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Accelerator card
refer to computer terms page
Access time
refer to computer terms page
ADC (Acronym for Analogue-Digital-Convertor)
refer to computer terms page
ANALOGUE:
(of a quantity or device) Changing continuously; by contrast
a digital quantity or device varies in series of distinct steps. For example,
an analogue clock measures time by means of a continuous movement of hands
around a dial, whereas a digital clock measures time with a numerical display
that changes in a series of discrete steps.
Most computers are digital devices. Therefore, any signals and data from a analogue device must be passed through a suitable analogue-to-digital converter before they can be received and processed by computer. Similarly, output signals from digital computers must be passed through a digital-to-analogue converter before they can be received by an analogue device.
Analogue to Digital convertor (ADC). A device to sample an analogue signal and give it a digital code. This can then be handled by a computer. The reverse process to digital to analogue conversion DAC.
ANSI (American National Standards
Institute)
Uses eight bits to represent each character. Used
by MS Windows to store documents (256 codes) the first 128 are the same
as ASCII with a zero in the eigth bit the other 128 bits include codes
for copyright, foreign symbols and letters with accents. see also character
sets
APPLICATION SOFTWARE is software used for a specific purpose, eg: textual document processing (word processing), text and graphic printed presentation (DTP), numerical analysis processing (spreadsheets), record and transaction processing (databases), computer-aided design and graphic drawing (vector graphics), graphics/artwork processing (bitmap graphics), accounts processing. Some software integrates several of the above uses in one package (integrated software)
APPLICATION SOFTWARE ROUTINES facilities providing user automation facilities such as macros, styles and templates.
Architecture refer to computer terms page
ASCII (American
Standard Code for Information Interchange)
is the most widely used coding scheme for character data.
The standard ASCII code uses seven bits to represent 128 symbols including
upper/lower case letters, special control codes, numerals etc. See
also character sets.
Asymmetrical Digital Subscriber Line (ADSL) an alternative system to ISDN to provide multimedia facilities over a copper PSTN line connection.
ASSEMBLER a readable form of low-level program language which converts an English-like code program into a machine code program which can be executed.
ASYNCHRONOUS:
Irregular or not synchronised. In computer communications,
the term is usually applied to data transmitted irregularly rather than
as a steady stream. Asynchronous communication uses start bits and stop
bits to indicate the beginning and end of each data block.
Related Articles: transmission
modes
start bit
stop bit
Asymmetric duplex see duplexhalf duplexsimplex transmission modes
ATTENUATION
The decrease in signal strength as it travels along a
media. The losses are caused by a number of factors involving the
resistance, capacitance and inductance
of the media.
BACKBONE: 1) In 10BASE5 Ethernet, the main network cable, to which nodes are attached through transceivers. 2) In packet-switched networks, the major transmission path for a PDN.
BACKBONE NETWORK: A transmission facility designed to interconnect low-speed distribution channels or clusters of dispersed user devices.
BACKGROUND PRINTING: A feature that allows you to keep using your computer while it's sending a document to the printer.
BANDWIDTH: The data-carrying capacity of a communications channel; measured (in Hertz) as the difference between the highest and lowest frequencies of the channel. Bandwidth varies depending on the transmission method.
BASEBAND: A transmission method, typically for shorter distances, in which the entire bandwidth of the cable is required to transmit a single digital signal. (Compare broadband.) Digital signals are put onto the cable without modulation and are transmitted one at a time, making baseband a simpler and cheaper way to transmit data. Simultaneous transmissions can be achieved through time-division multiplexing. TDM
BATCH FILE an operating system program file which is created to automate a processing procedure.
BATCH PROCESSING this type of processing involves collecting jobs or material to be processed over a period of time, and creating a schedule followed by one complete processing session. The user has very little interaction with the process.
BAUD: unit of signalling speed. The speed in baud is the number of line changes (in frequency, amplitude, etc.) or events per second. At low speeds, each event represents only one bit condition, and baud equals bps. As speed increases, each event represents more than one bit, and baud rate does not truly equal bps. But in common usage, baud rate and bps are often used interchangeably.
BBS (Bulletin-Board Service): An on-line service from which users can get information or files, or exchange messages, via modem.
BCD (Binary-Coded Decimal): A digital system that uses binary codes to represent decimal digits.
Biometrics Userrecognitionsystems
to access logical security built into networks using parts of the body
instead of passwords. Input devices such as hand geometry readers
(identify finger prints) and retina scanners (identify an individuals eyeball
pattern).
see retina
scanner hand geometry
BITMAP GRAPHICS a graphic image or text formed by a pattern of dots or pixels. Examples include scanned documents and printed newspaper pictures. An electronic graphic file where each minute item (dot) in the graphic picture is represented by a single (or several for colour) bit of information in the file. Thus a picture with 8000 bits of information would produce a 1000 byte file. (1 byte = 8 bits). These files cannot be scaled in the way that vector-based images can.
There are a number of forms of bitmap images usually known
by the file extension.
BOOLEAN OPERATOR: Also called a logical operator. Used in programming to identify AND, OR, NOR, XOR, and NOT operations.
BRIDGE: In computing, a device that connects two similar local area networks LANs (often called subnetworks) that are running the same protocols and cabling.. Bridges transfer data in packets between the two networks, without making any changes or interpreting the data in any way. This arrangement creates an extended network, in which any two workstations on the linked LANs can share data. Bridges use only the bottom two layers of the OSI model. Compare router, gateway.
BROADBAND: A transmission method that uses a bandwidth greater than a voice-grade channel's, andpotentially capable of much higher transmission rates; also called wideband. In broadcasttransmission, multiple channels access a medium (usually coaxial cable) that has a large bandwidth,using radio-frequency modems. Each channel occupies (is modulated to) a different frequency sloton the cable, and is demodulated to its original frequency at the receiving end. Cable television is an example, with up to 50 channels occupying one coaxial cable.
BROADCAST: A method of transmitting messages to two or more stations at the same time, such asover a bus-type local area network or by satellite; a protocol mechanism that supports group and universal addressing. Any simultaneous transmission to many receiving locations. One example is a message sent over a multipoint line to all terminals that share the line.
BUFFER: A device that temporarily stores data from a faster device, then sends it out to a slave device. The faster device can thus move on to another task while the slower device is still accepting data. A buffer between your PC and your printer lets you get back to work quickly after you send afile to be printed. Buffers are also called spoolers.
BUFFERING: The process of temporarily storing data in a software program or in RAM, to allow transmission devices to accommodate differences in data transmission rates.
BYTE: A unit of information, used mainly in referring to data transfer, semiconductor capacity, and data storage; also referred to as a character; a group of eight (sometimes seven) bits used to represent a character.
CACHE: The portion of a computer's RAM reserved to act as a temporary memory for the last items read from a disk. This removes the need to repeatedly go to the hard drive for regularly used data.
CAD (Acronym for Computer Aided Design)
CAPACITANCE:
the ability of a media to store up an electrical charge
it can cause problems when not wanted but can be used to good effect in
storage devices such as some forms of RAM. see
crosstalk
and noise
CAL (Acronym
for Computer Aided Learning) refer to computer
terms page
CAM (Computer
Aided Manufacturing) refer to computer
terms page
CARRIAGE RETURN: see CR
CARRIER: A continuous signal modulated with a second, information-carrying signal.
CATEGORY3: A cabling standard for UTP horizontal wiring. Used for 10-Mbps 10BASE-T Ethernet networks or 4-Mbps Token Ring networks. see 10base-t and UTP
CATEGORY4: A cabling standard for UTP wiring with a bandwidth of 20 MHz. Commonly used for 16-Mbps Token Ring networks. see 10base-t and UTP
CCITT STANDARDS (Comite
Consultatif Internationale de Telegraphique et Telephonique): An international
consultative committee that set world-wide communications standards (such
as V.21, V.22, and
X.25). CCITT are accepted standards and include the following
modem
connection speeds:
Speed (bps)
CCITT Standard
1200
V22
2400
V22bis
9600
V32
14400
V32bis
28800
V34
where bis indicates an second improvement on the previous standard
CD (Carrier Detect): An RS-232 control signal (on pin 8) which indicates that the local modem is receiving a signal from the remote modem. Also called Received Line Signal Detector (RLSD) and Data Carrier Detect (DCD).
