DAC: Discretionary Access Control
Discretionary Access Control (DAC) is an access control service that enforces a security policy based on the identity of system entities and their authorizations to access system resources. This includes setting permissions on files, folders and shared resources.

DAC: Dual-attached Concentrator
Dual-attached Concentrator(DAC) is the FDDI or CDDI concentrator capable of attaching to both rings of a FDDI or CDDI network. It also can be dual-homed from the master ports of other FDDI or CDDI concentrators.

DACS: Digital access and Cross-Connect System
Digital access and Cross-Connect System(DACS) is a digital system in which access is performed by T-1 hardware architecture in private and public networks with centralized switching and cross-connection is performed by D3/D4 framing for switching digital-signal-0 (DS-0) channels to other DS-0 channels.

DARPA: Defense Advanced Research Projects Agency
Defense Advanced Research Projects Agency (DARPA), evolved from ARPA, is a U.S. government agency that provides research funds for and experimentation with the Internet.

DAS: Dual Attachment Station
Dual Attachment Station (DAS), also called a Class A station, is a device attached to both the primary and the secondary FDDI rings. Dual attachment provides redundancy for the FDDI ring: If the primary ring fails, the station can wrap the primary ring to the secondary ring, isolating the failure and retaining ring integrity.

DAS: Dynamically Assigned Socket
Dynamically Assigned Socket (DAS) is assigned dynamically by Datagram Delivery Protocol (DDP) upon request by a client in the AppleTalk network. In an AppleTalk network, the sockets numbered 128 to 254 are allocated as DASs.

Data Direct VCC
Data Direct VCC(Virtual Channel Connection ) is a bidirectional point-to-point VCC setup between two LECs (LAN Emulation Clients) in an ATM network. It is one of three data connections defined by Phase 1 LANE. Data direct VCCs do not offer any type of QOS guarantee, so they typically are used for UBR and ABR connections.

Data Encapsulation
Data Encapsulation is a process of taking one Protocol Data Unit (PDU) and enveloping it within a set of protocol header and trailer. In the OSI 7 layers model, each layer is primarily responsible for communicating with a peer layer on another machine. This communication between peers is done in "Protocol Data Units" (PDU), which consists of this layer's Header, Trailer and Data encapsulated.

Data Flow
Data flow is a group of traffic, identified by a combination of source address/mask, destination address/mask, IP next protocol field, and source and destination ports, where the protocol and port fields can have the values of any. In effect, all traffic matching a specific combination of these values is grouped logically together into a data flow. A data flow can represent a single TCP connection between two hosts, or it can represent all the traffic between two subnets. IPSec protection is applied to data flows.

Data Flow Control Layer
Data Flow Control Layer is the layer 5 (corresponds to the session layer of the OSI model) of the IBM SNA architectural model, which determines and manages interactions between session partners, particularly data flow.

Data Link Control
Data Link Control (DLC) is the service provided by the Data Link Layer at the second lowest layer in the OSI Reference Model. Every network interface card (NIC) has a DLC address or DLC identifier (DLCI) that uniquely identifies the node on the network. For example, Ethernet and Token-Ring use the DLC addresses with a 48 bits number and it is also called MAC address.

Data Link Layer
The data link layer is the layer two of the seven-layer OSI model, which provides reliable transit of data across a physical link. The data-link layer is concerned with physical addressing, network topology, line discipline, error notification, ordered delivery of frames, and flow control.

Data packet
Data packet, also called packet, is a format in which data is transmitted over a network. A packet contains the data itself as well as addresses, error checking, and other information necessary to ensure the packet arrives intact at its intended destination.

Data Sink
Data sink refers to a computer or any other network equipment capable of receiving data.

Data Stream
Data Stream or datastream refers to all data transmitted through a communications line in a single read-or-write operation. A data stream can be distinguished in practice from a block transfer, although the moving of blocks could itself be considered a “stream” (of coarser granularity).

Datacasting
Datacasting is broadcast of digital information over networks to receivers and players. Datacasting is sometimes used as an alternative to traditional video broadcasting, because the receivers and player units can have the "intelligence" to customize their playback programming for the location and intended audience. The system of receivers set up to receive messages from a particular datacasting is known as Datacasting Networks, which are a venue for advertisers.

Datagram
Datagram is a self-contained, independent entity of data carrying sufficient information to be routed from the source to the destination computer without reliance on earlier exchanges between this source and destination computer and the transporting network. IP datagrams are the primary information units in the Internet. User Datagram Protocol is the most popular protocol to route datagrams connectionless over the IP network, while TCP is for reliable data transmission. The terms cell, frame, message, packet, and segment also are used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.

Datakit
Datakit is proprietary packet switching system similar to X.25 widely deployed by the RBOCs. Datakit supports host-to-host connections and EIA-232 connections for terminals, printers, and hosts.

Data-link Control Layer
Data-link Control Layer is the layer 2 (corresponds roughly to the data-link layer of the OSI model) in the IBM SNA architectural model. It is responsible for the transmission of data over a particular physical link.

Datex
Datex is a circuit-switched network for data communications in Scandinavia, interconnected with similar networks around the world. Due to its limitation in data transfer speed, most businesses have switched to more efficient media like the Internet.

DAVIC: Digital Audiovisual Council
Digital Audiovisual Council(DAVIC), dissolved in 2001, was an industrial organization to promote interactive digital audio-visual applications and services by promulgating specifications of open interfaces and protocols that maximize interoperability, not only across geographical boundaries but also across diverse applications, services, and industries.

DB Connector: Database Bus connector
Database Bus connector (DB connector), defined by various EIA/TIA standards, is a type of connectors used to connect serial and parallel cables to a data bus. DB connector names are in the format of DB-x, where x represents the number of (wires) within the connector. Each line is connected to a pin on the connector, but in many cases, not all pins are assigned a function. DB connectors come in 9, 15, 25, 37 and 50-pin sizes. The DB connector defines the physical structure of the connector, not the purpose of each line. For example, DB-9 connectors have 9 pins and are used to connect a mouse. DB-25 connectors have 25 pins and are used to connect a printer.

