Telecommunication Essay Research Paper Telecommunication1 IntroductionComputer and — страница 3

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upper- and lowercase, numbers from 0 to 9, and various punctuation marks) and several control characters such as carriage return, line feed, backspace etc. ASCII also included an error checking mechanism. An extra bit, called the parity bit, is added to each character. When in even parity mode, the bit would have a value of one if there was an even number of ones and zero if there was an odd number of ones. IBM invented it’s own code which used 8 bits of code giving 256 character possibilities. The code was called EBCDIC, for Extended Binary Coded Decimal Interchange Code and was not sequential. The Extended ASCII was designed so that PCs could again attain compatibility with the IBM machines. The other upper 128 characters of the EASCII code include pictures such as lines,

hearts and scientific notation. In 1969 guidelines were set for the construction of serial ports. The RS-232C standard was established to define a way to move data over a communications link. The RS-232C is commonly used to transmit ASCII code but can also transmit Baudot and EBCDIC data. The connector normally uses a 25 pin D shell connector with a male plug on the DTE (Data Terminal Equipment) and a female plug on the DCE (Data Communications Equipment). 3.3. Hello Joshua, Would You Like To Play A Game… In the 1950s a need arose to connect computer terminals across ordinary telephone lines. This need was fulfilled by AT&T’s Bell 103 modem. A modem (modulator/demodulator) is used to convert the on-off digital pulses of computer data into on-off analog tones that can be

transmitted over a normal telephone circuit. The Bell 103 operated at a speed of 300 bits per second, which at that time was more than ample for the slow printing terminals of the day. The Bell 103 used two pairs of tones to represent the on-off states of the RS-232C data line. One pair for the modem that is calling and the other pair for the modem answering the call. The calling modem sends data by switching between 1070 and 1270 hertz, and the answering modem by switching between 2025 and 2225 hertz. The principle on which the Bell 103 operated is still in use today. During the sixties and seventies the concept of mainframe networks arose. A mainframe consisted of a very powerful computer to which thousands of terminals were connected. The mainframe worked on a timesharing

process. Timesharing was when many users on terminals could use limited amounts of the host computer’s resources, thus letting many parties access the host at the same time. This type of network, however, was very expensive, and since on time sharing you could only use small amounts of the host’s total computing power (CPU), the use of the terminal was slow and sluggish. In the late seventies the personal computer was introduced to the public. A personal computer consisted of a monitor, a keyboard, a CPU (Central Processing Unit), and various other connectors and memory chips. The good things about PCs were that they did not have to share their CPU and that the operating costs of these systems were much less that that of their predecessors. The computers could, with a

software package, emulate terminals, and be connected to the mainframe network. Bell laboratories came up with the 212a unit which operated at the speed of 1200 bits per second. This unit, however, was very susceptible to noise interference. 3.4. Hey Bell! I Can Hang Myself Up! After the breakup of the AT&T empire that controlled the modem industry, many other companies started to create new designs of modems. Hayes Microcomputer Products, took the lead in the PC modem business. Hayes pioneered the use of microprocessor chips inside the modem itself. The Hayes Smartmodem, introduced in 1981, used a Zilog Z-8 CPU chip to control the modem circuitry and to provide automatic dialing and answering. The Hayes unit could take the phone off the hook, wait for the dialtone, and dial

a telephone number all by itself. The Hayes Smartmodems sometimes had more powerful CPUs than the computers that they were connected to. The next advancement was the invention of the 2400 bits per second modem. The specifications came from the CCITT, an industry standard setting organization composed of hundreds of companies world wide. The new standard was designated as V.22bis and is still in use today. Other CCITT standards that followed were the V.32 (9600 bps), the V.32bis (14400 bps), the V42 (error control), and the V42bis (data compression). Virtually all modems today conform to these standards. The next big computer invention was the fax modem. It uses the on-off data transmission just as a modem but for the purpose of creating a black and white image. Each on-off signal