D3/D4 - Refers to compliance with AT&T TR (Technical Reference) 62411 definitions for coding, supervision, and alarm support. D3/D4 compatibility ensures support of digital PBXes, M24 services, Megacom services, and Mode 3 D3/D4 channel banks at DS-1 level.

D4 Channelization - Refers to compliance with AT&T TR (Technical Reference) 62411 in regards to the DS1 frame layout (the sequential assignment of channels and time slot numbers within the DS1).

D4 Framed/Framing Format - In T1, a 193 bit frame format in which the 193^rd bit is used for framing and signaling information (the frame/framing bit). To be considered in support of D4 Framing, a device must be able to synchronize and frame-up on the 193^rd bit.

D/A - Digital to analog conversion.

DACS (Digital Access and Cross-Connect System) - Digital switching system for routing T1 lines, and DS-0 portions of lines, among multiple T1 ports.

Daisy Chain - A method of sending data signals along a bus. Any devices which do not need the signal pass it on until it reaches the device which does want it. This device then breaks the daisy-chained signal continuity.

DARPA (Defense Advanced Research Projects Agency) - the US government agency that funded the ARPANET.

Data Link - Communications connection which is used to transmit data from a source to a destination.

Data Link Layer - Layer 2 of the OSI model, responsible for encoding data and passing it to the physical medium. The IEEE divides this layer into the LLC (Logical Link Control) and MAC (Media Access Control) sublayers.

Datagram - A packet of information used in a connectionless network service that is routed to its destination using an address included in the datagram's header.

dB (Decibel) - A unit of measure of signal strength, usually the relation between a transmitted signal and a standard signal source.

dBrnC (Weighted Electrical Noise) - Power level in dB relative to a noise reference of -90dBrn, as measured with a noise meter weighted by a special frequency function call C message eighting, which expresses average subjective reaction to interference in telephone sets as a function of frequency band. In power circuits, it is a measure of the interfering effect on the noise transmission band (about 200 to 3200 Hz) caused by the noise.

DC (Direct Current) - The flow of free electrons in one direction within an electrical conductor, such as a wire.

DCC (Data Country Code) - This specifies the country in which an address is registered. The codes are given in ISO 3166. The length of this field is two octets. The digits of the data country code are encoded in Binary Coded Decimal (BCD) syntax. The codes will be left justified and padded on the right with the hexadecimal value "F" to fill the two octets.

DC-DC Converter - Device which converts DC at one voltage to DC at another voltage.

DCE (Data Communications Equipment) - A definition in the RS232C standard that describes the functions of the signals and the physical characteristics of an interface for a communication device such as a modem.

DC Power Plant - The equipment that converts AC power to DC and stores and distributes this DC power to operate the telephone plant. Nominal DC voltage levels may be 12, 24, 48, 130, or 140 volts. Other voltage levels are provided by DC to DC converters.

DC Ringing - Activation of a ring key at a switchboard which operates a normal DC relay circuit, the contacts of which in turn send AC ringing out to the line.

DCS (Digital Cross-connect System) - An electronic patch panel used to route digital signals in a central office.

Dead Short - A short circuit.

Debugging - The technique of detecting, diagnosing and correcting errors (bugs) which may occur. Bugs can exist both in programs, e.g. failure to understand and describe a process, or in hardware, e.g. in the way a hardwired logic unit has been strapped.

DECnet -Digital Equipment Corporation's proprietary LAN.

De-Energized - The condition of a relay when it is deactivated. Also a circuit that has had its power source removed.

Default Node Representation - A single value for each nodal state parameter giving the presumed value between any entry or exit to the logical node and the nucleus.

Demultiplexing - A function performed by a layer entity that identifies and separates SDUs from a single connection to more than one connection (see multiplexing).

DES (Destination End Station) - An ATM termination point which is the destination for ATM messages of a connection and is used as a reference point for ABR services. See SES.

Designated Transit List - A list of node and optionally link Ids that completely specify a path across a single PNNI peer group.

Designation Strip - Strip on a manual switchboard or distribution panel into which circuit designation labels are placed.