DB: Decibels
Decibels (db) is an unit for measuring relative power ratios in terms of gain or loss. The units of db are expressed in terms of the logarithm to base 10 of a ratio and typically are expressed in watts. For example, a -3dB loss indicates a 50% loss in power; a +3dB reading is a doubling of power; 10 dB indicates an increase (or a loss) by a factor of 10; 20 dB indicates an increase (or a loss) of a factor of 100; 30 dB indicates an increase (or a loss) by a factor of 1000.

DBA: Dynamic Bandwidth Allocation
Dynamic Bandwidth Allocation (DBA) refers to the process of automatically increasing or decreasing the bandwidth of the channel in real time without terminating the link to optimize the utilization of bandwidth efficiency.

dBm: Decibels per milliwatt
Decibels per milliwatt (dbm) is power measurement relative to 1 milliwatt. It is used in radio, microwave and fiber optic work as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form. dBm (or dBmW) and dBW are independent of impedance. Since it is referenced to the watt, it is an absolute unit, used when measuring absolute power. It should not be confused with dB, a dimensionless unit, which is used when measuring the ratio between two values. 0 dBm is defined as 1 mw at 1 kHz of frequency at 600 ohms of impedance.

dBmV
dBmv refers to decibels with respect to one millivolt in a 75-ohm system. The measure dBmV is a power ratio with reference to 13.33 nanowatts of power. dBmv is the unit of RF power used in CATV work in North America.

dBW
dBW is the ratio of the power to one Watt expressed in decibels. dBW measures the strength of a signal expressed in decibels relative to one Watt.

DCA: Defense Communications Agency
Defense Communications Agency (DCA), now known as Defense Information Systems Agency (DISA), is a combat support agency of the United States Department of Defense (DoD) responsible for planning, developing, fielding, operating, and supporting command, control, communications, and information systems that serve the needs of the President, the Secretary of Defense, the Joint Chiefs of Staff, the Combatant commanders, and other Department of Defense components under all conditions of peace and war.

DCC: Data Communications Channel
Data Communications Channel (DCC) is the inband data communication channel in SONET/SDH communication. Each SONET/SDH frame includes two Data Communication Channels (DCC) called Section DCC and Line DCC for transporting management messages between NEs and between NEs and Management systems. DCC enables service providersOperation Support Systems (OSS) to manage SONET/SDH Network Elements (NE) without the need for an expensive out-of-band data communication network.

DCC: Data Country Code
Data Country Code (DCC) is one of two ATM address formats developed by the ATM Forum for use by private networks. DCC is adapted from the subnetwork model of addressing in which the ATM layer is responsible for mapping network layer addresses to ATM addresses.

DCE: Data Communications Equipment
Data Communications Equipment (DCE) is a device that communicates with a Data Terminal Equipment (DTE) device. The DCE provides a physical connection to the network, forwards traffic, and provides a clocking signal used to synchronize data transmission between DCE and DTE devices. Modems and interface cards are examples of DCE.

DCE: Distributed Computing Environment
Distributed Computing Environment (DCE) is a group of open technologies developed by The Open Group for creating distributed applications that run on different platforms. DCE services include:
Remote Procedure Calls (RPC)
Security Service
Directory Service
Time Service
Threads Service
Distributed File Service

DCF: Dispersion Compensating Fiber
Dispersion Compensating Fiber is a fiber that has the opposite dispersion of the fiber being used in a transmission system. It is used to nullify the dispersion caused by that fiber.

DCN: Data Communications Network
In telecommucation, Data Communications Network (DCN) is an out-of-band network based on the configuration of telecommunication facilities for the purpose of transmitting data, as opposed to analog signals. DCN provides connectivity between network elements and their respective operations support systems (OSSs). Its primary function is enabling the surveillance and the status of a telco/PTT network but it also facilitates network operations and management, such as provisioning, billing, planning, and service assurance.

DCOM: Distributed Component Object Model
Distributed Component Object Model (DCOM), developed by Microsoft and previously called Network OLE, enables software components to communicate directly over a network. DCOM is designed for use across multiple network transports, including such Internet protocols as HTTP.

DCS: Digital Crossconnect System
Digital Crossconnect System (DCS) is a network element providing automatic cross-connection of a digital signal or its constituent parts.

DCT: Discrete Cosine Transform
The discrete cosine transform (DCT) is a Fourier-related transform similar to the discrete Fourier transform (DFT), but using only real numbers. It is equivalent to a DFT of roughly twice the length, operating on real data with even symmetry (since the Fourier transform of a real and even function is real and even), where in some variants the input and/or output data are shifted by half a sample.

DDIC: DVB/DAVIC Interoperability Consortium
DVB/DAVIC Interoperability Consortium (DDIC) is an industrial organization to promote and support product interoperability between member vendors employing the DVB-RCCL/DAVIC international standards. An independent verification process allows DDIC members to offer tested and documented interoperable products, thereby enable cable network operators to choose reliable and highly intergratable, standards-based DVB solutions.

DDM: Distributed Data Management
Distributed Data Management (DDM) is one of three SNA transaction services in an IBM SNA environment that provides peer-to-peer communication and file sharing.