CELLULAR: Based on the use of cells. Cellular telephone and data networks divide an area into regional cells, each of which has its own central transmission facilities; that way, every point in the area is within range of some transmission facility.
CENTRALISED PROCESSING:see Networking terminology
CENTRONICS(R) parallel: The de-facto standard for personal computer printers. This 36-pin parallel interface allows the connection of printers and other devices to a computer.
CGA (Color Graphics Array): A graphics standard for IBM PC, PC/XT(tm), and AT(R) computers or compatibles, introduced by IBM in 1981. CGA can display a maximum of 16 colors in two graphics modes: High Resolution which has 640 x 200 pixels in black and white, and Low Resolution, which has 320 x 200 pixels in four colors.
CHARACTER SETS
Standard character representation codes can usually be
recognised across different computer platforms. Examples are ASCII,
ANSI,
EBCDIC
and
UNICODE
CHECKSUM: The total of a group of data items or a segment of data that is used for error-checking purposes. Both numeric and alphabetic fields can be used in calculating a checksum, since the binary content of the data can be added. If the checksum calculated does not match the original checksum, something in the data has changed. Checksums can detect single-bit errors and some multiple-bit errors.
CLIENT-SERVER:see network terminology page
CLUSTER: A collection of terminals or other devices in a single location.
CLUSTER CONTROLLER: A device that manages the input/output operations of a group (cluster) of terminals (usually dumb) or workstations.
COAXIAL:
Coaxial cable (widely used for TV aerials) comprises
od an inner conductor surrounded by a copper web outer conductor.
The separation of the two conductors is evenly maintained by either regular
spacers/air or more commonly insulating material. It is less flexible
than twisted pair, particularly thick ethernet (RG-8
50-ohm),thin ethernet (RG-58
A/U or CU 50-ohm) and a third type used by ARCNET (RG-62 A/U 93-ohm)
CODEC: A device that encodes and decodes signals.
COLLISION: 1) In LAN technology, two stations attempting to use the same transmission medium at the same time.2) In a half-duplex system, the result of both ends trying to transmit at the same time. see CSMA/CD
COLLISION DETECTION: In LAN technology, the act of detecting when a collision has occurred; typically occurs when a workstation does not receive an acknowledgment from a receiving station. Collision detection is an integral part of the CSMA/CD access method.
COMMUNICATION SERVER: acomputer designated with the task of organising communication flow within a network. see also printer server
COMMUNICATIONS SETTINGS can be set in Windows and include Baud rate, Data bits (5,6 7 or 8), Stop bits (1, 1.5 or 2), Parity (none, odd, even, mark or space), Parity check, Carrier detect, Connector (none,COM1 or COM2) and Flow control (X-on, X-off or none).
COMMUNICATION SOFTWARE software is required for all types of computer-based communication. The main types of software are: system, user interface and communications, eg Terminal in Windows. (see communications settings)
COMPILER a program which translates a complete high-level language program into machine code to create a fully executable version.
COM PORT an I/O port for connecting serial deivices to a computer. Eg a mouse or a modem. Most computers have a COM1 and a COM2 port and if COM1 is allocated for use by a mouse then it would be necessary to use COM2 when setting up an external modem.
COMPRESSION
is a technique used for reducing the size of files.
It can be applied to data, text, graphics or video. Modems
also have facilities for compression.The technique is used to increase
the number of bits per second sent over a data link by replacing often-repeated
characters, strings, and command sequences with electronic code. When this
compressed data reaches the remote end of the transmission link, the coded
data is replaced with the actual data. Also called "compaction."
CONFERENCING video conferencing and tele-conferencing enable two or more people to have a conference. Such facilities are available through several network service providers.
CONTROL CHARACTER: A non-printing character used to start, stop, or modify a function. CR is an example of a control character.
CONTROL SIGNAL: A modem interface signal used to announce, start, stop, or modify a function; for example, CD is an RS-232 control signal that announces the presence of a carrier.
CONTROL SYSTEM a computer system which automatically controls a process or mechanical device by sensing the need to vary the output. Examples of sensors are light, heat, humidity and Ph. A control system is said to have feedback when it is the output of the controlled device which is sensed and fed back to the computer.
CPU (Central Processing Unit): The heart of a computer; the component that does the computing. In a PC, it might be contained in a single microprocessor; in a minicomputer, on one or several printed circuit boards; in a mainframe, on many printed circuit boards.
CR (Carriage Return): An ASCII or EBCDIC control character used to position the print mechanism at the left margin on a printer - or the cursor at the left margin on a display terminal.
CRC (Cyclic Redundancy Check): A basic error-checking mechanism for link-layer data transmissions; a characteristic link-layer feature of (typically) bit-oriented data-communications protocols. The data integrity of a received frame or packet is checked via a polynominal algorithm based on the content of the frame, and then matched with the result that is performed by the sender and included in a (most often 16-bit) field appended to the frame.
CROSSTALK (noise):
This is the result induction, where for example a copper
wire is in the magnetic field caused by another copper wire carrying a
signal. (ie wires carrying electrical signals form a magnetic field
around them. The strength of the magnetic field is dependant upon
the strength and frequency of the electrical signal being carried).
This is common in twisted pair cable. The twist in the cable is designed
to overcome this problem. Shielded twisted pair cuts down this interference
even more. Cabling should not be run close to sources such as radio
transmissions, flourescent lights, fan motors or speakers. see
attenuation,
CSMA/CD (Carrier Sensing Multiple Access with Collision Detection) A LAN contention protocol by which workstations connected to the same channel are able to sense transmission activity on that channel and so defer their own transmission while the channel is active. If two nodes try to transmit simultaneously a collision is detected and both stations stop transmission for a period. After a random period of time each station will make a repeat attempt. It is unlikely that they will attempt to transmit at the same time and further collision usually does not occur. seecollision
CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance): A LAN access method in which contention between two or more stations is avoided. When a station is prepared to transmit, it first listens to see if the channel is clear.
CTS (Clear To Send): An RS-232 modem interface control signal used in a switched half-duplex circuit to notify the computer that it has line control. In a full-duplex circuit, this signal is constant.
DAT (Digital Audio Tape)
DATA COMPRESSION this feature enables a modem to transmit data at a quicker rate than its connect speed. The number of bits does not alter, but by using fewer bits to represent the data, the effective transmission rate is increased. Data compression can only be used if supported by modems at each end of a communications link. CCITT's data compression standard is V42bis and is built into most modern modems. This standard compresses standard text to 25% but program .exe will only reduce to 50 or 75%.
This means that, for example, a 2400bps file can transmit text at an effective rate of 9600bps (4 x 2400). If file compression has been used to save space on computer storage by using fewer bits to represent the data contained in within it .zip (Zipped) then the file cannot be compressed further and will be transferred at the modems connect speed. see also compression
DATA TYPE the characteristics of the data used. These may differ for each type of system. Typical data types are: character, number (integer, real), graphic, logic (boolean), date.
(DCC) Data circuit connection referred to by the GNVQ as the standards required electronic communications eg RS232C and V24. These two standards, (from different standards organisations) are the same and allocate signals to the pin layout in a cable connecting a computer to a modem. A null modem is a special type of RS232C cable with a cross over between the TX (transmit) and the RX (receive) used when connecting two computers together.
(DCE) Data circuit terminating equipment the connections of a network are leased from a main telecommunications provider such as British Telecom. The equipment that allows the DTE to be connected to and interfaced with the network is known as DCE. A modem is an example of such a device. DCE is the equipment that enables a DTE to communicate over a telephone line or data circuit. The DCE establishes, maintains, and terminates a connection, and performs the conversions necessary for communications. In RS-232, designation as either DCE or DTE determines the signalling role in handshaking. see DCC
DIGITAL SAMPLING:
Electronic process used in telecommunications for transforming
a constantly varying (analogue) signal into one composed of discrete units,
a digital signal. In the creation of recorded music, sampling enables the
composer, producer, or remix engineer to borrow discrete vocal or instrumental
parts from other recorded work (it is also possible to sample live sound).
