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Glossary of broadband terminology: D

D Channel: The "data" signaling channel, in the ISDN scheme; this is the channel that carries signaling and limited packet communications. In basic rate ISDN, this is a full-duplex 16 Kbps channel; in primary rate ISDN, this is a full-duplex 64 Kbps channel. See Basic Rate Interface, Integrated Services Digital Network, and Primary Rate Interface.
Data Circuit Terminating Equipment: Any device that is connected to the subscriber end of a transmission circuit and provides the appropriate termination functions for that connection. A modem or DSU/CSU are considered DCE. Also called Data Communications Equipment.
Data Link Connection Identifier: The frame relay virtual circuit number used in internetworking to denote the port to which the destination LAN is attached.
Data Service Unit: A digital interface device that connects end user data communications equipment to the digital access lines and provides framing of sub-64 Kbps customer access channels onto higher-rate data circuits. ADSU may be combined with a CSU into a single device called a CSU/DSU. See Channel Service Unit/Data Service Unit.
Data Terminal Equipment: Typically the device that transmits data such as a personal computer or data terminal.
Dedicated Line: A transmission circuit that is reserved by the provider for the full-time use of the subscriber. Also called a Private Line.
Demodulation: Conversion of a carrier signal or wave form (analog) into an electrical signal (digital).
Dense Wave Division Multiplexing: A SONET term. High-speed versions of WDM, which is a means of increasing the capacity of SONET fiber optic transmission systems through the multiplexing of multiple wavelengths of light. Each wavelength channel typically supports OC-48 transmission at 2.5 Gbps. A 32-channel system will support an aggregate 80 Gbps.
Dial Up: The process of initiating a switched connection through the network; when used as an adjective, this is a type of communication that is established by a switched-circuit connection.
Digital Access and Cross-Connect System: A digital cross-connect device for routing lines among multiple ports.
Digital Data Service: Private line digital service with data rates of 56/64 Kbps.
Digital Equipment Corporation Network: Digital Equipment Corporation's proprietary network architecture.
Digital Hierarchy: The progression of digital transmission standards typically starting with DS-0 (64 Kbps) and going up through at least DS-3. Twenty-four DS-0s make up a DS-1; 28 DS-1s make up a DS-3. There are other links (including a DS-2), but these are less common.
Digital Loop Carrier: Equipment used to concentrate many local loop pairs onto a few high-speed digital pairs or one fiber optic pair for transport back to the CO.
Digital Signal 0: In the digital hierarchy, this signaling standard defines a transmission speed of 64 Kbps.
Digital Signal 1: In the digital hierarchy, this signaling standard defines a transmission speed of 1.544 Mbps; a DS-1 is composed of 24 DS-0signals; this term is often used interchangeably with T-1.
Digital Signal 3: In the digital hierarchy, this signaling standard defines a transmission speed of 44.736 Mbps; a DS-3 is composed of 28 DS-1signals; this term is often used interchangeably with T-3.
Digital Signal Processor: A specialized computer chip designed to perform speedy, complex operations on digitized wave forms.
Digital Subscriber Line: A general term for any local network loop that's digital in nature; technically, DSL equates to ISDN, but this is decreasingly enforced terminology. See Asymmetrical DSL, High Bit Rate DSL, Symmetric DSL, Very High Data Rate DSL, and xDSL.
Digital Subscriber Line Access Multiplexer: A CO device used to aggregate data traffic from many DSL subscribers into one high-speed signal for hand-off to the data communications network.
Discrete Wavelet Multi-Tone: A multicarrier modulation system pioneered by Aware Inc. that, according to the vendor, isolates its subchannels in a method that is superior to conventional DMT modulation. In the vendor's own words, "DWMT is able to maintain near optimum throughput in the narrowband noise environments typical of ADSL, VDSL, and Hybrid Fiber Coax, while DMT systems may be catastrophically impaired."
Distribution: Portion of the telephone cabling plant that connects subscribers to feeder cables from the CO.
Downstream: In xDSL, the communications from the network towards the customer premises.
DS-0: Basic North American 64 Kbps digitized voice channel.
DS-1: First level in North American digital hierarchy; the 1.544 Mbps signal consists of 24 DS-0s multiplexed together.
DS3: .
DMT: Discrete Multitone Transmission, as used for FasTrak DSL service from Pacific Bell. See also g.lite and ADSL
Downstream/Upstream: Downstream refers to data flowing from the source such as a corporate host or Internet service provider (ISP) to the end user.
Upstream refers to data flowing from the end user up to the corporate host or ISP.
DSL: Digital Subscriber Line: Modems attached to twisted pair copper wiring that transmits (on average) from 1.5 Mbps to 7 Mbps downstream (to the subscriber) and from 16 kbps to 800 kbps upstream, depending on line distance from the CO (Central Office).
DSLAM: Digital Subscriber Line Access Multiplexer - specifically, a device that takes a number of DSL subscriber lines and concentrates these to a single ATM line.
DSL Modem: Short for MOdulator-DEModular, this hardware device converts ATM cells to Ethernet packets and visa versa in the use of DSL.
Dynamic Host Control Protocol: A TCP/IP protocol.
Dynamic Random Access Memory: Memory used to store data in PCs and otherdevices.