Telegraph Networks and Message Switching

In 1837 Samuel B. Morse demonstrated a practical telegraph that provided the basis for telegram service, the transmission of text messages over long distances. The text was encoded using the Morse code into sequences of dots and dashes. Each dot or dash was communicated by transmitting short and long pulses of electrical current over a copper wire. By relying on two signals, telegraphy made use of a digital transmission system. The time required to transmit a message is minimized by having more frequent letters assigned to strings that are shorter in duration. The Morse telegraph system is a precursor of the modern digital communication system in which all transmission takes place in terms of binary signals and all user information must first be converted to binary form.

In early telegraph networks a message or telegram would arrive at a telegraph-switching station, and an operator would make a routing decision based on destination address information. The operator would then store the message until the communication line became available to forward the message to the next appropriate station. This process would be repeated until the message arrived at the destination station. Message switching is used to describe this approach to operating a network. Addressing, routing, and forwarding are elements of modern computer networks.

The information transmission rate (in letters per second or words per minute) at which information could be transmitted over a telegraph circuit was initially limited to the rate at which a single human operator would enter a sequence of symbols. An experienced operator could transmit at a speed of 25 to 30 words per minute, which, assuming five characters per word and 8 bits per character, corresponds to 20 bits per second (bit/s).

A subsequent series of inventions attempted to increase the rate at which information could be transmitted over a single telegraph circuit by multiplexing the symbols from several operators onto the same communication line. One multiplexing system, the Baudot system, used characters, groups of five binary symbols, to represent each letter in the alphabet. The Baudot multiplexing system could interleave characters from several telegraph operators into a single transmission line.

The Baudot system eventually led to the modern practice of representing alphanumeric characters by groups of binary digits as in the ASCII code (short for American Standard Code for Information Interchange). The Baudot system also eventually led to the development of the teletype terminal, which could be used to transmit and receive digital information and was later used as one of the early input/output devices for digital computer systems. A Baudot multiplexer telegraph with six operators achieved a speed of 120 bit/s.

Another approach to multiplexing involves modulation, which uses a number of sinusoidal pulses to carry multiple telegraphy signals. For example, each of the binary symbols could be transmitted by sending a sinusoidal pulse of a given frequency for a given period of time, say, frequency f0 to transmit a "0" and f1 to transmit a "1". Multiple sequences of binary symbols could be transmitted simultaneously by using multiple pairs of frequencies for the various telegraphy signals. These modulation techniques formed the basis for today's modems.

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