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D/A - Digital to analog.

DAC (digital to analog converter) - A device that converts a digital input to an analog output signal carrying equivalent information.

DACS - Digital access and cross-connect system.

Daisy chaining - The connection of multiple devices in a serial fashion. An advantage of daisy chaining is a savings in transmission facilities. A disadvantage is that if a device malfunctions all of the devices Daisy chained behind it are disabled.

DASD - Direct attached storage device.

DATA - Digitally represented information including voice, text, images and video.

Database - A large, ordered collection of information.

Data center - A building or a portion of a building whose primary function is to house a computer room and its support areas.

Data compression - Packing data into a reduced format. Compressed data are in “short-hand” form and must be decompressed before it can be used by the receiving computer.

Datagram - A packet that includes a complete destination address specification (provided by the user, not the network) along with whatever data it carries.

Data integrity - A measure of data communications performance, indicating a scarcity (or, ideally, the absence of) undetected errors.

Data rate - A measure of the signaling rate of a data link.

dB - Decibel. The standard unit used to express the relative strength of two signals. When referring to a single signal measured at two places in a transmission system, it expresses either a gain or loss in power between the input and output devices. The reference level must always be indicated, such as one mill watt for power ratio.

DBC - A measure of spurious signal level. The level is measured relative to the nominal un-modulated carrier level.

DBM - Absolute measure of signal power where 0 DBM is equal to one mill watt. Contrast with dB.

dBmV (decibel mill volt) - The level at any point in a system expressed in dBs above or below a one mill volt/75 ohm standard is said to be the level in decibel- mill volts or dBmV. Zero dBmV is equal to one mill volt across 75 ohms.

DB-9 (RS-449) - A 9-conductor EIA cable assembly used for interfacing VF lines to printers, PCs and all data compatible operations.

DB-15 (RS-422/RS-423) - A 15-conductor EIA cable assembly used as drop cable for Ethernet local area networks and for interfacing all freestanding a sync peripheral equipment, such as PCs, controllers, etc.

DB-25 (RS-232) - A 25-conductor round or flat EIA cable assembly used to interface most
IBM DEC, local area networks and all a sync compatible systems, such as connects mainframes, controllers, PCs, modems, etc.

DB-37 (RS-449) - A 37-conductor EIA cable assembly that connects data terminal equipment (DTE) and data circuit terminating equipment (DCE) employing binary serial data interchange.

DB-50 (RS-422/RS-423) - A 50-conductor EIA cable assembly meets interface needs of all high-volume circuit systems. Same standards as RS-232 connect controllers, PCs, modems, etc.

DC - Direct current. (See CURRENT, DIRECT.)

DC resistance - See RESISTANCE.

DC type lens - An auto iris lens with internal circuit that receives voltage and a video signal from the camera to adjust the iris.

DCE (data communications equipment) - In common usage, synonymous with modem; the equipment that provides the functions required to establish, maintain, and terminate a connection as well as the signal conversion required for communications between the DTE and the telephone line or data circuit.

DCL - Data carrier level.

DCS - Distributed control system. A type of industrial automation system in which the processors are distributed in various locations though out the facility.

DDA (direct digital access) - A 56 kbps digital data access through a 4 ESS switch.

DDNS (dynamic DNS) - A service that allows you to automatically (dynamically) upload your current IP info that has been assigned via DHCP.

DDS (digital data system) - A network that transmits data signals point-to-point.

DecaBDE - Decabromodiphenyl ether. A type of brominated flame retardant sometimes used in wire and cable and other products. A type of polybrominated biphenyl ether (PBDE).

Decibel (dB) - One-tenth of a bell. It is equal to 10 times the logarithm of the power ratio, 20 times the log of the voltage ratio or 20 times the log of the current ratio. One decibel is the amount by which the pressure of a pure sine wave of sound must be varied in order for the change to be detected by the average human ear. The decibel can express an actual level only when comparing with some definite reference level that is assumed to be zero dB.

Delay skew - The difference in propagation delay between the fastest and slowest pair within the same cable sheath. Usually expressed in nanoseconds.

Demand – (1) The measure of the maximum load of a utility’s customer over a short period of time. (2) The load integrated over a specified time interval.

Demarcation point - A point where the operational control or ownership changes.

