D
D channel	Data channel. Full-duplex, 16-kbps (BRI) or 64-kbps (PRI) ISDN channel. See also B channel.
DCN message	Disconnect message. Message sent by a calling or called fax machine at the end of a fax call. On receipt of the message, the devices tear down the call and release the circuit. DCN messages do not require a response from the opposite device.
DCS	Distributed Call Signaling or Digital Command Signal. Distributed Call Signaling. A set of proposals for extending SIP.
DCS message	Digital Command Signal message. Message sent by a calling fax machine during capabilities exchange that tells the called device which facilities to select for the reception of a fax transmission.
DDR	Dial-on-demand routing.
default configuration file	XML file that contains configuration details for IP phones. There is one shared default XML configuration file for each type of phone. When a phone goes online or is rebooted, it automatically gets information about itself from the default configuration file.
delta	Incremental value, such as the difference between the current time and the time when the response occurred.
DFC	Dial-feature card.
DHCP	Dynamic Host Configuration Protocol. Protocol for allocating IP addresses dynamically so that addresses can be reused when hosts no longer need them.
dial peer	Addressable call endpoint that contains configuration information including voice protocol, codec type, and associated phone number. VoIP has three types of dial peers: POTS, VoIP, and MMoIP.
DID	Direct Inward Dial (or Dialing). Service that allows external callers to dial an internal extension number and reach the called party without passing through an operator or attendant.
DID/DNIS	Direct Inward Dial (or Dialing)/Dialed Number Identification Service. When a call arrives at an ACD or PBX, the carrier sends a digital code on the trunk line. The switch reads this code—typically the specific number dialed by the user—to determine how to dispatch the call. By mapping each possible code with an internal extension, the switch can provide DID service.
direct station select	Ability to use the Transfer soft key and a monitored line button to quickly transfer a call.
DIS message	Digital Information Signal message. Message sent by a called fax machine during capabilities exchange that describes the reception facilities of the called machine, such as maximum page length, scan-line time, image resolution, and error-correction mode.
disconnected procedure	Procedure that the gateway may initiate when an endpoint attempts to communicate with its call agent and fails. The disconnected timer starts and RSIP messages are sent to the call agent at prescribed intervals until contact is established. This procedure ensures that an RSIP message is the first message to reach the call agent after communications are reestablished.
DN	Directory number.
DND	Do not disturb. Feature that causes incoming calls to not ring on an soft-key IP phone, but provides visual alerting and call information so that the call can be answered if desired.
DNIS	Dialed-number identification service. Use of a called-number identifier to route a call appropriately, as when calls placed to specific toll-free numbers are routed to the appropriate area within a company to be answered. Sometimes referred to as called number.
DNIS map	Table containing multiple called numbers, each individually linked to the URL of a VoiceXML document. Can reside on the gateway or an external server.
DNS	Domain Name (or Naming) System. Mechanism for translating H.323 IDs, URLs, or e-mail IDs into IP addresses. Also used to assist in locating remote gatekeepers and to reverse-map raw IP addresses to hostnames of administrative domains.
DNS SRV	Domain Name System server. Server that provides DNS service.
document	Textual object. In HTML, documents (or pages) are single files containing HTML. In XML, documents contain markup structures and may contain content from several files.
DOD	Direct Outward Dial (or Dialing).
domain	1. On the Internet, a portion of the naming-hierarchy tree that refers to general groupings of networks based on organization type or geography. 2. In security, an environment or context that is defined by a security policy, model, or architecture to include a set of system resources and the set of entities that have the right to access the resources. Sometimes used interchangeably with ITAD. In that environment, in most cases the service provider configures the entire network to act as a single domain so that routes and routing policies are consistent across the network. Alternatively, the service provider might configure multiple domains within the network to represent major geographical regions.
DRQ message	Disengage-Request RAS message.
DS0	Digital signal level 0. Single time slot on a DS1 (also known as T1) digital interface; that is, a 64-kbps, synchronous, full-duplex data channel, typically used for a single voice connection on a PBX. Alternatively, framing specification used in transmitting digital signals over a single channel at 64-kbps on a T1/E1 line.
DS3	Digital signal level 3. Framing specification for sending digital signals at 44.736 Mbps on a T3 facility.
DSL	Digital subscriber line. Public-network technology that delivers high bandwidth over conventional copper wiring at limited distances. There are four types of DSL: ADSL, HDSL, SDSL, and VDSL. All are provisioned via modem pairs, with one modem located at a central office and the other at the customer site. Because most DSL technologies do not use the whole bandwidth of the twisted pair, there is room remaining for a voice channel.
DSN message	Delivery Status Notification message. Message returned to the originator indicating the delivery status of an e-mail message. A sender can request three types of delivery status notifications: delay, success, and failure.
DSP	Digital signal processor. Specialized electronics on a router that provide stream-to-packet signal processing functionality that includes voice compression, echo cancellation, and tone- and voice-activity detection.
DSP resource pool	Collection of DSP resources available to a network module or VWIC on the router.
DSU	Data service unit. Digital-transmission device that adapts the physical interface on a DTE device to a transmission facility such as T1 or E1. Also responsible for such functions as signal timing. Often referred to together with CSU as CSU/DSU.
DTD	Document Type Definition. Set of rules for XML document construction. Applications use DTDs to identify the parts required of a particular document type.
DTE	Data-terminal equipment. Device at the user end of a user-network interface that serves as a data source, destination, or both. Connects to a data network through a DCE device (such as a modem) and typically uses clocking signals generated by the DCE. Includes such devices as computers, protocol translators, and multiplexers.
DTMF	Dual-tone multifrequency. Type of signaling that combines two distinct frequencies to generate a distinctive tone for each keypad digit or character. Sometimes referred to as "touchtone."
DTMF-relay	Mechanism for providing a reliable digit relay between VoIP gateways when a low-bandwidth codec is used. Provides a standardized means of transporting DTMF tones in RTP packets and is identified by dynamic payload types in the SDP.
dual-line ephone-dn	An ephone-dn with one voice port and two channels to handle two independent calls. See also ephone-dn.
DVMRP	Distance Vector Multicast Routing Protocol. Internetwork gateway protocol, largely based on RIP, that implements a typical dense-mode IP multicast scheme.
dynamic payload	Payload that is defined at session initiation and is not assigned a specific RTP format. See also static payload.