A telephone microphone changes sound waves into an analogue signal that fluctuates up and down like a wave. In the digitizing process the waveform is sampled thousands of times a second and each part of the sampled wave is given a binary code number (made up of combinations of the digits 0 and 1) related to the height of the wave at that point, which is transmitted along the telephone line. Using digital signals, messages can be transmitted quickly, accurately, and economically. see PCM
Digital to Analogue convertor (DAC) see also Analogue to Digital convertor
DISTRIBUTED SYSTEMS systems in which processing is spread over several computer systems which are connected and share resources. One computer may have data relating to goods and another may have data relating to customer; as a system they provide facilities for combining this data. Distributed processing allows separate CPUs to share work on the same application program, thus allowing each CPU to perform a certain task. Often used to mean distributed computing.
(DTE) Data terminal equipment all external devices attached to a network can be referred to as data terminal equipment. Eg. computer terminals, EFT cash dispensers, EPOST supermarket cashout or printers etc.
DTR (Data Terminal Ready): An
RS-232
modem interface control signal (sent from the DTE to
the modem on pin 20) that indicates that DTE is ready
for data transmission, and requests that the
modem be connected to the telephone circuit.
DUMB TERMINAL: see Network terminology
DUPLEX (sometimes called Full
Duplex)
allows transmission of data in both directions simultaneously.
see
also asymmetric duplex half
duplex, simplex and
echoflex
EBCDIC (Extended
Binary Coded Decimal Interchange Code)
is an eight bit code developed by IBM for it's mainframe
computers. It does not use the same representation as ASCII or ANSIIfor
the initial 128 characters. (see ANSII)
this produces problems when trying to communicate with computers which
use other character sets unless translation software is used. see also
character
sets
ECHO: in electronics the echo is the distortion created when a signal is reflected back to the originating station.
ECHO (also called echoplex) is the term referred to by GNVQ meaning a method of checking data integrity by returning characters to the sending station for verification of data integrity, channels send information to the receiver and then echo it back to the sender as an accuracy check. Echo is useful in situations when it is important to be absolutely sure that data is transmitted accurately. see also Asynchronous transmission duplex , half duplex and simplex
ECHO CANCELLATION: is the technique used in modems to filter out unwanted signals.
ECHO SUPPRESSOR: A device used by telcos or PTTs that blocks the receive side of the line during the time that the transmit side is in use.
EEPROM (Electrically Erasable Programmable Read-Only Memory): Similar to EPROM, except that chips can be erased with electrical signals (rather than ultraviolet light) while installed in a device, and then reloaded with firmware. Often used to store system configuration data.
EGA (Enhanced Graphics Adapter): A video standard for IBM PC, PC/XT, and AT computers or compatibles. EGA retains compatibility with CGA, but has more graphics modes. Its resolution, at 640 x 350 pixels, is better than that of CGA.
EISA (Extended Industry Standard Architecture) Bus: A 32-bit adaptation of the 8- and 16-bit buses originally developed by IBM. It was designed for use with Intel microprocessors. The EISA bus was a joint development of COMPAQ(R) and other PC manufacturers as an alternative to the proprietary IBM Micro Channel bus. Electronic mail, email: Messages sent between subscribers electronically via a public or private data communications system.
ELECTRONIC MAIL e-mail is the sending of messages, letters and other documents through an electronic system instead of using the postal service (sometimes referred to as "snail mail") This can be undertaken between users of a networked system, through network service providers. When mail is sent the user is informed that it has arrived as soon as they login to the system providing the service.
EMI (ElectroMagnetic Interference): Unwanted electromagnetic emissions, generated by lightning or by electronic or electrical devices, that degrade the performance of another electronic device. Interference may be reduced by shielding. Maximum acceptable levels of EMI from electronic devices are detailed by the FCC. see also noise
EMPTY-SLOT RING: In LAN technology, a ring LAN in which a free packet circulates through every station; a bit in the packet's header indicates whether it contains any messages (if it contains messages, it also contains source and destination addresses).
EMULATION: The imitation of all or part of one device, terminal, or computer by another, so that the emulating device accepts the same data, performs the same functions, and appears to other network devices as if it were the emulated device.
ENCODING/DECODING: The process of organizing information into a format suitable for transmission, and then reconverting it after transmission; for pulse-code-modulated voice transmission, the generation of digital signals to represent quantified samples, and the subsequent reverse process.
ENCRYPTION method by which data is made unintelligible to protect its confidentiality.
EPROM (Erasable Programmable Read-Only Memory): A nonvolatile semiconductor PROM that can be erased for reuse by exposing it to intense ultraviolet light.
ERGONOMICS the relationship between people and their working environment. Study of factors affecting the health & safety of IT workers.
ERROR CONTROL: An arrangement that combines error detection and error correction.
ERROR CORRECTION: An arrangement that restores data integrity in received data, either by manipulating the received data or by requesting retransmission from the source.
ERROR DETECTION: An arrangement that senses flaws in received data by examining parity bits, verifying block check characters, or using other techniques.
ETHERNET: A network standard first developed by Xerox, and refined by DEC and Intel. Ethernet interconnects personal computers and transmits at 10 megabits per second. It uses a bus topology that can connect up to 1,024 PCs and workstations within each main branch. Ethernet is codified as the IEEE 802.3 standard.see also 10base-2, 10base-5ethernet
ERROR CHECKS: see CRC
EVEN PARITY: A dumb-terminal
data-verification method in which each character must have an even
number of one bits.
EXPERT SYSTEMS an application which enables users to benefit from knowledge of human experts. The knowledge is built into the system as a set of rules which may be updated with use. Allows diagnostics to be carried out by people who are not necessarily experts themselves.
FAST ETHERNET:Any 100-Mbps Ethernet-based networking scheme.
FDDI (fibre-optic Distributed Data Interface) In computing, a series of network protocols, developed by American National Standards Institute (ANSI), concerned with high-speed networks using fibre optic cable.
FDDI supports data transmission rates of up to 100 Mb
per second and is being introduced in many sites as a replacement for Ethernet.
FDDI not only makes possible transmission of large amounts of data, for
example colour pictures, but also allows the transmission of voice and
video data. See also optical fibres. The
standard
specifies: multimode fiber; 50/125, 62.5/125, or 85/125
core cladding; an LED or laser light source; and 2 km for unrepeated data
transmission at 40 Mbps.
FEEDBACK the process where part of an output is fed back into the input to enable action to be taken to increase or reduce the output. Positive feedback results in increased output whereas negative feedback results in reduced output.
FIBRE OPTIC CABLE is a bundle of extremely thin glass strand. Each strand is much thinner than a human hair and is extremely transparent to prevent attenuation of the light pulses that are transmitted along it. Each strand is coveredby plastic insulation and strenghthening materials. The strand acts as a wave guide and uses total internal reflaction to steer the light pulses (usually produced by miniature lasers or LEDs (light emitting diodes) . Each fibre is one way and two fibres are needed to provide duplex links. Each strand has a very high bandwidth and is capable of transmitting large amounts of data. Fibre optics do not transmitt electrical currents and are not susceptible to elctro-magnetic interference. see also FDDI , transmission media optical fibre graded-index fibreBT Factfile No. 9
FILE PROTECTION a facility offered on most LANs to enable users to set rights to their own files and subdirectories for other users, eg read, copy and, write. These rights may also be set in most systems by adjusting the attributes of the file itself, ie read only, hidden. see 'Local Area Network'
FILE SERVER: see Network terminology page
Floppy Disk see computer terms page
FLOW CONTROL it is necessary to ensure that the data receiver is not overloaded with incoming data and is able to tell the transmitter to wait while it catches up with transmission. One common method used is to enable the computer to issue stop and start commands, typically X-off and X-on. The procedure for regulating the flow of data between two devices; prevents the loss of data once a device's buffer has reached capacity.
FRAME RELAY: A packet network service, relying on the data integrity inherent in digital transmissions to speed up transmission. Unlike old X.25 networks, Frame Relay "assumes" the data is correct and starts checking as soon as it receives the header, in a half-dozen error-checking steps. Frame-relay services are offered with T1 and DDS connections.
FRAMING: A control procedure used with multiplexed digital channels, such as T1 carriers, whereby bits are inserted so that the receiver can identify the time slots that are allocated to each subchannel; framing bits may also carry alarm signals indicating specific alarm conditions.