DDN: Defense Data Network
Defense Data Network (DDN) is a U.S. military network composed of an unclassified network and various secret and top-secret networks. DDN is operated and maintained by the Defense Information Systems Agency (DISA) under the US Department of Defence (DOD). The four DDN subnetworks were:
Military Network (MILNET) for Unclassified traffic
Defense Secure Network One (DSNET 1) for Secret traffic
Defense Secure Network Two (DSNET 2) for Top Secret traffic
Defense Secure Network Three (DSNET 3) for Top Secret/Sensitive Compartmented Information

DDNS: Dynamic DNS
Similar to DNS, Dynamic DNS (DDNS) allows anyone hosting a Web or FTP server to advertise a public name to prospective users. Unlike DNS that only works with static IP addresses, DDNS works with dynamic IP addresses, such as those assigned by an ISP or other DHCP server. DDNS is popular with home networkers, who typically receive dynamic, frequently-changing IP addresses from their service provider. To use DDNS, one simply signs up with a provider and installs network software on their host to monitor its IP address.

DDR: Dial-on-demand Routing
Dial-on-demand Routing (DDR) is a technique that enables router automatically initiate and close a circuit-switched session as transmitting stations demand. The router spoofs keep alive so that end stations treat the session as active. DDR permits routing over ISDN or telephone lines using an external ISDN terminal adapter or modem.

De jure standard
De jure standards are standards that have been approved by standard bodies, professional organizations, industry groups, a government body, or other groups.

DE: Discard Eligible
Discard Eligible (DE) is a bit in the Frame Relay header that, if frames must be discarded, signals a switch to choose this frame to discard instead of another frame without the DE bit set.

De-activation
De-activation refers to the process of disabling network access and privileges for a subscriber device, and reclaiming device attributes for other subscriber devices. De-activation occurs as part of subscriber account de-provisioning, or as part of activation of a replacement subscriber device.

Deadlock
A deadlock is a situation wherein two or more competing actions are waiting for the other to finish, and thus neither ever does. In computing, deadlock refers to a specific condition when two or more processes are each waiting for another to release a resource, or more than two processes are waiting for resources in a circular chain. Deadlocks are a common problem in multiprocessing where many processes share a specific type of mutually exclusive resource known as a software.

DECnet
DECnet is a protocol suite developed and supported by Digital Equipment Corporation (Digital or DEC, now part of HP). Several versions of DECnet have been released. The original DECnet allowed two directly attached minicomputers to communicate. Subsequent releases expanded the DECnet functionality by adding support for additional proprietary and standard protocols. Currently, two versions of DECnet are in wide use: DECnet Phase IV and DECnet plus (DECnet V). The DECnet now is part of the HP OpenVMS.

DECnet Plus
The DECnet Plus, also known as DECnet/OSI or DECnet V, defines a layered model that implements three protocol suites: OSI, DECnet, and TCP/IP. DECnet plus conforms to the seven-layer OSI reference model and supports many of the standard OSI protocols. DECnet plus provides backward compatibility with DECnet Phase IV and supports multiple proprietary Digital protocols. DECnet plus supports functionality in the application, presentation, and session layers. The TCP/IP implementation of DECnet plus supports the lower-layer TCP/IP protocols and enables the transmission of DECnet traffic over TCP transport protocols.

DECnet Routing
DECnet Routing is a proprietary routing scheme introduced by Digital Equipment Corporation in DECnet Phase III. In DECnet Phase V, DECnet completed its transition to OSI routing protocols (ES-IS and IS-IS).

Dedicated LAN
Dedicated LAN is a network segment allocated to a single device, which is used in LAN switched network topologies.

Dedicated Line
Dedicated Line is a communications cable in telecommunications that is dedicated to a specific application, in contrast with a shared resource such as the telephone network. Practically, Dedicated Line may not be implemented by using a single, discrete, end-to-end cable, but they do provide guarantees of constant bandwidth availability and near-constant latency, properties that cannot be guarantied for more public systems. Such properties add a considerable premium to the price charged.

Default Gateway
A default gateway is a router or a switch in a network that serves as an access point to another network. In enterprises, the gateway is the computer that routes the traffic from a workstation to the outside network that is serving the Web pages.

Default Route
A default route is the network route used by a router when no other known route works for a given IP packet's destination address. A router determines which route is valid to reach an IP address by finding the "most specific match".

Defense Message System
The Defense Message System (DMS) is a deployment of secure electronic mail and directory services in the United States Department of Defense. DMS is usually operated in conjunction with DMDS a profiling system that takes a message and forwards it, based on message critera, to parties that are required to take action on a message. This combination has met success with the upper echelons of command, since parties do not have to wait for messaging center operators to route the messages to the proper channels for action.

Deflection routing
Deflection routing is a routing strategy for networks based on packet switching which does not require buffering of packets. Every packet has preferred outputs along which it wants to leave the router, and when possible a packet is sent along one of these outputs. However, two or more packets may want to leave along the same output (which is referred to as a contention among packets), and then only one of the packets may be sent along the link, while the others are sent along available outputs, even though the other links are not preferred by the packets .

Delay
Delay is the time between the initiation of a transaction by a sender and the first response received by the sender. Also, it is the time required to move a packet from source to destination over a given path.

DELNI : Digital Ethernet Local Network Interconnect
Digital Ethernet Local Network Interconnect (DELNI), similar to a hub, is an Ethernet communications product manufactured by Digital Equipment Corporation (DEC) in the 1980s. While it could be used in three different applications, the most popular one was connecting eight DS-200 Terminal Servers to a single Ethernet.

Demand Priority
Demand Priority is a media access method used in 100VG-AnyLAN that uses a hub that can handle multiple transmission requests and can process traffic according to priority, making it useful for servicing time-sensitive traffic such as multimedia and video. Demand priority eliminates the overhead of packet collisions, collision recovery, and broadcast traffic typical in Ethernet networks.

Demarc: Demarcation point
Demarcation point (Demarc), also known as Point of Demarcation (POD), is the network point or joint at which the public network of a telecommunications company ends and the private network of a customer begins. For example, the point between carrier equipment and CPE, at which the telephone company's local loop network ends, connects with the telephone system or wiring at the customer's premises. A demarcation point is also referred to as the minimum point of entry (MPOE).

Demodulation
Demodulation is the process of returning a modulated signal to its original form. Modems perform demodulation by taking an analog signal and returning it to its original (digital) form.

