................................52

ADDITIONAL TEXTS.................................................................................60