FDM (Frequency-division multiplexor): A device that divides the available transmission frequency range into narrower bands, each of which is used for a separate channel. see multiplexer
FTP (File Transfer Protocol): An
upper-level TCP/IP service that allows copying of files across a
network.
FULL DUPLEX (FDX): Simultaneous,
two-way, independent transmission in both directions (4-wire).
Compare with asymmetric duplexhalf-duplex.,
simplex
also known as just duplex
GATEWAY: A hardware-software combination that connects two LANs (or a LAN and a host computer) that run different protocols - for example, a TCP/IP LAN and an SNA mainframe. The gateway provides the protocol conversion. Gateways, because they operate on the top three layers of the OSI model, are much more complex than bridges. See bridge.
GOPHER:A browsing tool used on the Internet.
GRADED-INDEX FIBRE: An optical
fiber whose core, composed of concentric rings of glass, has a
nonuniform index of refraction. The refractive indexes
of the rings decrease from the center axis out,
in order to increase bandwidth. see fddi
, fibre optic , optical
fibre BT
Factfile No. 9
GRAPHICAL USER INTERFACE (GUI pronounced
"gooey")
an operating system, or an addition
to the operating system, which displays program choices and options on
the screen as icons, or picture symbols. Users enter commands by
pointing at icons with a mouse or other pointing device and clicking a
button on the pointing device. Some other GUIs are Microsoft Windows(tm),
OS/2(R), and X-Windows for UNIX(R).
HALF DUPLEX (HDX)
allows bothway transmission of data between two devices
but only in one direction at a time. (2-wire). see
also asymmetric duplex duplex
, echoplex and simplex
Hand Geometry Reader
is a device used to map the lines and finger prints of an individuals hand
allowing identification.
see retina
scanner biometric
HANDSHAKING: Exchange
of predetermined signals between two devices, establishing a connection
or providing flow control. Usually part of a communications protocol.
HYPERLINK: Link from one document to another or, within the same document, from one place to another. It can be activated by clicking on the link with a mouse. The link is usually highlighted in some way, for example by the inclusion of a small graphic. Documents linked in this way are described as hypertext. Examples of programs that use hypertext and hyperlinks are Windows help files, Acrobat, and
HYPERTEXT: ystem for viewing information (both text and pictures) on a computer screen in such a way that related items of information can easily be reached. For example, the program might display a map of a country; if the user clicks (with a mouse) on a particular city, the program will display some information about that city.
HYPOTHESIS TESTING (modelling) using computer models such as spreadsheets to test possible situations, eg modelling financial breakeven points for a business such as an airline or hotel, using 'what-if' queries such as changing fuel consumption etc.
INDUCTANCE: see crosstalk
INTERFACE: In computing, the point of contact between two programs or pieces of equipment. The term is most often used for the physical connection between the computer and a peripheral device, which is used to compensate for differences in such operating characteristics as speed, data coding, voltage, and power consumption. For example, a printer interface is the cabling and circuitry used to transfer data from a computer to a printer, and to compensate for differences in speed and coding.
Common standard interfaces include the Centronics interface, used to connect parallel devices, and the RS232 interface, used to connect serial devices. For example, in many microcomputer systems, an RS232 interface is used to connect the microcomputer to a modem, and a Centronics device is used to connect it to a printer.
ISDN (Integrated Services Digital Network): Internationally developed telecommunications system for sending signals in digital format along optical fibres and coaxial cable. It involves converting the ‘local loop’ - the link between the user’s telephone (or private automatic branch exchange) and the digital telephone exchange - from an analogue system into a digital system, thereby greatly increasing the amount of information that can be carried. The first large-scale use of ISDN began in Japan 1988.
ISDN has advantages in higher voice quality, better-quality faxes, and the possibility of data transfer between computers faster than current modems. With ISDN’s Basic Rate Access, a multiplexer divides one voice telephone line into three channels: two B bands and a D band. Each B band offers 64 kilobits per second and can carry one voice conversation or 50 simultaneous data calls at 1,200 bits per second. The D band is a data-signalling channel operating at 16 kilobits per second. With Primary Rate Access, ISDN provides 30 B channels.
A CCITT standard for a network that accommodates a variety of mixed digital-transmission services; the access channels are basic rate (144 Kbps) and primary rate (1.544 Mbps).
British Telecom began offering ISDN to businesses 1991, with some 47,000 ISDN-equipped lines. Its adoption in the UK is expected to stimulate the use of data-communications services such as faxing, teleshopping, and home banking. New services may include computer conferencing, where both voice and computer communications take place simultaneously, and videophones.see also BT Factfile No.10
ISO (International Standards Organization): An organization that promotes the development of standards for computers.
JANET(Joint Academic Network):. A network serving the Higher Education Funding Council(s) and the Research Councils and Institutions, with links to international networks.
JPEG (Joint Photographic Experts Group): A standard for lossy compression of graphic-image files.
JSP Jackson Structured Programming. A method of using structure diagrams for analysing data and programs.
KILOBYTE: A standard quantity measurement for disk and diskette storage and semiconductor circuit capacity: one kilobyte of memory equals 1024 bytes (8-bit characters) of computer memory.
LOCAL AREA NETWORK (LAN) the cable interconnection of items of computing equipment over a small local area such as a single building or site. Such systems enable the sharing of data, software and equipment resources.
LOGICAL OPERATOR the Boolean operations such as AND, OR etc.
Logical Security is the features built into a network to control access and the usage of network facilities, features include user permissions to access files and directories, password controls and more recently the use of biometrics to identify users.
See
MACRO a program written using application software tools to automate a sequence of keystrokes or events. Simplified ways of creating such programs are often provided (eg recording feature)
MAIL MERGING combining a master file with a secondary file containing variable data such as names an addresses, to produce multiple documents - each of which contains the same master information but is addressed to a different addressee.
MAINFRAME: A large-scale computer system that can house comprehensive software and several peripherals and also handle multiple users, usually in the hundreds.
MAN (Municipal Area Network): An extended network or cluster of networks serving a city, an academic or business campus, or any site featuring several widely separated buildings.
MAPPING: In network operations, the logical association of one set of values, such as addresses on one network, with quantities or values of another set, such as devices on another network (name-address mapping, internetwork route mapping, protocol-to-protocol mapping).
MARK: Presence of signal. In telegraph communication, a mark represents the closed condition or current flowing. A mark impulse is equivalent to a binary 1. Compare with space.
MEDIA: Anything used for the propagation or transmission of signals, usually in the form of electrons or modulated radio, light, or acoustic waves; examples include optical fiber, cable, wire, dielectric slab, water, air, or free space.see also Networking Terminology
MEMORY: see computer terms memory devices, memory access, memory hierarchy,
MINICOMPUTER: A small- or medium-scale computer, also called a "mini," usually operated with interactive dumb terminals. A mini can operate as a single powerful workstation or as a multiuser system.
MODE OF OPERATION (software) the way in which a software package enables user interaction, eg control key combinations, menu, object selection, function key control and the use of peripheral devices.
MODEL (computer) a software representation of a real situation or system which can be used for analysis of its operation. A simplified version of a process. Examples of models include: financial budgets with variable costs and profits, journey planning between geographical points using roads available, queues at checkout desks and the number of people waiting, traffic lights controlled by numbers of vehicles and pedestrians, producing three dimensional model of a building to investigate environmental effects on nearby surroundings, pilot simulation etc. see hypothesis testing. Programs used might be spreadsheet, CAD, Virtual reality and simulators
MODEM (modulate-demodulate) is a device that allows digital signals to be modulated by the transmitting modem for sending over the analogue PSTN telephone network and converted back to digital signals by the receiving modem. Its connect speed is measured in bps. Standard maximum speeds are 1200, 2400, 9600, 14400, 28800 and more recently 32K and 56K. ( see CCITT standards). Modems can be internal using a spare slot on the motherboard or connected externally via a serial COMS port. External modems often have status lights (led's) to monitor operations such as CD (carrier detect indicating that connection has been made), TD (transmitting data) and RD (receiving data).
