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    D-4 (or Super Frame SF)
         A data transmission format comprised of 12 frames of 192 bits each. A single 193rd bit is used for link control and error  checking. As an industry standard, D4, also known as SF, has been superseded by the Extended Super Frame (ESF)  format. However because ESF is not backward compatible and there continues to be a large installed base of channel banks  and DS-1 Multiplexers that are based upon D4, it is still the default private line formatting technique.

    D-4 Channel Bank
         A multiplexer used to convert 24 voice grade analog or data channels into a DS-1. The D-4 is not equipped for  software-controlled provisioning or remote circuit testing.

    DA - Directory Assistance
         Phone number lookup service.

    DACS - Digital Access Cross-Connect System
         A digital switching facility interconnection device. A DACS allows reconfiguration of lower levels of capacity, within a circuit,  without the need for manual changes in the interconnections. A DACS is similar to a Mux, but a DACS permits software  changes which would require physical changes with a MUX. Also known as DCS.

    DAL - Dedicated Access Line
         A non-switched access line that connects an inter-LATA or interstate user directly to his long
    distance carrier's point  of presence. The carrier performs no switching or call-screening functions on this line.

    Dark Fiber
         An inactive fiber-optic strand without electronics or optronics, (i.e., no connected transmitters, receivers, regenerators, etc.)

         A multiuse collection of information (on a computer). Often supports random access selectivity (via "Keys") and multiple  "Views" or levels of abstraction of the underlying data.

    Data Circuits
         Communication links that are optimized for digital transmission of computer information.

    Day of Week Routing
         Route calls based on day of week (e.g. single weekday, list of week days, range of weekdays or combinations).

    Day of Year Routing
         Route calls based on month/day of the year (e.g. single day, list of days, range of days or combination).

    DCE - Data Communications Equipment
        A generic definition of computing equipment that attaches to a network via a DTE.

        The signaling and data transmission channel specified in ISDN standards, which is packed switched and used to transmit network control signals for setting up hone calls.  It also carries information about incoming and outgoing calls that is displayed on the screens of ISDN desktop terminal devices. In the 1.544 Mbps ISDN primary rate interface standard, this channel occupies 64 Kbps of bandwidth.

    Digital European Cordless Telecommunications
         A wireless standard for communications, operating at 1,88-1,9 GHz. The system is developed for high traffic rates and small  cells. Applications are private, residential and business telephone and data transmissions.

    DDD - Direct Distance Dialing
         Any switched telecommunication service (like 1+, 0++, etc.) that allows a call originator to place long distance calls directly to telephones outside the local service area without an operator.

        A digital multiplexer manufactured by Lucent Technologies Network, that interfaces DS-1 signals with either electrical DS-3 signals and/or optical signals such as OC-3, OC-12, etc..

    DDS - Digital Data Service
        A dedicated digital data transmission service that offers transmission speeds of 2.4, 4.8, 9.6 or 56 Kbps over interconnected point-to-point digital private lines.

    DE - Discard Eligible
         Frame relay frame indicator that identifies that the frame may be discarded in the case of network congestion.

         A request to terminate service (or the process of terminating service)

    Decibel - Db - db
         A unit of power (sound, electrical or optical) increase (amplification) or loss (attenuation) that is proportional to the exponential  increase or decrease in power.

    Dedicated Line
         A private line leased from a telecommunications carrier for the exclusive use of a particular subscriber.

    Demarcation Point - Demarc
         A point (such as a jack or cross-connect panel) at which ownership or responsibility for operating and maintaining facilities  passes from one party to another.

    Design Layout Record - DLR
         Contains the layout (path through locations and equipment)  for the local portion of a circuit at a particular location.

         To place a call on a switched network. The term "dial" is obsolete - based on rotary dial phones and electromechanical relay switches (which are nearly non-existent in modern telephone systems.)
         Touch Tone service recognizes dual tones that are generated as each telephone key is pressed.  Where Touch Tone service is not available, telephones and switches electronically "pulse" signals that emulate the older rotary dial telephones. The terms "place" a call or "originate" a call are more accurate than "dial".