Demodulator
Demodulator is a device for assembling signals after they have been received by an antenna. A demodulator is typically the first major device downstream from an antenna receiving system. The corresponding device on the transmission side of a system is a modulator.

Demux: Demultiplexer
Demultiplexer (Demux) is a device used to separate two or more signals that previously were combined by a compatible multiplexer and are transmitted over a single channel.

DEN: Directory Enabled Networking
Directory-Enabled Networking (DEN) is an initiative to define specifications for how to construct and store information about a network's users, applications, and data in a central directory. DEN defines an object-oriented information model that is based on another recent standard initiative, the Common Information Model (CIM). Both models are being mapped into the directory defined as part of the Lightweight Directory Access Protocol (LDAP).

Dense Mode Multicast
Dense mode multicast is one mode which multicast can use to construct a tree for sending packets to the multicast subscribers. Its opposite is sparse multicast. Dense mode is ideal for groups where many of the nodes will subscribe to receive the multicast packets, so that most of the routers must receive and forward these packets (groups of a high density).

Deny
Deny is an action taken with an ACL that implies that the packet is discarded.

Deprovisioning
Deprovisioning is a process to eliminate of an existing subscriber account, which includes subscriber account deregistration and device de-activation.

DER: Distinguished Encoding Rules
Distinguished Encoding Rules (DER) is a subset of the Basic Encoding Rules, which gives exactly one way to represent any ASN.1 value as an octet string [X690].

Designated Bridge
A designated bridge is the bridge on each LAN that provides the minimum root path cost. A LAN's designated bridge is the only bridge allowed to forward frames to and from the LAN for which it is the designated bridge.

Designated Port
Designated Port is the port that connects it to the designated bridge. The designated port (interface) on a bridge or switch advertises the best spanningtree BPDU (hello message) onto a LAN segment.

Designated Router
Designated Router is a router running Open Shortest Path First (OSPF) that generates LSAs for a multiaccess network and has other special responsibilities in running OSPF. Each multiaccess OSPF network that has at least two attached routers has a designated router that is elected by the OSPF Hello protocol. The designated router enables a reduction in the number of adjacencies required on a multiaccess network, which in turn reduces the amount of routing protocol traffic and the size of the topological database

Destination Address
Destination Address is the address of a network device that is receiving data. Many protocols such as IP, TCP and UDP have a field for the destination address for packet routing and transportation.

Destination Node
Destination Node is a node having the network address, as determined by the release node, to be where an existing call should be routed. Destination Node is the termination of an end-to-end channel or virtual wavelength path (VWP). The destination node may or may not be a Signaling System 7 signaling point.

Destination User
Destination User refers to a user in an information transfer transaction that receives information from the source, i.e., from the originating user.

Deterministic Load Distribution
Deterministic Load Distribution is the technique for distributing traffic between two bridges across a circuit group. It guarantees packet ordering between source-destination pairs and always forwards traffic for a source-destination pair on the same segment in a circuit group for a given circuit-group configuration.

DFN: Deutsches ForschungsNetz e.V.
Deutsches ForschungsNetz e.V. (DFN) is the German computer network for academic and research purposes. DFN has been in operation since 1984. Its current network is called "G-WiN". It operates on Gigabit speeds, and is linked with 10G links to GEANT.

DIA: Document Interchange Architecture
Document Interchange Architecture (DIA) defines the protocols and data formats needed for the transparent interchange of documents in an SNA network. DIA is one of three IBM SNA transaction services.

Diag100
Diag100 is a diagnostic software utility that tests the card in a sample environment. It either comes with the NS4/100 or the FNIC 10/100. The phrase of "Provides Diag100 diagnostic software" means this product comes with Diag100 -- a diagnostic software utility, a program.

Dial Backup
Dial Backup is a feature that provides protection against WAN downtime by allowing the network administrator to configure a backup serial line through a circuit-switched connection.

Dial Peer
Dial Peer, also known as addressable call endpoint, is a device that can originate or receive a call in a telephone network. In voice over IP (VoIP) network dial peers include VoIP-capable computers, routers, and gateways within a network. POTS dial peers include traditional telephone network devices such as phone sets, cell phones, and fax machines.

Dial-up Line
Dial-up Line is a communications circuit that is established by a switched-circuit connection using the telephone company network.

DID/DNIS: Direct Inward Dialing/Dialed Number Identification Service
Direct Inward Dialing/Dialed Number Identification Service (DID/DNIS) refers to a service when a call arrives at an ACD or PBX, the carrier sends a digital code on the trunk line. The switch can read this code to determine how it should dispatch the call. Typically, this value is the specific number dialed by the user. By mapping each possible code with an internal extension, the switch can provide direct inward dialing (DID).The ICM uses the DID/DNIS value to specify the service, the skill group, or the specific agent to whom the switch should route the call. The switch reads the value from the trunk line when the call arrives and dispatches the call appropriately.

DID: Direct-inward-dial
Direct-Inward-Dial (DID) is a service offered by telephone companies that enables callers to dial directly into an extension on a PBX and not use an auto-attendant. Direct-inward-dial (DID) are the calls in which the gateway uses the number initially dialed (DNIS) to make the call, as opposed to a prompt to dial additional digits.

Differential Encoding
Differential Encoding is a digital encoding technique whereby a binary value is denoted by a signal change rather than a particular signal level.

Differential Manchester Encoding
Differential Manchester Encoding is a digital coding scheme where a mid-bit-time transition is used for clocking, and a transition at the beginning of each bit time denotes a zero. This coding scheme is used by IEEE 802.5 and Token Ring networks.

Differentiated Service
Differentiated Service refers to providing services at different QoS on the network by employing a small, well-defined set of building blocks from which a variety of services can be built.

