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Pulse Code Modulation (PCM) :
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The process of sampling and coding of a modulating signal, transforming each element of information into various kinds or numbers of pulses and spaces.
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QAM (Quadrature Amplitude Modulation) :
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The process of combining both amplitude modulation and phase modulation techniques to provide more bits per baud.
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Receive, Receiver :
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The process or device assigned to receive messages in a data communications network, usually, but not always, at a DTE device.
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Recovery :
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The procedure or process required to be performed to restore a computer system to a predetermined level of operation or availability after a failure.
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Remote, Remote Access :
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Accessing a computer system from a location of at least several hundred feet, and sometimes very many miles distant. See remote job entry.
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Repeater :
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Two definitions exist in normal usage: 1) A device used for signal shape and level restoration, for signals that have been distorted due to attenuation . 2) A device used to repeat signals from one circuit onto another circuit (s) usually in a reshaped / amplified form.
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Response Time :
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Response time is essentially the elapsed time between an event, and the computer systems response to the event, or, from the final character of a message at the terminal until the receipt of the first character of the reply.
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Return To Zero (RZ) :
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The opposite of NRZ, whereby the voltage levels return to zero after each encoded bit. Contrast with NRZ.
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Ring, Ring Network :
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A term used in a LAN environment to describe a network topology that is essentially a closed loop. Contrast with bus and star.
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Routing :
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The communications path assigned to a telephone call to reach the calls destination.
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RS-232, RS-232C :
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An EIA recommended interface standard between data terminal equipment ( DTE) and data communications equipment ( DCE) employing serial binary data interchange, and specifying a 25-pin connector (the DB-25 is almost always used) and essentially identical to the CCITT standards V.24 and V.28.
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RS-422 :
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An EIA recommended interface standard between DTE and DCE normally used when exceeding RS-232 's 50 foot maximum limitation and usually implemented of the unused pins of a DB-25 connector. RS-422 is essentially identical to the CCITT V.11 standard.
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RS-449 :
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A companion standard to RS-422 and RS-423 specifying two connectors, a 37-pin and a 9-pin connector. RS-449 is not widely used
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RS-423 :
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A companion standard issued along with RS-422, RS-423 is not widely used. Nevertheless, is electrically compatible with CCITT standard V.10
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RTS (Request To Send) :
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An RS-232 modem interface signal sent from the DTE to the modem on pin 4 indicating that the DTE has data to transmit.
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Satellite Microwave Radio :
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A communications satellite oriented microwave or beam radio system.
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SDLC (Synchronous Data Link Control) :
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A communications protocol used in the IBM SNA environment to control, check, initiate and terminate information sessions on transmission lines.
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Shared Access :
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A term used in a LAN environment to describe an access method that allows many terminals to share a transmission medium, as opposed to discrete access . Examples of shared access methods would be explicit access and contended access.
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Simplex, Simplex Circuit :
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Two definitions exist in normal usage: 1) The transmission of signals in one direction only. 2) A circuit allowing transmission of signals in either direction, but not both directions simultaneously, according to the CCITT.
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Simplex Mode :
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Data communications in only one direction, with no capability for reversing transmission.
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Sine Wave :
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A continuously variable and repeating signal, discovered by a man named Fourier. A sine wave signal is often used as the carrier in an analog modulation process, as well as being able to represent data via frequency and phase modulation. Sine waves can be generated by electronic oscillators and electromechanical generators.
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Source :
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The originator or transmitter of information in a data communications network or link.
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Spectrum, Frequency Spectrum :
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The continuous range of frequencies beginning with audible sounds in the 20 Hz range to cosmic rays occurring above 1000 gHz . Voice grade telephone channel frequency spectrum exists between 300 and 3300 Hz.
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CPS (Characters Per Second) :
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A data flow measuring unit where circuits carry bits forming a data character
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CSMA (Carrier Sense Multiple Access) :
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A contention-based LAN access method where terminal stations listen prior to transmitting, send a packet of data, then free the line for other stations to transmit . While stations do not transmit until the line is clear, transmission collisions still occur.
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Demodulation :
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The opposite of modulation, the process of retrieving data from a carrier signal.
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Demultiplexing :
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The opposite of multiplexing where a composite signal is broken into is component channels
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Digit, Digital :
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A discretely variable signal as compared with analog, which is continuously variable. Data signals are coded in discrete and separate pulses.
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FCS (Frame Check Sequence) :
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A method for error detection in bit-oriented protocols, normally consisting of a 16-bit fields
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FDM (Frequency Division Multiplexing) :
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A method of multiplexing where a data lines bandwidth is divided into channels and assigning a specific range of frequencies to each channel.
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FDX (Full Duplex) :
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Transmitting data in both directions simultaneously. FDX can occur on either two or four wire circuits.
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Frame :
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The sequence of bits and bytes in a transmitted block
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High Pass :
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A predetermined filter frequency level, above which all frequencies may pass.
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Horizontal Redundancy Checking (HRC) :
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A method for data error checking whereby redundant information would be included in the data to be checked. Contrast with LRC ( Longitudinal Redundancy Checking).
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Host, Host Computer :
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The central or controlling computer in a data communications network, usually providing database access, programming languages, etc.
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Hub :
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A DDS office designed to multiplex T1 data streams from a number of local offices into signals suitable for transmission.
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Hybrid :
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Any combination of two or more technologies. For example, DDCMP is a hybrid character/bit data communications protocol.
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File Server, File Server protocol :
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A LAN station or protocol designed to allow application programs to share and store data files.
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Physical Layer :
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The lowest or bottom layer of the OSI model concerned with the mechanical (physical connectors) and electrical (voltages, timing, etc.) and other related items, and interfacing with the layer above, the link layer.