MODEM ELIMINATOR: A device used to connect a local terminal and a computer port in lieu of the pair of modems that they would ordinarily connect to; allows DCE-to-DTE data and control signal connections otherwise not easily achieved by standard cables or connectors in a synchronous environment.
MODULATION: Varying the characteristics of a wave-frequency, amplitude, or phase - to match those of another wave. This is done by varying one or more of the signal's basic characteristics: frequency, amplitude, or phase.
MODES OF COMMUNICATION see 'Transmission modes'
MOTHERBOARD: The main board in a computer that holds the CPU chip, the ROM, the RAM, and sometimes coprocessors.
MULTIMODE FIBRE: An optical fiber designed to carry multiple signals, distinguished by frequency or phase, at the same time; compare with single-mode fiber. see fibre optic optic fibre FDDI
MULTIPLEXER: in telecommunications, a device that allows a transmission medium to carry a number of separate signals at the same time - enabling, for example, several telephone conversations to be carried by one telephone line, and radio signals to be transmitted in stereo.
In FDM (frequency-division multiplexing), by splitting the frequency band into narrower bands (see frequency-division multiplexor) signals of different frequency, each carrying a different message, are transmitted, or by allotting a common channel to several different transmitting devices one at a time (see time-division multiplexor).
Electrical frequency filters separate the message at the receiving station. In time-division multiplexing, the messages are broken into sections and the sections of several messages interleaved during transmission. Pulse-code modulation allows hundreds of messages to be sent simultaneously over a single link.
NETWORK: in computing, a method of connecting computers so that they can share data and peripheral devices, such as printers. The main types are classified by the pattern of the connections - star or ring network, for example - or by the degree of geographical spread allowed; for example, local area networks (LANs) for communication within a room or building, and wide area networks (WANs) for more remote systems. Internet is the computer network that connects major English-speaking institutions throughout the world, with around 12 million users. Janet (joint academic network), a variant of Internet, is used in Britain. SuperJanet, launched 1992, is an extension of this that can carry 1,000 million bits of information per second.
One of the most common networking systems is Ethernet,
developed in the 1970s (released 1980) at Xerox's Palo Alto Research Center,
California, by Rich Seifert, Bob Printis, and Dave Redell.
NIC (Network interface controller or Network interface card): Electronic circuitry that connects a workstation to a network; usually in the form of a card that fits into one of the expansion slots inside a personal computer. It works with the network software and computer operating system to transmit and receive messages on the network.
NETWORK LAYER (Layer 3 in the OSI model); the logical network entity that services the transport layer; responsible for ensuring that data passed to it from the transport layer is routed and delivered through the network.
NOS (Network operating system): The software used to connect devices, share resources, transfer files, and perform network activity. Usually, there are two parts to a network operating system: server and workstation (requester).
NETWORK TOPOLOGY: The physical and logical relationship of nodes in a network; the schematic arrangement of the links and nodes of a network; networks typically have a star, ring, tree, or bus topology, or some hybrid combination.
NETWORK UTILITIES: Programs that handle routine procedures such as troubleshooting.
NODE: A termination point for two or more communications links. The node can serve as the control location for forwarding data among the elements of a network or multiple networks, as well as performing other networking and, in some cases, local processing functions. A node is usually connected to the backbone network and serves end points and/or other nodes.
NOISE: Random electrical signals, generated by circuit components or by natural disturbances, that corrupt the data by introducing errors. Unwanted signals that disrupt transmission. Noise can be caused by EMI (electro-magnetic interference) or RFI (radio frequency interference) see crosstalk
NRZ (Non Return to Zero): A binary coding technique that does not return to a neutral state after each bit is transmitted.
NULL MODEM a cable used to enable two computers to communicate with each other by emulating a modem. The null modem cable is wired up so that the end connections from one to the other are: 2 connected to 3; 4 and 5 connected to 8; 6 connected to 20. (see RS232)
OPERATING SYSTEM the software program which provides the environment in which application programs can be used. The operating system controls the operations of handling: input, output, interrupts, storage and file management.In computing, a program that controls the basic operation of a computer. A typical OS controls the peripheral devices, organises the filing system, provides a means of communicating with the operator, and runs other programs.
Some operating systems were written for specific computers, but some are accepted standards. These include CP/M (by Digital Research) and MS-DOS (by Microsoft) for microcomputers. Unix (developed at AT&T's Bell Laboratories) is the standard on workstations, minicomputers, and supercomputers; it is also used on desktop PCs and mainframes. Windows 98 is now considered an operating system although windows was developed as a graphical user interface GUI to assist people use DOS. Dos is now integrated into windows to such a degree that windows is now classed as the operating system. Even so access can be made by opening a DOS window should the user need to access DOS.
In Nov 1994 IBM, Apple, and Motorola announced a joint
venture to produce a ‘common reference platform’ personal computer, capable
of running virtually all existing operating systems.
OPTICAL FIBRES: Very fine, optically pure glass fibre through which light can be reflected to transmit images or data from one end to the other. Although expensive to produce and install, optical fibres can carry more data than traditional cables, and are less susceptible to interference.
Optical fibres are increasingly being used to replace
metal communications cables, the messages being encoded as digital pulses
of light rather than as fluctuating electric current. Bundles of optical
fibres are also used in endoscopes to inspect otherwise inaccessible parts
of machines or of the living body (see endoscopy).
see also Total internal reflection,FDDIfibre
optic transmission mediagraded-index
fibreBT
Factfile No. 9
OSI (Open systems interconnection) is a recognised standard and comprises of seven layers:
PABX (Private Automatic Branch Exchange): A user-owned, automatic telephone exchange that accommodates the transmission of calls to and from the public telephone network.
PACKET: A sequence of data, with associated control information, that is switched and transmitted as a whole; refers mainly to the field structure and format defined with the CCITT X.25 recommendation.
PACKET HEADER: In packet-switched networks, the first three octets of an X.25 packet.
PACKET TYPE IDENTIFIER: In X.25 packet-switched networks, the third octet in the packet header, which identifies the packet's function and, if applicable, its sequence number.
PACKET-SWITCHED NETWORK:
A
data-communications network that transmits packets. Packets from
different sources are interleaved and sent to their destination
over virtual circuits. The term includes
PDNs and cable-based LANs.
PAD (Packet Access Device): An interface between a terminal or computer and a packet-switching network.
PAL (Phase Alternation by Line): A standard for color broadcasting used mostly in Europe. This system avoids the color distortion that appears in the NTSC systems.
PAM (Pulse amplitude Modulated) Sampling the amplitude of an analogue signal and giving it a digital code. see PCM
PARALLEL PORT provides a connection for transmitting 8-bits of data at simultaneously. Typically used to send data to a printer. Uses a 25-pin conector. Because the wires that carry data run parallel to one another for the full length of the cable, the signals tend to interfere with one another over long distances. Therefor the cables are only reliable over relatively short distances (from 10-50 ft). The speed that signals travel down each individual wire may also differ and therefore not always arrive at the distrant end simultaneously, this is called "Skew" and can introduce errors. IBM PCs generally allow up to 3 ports LPT1, LPT2 and LPT3.
PARALLEL PROCESSING: Concurrent or simultaneous execution of two or more processes, or programs, within the same processor, as contrasted with serial or sequential processing.
PARALLEL TRANSMISSION: Transmission mode in which a number of bits of information are sent simultaneously over separate lines (for example, eight bits over eight lines); usually unidirectional. Compare with serial transmission.
PARITY a method for detecting errors during transmission of data. The addition of an eighth bit to keep the number of binary ones in the envelope of transmission either even or odd. parity bit parity check
PARITY BIT: A bit that is set at 0 or 1 in a character to ensure that the total number of one bits in the data field is even or odd, as desired. parity check parity
PARITY CHECKING: An error-detection technique in which character bit patterns are forced into parity, so that the total number of one bits is always odd or always even. This is accomplished by the addition of a one or zero bit to each byte, as the byte is transmitted; at the other end of the transmission, the receiving device verifies the parity (odd or even) and the accuracy of the transmission.
PBX (Private Branch Exchange): A manual, user-owned telephone exchange.
PCI(R) (Peripheral Component Interconnect) bus: A local-bus standard created by Intel.
PCL(R) (Printer Control Language): A control language
for Hewlett-Packard and compatible
laser printers.