DIN Connector: Deutsche Industrie Norm connector
Deutsche Industrie Norm connectors (DIN Connector) are 13.2 mm in diameter, and are available in patterns with three to fifteen pins. DIN Connector are used in some Macintosh and IBM PC-compatible computers, and on some network processor panels. For example, the keyboard connector for PCs is a DIN connector. DIN 41612 connectors are used widely to connect network equipment, such as routers and switches.

DIN: Deutsche Industrie Norm
Deutsche Industrie Norm (DIN) is a German national standards organization.

Directed Broadcast Address
Directed Broadcast Address, the same as a broadcast address, is an Internet Protocol address that specifies "all hosts" on a specified network. A single copy of a directed broadcast is routed to the specified network, where it is broadcast to all terminals on that network.

Directed Search
A directed search is used to determine the continued existence of the resource and to obtain routing information specific to the node.

Directed Tree
Directed Tree is a logical construct used to define data streams or flows. The origin of a data stream is the root. Data streams are unidirectional branches directed away from the root and toward targets, and targets are the leaves of the directed tree.

Directory Service
A directory service is a type of application that stores and organizes information about a computer network's users and network shares, and that allows network administrators to manage users' access to the shares. Additionally, directory services act as an abstraction layer between users and shared resources.

DISA: Defense Information Systems Agency
Defense Information Systems Agency (DISA), formerly DCA, is a combat support agency of the United States Department of Defense (DoD) responsible for planning, developing, fielding, operating, and supporting command, control, communications, and information systems that serve the needs of the President, the Secretary of Defense, the Joint Chiefs of Staff, the Combatant commanders, and other Department of Defense components under all conditions of peace and war.

Discovery Mode
Discovery Mode, also called dynamic configuration, is a method by which an AppleTalk interface acquires information about an attached network from an operational node, and then uses this information to configure itself. The discovery mode is set and reset using the appletalk discovery interface subcommand.

Disengage Request
Disengage Request is a message in the H.323 protocol with the Billing Information Token (which contains the duration of the call) sent by the gateway to the gatekeeper when a call ends.

DISN: Defense Information System Network
Defense Information System Network (DISN) is a network that provides global voice services through the Defense Switched Network (DSN).

Distance Vector
Distance Vector is one of the two routing algorithms. The other type is Link State Routing. Basically, routing protocols based on distance vector algorithms determine best path on how far the destination is, while LinkState protocols are capable of using more sophisticated methods taking into consideration link variables, such as bandwidth, delay, reliability and load. Distance Vector protocols judge best path on how far it is. Distance can be hops or a combination of metrics calculated to represent a distance value.

Distance Vector Routing Algorithm
Distance Vector Routing Algorithm is a type of routing algorithm that iterate on the number of hops in a route to find a shortest-path spanning tree. Distance vector routing algorithms call for each router to send its entire routing table in each update, but only to its neighbors. Distance vector routing algorithms can be prone to routing loops, but are computationally simpler than link state routing algorithms. Also called Bellman-Ford routing algorithm.

Distortion Delay
Distortion Delay, also called group delay, refers to distortions caused by the difference in arrival times of the frequency components at the output of a transmission system in a waveform consisting of two or more wave components at different frequencies.

Distributed Computing Environment
The Distributed Computing Environment (DCE) is a software system developed in the early 1990s by a consortium that included Apollo Computer (now part of Hewlett-Packard), IBM, Digital Equipment Corporation, and others. The DCE supplies a framework and toolkit for developing client/server applications. The framework includes a remote procedure call (RPC) mechanism known as DCE/RPC, a naming (directory) service, an authentication service, and a distributed file system (DFS).

Distributed Director
Distributed Director is a method of distributing Web traffic by taking into account Web server availability and relative client-to-server topological distances in order to determine the optimal Web server for a client. Distributed Director uses the Director Response Protocol to query DRP server agents for BGP and IGP routing table metrics.

Distributed Processing
Distributed Processing is a technique of distributing the information-processing functions over a number of devices. The devices may be computers or data terminals with some level of intelligence. The devices may or may not be interconnected with communication facilities.

Distributed Switching
Distributed Switching is a technique of distributing the switching function of a communications network over a number of switching or routing devices. There may or may not be some central controls over the switching process. There will usually be some forms of signaling information passed between the switches.

DIT: Directory Information Tree
Directory Information Tree (DIT) is a global tree of entries corresponding to information objects in the OSI X.500 Directory.

DIX: DEC, Intel, and Xerox
DIX, abreviated from three company names: DEC, Intel, and Xerox, refers to the the Ethernet Version 2 or Ethernet II frame, the so-called DIX frame. Ethernet v2 was defined by the three leading companies and is the most commonly used Ethernet today. Ethernet v2 framing differs from the original framing used in IEEE 802.3 by its interpretation of the 2-octet field following the destination and source addresses. With Ethernet v2 framing, that field is treated as an EtherType and immediately identifies an upper-layer protocol. For example, an EtherType value of 0x0800 signals that an IP datagram is encapsulated. Likewise, an EtherType of 0x0806 indicates an ARP frame. With the original framing used in IEEE 802.3, that field is treated as a packet length, and an IEEE 802.2 header is expected to follow the length field.

DLCI: Data-link Connection Identifier
Data-link Connection Identifier (DLCI) is the value that specifies a PVC or SVC in a Frame Relay network. In the basic Frame Relay specification, DLCIs are locally significant (connected devices might use different values to specify the same connection). In the LMI extended specification, DLCIs are globally significant (DLCIs specify individual end devices).

DLL: Delay Locked Loop
Delay-Locked Loop (DLL) supports high-bandwidth data rates between devices. These DLLs are circuits that provide zero propagation delay, low-clock skew between output clock signals throughout a device, and advanced clock domain control. These dedicated DLLs can be used to implement several circuits that improve and simplify system-level design.