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Star, Star Network :
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A term used in a LAN environment to describe a topology configured like a "star" in which the center of the star would be the control point linked individually to all stations.
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Start Bit, Start Element :
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A term used in asynchronous transmission to describe the first bit used to indicate the beginning of a character. Usually, a space condition would be used to alert a receiving devices of the reception of a character as in Baudot teletypewriter operation.
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Synchronous Modem :
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A type of modem designed to provide a clocking signal to perform bit synchronization.
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Terminal :
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A device designed to allow users of a data processing system to gain access to information in a more conventional manner than through the input/output devices local to that system. Often, computer terminals are located away from the host, at locations convenient to the users.
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Terminal Server :
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A device used in an Ethernet LAN environment allowing one or more devices to be connected to the Ethernet network.
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Transmit :
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To send out a signal over data communications media such as optical fiber, copper wire, or via radio waves through the atmosphere, from one station to another.
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Transport Layer :
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The fourth layer in the OSI model responsible in conjunction with the network, data link, and physical layers for error-free, end-to-end message delivery.
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Video Conferencing :
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A method of audio/video communications, usually employing satellite transmission over a wideband range spanning between 56 kbps to 1.544 mbps (T1 circuit speed).
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Voice Digitizing :
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The process of converting an analog voice signal into digital signals for transmission.
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WAN (Wide Area Network) :
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A term used to describe a data communications network using common carrier circuits to connect stations and processors. Contrast with LAN.
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Workstation :
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The terminal or other device, at which a computer operator works, capable of sending and receiving data needed to perform a specific task. An Input/Output terminal.
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WPM (Words Per Minute) :
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A term used in the telegraph environment to measure transmission speed.
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XMODEM :
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A communications protocol devised by Ward Christiansen to perform simple error checking between microcomputers . XMODEM is a half-duplex protocol, used on full-duplex circuits transmitting 128 characters per block. After information is sent, the sender waits for a reply before transmitting the next message.
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YMODEM :
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A communications protocol very similar to XMODEM designed to perform simple error checking between microcomputers . YMODEM is a half-duplex protocol, used on full-duplex circuits transmitting a 1 kilobyte (1,024) characters per block. After information is sent, the sender waits for a reply before transmitting the next message.
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Concentrator :
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A device which connects a number of circuits which are not all used at once to a smaller group of circuits for cost effective transmission. The data channel is split into two or more channels of average lower speed while the concentrator dynamically allocates space according to the demand in order to allow the greatest possible throughput. Also known as a statistical multiplexer, ATDM or TDM.
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Diagnostics, Diagnostic Programs :
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Programs or procedures used to check equipment, communications links or networks and pinpoint faulty components. Diagnostics may be used by the computer engineer or may be called in by the supervisory programs automatically.
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DSR (Data Set Ready) :
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An RS-232 modem interface control signal which indicates to the attached terminal the modem is connected, powered-on, and ready.
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Dumb Terminal :
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Any terminal unable to perform error detection or use a communications protocol, such as a Teletype or Teletype compatible terminal. Usually an asynchronous, ASCII device operating at 9600 bps or higher. Contrast with intelligent terminal
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Duplex :
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Any communications facility providing transmission in both directions simultaneously, often referred to as "full duplex". Contrast with "half-duplex".
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Duplexing :
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A process using redundancy, whereby duplicate processors, files or circuitry would be available to enable the system to carry on its work. Providing an alternate system in this way is also called "Tandem".
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EBCDIC :
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(Extended Binary Coded Decimal Interchange Code) - An 8- bit character code, standard for many IBM systems offering 256 possible combinations of characters.
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Echo :
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The reflection or return of transmitted data.
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Encryption :
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A method of data protection whereby a bit stream would be changed to include additional bits, or appear as a random sequence of bits to an unauthorized observer.
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Error Correction :
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A method to insure data integrity in received data, performed by retransmission requests to the sending station (source), or by manipulating the received data. See ARQ and FEC.
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FM (Frequency Modulation) :
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One of three different methods of transmit ting digital information on an analog line. FM changes the carrier frequency to different values. Also see AM and PM.
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Teleconferencing :
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Engaging in a conference between remote stations linked by a telecommunications medium.
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Cluster :
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A group of user terminals located near each other and connected to a single controller. Terminals may consist of printers, keyboard devices, microfiche printers, etc.
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Data Channel :
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The data transmission path between two or more stations
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Data Communication :
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The processes, facilities and equipment used to transport encode d information from one point to another.
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Datagram :
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A capability in a packet-switched network where a complete message may be contained in the data field of a packet, not usually implemented on today's packet data networks (PDN). See minicall .
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Data Integrity :
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The performance of a data communications system, ideally indicating an absence of undetected errors.
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Data Link :
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The physical connection that includes all necessary equipment for two devices to communicate
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Data Link Layer :
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The second layer in the OSI model that establishes, maintains and released data link connections between the network layer and physical layer. While the data link layer is not responsible for error correction, it is responsible for error detection, transmission and reception of datagrams, packet reception and local addressing.
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Data Network :
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A telecommunications system consisting of a number of terminals able to access each other via communication lines and switching methods.
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Data Stream :
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The transmission of characters and data bits through a channel
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Data Switch :
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A device used to connect data processing equipment to network lines, offering flexibility in line /device selection.
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Data Terminal Equipment (DTE) :
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A term used to describe numerous data processing equipment such as computers, terminals, controllers and printers.
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Data Transfer Rate, Data Rate :
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The measure of the speed of data transmission, usually expressed in bits per second. Synonymous with speed, the data rate is often incorrectly expressed in baud .
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