PCM (Pulse
Code Modulation): A modulation technique used
to convert analogue voice signals into
digital form. The signal amplitude is sampled and coded
(PAM) Used for voice multiplexing on T1 circuits.
see digitiseAnalogue-Digital
convertor
PCMCIA (Personal Computer Memory Card International Association): Defines the format and interface for a credit card six device for use with portable or laptop computers. There are 3 types. Type 1, the thinnest at 3.3 mm, is normally used for memory type enhancements. Type 2, which is 5 mm thick, is for modem and LAN enhancements. Type 3, 10.5 mm thick, is for mass storage I/O.
PCS (Personal Conferencing Specification): A standard for desktop videoconferencing system developed by Intel. Uses the Indeo algorithm.
PDN (Packet Data Network): A network established and operated by a PTT, common carrier, or private operating company for the specific purpose of providing data communications services to the public. May be a packet-switched network or a digital network such as DDS.
PEER-to-PEER; networking, see Networking terminology page
PGA (Professional Graphics Adapter): A video standard for IBM PC/XT and AT or compatibles. It offered better relolution than EGA, but has been largely superseded by VGA.
PHASE MODULATION: One of three ways (see AM and FM) of modifying a sine wave signal to make it carry information. The sine wave or carrier has its phase changed in accordance with the information to be transmitted.
PHYSICAL LAYER (Layer 1 in the OSI model): Within the OSI model the lowest level of network processing, below the link layer; concerned with the electrical, mechanical, and handshaking procedures over the interface that connects a device to a transmission medium; an electrical interface.
PIXEL the smallest element of an image which can be displayed on a visual display screen.
POLLING: A communications technique that determines when a terminal is ready to send data. The computer continually interrogates all of its attached terminals in a round-robin sequence. A terminal acknowledges the poll when it has data to send.
PORT the external connection
point on a computer to which peripheral devices are attached.
See also Serial, Parallel,
mouse and SCSI (scuzzy).
POTS (Plain Old Telephone Service): The basic service provided by the public telephone network, without any added facilities such as conditioning.
PRESENTATIONAL GRAPHICS bit mapped graphic images which are used to present information such as pictures or electronic artwork. Elements of an image such as lines cannot be edited as an entity, ie moved and rotated etc.
PRESENTATION LAYER: In the OSI model, the layer of processing that provides services to the Application Layer, allowing it to interpret the data exchanged, as well as to structure data messages for transmission in a specific display and control format.
PRIMARY STORAGE storing data and instructions in a computer's ROM and RAM.
PRINT DRIVER: A driver that manages an application's communication with a printer
PRINTER SERVER the computer which contains the printer server software for a LAN and controls the printer queue. see also communications server
PRIVATE WIDE AREA NETWORK a network which uses privately leased or owned lines and does not offer the general public connection facilities, although it may provide network services to subscribers. Many of the well-known service providers offer these facilities and enable access through gateways from networks, eg Internet, JANET, Compuserve.
PROGRAMS see Sue's page
PROM (Programmable Read-Only Memory):
A
nonvolatile memory chip that allows a program
to reside permanently in a piece of hardware; compare
with volatile memory.
PROTOCOLS is an agreed
set of rules and can relate to hardware as well as software. OSI
The procedures used to control the orderly exchange of information between
stations on a data link or on a data-communications network or system.
Protocols specify standards in three areas: the code set, usually ASCII
or EBCDIC; the transmission mode, usually asynchronous
or synchronous; and the non-data exchanges of information by which the
two devices establish contact and control, detect failures or errors, and
initiate corrective action.
I
PSEUDO CODE see
'Structured English'
PSTN (Public Switched Telephone Network): The telephone system over which calls may be dialed.
PUBLIC NETWORK: A network
operated by common carriers or telecommunications administrations
for the provision of circuit-switched, packet-switched,
and leased-line circuits to the public.
PUBLIC SWITCHED NETWORK
: Any switching communications system, such as Telex, TWX, or public
telephone networks, that provides circuit switching to
many customers.
PUBLIC WIDE AREA NETWORK a network which is generally intended to be accessible to the general public, eg telephone and cable TV networks which are wired up to many homes and may be used in a variety of ways. See 'Public switched telephone network', 'Public switched data network' and 'Integrated digital services network'
Pulse-Code Modulation (PCM)
(PCM) In physics, a form of digital
modulation in which microwaves or light waves (the carrier waves) are switched
on and off in pulses of varying length according to a binary code. It is
a relatively simple matter to transmit data that are already in binary
code, such as those used by computer, by these means. However, if an analogue
audio signal is to be transmitted, it must first be converted to a pulse-amplitude
modulated signal (PAM) by regular sampling of its amplitude.
The value of the amplitude is then converted into a binary code for transmission
on the carrier wave.
QBE query by example (relational).
A simplified way of entering SQL queries, enabling users to enter the query
using a menu or keystroke sequence which is automatically converted inot
an SQL command. see 'SQL'
RAID (Redundant Array of Inexpensive Disks): A method of storing data on multiple hard-disk drives, for faster access, greater reliability, or both. There are six officially defined "levels," each designed for a specific kind of application.
RAM (Random-Access
Memory): Semiconductor read/write volatile memory.
Data stored is lost
if power is turned off. It is where data is stored while
being processed by the microprocessor.
RD (Received Data): An RS-232 data signal (received by DTE from DCE on pin 3).
REPEATER: In digital transmission, equipment that receives a pulse train, amplifies it, retimes it, and then reconstructs the signal for retransmission; in fiberoptics, a device that decodes a low-power light signal, converts it to electrical energy, and then retransmits it via an LED or laser source. Also called a "regenerative repeater."
RGB: Red, Green, Blue; a video standard in which the color signals for red, green, and blue are carried on separate lines, then combined to form a color video picture. Horizontal and vertical sync are imposed on one of the colors, usually green.
Retina Scanner a device for identifying a user by mapping the shape of their eyeball.
RISC (Reduced Instruction Set Computing): Internal computing architecture in which processor instructions are pared down so that most can be performed in a single processor cycle, theoretically improving computing efficiency.
A Null modem cable is an RS232 cable with modified pin functions (see null modem). It allows communications to be setup between two machines in the same room. Each machine requires terminal software (eg. Laplink)
ROUTER: Device that enables dissimilar networks (such as Ethernet and Token Ring) to interconnect, provided there is a common protocol. Routers can be programmed to determine the fastest or most cost-effective route for data to travel. Operates at the Network Layer of the OSI model. See also brouter bridgegateway
Routing: The process of selecting the correct circuit path for a message.
RS-422 , RS-423: EIA serial transmission standard that extends transmission speeds and distances beyond those of RS-232. RS-422 is a balanced system with a high level of noise immunity. RS-423 is an unbalanced version of RS-422.
RS 232C an RS232 cable is a serial cable, which connects on of the computer's serial I/O ports (RS232 ports) to a modem. The connector at the modem end is usually a 9-pin D-connector, whilst the computer connector could be either a 9-pin or 25-pin D-connector. see carrier detect control signalclear to send data terminal readyTransmitted data receive data null modem
RS-449: RS-449 specifies the pinning for RS-422 and RS-423 when a DB37 or DB9 connector is used.
RS-485: EIA serial interface standard for multipoint lines. Unlike its predecessors (RS-232, RS-422, etc.), RS-485 uses a tri-state driver. The third state the other interfaces lack is an off condition that allows equipment to communicate over shared wiring while another device sits quietly. It's tolerant to electrical noise, which makes it ideal for industrial users like manufacturing facilities and airports. And RS-485 supports long cabling distances up to 4000 feet (1219.2 m) at 100 Kbps.
RS-530: Specifies the pinning for balanced interfaces such as RS-422, when a DB25 connector is used. Designed to replace RS-449.
RTS (Ready To Send): An RS-232 signalling lead generated by the DTE. In a switched circuit (half-duplex) the DTE raises RTS in a bid for line control. It then receives a verify signal (CTS) from the DCE when it is Clear To Send (CTS).
SCSI (Small Computer System Interface pronounced scuzzy) provides a connection for up to seven devices and is used for attaching hard disk drives, CD-ROMs, scanners and tape drives.