DLSw+: Data-link Switching Plus
Data-link Switching Plus (DLSw+) refers to the Cisco implementation of the DLSw standard for SNA and NetBIOS traffic forwarding. DLSw+ goes beyond the standard to include the advanced features of the current Cisco RSRB implementation, and provides additional functionality to increase the overall scalability of data-link switching.

DLU: Dependent LU
Dependent LU (DLU), a term used in the IBM SNA APPN, is the logic unit (LU) that depends on the SSCP to provide services for establishing sessions with other LUs.

DLUR Node
DLUR Node is an end node (EN) or an network node (NN) that implements the Dependent LU Requester (DLUR) component in APPN networks.

DLUR: Dependent LU Requester
Dependent LU Requester (DLUR) is the client half of the Dependent LU Requestor/Server enhancement to the IBM APPN. The DLUR component resides in APPN ENs and NNs that support adjacent DLUs by securing services from the DLUS.

DLUS Node
DLUS Node is an network node (NN) that implements the Dependent LU Server (DLUS) component in APPN networks.

DLUS: Dependent LU Server
Dependent LU Server (DLUS) is the server half of the Dependent LU Requestor/Server enhancement to the IBM APPN. The DLUS component provides SSCP services to DLUR nodes over an APPN network.

DMA: Direct Memory Access
Direct Memory Access (DMA) refers to the transfer of data from a peripheral device, such as a hard disk drive, into memory without that data passing through the microprocessor. DMA transfers data into memory at high speeds with no processor overhead.

DMAC: Destination MAC
Destination MAC (DMAC) is the MAC address specified in the Destination Address field of a packet.

DMP: Data Movement Processor
Data Movement Processor (DMP) is the processor on the Cisco Catalyst 5000 that, along with the multiport packet buffer memory interface, performs the frame-switching function for the switch. The DMP also handles translational bridging between the Ethernet and FDDI interfaces, IP segmentation, and intelligent bridging with protocol-based filtering.

DMTF: Distributed Management Task Force
Distributed Management Task Force (DMTF, formerly "Desktop Management Task Force") is an industry consortium that develops, supports, and maintains standards for systems management of PC systems and products, to reduce total cost of ownership. These include the Desktop Management Interface (DMI), the most-widely used management standard today.

DN: Dialed Number
Dialed Number (DN) is the telephone number that a caller dialed to initiate a call.

DN: Distinguished Name
Distinguished Name is the global,authoritative name of an entry in the OSI Directory based on the X.500 protocol.

DNA: Digital Network Architecture
Digital Network Architecture (DNA), defined by Digital Equipment Corporation now part of HP, is essentially a set of documents which define the network architecture in general, states the specifications for each layer of the architecture, and describes the protocols which operate within each layer.

DNA: DoNotAge
DoNotAge (DNA) is the most significant bit of the LS Age field. LSAs having the DoNotAge bit set are not aged as they are in the link-state database of the OSPF router, which means that these LSAs need not be refreshed every 30 minutes.

DNIC: Data Network Identification Code
Data Network Identification Code (DNIC) is the part of an X.121 address. DNICs are divided into two parts: the first specifying the country in which the addressed PSN is located and the second specifying the PSN itself.

DNIS: Dialed Number Identification Service
Dialed Number Identification Service(DNIS) is the feature of trunk lines where the called number is identified; this called number information is used to route the call to the appropriate service. DNIS is a service used with toll-free dedicated services whereby calls placed to specific toll-free numbers are routed to the appropriate area within a company to be answered.

DNS zone: Domain Name Server zone
Domain Name Server zone (DNS zone) is the point of delegation in the DNS tree. It contains all names from a certain point downward except those for which other zones are authoritative. Authoritative name servers can be asked by other DNSs for name-to-address translation. Many name servers can exist within an organization, but only those known by the root name servers can be queried by the clients across the Internet. The other name servers answer only internal queries.

DNSIX: DOD Network Security for Information Exchange
Department of Defense Intelligence Information System Network Security for Information Exchange (DNSIX) is a collection of security requirements for networking defined by the U.S. Defense Intelligence Agency.

DOCSIS CM: DOCSIS Cable Modem
DOCSIS Cable Modem (DOCSIS CM) is a cable modem based on the DOCSIS standards that connects to the Internet via TV cable network from homes.

DOCSIS CMTS: DOCSIS cable modem termination system
DOCSIS cable modem termination system (DOCSIS CMTS), also known as Cable Modem Termination System (CMTS), is a specially designed router or a bridge for data networking. CMTS is located at the headend of service providers and used to aggregate traffic from multiple Cable Modems and then communicate with the backbone network.

DOCSIS Configuration File
DOCSIS Configuration File is the file containing configuration parameters for a DOCSIS cable modem. The cable modem obtains this file at boot time using the TFTP protocol.

DoD: Department Of Defense
Department of Defense (DoD) is the U.S. government organization that is responsible for national defense. The DoD frequently has funded communication protocol development.

Domain
Domain means many different things in networking. 1. In the Internet, domain refers to a URL for a web site. 2. In VLAN, it is a portion of the spanning hierarchy tree that refers to general groupings of networks based on organization type or geography. 3. In SNA, it is an SSCP and the resources it controls. 4. In IS-IS, it is a logical set of networks.

Domain Name
Domain Name is the unique name that identifies a web site on the Internet. Domain Names always have 2 or more parts, separated by dots. A given machine may have more than one Domain Name but a given Domain Name points to only one machine. For example, the domain names are: javvin.com, www.networkdictionary.com, but each domain name can refer to no more than one machine.

Dongle
In computer networking, a dongle is a short network cable that joins a PCMCIA adapter to a network cable. Dongles typically attach to either a RJ-45 connector for Ethernet networking or an RJ-11 connector for dial-up networking. Dongles tend to run no longer than about six inches.
The term "dongle" also has become popular in USB networking, referring to the USB cable that extends from a USB peripheral.