DFA (DXI Frame Address) - A connection identifier associated with ATM DXI packets that serves the same functions as, and translates directly to, the VPI/VCI on an ATM cell.

Dial Tone - Tone which indicates to the caller that equipment is ready to receive dialed information. A dial tone is usually 350 Hz plus 440 Hz at -13dBm.

Dielectric Strength - Potential gradient at which electric breakdown occurs.

Diesel Engine - An internal combustion engine in which the air fuel combustion mixture ignites entirely by the heat of compression.

Digital Microwave - A microwave system in which the modulation of the radio frequency carrier is digital. The carrier is still a standard microwave radio wave. The digital modulation may be frequency or phase shift, but the control of that modulation is the digital bit stream.

Digital Modulation - A new method of encoding information for transmission that is replacing standard standard analog. The information, or in this case, voice conversation, is turned into a series of digital bits — the 0’s and 1’s of computer binary language. At the receiving end, the information is reconverted. Digital transmission offers a cleaner signal, virtually immune to the problems that plague analog modulation such as fading and static. Digital transmission also has been embraced by the cellular industry because it offers major gains in capacity compared to analog.

Digital-To-Analog Conversion - A circuit that accepts digital signals and converts them into analog signals. A modem typically has such a circuit. Diode: A semiconductor device which allows electricity to pass through it in only one direction and stops it flowing the other way.

Dijkstra's Algorithm - An algorithm that is sometimes used to calculate routes given a link and nodal state topology database.

Diode Bridge - A bridge which permits polarity sensitive devices to be used across a line.

Diode Rectifier - Rectifier utilizing a diode, either of electron tube or semiconductor type, as the rectifying element.

DIP Switch (Dual In-line Package) - A device that has two parallel rows of contacts that let the user switch electrical current through a pair of those contacts to on or off. They are used to reconfigure components and peripherals.

Direct Current Generator - Rotating machine which produces a direct current output.

Discharge - Conversion of a storage battery’s chemical energy into electrical energy, or of a capacitor’s dielectric stress into an electric current.

Discharge Current - The rate at which a battery string can be safely discharged or the current that a battery is providing to a load.

Disconnect - The breaking or release of a circuit connecting two telephones or data devices.

Disconnecting Switch - In a power system, a switch used for closing, opening, or changing the connection in a circuit or system or for isolating purposes. It has no interrupting rating and is intended to be operated only after the circuit has been opened by some other means.

Discrete Alarm - A functional or distinctive alarm such as a low fuel, high voltage or rectifier failure alarm.

Disk - A memory system based on rotating disks coated with a magnetic recording medium.

Diskette - Floppy disk used as a magnetic store.

Display - The visual presentation of information, usually a TV-like screen or an array of illuminated digits.

Display Device - An output unit that gives a visual representation of data.

Distribution - The arrangement of premises wiring runs, and their associated hardware, required to implement the planned customer premises wiring system extending from the Network Interface Jack to each Communications Outlet.

Distribution Cable - Secondary cable in a city’s distribution network which has terminals from which leads or drops go to subscribers’ premises.

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

Distribution Network - External plant network connecting subscribers’ premises with a telephone exchange/central office.

Distribution Voltage Drop - The voltage drop between any two defined points of interest in a power distribution system.

Distribution Wire Tap - Terminal box used for joining drop wires to rural distribution wires without breaking and terminating the latter.

DLC (Data Link Control) - Characters used in data communications that control transmission by performing various error checking and housekeeping functions —connect, initiate, terminate. Also Digital Loop Carrier systems such as DISC*S.

DLCI (Data Link Connection Identifier) - A connection identifier associated with frame relay packets that serves the same functions as, and translates directly to, the VPI/VCI on an ATM cell.

DMS - (1) Digital Multiplex System and (2) the name of a line of digital central office switches from Northern Telecom.

DNS (Domain Name System) - The distributed name and address mechanism used in the Internet.

Documentation - Written text describing the system, how it works, and how to work it.

Domain - A group of devices on a network that are administered as a unit with common rules and procedures.

Domain Name Server - A computer that converts names to their corresponding Internet numbers. It allows users to telnet or FTP to the name instead of the number.