Secondary Storage see also primary storage
SERIAL PORT sometimes referred to as the RS232C port provides a connection for transmitting data one bit at a time. IBM computers allow for up up to four ports COM1, COM2, COM3 and COM4, which may be built into the main board or mounted on an expansion card. (either 9-pin or 25-pin). Because a serial cable is less susceptible to interference than prallel cable it is used to connect cables that are located some distance apart.
SERVER see Network terminology page, printer server and communication server
SIMPLEX: only allows the transmission of data in one direction between two devices. see also duplex half duplex and echoplex
SOFTWARE the programs which enable computers to operate; instructions to a computer. Software can generally be classified according to the following different types: operating systems, applications, utilities (editors, diagnostics, file management), use interface, program generators, system services (database management, translators), videotex systems, coding and programming languages, system analysis (CASE), local and wide area networking, training and learning (CBL), testing and assessment, games and leisure applications.
SOFTWARE FACILITIES facilities within software packages which enable users to perform tasks more effectively and efficiently. Examples include macros, mail merge, pagination and page headers or footers.
SQL structured query language. A special language designed to enable trained users to enter English-type queries onto an electronic database to extract the information they require.
Storage see primary storagesecondary storage
STRUCTURED ENGLISH a shorthand form of the English language which is used to define elements of a computer program. Sometimes called 'Pseudo code'
STRUCTURED PROGRAMMING a method of programming which uses the principles of a hierarchy of components related to the three basic constructs, a modular approach and modules which have restricted entry and exit points.
SYNCHRONOUS: Regular. Most communication within a computer system is synchronous, controlled by the computer's own internal clock, while communication between computers is usually asynchronous. Synchronous telecommunications are, however, becoming more widely used.
SYSTEM CONFIGURATION (LANs) the setting of passwords, rights, access to servers and printer queues.
TCP/IP
(Transmission Control Protocol/Internet Protocol): A
layered set of protocols that
allows sharing of applications among PCs, hosts, or workstations
in a high-speed communications
environment. Because TCP/IP's protocols are standardized
across all its layers, including those that
provide terminal emulation and file transfer, different
vendors' computing devices (all running
TCP/IP) can exist on the same cable and communicate with
one another across that cable.
Corresponds to Layers 4 (Transport) and 3 (Network) of
the OSI reference model.
TD (Transmitted Data): An RS-232 data signal (sent from DTE to DCE on pin 2).
TDM (Time Division Multiplexor): A device that accepts multiple channels on a single transmission line by connecting terminals, one at a time, at regular intervals, interleaving bits (Bit TDM) or characters (Character TDM) from each terminal. see fdm
TDMA (Time-Division Multiple Access): A high-speed, burst mode of operation that can be used to interconnect LANs; first used as a multiplexing technique on shared communications satellites.
TECHNICAL DRAWING a graphic representation produced using vector-based software which enables entities to be edited individually and the whole drawing to be scaled and manipulated. Such drawings could be layouts such as a building floor or car park layout or detailed drawings of artefacts used for manufacture.
TELNET(tm): A virtual terminal service available through the TCP/IP protocol suite. Allows remote operation of another server. Terminal emulation.
TEMPLATE an electronic file which holds a standardised document layout or screen format, eg letter type, position of references and location of addressee details. The template can also hold style data, variable data or macros. Templates also refer to overlays for keyboard keys to indicate their action when used with a particular application.
ter: The third version of a CCITT recommendation. see CCITT
TERMINAL EMULATION the type of terminal selected will set the character and control codes used in transmission and reception. Three or four standards are commonly used. Telnet
TERMINATORS:
Terminators are required on networks to prevent an echo
effect from signals. As an electrical signal travels along a media
it can reflect back along the media causing unwanted signals (noise).
Usually networks are terminated with a 50-ohm resistor which dampens any
such reflections. (see echo)
THICK ETHERNET, Thicknet: Standard 10BASE5 Ethernet; see 10BASE5.
THIN
ETHERNET: An Ethernet LAN or IEEE, 802.3 LAN
that uses a smaller-than-normal diameter
coax; often used to link IBM personal computers together.
Operates at same frequency as Ethernet
but at smaller distances. Also known informally as "Cheapernet"
TIME SLOT: An assigned period of time or an assigned position in a sequence.
TOKEN: A continuously repeating frame, transmitted onto the network by the controlling computer; the frame that polls for network transmissions. See token bus, token passing, token ring.
TOKEN BUS: A LAN-access mechanism and topology in which all stations actively attached to the bus listen for a broadcast token or supervisory frame. Stations wishing to transmit must receive the token before doing so. The next physical station to transmit is not necessarily the next physical station on the bus, bus access is controlled by preassigned priority algorithms.
TOKEN PASSING: A LAN protocol in which the network continuously circulates a bit pattern known as a token. Only the workstation holding the token can put a message onto the LAN.
TOKEN RING: A network access mechanism and topology in which a supervisory frame or token is passed from station to station in sequential order. Stations wishing to gain access to the network must wait for the token to arrive before transmitting data. In a token ring, the next logical station receiving the token is also the next physical station on the ring. The standard was developed by IBM and ratified as an IEEE standard, 802.5. Token Ring interconnects PCs via special twisted-wire cable in a star topology, connecting all computers to a central wiring hub.
TOPOLOGY the topology of a network refers to the way it is organised. Examples are bus, ring, star, mesh etc.
TOTAL INTERNAL REFLECTION; the complete reflection of a beam of light that occurs from the surface of an optically ‘less dense’ material. For example, a beam from an underwater light source can be reflected from the surface of the water, rather than escaping through the surface. Total internal reflection can only happen if a light beam hits a surface at an angle greater than the critical angle for that particular pair of materials.
Total internal reflection is used as a means of reflecting light inside prisms and optical fibres. Light is contained inside an optical fibre not by the cladding around it, but by the ability of the internal surface of the glass-fibre core to reflect 100% of the light, thereby keeping it trapped inside the fibre. see optical fibre
TRANSACTION PROCESSING; the type of 'real time' data processing system which handles one transaction at a time. The system ensures that other users are locked out of the records being used, so that when a transaction is completed it is secure.Data (transactions) are processed as they occur - with no primary editing or sorting.
TRANSMISSION MEDIA the form of communication link used to communicate between two or more stations. EG. Radio/TV broadcasts, copper cable (twisted pair/coax), fibre optic (FDDI), microwave, infra-red, satellite, cellular telephone network. see also Networking terminology
TRANSMISSION MODES there are several configuration settings which affect the mode of transmission.
Flow control and cable provisions affect the type of exchanges between stations. Simplex is a one-way mode of transmission; duplex is two way; half duplex enables two-way transmission but not for each station simultaneously; asymmetric duplex is two-way but with different speeds each way. Asynchronous transmission sends a character at a time with markers either end; synchronous systems are used to speed up this process and send whole blocks of data at a time by timing the start and end of the block. Cable types and facilities affect the type of transmission. Serial transmission sends each bit of data down the same cable in sequence; parallel transmission enable the eight data bits to travel along separate cables simultaneously.
TRANSMISSION RATE expressed in bits per second (bps), this rate depends on transmission media, eg cable type (copper/fibre) or modem facilities. Baud is the signalling rate and is sometimes the same as the bps, but the bps can be three or four times the baud rate.
TRANSPORT LAYER: In the OSI model, the network processing entity responsible, in conjunction with the underlying Network, Data Link, and Physical Layers, for the end-to-end control of transmitted data and the optimized use of network resources.
TWISTED PAIR copper cable. The twist is to cut down on the interference normally experienced when wires run adjacent to one another over relatively long distances. This type of cable is used in telecommunications for local distribution. Twisted pair cable is usually connected by an RJ45 plug. One of the main features of FDDI fibre optic is that as it uses light, the signal is not affected by electro-magnetic radiation as in copper cable. see also UTP, transmission media, 10baseT , coaxial cable, fibre optic cable, category3/4,capacitance, attenuation, crosstalk
Type 1 cable: Two twisted pairs of 22 AWG solid conductors, enclosed in a braided cable shield; associated with the IBM cabling system.