Dot Address
Dot Address, also called dotted notation and four-part dotted notation, refers to the common notation for IP addresses in the form n.n.n.n where each number n represents, in decimal, 1 byte of the 4-byte IP address.

Dot-decimal Notation
Dot-decimal notation, also called Dotted decimal notation or dotted quad notation, is a method of writing down octet strings using base-10 (decimal) rather than hexadecimal numbers. Adding dots allows for easy separation of the individual octet values. For example, 255.255.255.255 is in dotted-decimal notation. IPv4 addresses are almost universally written or typed in dot-decimal notation.

Dotted Decimal Notation
Dotted Decimal Notation, also called Dot-decimal notation or dotted quad notation, is the syntactic representation for a 32-bit integer that consists of four 8-bit numbers written in base 10 with periods (dots) separating them rather than hexadecimal numbers. It is used to represent IP addresses on the Internet, as in 192.67.68.35.

Download
Download refers to the process of moving a file from a remote network location to a local computer. There are a few methods of doing this on the Internet. HTTP, FTP and e-mail attachments are the most common.

Downstream
In Internet access, from the user's perspective, downstream refers to traffic handled at the service provider end and terminating to the customer. In other words, downstream traffic flows to the user's computer. On the other hand, upstream network traffic flows away from the local computer toward the remote destination. Traffic on most networks flows in both upstream and downstream directions simultaneously.

DPM: Defect Per Million
Defect Per Million (DPM), alos called Defects per million opportunites (DPMO), constitutes a standard of measure that correlates failures of expected norms in any process. One failure per every million normative outcomes constitutes 1 DPMO.

DPMO: Defects per million opportunites
Defects per million opportunites (DPMO), also called Defects per million (DPM), constitutes a standard of measure that correlates failures of expected norms in any process. One failure per every million normative outcomes constitutes 1 DPMO.

DRDA: Distributed Relational Database Architecture
Distributed Relational Database Architecture is an IBM proprietary architecture for relational database.

Drop
A drop is a point on a multipoint channel where a connection to a networked device is made

Drop Cable
Drop Cable is the cable that connects a network device (such as a computer) to a physical medium such as Ethernet transceiver. The maximum length if full-spec is 47m.

Drop-and-Insert
Drop-and-Insert is a feature that enables a single T1 access line to carry both data and voice traffic. The MAX uses a pre-allocated portion of the T1 access line to use both nailed-up and switched circuits for LAN internetworking. The remaining portion of the line can go to a PBX with a T1 interface; the PBX can access both nailed-up and switched circuits for voice purposes. Drop-and-Insert can also be used to share access line bandwidth between the MAX and equipment other than a PBX, such as a channel bank or T1 multiplexer.

DSA: Directory System Agent
Directory System Agent (DSA) is a piece of software that provides the X.500 Directory Service for a portion of the directory information base. Generally, each DSA is responsible for the directory information for a single organization or organizational unit.

DSAP: Destination Service Access Point
Destination Service Access Point (DSAP) is the individual or group address for the address into the upper layers of the network protocol stack. DSAP is an eight bit field that allows the LLC layer to keep track of multiple connections across an IEEE 802 LAN environment.

DSCP: Differentiated services code point
Differentiated Services Code Point (DSCP) is a field in an IP packet that enables different levels of service to be assigned to network traffic. DSCP is a 6 bits field that is the combination of IP Precedence (3bits) with a 3 bits expansion, as defined in RFC 2474. DSCP can be marked for each packet on the network with a DSCP code and appropriating to it the corresponding level of service. In order to work with legacy routers that only support IP Precedence, DSCP values are used because they are compatible with IP Precedence fields.

DSF: Dispersion-shifted fiber
Dispersion-shifted fiber (DSF) is a type of single-mode fiber designed to have zero dispersion near 1550 nm.

DSLAM: Digital Subscriber Line Access Multiplexer
Digital Subscriber Line Access Multiplexer (DSLAM), a system in the DLS technology architecture, links and aggregates many customer DSL connections to a single high-speed ATM line. To enable DSL technology, service providers must have a DSLAM located in their network to interact with the customer premises equipment (CPE) at the end-user location. When the phone company receives a DSL signal, an ADSL modem with a POTS splitter detects voice calls and data. Voice calls are sent to the PSTN, and data are sent to the DSLAM, where it passes through the ATM to the Internet, then back through the DSLAM and ADSL modem before returning to the customer's PC.

DSn: Delivery Status notification
Delivery Status notification (DSn) is the 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: Domain Specific Part
Domain Specific Part (DSP) is the part of an NSAP-format ATM address that contains an area identifier, a station identifier, and a selector byte.

DSPU concentration
DSPU concentration is a Cisco IOS feature that enables a router to function as a PU concentrator for SNA PU 2 nodes. PU concentration at the router simplifies the task of PU definition at the upstream host while providing additional flexibility and mobility for downstream PU devices.

DSPU: Downstream Physical Unit
Downstream Physical Unit (DSPU), a term in the IBM SNA, is a PU that is located downstream from the host.

DSR: Data Set Ready
Data Set Ready(DSR) refers to the EIA/TIA-232 interface circuit that is activated when DCE is powered up and ready for use

DSRFLOW
DSRFLOW, the Flow-State extensions to Dynamic Source Routing (DSR), provides all of the benefits of source routing, without most of the per-packet overhead that is associated with source routing. It works by allowing most packets to be sent without a source route header, thus substantially reducing overhead. Indeed, one of the disadvantages of DSR was that the longer the source route of the packet was, the bigger the packet header became. The technique used is called implicit source routing.

DSU: Data Service Unit
Data Service Unit(DSU), often referred to together with CSU, as CSU/DSU, is the device used in digital transmission that adapts the physical interface on a DTE device to a transmission facility, such as T1 or E1. The DSU also is responsible for such functions as signal timing.

DSX-1
Digital Signal Cross-connect Level 1(DSX-1) is a standard that defines the voltage, pulse width and plug and socket for connecting DS-1 (T1) signals.