Demarcation strip - The terminal strip or block (typically a 66-block) that is the physical interface between the phone company’s lines and the lines going directly to your own phone system.

Demodulation - The process of separating a data (digital) signal from an analog carrier signal. Opposite of MODULATION.

Demultiplexing - The process of breaking a composite signal into its component channels; the reverse of multiplexing.

DEMUX - Demultiplexer.

Depth of field - The front to back zone in a field of view that is in focus in the televised scene.
With a greater depth of field, more of the scene, near too far, is in focus. Increasing the f-stop number increases the depth of field of the lens. Therefore, the lens aperture should be set at the highest f-stop number usable with the available lighting. The better the lighting the greater the depth of field possible. In other words, the depth of field is the area in front of the camera that remains in focus. The larger the f-number the greater is the depth of field.

Derating factor - A factor used to reduce the current carrying capacity of a wire when used in environments other than that for which the value was established.

DES (data encryption standard) - An encryption method originally developed by IBM in the 1970s.

Destination - Receiver of data; data sink.

Detector - A fiber optic device that picks up light from the fiber and converts the information into an electrical signal.

Device, as related to a workstation - An item such as a telephone, personal computer, or graphic or video terminal.

Device, as related to protection - A protector, a protector mount, a protector unit or a protector module.

Dew point-The temperature at which vapor starts to condense (liquefy) from a gas-vapor mixture at constant pressure.

DHCP (Dynamic Host Configuration Protocol) - A protocol that automatically assigns an IP address to a device requesting an IP address. A DHCP server automatically issues network devices with IP addresses when they connect to the network.

Diagnostics - Programs or procedures used to test a piece of equipment, a communications link or network, or any similar system.

Dial-up line - Your average everyday home or business phone line. See also LEASED LINE.

Dielectric - An insulating (non-conducting) medium.

Dielectric breakdown - Any change in the properties of a dielectric that causes it to become conductive. Normally the failure of insulation is because of excessive voltage.

Dielectric constant - The property of an insulation that determines the electrostatic energy stored per unit volume for unit potential gradient. It is expressed as a ratio. K for air is 1.0, while that for polyethylene is 2.2. Therefore, the capacitance of polyethylene is 2.2 times that of air. It is also referred to as specific inductive capacity or permittivity.

Dielectric dispersion - The change in relative capacitance due to a change in frequency.

Dielectric heating - The heating of insulating material when placed in a radio-frequency field, which is caused by internal losses during the rapid polarization reversal of molecules in the material.

Dielectric loss - The power dissipated in a dielectric as the result of the friction produced by molecular motion when an alternating electric field is applied.

Dielectric strength - The maximum voltage that insulation can withstand without breaking down; usually expressed as a gradient in V/mil (volts per mil). Polyethylene for example has a dielectric strength of about 800 V/mil.

Dielectric strength testing - A common test performed on electrical products that is often called hi-pot testing. A voltage higher than normal operating voltage is applied across the insulation. This test can increase product reliability by detecting faulty workmanship.

Differential amplifier - One that has two input signal connections in addition to the zero signal reference lead. The output is the algebraic sum of the instantaneous voltages appearing between the two input signal connections.

Digital - Discretely variable as opposed to continuously variable. Data characters are coded in discrete, separate pulses or signal levels. Contrast with ANALOG.

Digital line - The facility that carries the digital bit stream from one location to another.

Digital loopback - A diagnostic test that forms the loop at the modem’s DTE interface.

Digital PBX - A PBX that switches voice and data traffic as digital signals.

Digital signal - An electrical signal consisting of two discrete voltage levels, encoding information as a series of ones and zeros.

Digroup - A group of 24 customer channels.

DIN (Deutsches Institut fur Normung) - The German standard for many products.

DIN connected cable assembly 5 and 8 conductors - Used for interfacing the IBM keyboards and other compatible systems.

DIP (dual in-line pins) - Term used to describe the pin arrangement on an integrated circuit (IC) or a multiple (electric) switch.

DIP coating - An insulating coating applied to the conductor by passing the conductor through an applicator containing liquid insulating medium.

Diplex filter - A passive filter that combines and separates the inbound and outbound pass bands in a broadband system.

Direct burial cable - A cable installed directly in the earth.