Type 3 cable: 22 or 24 AWG with at least two twists
(unshielded) per linear foot. Four twisted pairs are recommended when installing
a new wire. Associated with the IBM cabling system.
UNICODE
A 16-bit code that represents over 65,000 different characters.
Theoretically unicode can represent every character in every current day
language as well as languages that are no longer used. Whereas
ASCII defines 128 characters used in English; ANSI represents additional
characters for many of the European character accents; EBCDIC represents
the alphabet and an assortment of codes; however non of these codes support
alternate character sets such as Hebrew, Cyrillic, Arabic, Japanese, chinese
etc. see also other character sets
UNIX(R): An operating system originally designed by AT&T for communicating multiuser, 32-bit minicomputers; has come into wide commercial acceptance because of its predominance in academia and its programming versatility. Other versions include AIX(R) and XENIX(R).
UTP (Unshielded Twisted Pair)
- see twisted pair cabling
for connecting workstations on a network. 22-24 gauge, inexpensive,
150 ohm cable. Ideal for short distances but is subject to crosstalk,
capacitance
and inductance. see attenuation.
V.10: A CCITT interface recommendation; electrically similar to RS-423 (unbalanced, high speed).
V.11: A CCITT interface recommendation; electrically similar to RS-422.
V.17: A CCITT 14,400-bps leased-line recommendation; used for 14,400-bps fax communication.
V.21: A CCITT 300-bps dial-line modem recommendation; similar to RS-422.
V.22: A CCITT 1200-bps dial- or 2-wire leased-line modem recommendation; similar to Bell 212.
V.22 bis: A CCITT 2400-bps dial or 2-wire leased-line modem recommendation.
V.23: A CCITT 600/1200-bps dial-line modem recommendation; similar to Bell 202.
V24 is the CCITT's version of RS232C. The V24 standard defines the flow control standards but does not define the physical connection whereas the RS232 standard does.
V.25: A CCITT dial-line parallel-interface recommendation.
V.25 bis: A CCITT dial-line serial-interface recommendation.
V.26: A CCITT 2400/1200-bps leased-line modem recommendation; similar to Bell 201 B.
V.26 bis: A CCITT 2400/1200-bps dial-line modem recommendation; similar to Bell 201 C.
V.26 ter: A CCITT 2400-bps dial- or 2-wire leased-line modem recommendation.
V.27: A CCITT 4800-bps leased-line modem recommendation with manual equalizer; similar to Bell 208 A.
V.27 bis: A CCITT 4800-bps dial-line modem recommendation with automatic equalizer.
V.27 ter: A CCITT 4800-bps dial-line modem recommendation; similar to Bell 208 B; used for 4800-bps fax communication.
V.28: A CCITT interface recommendation that defines electrical characteristics for the interchange circuits defined by V.24; similar to and operationally compatible with RS-232.
V.29: A CCITT 9600-bps leased-line modem recommendation; similar to Bell 209; used for 9600-bps fax communication.
V.32: A CCITT 9600-bps dial- or 2-wire leased-line modem recommendation.
V.32 bis: A CCITT 14.4-Kbps dial- or 2-wire leased-line modem recommendation.
V.34: A CCITT modem standard (not yet ratified as this is written) for data transmission at 28.8 Kbps.
V.35: A CCITT interface standard for high-speed communication. V.35 specifies a 34-pin connector and can transmit at speeds into the millions of bits per second. It can't connect, physically or electrically, to any other interface without the aid of a converter.
V.36: A CCITT interface for 4-wire data communication at speeds of 48 Kbps and up; designed to replace V.35.
V.42: A CCITT standard for an error-correction protocol for modems using async-to-sync conversion; defines LAP-M protocol.
V.42 bis: A CCITT standard for a protocol, built into a modem's firmware, that provides both error correction and data compression.
V.54: A CCITT standard for modem loopback tests.
V.110: Async rate adaption (for ISDN) at speeds up to 19.2 Kbps.
V.120: Async rate adaption (for ISDN) at speeds up to 64 Kbps; uses statistical multiplexing.
VAN (Value Added Network): A network whose services go beyond switching.
VECTOR GRAPHICS images are drawn by setting coordinate points, or vectors. Two-dimensional (2D) drawings require x-coordinates (horizontal position) and y-coordinates (vertical position). Three-dimensional (3D) drawings require a third z-coordinate (to provide depth)
When a vector image is re-scaled there is no loss of definition.
VESA (Video Electronic Standards Association): A video-industry group that promotes standardization.
V.Fast: A preliminary version of V.34.
VGA (Video Graphics Array): A video standard for IBM PC and compatible computers. Standard VGA has a resolution of 640 x 480 and supports 16 colors.
Video teleconferencing: The use of "television-type" transmissions to allow people at two or more locations to communicate as if they were in the same meeting.
Videotex: A data-communication service that transmits on TV channels.
Virtual Realityis one of the hottest research and development areas in the computer industry today. Its potential applications range from medical imaging and interior design to intercontinental video-conferencing and the exploration of future worlds. There are a number of ways in which virtual reality technology can be employed; its underlying premise, however, is to create more intuitive ways for humans and computers to work together ....................click on link to topic
Voice-frequency: The part of the audio frequency range that can transmit commercial-quality speech.
Volatile memory: A storage medium that loses all data when power is removed.
VRAM (Video RAM): RAM optimized for video processing.
WAN (Wide-Area Network):A network that serves an area of hundreds or thousands of miles, using common- carrier-provided lines; contrast with LAN. Wavelength: Distance between successive peaks of a sine wave.
WAVELENGTH: Distance between successive peaks of a sine wave.
WIDEBAND: A system in which multiple channels access a medium (usually coaxial cable) that has a large bandwidth, greater than that of a voice-grade channel; typically offers higher-speed data-transmission capability. Also see broadband.
WORKGROUP: A group of people working together on the same project, especially when they have their own dedicated network.
WORKSTATION: 1) Input/output equipment at which an operator works. 2) A high-end desktop computer using RISC processors, often running the UNIX or XENIX operating system.
WWW (World Wide Web): A type of Internet browsing tool.
WORM: A self-replicating program that consumes processor time but can't destroy data, software, or other sytem resources.
WORM (Write Once, Read Many): A common type of optical disk drive. The disk can be written to only once; after that, the data is permanently stored there.
WYSIWYG: What You See Is What You Get - a description
of computer software whose screen
display is nearly identical to its printed output.
X Terminal: A dedicated platform (terminal) that is designed to run X-server software; used with powerful machines that run 680 x 0 processors such as RISC computers. X is a standard baselevel windowing architecture for UNIX machines and their terminals.
X.121: A CCITT recommendation that defines general-purpose interface between data terminal equipment for synchronous operation on public data networks.
X.21: A CCITT standard governing the interface between DCE and DTE for synchronous operation on public data networks.
X.21 bis: In PDNs, a CCITT recommendation that defines the most popular digital interface; it's equivalent to RS-232 and V.24.
X.25: The standard interface for packet-switched data communications networks, as designated by the CCITT. see also packet switched network packet packet header packet type identifier
X.25 Packet Assembler/Disassembler (PAD): A device that permits communication between non-X.25 devices and the devices in an X.25 network.
X.26: Same as V.10.
X.27: Same as V.11.
X.28: In packet-switched networks, a CCITT recommendation that defines the interchange of commands and responses between a PAD and its attached asynchronous terminals.
X.29: In packet-switched networks, a CCITT recommendation that defines the use of packets to exchange data for control of remote PADs.
X.3: In packet-switched networks, a CCITT recommendation that defines the parameters that determine the behavior of the interface between a PAD and its attached asynchronous terminals.
X.400: A CCITT standard for a messaging and document distribution protocol to connect different email systems.
XENIX: Microsoft trade name for a 16-bit microcomputer operating system derived from UNIX.
XGA (Extended Graphics Array): A video standard for Micro
Channel 386SX or better PCs.
Resolution is 1024 x 768 pixels, and 256 colors are supported.
XGA's busmastering boosts
performance by giving its graphics coprocessor access
to system RAM, not just video RAM.
X-on X-off Software data flow controls. X-ON/X-OFF (Transmitter On/Transmitter Off): Control characters used for flow control, instructing a terminal to start transmission (X-ON) and stop transmission(X-OFF).
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