    Dial Tone
         When the off hook indication is received at a central office, a dial tone signal is sent to the originating caller on a switched network to indicate that the switch is ready to accept a number. At ELI, this equals Local Telephone Service (LTS).

    Dialed Number Identification Service - DNIS
         An optional feature on dedicated access lines (DALs). DNIS delivers identifying digits (up to 14 digits) that distinguishes  incoming calls. This allows CPE to identify which number was dialed when multiple numbers terminate on the same trunk  group.

         Equipment that pulses out a standard dial protocol signal.

         A device or method that uses discrete variations in voltage, frequency, amplitude, location, etc. to encode, process, or carry  binary (zero or one) signals for sound, video, computer data or other information. For example, a digital clock displays the  time as discrete numeric values, rather than angular displacement of analog hands. Digital communications technology  generally permits higher speeds of transmission with a lower error rate than can be achieved with analog technology. When  analog signals are received and amplified at each repeater station, any noise is also amplified. A digital signal, however, is  detected and regenerated (not amplified). Unlike amplification, any noise (less than a valid signal) is eliminated by digital  regeneration.

    Digital Loop Carrier - DLC
        Equipment outside the central office that concentrates analog local loop lines, and digitizes and multiplexes the calls for transport to the central office.

    Digital Switching System
        Newer switching systems that use Time-Slot-Interchange as opposed to older analog switching systems that use Space-Division-Multiplexing.

    Digital Systems Access Bay - DSAB
        An interconnection bay-frame designed specifically to provide a cabling interface to DACS.  Usually one DSAB bay is installed for each DACS interface bay in separate lineups.

    Digital Transmission System
        A set of equipment and facilities used to transmit digital signals in both the loop and interoffice plant.  Repeaters, which provide signal equalization and regeneration, may be deployed in these systems.

    Digit Stream
         The switch signaling protocol that contains information such as information digits, destination number, originating ANI, etc.  The digit stream is defined by Bellcore standards.

    Dim Fiber
         A fiber optic system in which the carrier provides regenerators, but does not originate the optical signals at one-or-both ends.

        Refers to database queries in the LNP process.  Query by a local switching office or toll switching office to a remote database.

    Direct Inward Dialing - DID
         A PBX or Centrex optional feature that permits outside calls to be placed directly to a station line without use of an operator.

    Direct Inward System Access - DISA
         The ability to access a PBX from an outside telephone number. For example, a caller might dial an assigned number, input a  security code and then use the PBX to place a long distance call.

    Directory Assistance - DA
         An information service whereby operators assist customers in obtaining the telephone number(s) they wish to call.

    Direct Trunk Transport
        A private-line (DS-3, etc.) that an IXC will lease so they can avoid the usage charges coming from the LEC's Access Tandem.  Usually runs from the IXC's POP to the co-locate space at the LEC's End Office.

         Call termination and the information associated with it such as time of day, who disconnected (originator, destination, etc.)

         Special pricing flexibility used to encourage increased use of products, or to improve competitive marketing.

         A change (other than attenuation or noise) in the characteristics of a signal due to the distance of transmission or equipment  problems or inefficiencies.

    Distributed Network Computing - DNC
         Separation of the elements of an application into sub component objects that can be implemented on multiple collaborative  computers on a common network. Objects are designed to optimize local processing and storage capabilities and to  minimize the slower overhead of remote communication.

    Distribution Frame
        A manual connection system for physically connecting customers' phone lines to a switch in a central office; or, for making cable connections between switches in a central office. Designed to make it easy for telco craftspeople to make, break or rearrange wiring connections in a central office. May also describe the Fiber Distribution Frame.

    DLCI - Data Link Connection Identifier
         A frame relay header field that identifies the destination of the packet. A unique number assigned to a PVC end point in a frame relay network.

