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Discuss the transition from an analogue to digital PSTN environment and give rough dates for each development.

The first electrical telecommunications were transmitted by the wire telegraph, invented by Samuel Morse in the 180s. The telegraph utilized the interruption or reversal of a direct current electrical circuit to cause a remote sounder to produce a click at the beginning and end of each interruption or reversal. A short interval between clicks was called a “dot,” and a longer interval (about three dot intervals in length) was called a “dash.”

Messages were transmitted by sending dots and dashes in conformance to the Morse code, which represented letters of the alphabet, numbers, and punctuation marks by combinations of dots and dashes. As many as six dots and dashes in combination were used to represent one character, and maximum sounder speed was around four clicks per second, resulting in a maximum transmission rate of about 0 words per minute. The messages were decoded by telegraph operators and initially recorded as handwritten copy.

The telegraph had a tremendous impact on the United States and the world. For the first time ever, reports of events from great distances were available almost instantaneously. The railroads granted right-of-way to telegraph companies and negotiated service contracts with them.

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The railroads used the telegraph systems to control the movement of trains, and the telegraph companies provided telegraph service to the public. The newspapers used the telegraph to gather news of happenings around the country. During the Civil War military orders, information on troop movements, and reports on the outcomes of battles were transmitted by telegraph.

In 1850, Western Union was formed in Rochester, New York, to provide message service to the general public over its privately controlled but publicly accessible network. By 1851, more than 50 telegraph companies were in operation in different regions of the United States.

Western Union profited from the wartime business. Over a short period of time it absorbed the independent telegraph companies and became one of the largest corporations in the United States.

In 1850, the first undersea telegraph cable was laid between France and England, extending wire line telegraphic communication internationally. With manual operation, the transmission rate was still slow.

Printing Telegraphs and the Teletypewriter

David E. Hughes first introduced a printing telegraph in 1855. A continuously rotating wheel that had letters of the alphabet, numbers, and other characters on it. Keys at the sending end were operated to send electrical impulses that would stop the wheel and print when the desired character was over a moving strip of paper. The Hughes tele-printer increased the maximum transmission speed to between 40 and 45 words per minute.

In 1874 J. M. E. (Emile) Baudot invented a code that represents each alphabetical character, number, punctuation mark, or control command by a group of five equal-length units. Each unit could be in one of two possible states voltage present on the line representing a 1, or no voltage on the line, representing a 0. Alternatively, a 1 can be represented by a positive or negative voltage and a 0 by the opposite polarity. Systems that consist of only two digits are called binary systems, and bit is an abbreviation of the term “binary digit.”

Baudot developed prototypes of tele-printers (also called teletypewriters) during the 1870s, and by the early 100s teletypewriters that used a version of the Baudot code began replacing manual telegraphy. Start and Stop bits were added to denote the beginning and end of each character.

The teleprinters operated in the range of 100 to 600 baud, which is equivalent to 0 to 100 words per minute, a significant improvement over manual telegraphy.

The Telephone

With the invention and demonstration of the telephone by Alexander Graham Bell in 1876, the electrical transmission of sound became possible. Sound waves were converted to electrical waves by a microphone at the sending end. The electrical waves were transmitted over a pair of copper wires and reconverted to sound waves at the receiving end. The electric waves are analogous to the sound waves. Thus, this type of transmission is called analogue transmission.

The human ear can recognize sounds of frequencies between 0 and 16,000 Hz, with maximum sensitivity around ,000 Hz. The maximum hearing range will vary from person to person and response to the higher frequencies diminishes with increased age.

The human voice includes frequencies in the 00- Hz to 5,000-Hz range. Telephone company personnel determined that intelligible speech could be transmitted over circuits that pass the band of frequencies from 00 to ,000 Hz and established this range as the requirement for voice circuits.

The Bell Telephone Company was formed in 1877 to produce telephones commercially. Western Union created the American Speaking Telephone Company, a competitor to Bell, in the same year.

In 1878, the Bell Company hired Theodore N. Vail, a former superintendent in the U.S. Post Office, to manage the organization, and it filed suit against the Western Union Telegraph Company for infringement of patents. In 187, an out-of-court settlement was reached wherein Western Union agreed to give up the telephone business and Bell agreed to stay out of the telegraph business. In 1880, the American Bell Telephone Company was organized, and years later Bell purchased the Western Electric Company to ensure a ready supply of telephone equipment.

The first long distance telephone service, between Boston, Massachusetts, and Providence, Rhode Island, began on January 1, 1881. Vail, now the president of American Bell, realized that the long distance lines were the key to monopolize the telephone network and began to acquire control. In 1885, the company was incorporated as the American Telephone and Telegraph Company (AT&T) with Vail as president. Vail left AT&T in 1887.



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