Downline Loading - A system in which programs are loaded into the memory of a computer system via the same communication line(s) the system normally uses to communicate with the rest of a network.

Downloading - The act of receiving data from another computer into your computer.

Downtime - The total time during which a telephone or data system is not working due to some software or hardware failure.

Drain - Total current drawn by a load.

DRAM (Dynamic Random Access Memory) - The most common type of computer memory.

Dropping Resistor - Resistor used to achieve a specific voltage drop in a circuit.

Dry Contacts - A set of fixed/movable contacts on a relay which are isolated. Utilization of such contacts is an ideal method of separating alarms for a given discrete grounded or looped pick-up. Can also be referred to as spare contacts.

DS- - A hierarchy of digital signal speeds used to classify capacities of lines and trunks. The fundamental speed level is DS-0 (64 kilobits per second) and the highest is DS-4 (about 274 million bits per second). They are defined in Mbps as: DS-1, 1.544; DS-1c, 3.152; DS-2, 6.312; DS-3, 44.736; and DS-4, 274.176.

DSn (Digital Standard n (0, 1, 1C, 2, and 3)) - A method defining the rate and format of digital hierarchy, with asynchronous data rates defined as follows:

DS-0	64kb/s	1 voice channel
DS-1	1.544Mb/s	24 DS-0s
DS-1C	3.152 Mb/s	2 DS-1s
DS-2	6.312 Mb/s	4 DS-1s
DS-3	44.736 Mb/s	28 DS-1s

Synchronous data rates (SONET) are defined as:

STS-1/OC-1	51.84 Mb/s
STS-3/OC-3	155.52 Mb/s	3 STS-1s byte interleaved
STS-3c/OC-3c	155.52 Mb/s	Concatenated, indivisible payload
STS-12/OC-12	622.08 Mb/s	12 STS-1s, 4 STS-3cs, or any mixture
STS-12c/OC-12c	622.08 Mb/s	Concatenated, indivisible payload
STS-48/OC-48	2488.32 Mb/s	48 STS-1s, 16 STS-3cs, or any mixture

DSP (Domain Specific Part) - Specifies the company and specific switching product.

DSR (Data Set Ready) - An RS-232 modem interface control signal (sent from the modem to the DTE on pin 6) which indicates that the modem is connected to the telephone circuit. Usually a prerequisite to the DTE issuing RTS.

DTE (Data Terminal Equipment) - generally user devices, such as terminals and computers, that connect to data circuit-terminating equipment. They either generate or receive the data carried by the network.

DTL (Designated Transit List) - A list of node and optionally link Ids that completely specify a path across a single PNNI peer group.

DTL Originator - The first lowest-level node within the entire PNNI routing domain to build the initial DTL stack for a given connection.

DTL Terminator - The last lowest-level node within the entire PNNI routing domain to process the connection (and thus the connection's DTL).

DTMF (Dual Tone Multi Frequency) - (Touch Tone) A type of dialing in which, when you touch a push button, it makes a tone which is actually the combination of two tones, one high frequency and one low frequency. In U.S. telephony, there are actually two types of DTMF signaling, one that is used on normal business or home push button phones, and one that is used for signaling within the telephone network itself.

DTR (Data Terminal Ready) - An RS232 modem interface control signal (sent from the DTE to the modem on pin 20) which indicates that the DTE is ready for data transmission and which requests that the modem be connected to the telephone circuit.

Due Date - The date an event is to occur, i.e. an installation, a change or a connection.

Dumb Terminal - A computer terminal with no processing or no programming capabilities which derives all its power from the computer to which it is attached, typically over a local hardwire or a phone line. Dumb terminals are typically used for simple data entry and data retrieval tasks.

Dummy Fuse - Device the same physical shape as a fuse but which plays no active role.

Duplex - Two way communication.

Duration of Discharge - The amount of time it takes a given battery plant to totally discharge during normal usage.

DXI - A generic phrase used in the full names of several protocols, all commonly used to allow a pair of DCE and DTE devices to share the implementation of a particular WAN protocol. The protocols all define the packet formats used to transport data packets between DCE and DTE devices.

Dynamic Allocation - A technique in which the resources assigned for program execution are determined by criteria applied at the moment of need.