Direct capacitance - The capacitance measured directly from conductor to conductor through a single insulating layer.

Direct connection - A connection between a terminal and a host computer that does not use terminal servers and Ethernet. Direct connections use the RS-232 or DEC423 interface.

Direction of lay - The lateral direction, designated as left-hand or right-hand, in which the wires of a conductor run over the top of the conductor as they recede from an observer looking along the axis of the conductor.

Directional coupler - A passive device used in a cable system to divide or combine unidirectional RF power sources.

Discrete access - In LAN technology, an access method used in star LANs; each station has a separate (discrete) connection through which it makes use of the LAN’s switching capability. Contrast with SHARED ACCESS.

Dispersion - The variation of the refractive index of an optical fiber with wavelength, causing light of different wavelengths to travel at different velocities in the fiber.

Display station, display terminal - A device consisting of a keyboard and video or CRT display.
In the IBM 3270 Information Display System, a 3278 is an example of a display station; in an
ASCII CRT terminal, it is an example of a display terminal.

Dissipation factor - Energy lost when voltage is applied across insulation. The cotangent of the phase angle between voltage and current in a reactive component. Dissipation factor is quite sensitive to contamination and deterioration of insulation. Also known as power factor (of dielectrics).

Distortion factor - An undesired change in waveform as the signal passes through a device.

Distortion level - (1) The ratio, measured in dB, of unwanted distortion to desired carrier. (2) Any unwanted electromagnetic component present on the desired RF modulated carrier. (3) The unwanted changes in signal or signal shape that occur during transmission between two points.

Distributed architecture - In LAN technology, a LAN that uses a shared communications medium; used on bus or ring LANs; uses shared access methods.

Distributed computing - The name of the trend to move computing resources such as minicomputers, microcomputers or personal computers closer to individual workstations. See also DISTRIBUTED PROCESSING.

Distributed processing - An arrangement that allows separate computers to share work in the same application program. Often erroneously used to mean distributed computing.

Distribution amplifier - A device that accepts a (video) signal and sends it out to a number of independent outputs.

Distribution cable - In a CATV system, the transmission cable from the distribution amplifier to the drop cable. In an electric power system, provides low-voltage service to the customer.

Distribution frame - A structure with terminations for connecting the permanent wiring of a facility in such a manner that interconnection by cross-connections may be readily made.

Distribution panel - A wiring board that provides a patch panel function and mounts in a rack.

Disturbed conductor - A conductor that receives energy generated by the field of another conductor or an external source such as a transformer.

Disturbing conductor - A conductor carrying energy whose field(s) creates spurious energy in another conductor.

DIW (D-Inside Wire) - Also called unshielded twisted pair (UTP). The standard wire originally designated for voice communications. Typically, DIW consists of four pairs of copper wire in the same sheath. Each pair is twisted around one another.

DLC - Digital loop carrier.

DNIS - Dialed number identification service.

DNS (Domain Name System [or Server/Service]) - A service that assigns and translates registered friendly names (registered domain names) into IP addresses. In large networks a domain name server is literally a “name” server. It associates and remembers given names to corresponding IP addresses. (i.e., a more easily remembered name such as “MDF camera” rather than by using a long IP address number like

DOCSIS (Data Over Cable Service Interface Specification) - A standard for cable modem products drafted in 1996. It was developed to ensure that cable modem equipment built by a variety of manufacturers is compatible, as dial-up modems are.

DOD (Department of Defense) - Part of the U.S. government executive branch that handles military matters, including data communications; responsible for some LAN associated protocols and standards such as TCP/IP.

Dose, integrated (nuclear power) - Cumulative radiation dosage over a given period of time.

Download - The process of loading software into the nodes of a network from one node or device over the network media.

Downstream - (1) On a ring network, the direction of data flow. (2) The direction on the cable from the head end to the modems.

Drain wire - An un-insulated wire in contact with a shield throughout its length, used for terminating the shield.

Drawing, wiring diagram - In wire manufacturing, pulling the metal through a die or series of dies to reduce diameter to a specified size. Shows how the devices are interconnected.

