The purpose of data communication is to transport symbolic or abstract information across great distances. The early beginnings of information transmission go back a long way. From the beginning of written history and before, humans have been working on ways to transmit information across great distances. The early methods used visible smoke or light flashes that could be observed by people watching from a distance. From the start, communication methods required ways to represent information as signals or messages that could be understood by both the transmitter and receiver of the information. All the early formal ways of encoding messages involved breaking down the information into some sort of easy-to-represent pattern. Of course,these early patterns were not transmitted electrically. Instead, the messages were transmitted as timed bursts of smoke or flashes of light that could be observed from a great distance. However, despite the simplicity, this was the beginning of the evolutionary process that led to modern electronic and optical data communication. Along the way, each method of data communication improves something but introduces new problems or challenges. Attempts to find solutions to the problems lead to innovations that are incorporated in the next generation of protocols. Recent data communication methods are the result of an evolutionary process that can be traced back to early and primitive nonelectrical methods.
The Evolution of Data Communication Methods
Modern data transmission methods actually evolved from primitive methods of signaling used since the earliest days of written history. However, the first common example of the use of electrical data communications was the telegraph. The early telegraphs consisted of a simple on and off modulation of a continuous DC current. When pressed, the telegraph operator’s key connected this circuit. Pressing the key for alternating short and long lengths of time form what was known as “dots” and “dashes.” The dot was a short duration current, and the dash was a longer duration current [ENCBRIT04a]. This system was invented by Samuel Morse in 1832 and came to be known as Morse Code. All methods of data communication in use at that time involved manual intervention. Telegraph operators were trained to key messages onto the wire as fast as possible and to interpret received messages just as quickly. Eventually, however, it became obvious that there was a need for even more rapid interpretation of the telegraph methods.
Coded Transmission and the Printing Telegraph
Once telegraph lines began to stretch from town to town, all over the continent and beyond, there was a need for faster and more reliable message transmission. Most of these improvements involved, in one form or another, what was known as a printing telegraph. There were various systems of printing telegraphs tried in the mid- and late nineteenth century. The early printing telegraphs actually printed the dots and dashes on paper as a sequence of lines that could be read by the human eye, but only an experienced telegrapher was able to interpret the printouts.
Most of the early methods used in printing telegraphy required the use of multiple circuits, multiple current directions, or some combination of the two. In those early days, the theoretical advantages of different encodings or of using current directions could not be realized. The methods were not practical because of grounding problems in the single-wire circuits. All this was about trying to increase the capacity of the communications channels. Printing telegraphs needed more capacity but were hampered by the scarcity and expense of multiple wires because inter-urban wiring was expensive and required a separate physical wire for each circuit. These limitations inspired the search for a completely automatic mechanism for data communication. Refer to table for a chronology of some of the key events in the early history of data transmission. For excellent source materials and information about the history of telecommunications, visit the North American Data Communications Museum [HOUSE].
Events in the History of Data Communication
Character Coded Transmission
In 1874, the French inventor Baudot devised a method of transmitting the characters directly over the circuit rather than having a skilled human operator key the transmissions. He came up with what was the first system of encoding Roman characters directly in a telegraph transmission.
A skilled telegraph operator would no longer be needed to interpret the dots and dashes in the message. His idea, with a few variations, dominated data communications for almost 100 years. The invention consisted of a 5-bit code called Baudot code. The code used five bits to represent 26 uppercase letters, plus the characters SPACE, CR (Carriage Return), LF (Line Feed), BELL, and 14 punctuation characters. Out of the five bits, two are designated as shift bits, which are used by the receiving machine to differentiate among groups of characters. They are used to differentiate between letters, control characters, and numbers depending on the value represented by the two shift bits. In the first implementations, the Baudot code was punched into paper tape at the sending machine prior to transmission. On the receiving machine it could be directly punched into paper tape and converted into printed text later using a paper tape reader. The tape was punched with holes where a hole represented a digital one and an absence of a hole known as a space represented a digital zero [ENCBRITa].
The earliest methods of automatic data synchronization required Baudot or some other similar character-coding scheme. When it was transmitted, the Baudot bit code was preceded by a start bit and followed by a stop bit. These bits were the first system of synchronizing the sending and receiving machine. They were called start and stop bits because the receiving machine was a mechanical device that would start interpreting bits when the start bit was received, and end its interpretation when the stop bit was received. Transmission of each character was separate, and the receiving machine had to wait for the start bit to arrive before beginning to decode the character and would end the decoding of the character when the stop bit was received. On later machines called teletypes, the reading of the character from the transmission line was done with an electro-mechanical system. This device consisted of a rotating wheel connected by an electrical clutch to a continuously rotating motor. Once a start bit was received, the clutch would engage, the wheel would start rotating, and it would “read” each bit until the stop bit was received, at which point the clutch would disengage. The machine would sit idly humming away waiting for the beginning of the next character. This method became known as asynchronousdata transmission. Some of you who have actually seen an old teletype machine will notice that the data rate is slow enough that the character reception can be observed. Notice that the wheel starts rotating with the beginning of the train of bits, and stops at the end. As each bit is interpreted, the machine aligns a group of electromagnets to rotate the print wheel to the correct position to print the received character after the stop bit is received and the wheel stops turning.