        In SS7, a digital circuit interconnecting mated pairs of signal transfer points that are at different hierarchical levels (i.e. local and regional D-links now operate at a transmission speed of 56 Kbps).

         Switch Profile Loading. Northern Telecom DMS-250 process for performing Network Database Updates. Defines the network  configuration, valid user ANIs, alternate routing tables, service classes, etc.

    DMS - Digital Multiplex System
        The overall product name for Northern Telecom's family of digital central office switches.

         Programmable communications switch which contains both the DMS-100 local telephone service switch and the  DMS-250 long distance switch.

    DN - Directory Number
         Usually 7 or 10 digits and equates to your telephone number.

    DNIC - Data Network Identification Code
         A four digit number used to identify public data networks.

    Dominant Carrier
         A carrier with marketing power and the ability to control overall price levels. LECs and AT&T, are considered dominant.

    DPC - Destination Point Code
        The part of a routing label that identifies where the CCS/SS7 signaling message should be sent.

    Drop and Insert
         The ability to add and remove lower-level capacity from a higher-level circuit at an equipment location. Lower-level capacity  may be interconnected at an intermediate point of a higher-capacity circuit. For example, a Chicago to Los Angeles DS-3  might have DS-1 level drop and insert capability at Salt Lake City. this would provide DS-1 service from Los Angeles to Salt  Lake and from Salt Lake to Chicago.

    DS - Digital Signal
        Well defined transmission rates and coding schemes in the time-division multiplex hierarchy.
    DS-0 (Facility)
         The base unit of digital transmission capacity. 1 communication channel = 1 simultaneous voice grade equivalent with a  communication capacity of 64 thousand bits per second (64 Kbps).

    DS-1 (Facility)
         T1- The equivalent of 24 multiplexed voice grade channels (DS-0s). 1.544 million bits per second (1.5Mbps).

    DS-2 (Facility)
         T2 - The equivalent of 4 multiplexed T1 channels. 6.312 million bits per second (6.3Mbps).

    DS-3 (Facility)
         T3 - The equivalent of 28 multiplexed T1 channels. 44.736 million bits per second (45Mbps).

    DS-4 (Facility)
         T4 - The equivalent of 6 multiplexed T-3 channels. 274.176 million bits per second (274Mbps).

    DSU - Digital Service Unit
         A network interface device between the CSU and the customer's DTE (Digital Terminal Equipment).  Basically, equipment used to attach users' computing equipment to a public network.  Usually associated with a CSU, the DSU has a V.35, RS232, RS449, or other interface to the customer equipment. The DSU is also a  multiplexer for FT1 circuits.

    DSX - Digital Signal Cross-Connect
        A centralized termination, interconnection, and test point for digital equipment at a particular digital signal bit rate. These capabilities enable a DSX frame to provide several operational functions, including equipment interconnection, test access and patching. DSX-1 handles DS-1 signals and DSX-3 handles DS-3 signals.

    DSX Lineup
        One or more contiguous DSX bays that are equipped with the necessary jumper pathways to provide the ability to do all required DSX functions.

    DTE - Data Terminal Equipment
        A generic definition of external networking interface equipment such as a modem.

    DTMF - Dual Tone Multi Frequency
        The signal to the phone company that you generate when you press an ordinary telephone's touch keys. Each key you press on your phone generates two tones of specific frequencies.  So that a voice can't imitate the tones, one tone is generated from a high-frequency group of tones and the other from a low frequency group.

         The length of a call (switched communication connection) or the effective length of a Contract, Product Offering, etc.

    DXI - Data eXchange Interface
         In the ATM forum, a specification developed by the SMDS interest group to define the interaction between inter networking devices and DSUs/CSUs that are transmitting over an SMDS access line.


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Since November 20, 1999

Created by Jim Needham

İVector Graphics 1995

Revised: February 16, 2011
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