Drop- (1) The physical location of the end of an Ethernet transceiver cable. (2) A cable that leads from a faceplate to the distribution panel in a wiring closet. When the IBM Cable System is used with the IBM token-ring network, a drop may form part of a lobe. See LOBE. (3) Individual connections (sometimes called nodes) on a multipoint (also called multi-drop) circuit. (4) The coaxial cables which connect the taps to the devices on the plant floor.

Drop cable – (1) In a CATV system, the transmission cable from the distribution cable to a dwelling. (2) The smaller diameter flexible coaxial cable used for drops (RG-59, RG-6, RG-11).

DSF - Dispersion shifted fiber.

DSLAM (digital subscriber line access multiplexer) - On a DSL service it separates voice and data traffic at the central office.

DSR - Data Set Ready. One of the control signals on a standard RS-232-C connector. It indicates whether the data communications equipment is connected and ready to start handshaking control signals so that transmission can start.

DSS/BLF - Direct station selector/busy lamp field.

DSU (data service unit) - Device designed to transmit digital data on transmission facilities. Typically a device that interfaces DTE (data terminal equipment) to a line contacting a data port channel to allow digital communications without a modem. It is used with a CSU when the DTE lacks complete digital line interface capability or alone (i.e., without a CSU) when the DTE includes digital line interface capability.

DS-0 (Digital Service, level 0) - It is 64,000 bps, the worldwide standard speed for digitizing one voice conversation.

DS-1 (Digital Service, level 1) - It is 1.544 Mbps in North America and 2.048 Mbps elsewhere.
1.544 Mbps is the old Bell System standard and 2.048 is the CCTIT standard.

DS-1C (Digital Service, level 1C) - It is 3.152 Mbps in North America.

DS-2 (Digital Service, level 2) - It is 6.312 Mbps in North America.

DS-3 (Digital Service, level 3) - Term referring to the signaling rate of a T3 network: 44.736 Mbps.

DTE (data terminal equipment) - User equipment. The end-user machine (terminal, computer, controller, etc.) that plugs into a unit which is the termination point of the communications circuit (DCE).

DTR - Data terminal ready. An RS-232 modem interface control signal (sent from the DTE to the modem on pin 20) that indicates that the DTE is ready for data transmission and which requests that the modem be connected to the telephone circuit.

Dual cable - A two-cable system in broadband LANs in which the coaxial cable provides two physical paths for transmission, one for transmit and one for receive, instead of dividing the capacity of a single cable.

Duct - A pipe, tube or conduit through which cables or wires can be passed. Duct space is always at a premium. If you ever install a duct, make sure it is twice the diameter you ever think you need. If you’re lucky, it will last a couple of years. The cost of putting in thicker or extra ducts is peanuts compared to the cost of having to install additional ones later.

Dumb terminal - The dumb terminal is an asynchronous terminal that may operate at speeds as high as 9,600 bps or higher. The dumb terminal is an ASCII terminal that, although it may be “intelligent” in many of the functions it provides, it uses no communications protocol.

Duofoil - Belden trademark for a shield in which metallic foil is applied to both sides of a supporting plastic film.

Duplex - Two-way data transmission on a four-wire transmission cable.

Duplex (multiplexer) - A multiplexer that allows the user to look at multi screen images while performing time multiplex recording.

Duplex cable - A cable composed of two insulated single conductor cables twisted together

DUT - Device under test.

DVI (digital visual interface or digital video interface) - Video standard and connector for digital and analog monitor connections. DVI-A: analog monitor only; DVI-D: digital video only. Works up to 16.5 feet (5 m).

DWDM - Dense wavelength division multiplexing.

Dwell time - The length of time a switcher holds on a camera before moving on to the next in sequence.

DVR (digital video recorder) - A device that records video in a digital format on an internal (or external) hard disk drive. This is in contrast to a traditional VCR (videocassette recorder) that records video in an analog format on tape media. Some DVRs are combined with a multiplexer in the same unit, which allows multiple cameras to be simultaneously recorded, played back or viewed. Some common features of DVRs are fast picture retrieval, date and time search, maintenance free, no tapes to change or heads to clean, time lapse and event alarm recording, adjustable recording rates per video input, programmable timer, audio recording, alarm event logs and water-marking of images to prevent tampering. (Not all DVRs have the same features.)

DX (duplex signaling) - Signaling system that occupies that same cable pair as the voice path, yet does not require filters.