Measuring Data Communication Speeds
For many years, data communication speed was measured in baud rate rather than bits or words per second. The origin of this word has an interesting history. The word baud is actually short for Baudot, who invented the 5-bit code that became known as Baudot code. This code was used for many years in telegraphy. Baud rate is a way of measuring the speed of asynchronous (characteroriented) data transmission. People often incorrectly use this term as if it were synonymous with character transmission rate provided by any type of data transmission. Baud rate is actually the number of 0 to 1 stat transitions per second. This concept does not directly calculate out as characters per second. Characters per second can’t be determined precisely by dividing the baud rate by the number of bits per character. This is because baud rate includes overhead due to the presence of the stop and start bits. To translate baud rate to character rate, you have to take into account the number of bits used to encode each character. For example, data transmission was at one time limited to at most 110 baud, which was the fastest rate at which the electro-mechanical teletypewriters could receive the transmission. However, the maximum character rate was really much less than 10 characters per second because the 110 baud number includes overhead for the start and stop bits in addition to the 8-bit characters.
Data Transmission Over the Telephony Voice Network
The telegraph was in common use for less than 20 years when the telephone was invented in 1876. Then, for more than 100 years from 1876 until at least 1976, the primary user of long distance bidirectional information exchange of any type was voice telephony. The familiar system of telephones and voice transmission is known as the Plain Old Telephone System (POTS), where the voice signal was transmitted by analog modulation of a carrier tone. This system is based on assigning a real twisted pair of wires, or later a conceptual circuit, at the start of each telephone call and maintaining the circuit while the call is in progress. In the earliest systems, these circuits were real pairs from the bundle or trunk and the circuits were established by a human operator in the telephone company central office, who would physically connect the phones based on the request of the caller[WIKIPEDb].
Late in the nineteenth century and early in the twentieth century, the number of telephones started to grow rapidly. There were too many phone calls to establish the connections by hand. A faster method was needed to construct the circuit connecting the phones engaged in the call. The improved method was a mechanism to make and break the circuit repeatedly, the dial or pulse generator. When a caller dials the phone, a sequence of pulses is emitted called dial pulses. Now, instead of patch panels, the circuit can be created automatically by electromechanical equipment at each switching point between the two phones. This switching point is called a central office. The request to construct the circuit begins at the central office nearest to the phone initiating the call. Relays at the central office forward the request to the next switching station by coupling the input circuit to a specific outgoing circuit, extending the circuit to the next switching station, and eventually to the called party. The first telephone switch was known as the Strowger Switch after Almon Strowger [RBHILL].
Multiplexing Data Communication Channels
It is interesting to note that early developments in nineteenth-century mathematics form a basis for twentieth-century data communications theory. This is not a single event that can be placed in the timeline presented in Table, but instead is a critical evolutionary step toward the development of mid-twentieth-century developments in data communication. The theory shows that a waveform can be thought of as a periodic continuous function. This complex waveform or function can be represented by a series of trigonometric functions called the Fourier series. The nineteenth-century French mathematician, Joseph Fourier (1768–1830), developed this idea during his search for a solution for a partial differential equation describing heat diffusion. This is called the Fourier series and allows a continuous function or waveform to be thought of as component sinusoidal functions. The Fourier transform converts the complex function into its components. Application of Fourier transform allows the breaking up of available bandwidth of a carrier to individual components or channels [GALENETb].
Eventually, as technology progressed, voice traffic was aggregated onto common lines called trunks, named after the cables of bundles of twisted pairs of wires. These new trunks were using Frequency Division Multiplexing (FDM). With the FDM method, an analog broadband carrier is modulated with separate bands of 3-KHz voice channels with a 1-KHz separation. Along with FDM came automatic switching equipment that instead of using electromechanical relays could electronically switch circuits using Dual-Tone Multi-Frequency (DTMF) tones generated by “touch-tone” phones. As computers and peripheral equipment became more spread out, there was increasing interest in data communications. A need was becoming prominent for a method of data transmission that could use the ordinary and ubiquitous POTS lines. If telephone lines could be used for data transmission, data could be sent from or received anywhere there was an available telephone connection, and telephones were becoming ubiquitous. The first modems were developed in the late 1960s. They modulated the 3-Khz voice band with digital data. Modems permitted data to be transmitted through long distance lines with inductive loading characteristics. The digital signals could then be sent over lines originally intended only for transmission of analog voice signals.
The first commercially available modems for use with POTS could transmit at only 300 baud. Modems got their name because they originally were responsible for the modulation and demodulation of the analog frequency with digital data. Modems modulate the voice band with data that is encoded as sequences of 8-bit characters. This encoding could be in various methods, but in modern use is almost always ASCII, which is very similar to the 5-bit Baudot code used since the telegraph era [ENCBRITc] [ITUTTV21].
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