DTE: Data Terminal Equipment
Data Terminal Equipment (DTE) refers to a user end node or device in a network. DTE may serve as a data source, destination, or both, which provides for the data communication control function to be performed in accordance with link protocol. DTE connects to a data network through a DCE device and typically uses clocking signals generated by the DCE. DTE includes devices such as computers, protocol translators, and multiplexers.

DTL: Designated Transit List
Designated Transit List (DTL) is a list of nodes and optional link IDs that completely specify a path across a single PNNI peer group.

DTMF relay: Dual-tone Multifrequency Relay
Dual-tone Multifrequency Relay (DTMF) is the mechanism whereby a local Voice over IP (VOIP) gateway listens for DTMF digits (during a call), and then sends them uncompressed as either RTP or H.245 packets to the remote VOIP gateway, which regenerates DTMF digits and prevents digit loss due to compression.

DTMF: Dual Tone Multifrequency
Dual Tone Multifrequency(DTMF), also known as Touch Tone or Tone Dialing, is used for telephone signaling over the line in the voice frequency band to the call switching center. DTMF is the tones generated when a button is pressed on a telephone, primarily used in the U.S. and Canada. DTMF is an example of a multifrequency shift keying (MFSK) system and is standardised by ITU-T Recommendation Q.23.

DTR: Data Terminal Ready
Data Terminal Ready (DTR) refers to EIA/TIA-232 circuit that is activated to let the DCE know when the DTE is ready to send and receive data.

DUA: Directory User Agent
Directory User Agent(DUA) is a piece of software that accesses the X.500 Directory Service on behalf of the directory user. The directory user can be a person or another software element.

Dual Counter-rotating Rings
Dual Counter-rotating Rings is a network topology in which two signal paths, whose directions are opposite each other, exist in a token-passing network. FDDI and CDDI are based on this concept.

Dual Homing
Dual Homing is a network topology in which a device is connected to the network by way of two independent access points (points of attachment). One access point is the primary connection, and the other is a standby connection that is activated in the event of a failure of the primary connection.

Dual Stack
Dual stack hosts are Internet hosts providing two discrete network layers. The term stack here refers to the protocol stack or suite of protocols used in computer networking software. Dual stack hosts run both the current standard, IPv4 and the next generation Internet layer, IPv6.

DUAL: Diffusing Update Algorithm
Diffusing Update Algorithm(DUAL) is a convergence algorithm used in Enhanced IGRP that provides loop-free operation at every instant throughout a route computation. Allows routers involved in a topology change to synchronize at the same time, while not involving routers that are unaffected by the change.

Dual-homed Station
Dual-homed Station is a device attached to multiple FDDI rings to provide redundancy.

Dumb network
A "dumb network" provides the physical interconnection between nodes but not much processing to support signaling. The Internet is often cited as a dumb network relative to the public switched telephone network(PSTN). The PSTN is considered an "intelligent network" because the intelligence required for operation is carried within the network, while the end devices (telephones) are simple devices. The Internet takes the opposite approach: the network simply transports packets of data and the end devices (computers, for example) contain the intelligence to process the data. This approach is sometimes referred to as "dumb network, smart devices."

DVB: Digital Video Broadcasting
Digital Video Broadcasting (DVB) is a suite of internationally accepted, open standards for digital television maintained by the DVB Project and published by the Joint Technical Committee (JTC) of European Telecommunications Standards Institute (ETSI), European Committee for Electrotechnical Standardization (CENELEC) and European Broadcasting Union (EBU). DVB standards define the physical layer and data link layer of a distribution system. Devices interact with the physical layer via a synchronous parallel interface (SPI), synchronous serial interface (SSI), or asynchronous serial interface (ASI). All data is transmitted in MPEG-2 transport streams with some additional constraints (DVB-MPEG). A standard for temporally compressed distribution to mobile devices (DVB-H) has been published in November, 2004.

DVB-C
DVB-C stands for Digital Video Broadcasting - Cable and it is the DVB European consortium standard for the broadcast transmission of digital television over cable. This system transmits an MPEG-2 family digital audio/video stream, using a QAM modulation with channel coding.

DVB-S
DVB-S, standing for Digital Video Broadcasting-Satelite, is the original Digital Video Broadcasting (DVB) forward error coding and modulation standard for satellite television and dates from 1995. It is used via satellites serving every continent of the world. DVB-S is used in both MCPC and SCPC modes for broadcast network feeds, as well as for direct broadcast satellite services like Sky TV (UK) via Astra in Europe, Dish Network in the U.S., and Bell ExpressVu in Canada. The transport stream delivered by DVB-S is mandated as MPEG-2.

DVB-S2
DVB-S2 is an improved and updated specification to replace the DVB-S (Digital Video Broadcasting-Satelite) standard, ratified by ETSI in March, 2005. The main use for this standard is the distribution of HDTV, while the original standard DBV-S was mainly applied to SDTV services. The development of DVB-S2 coincided with the introduction of HDTV and H.264 (MPEG-4) video codecs.

DVB-H
DVB-H stands for Digital Video Broadcasting - Handheld. DVB-H is a technical specification for bringing broadcast services to handheld receivers and was formally adopted as ETSI standard EN 302 304 in November, 2004. The DVB-H specification (EN 302 304) can be downloaded from the DVB-H Online website ([1]). The major competitor of this technology is DMB.

DVB-T
DVB-T stands for Digital Video Broadcasting - Terrestrial and it is the DVB European consortium standard for the broadcast transmission of digital terrestrial television. This system transmits an MPEG-2 family digital audio/video stream, using OFDM modulation with concatenated channel coding (i.e. COFDM).

DVVI: Data, Voice, Video Integration
Data, Voice, Video Integration (DVVI) refers to providing data voiceand video services over one single network. This term term is often used for an Enterprise network. In the case of a Service Provider, it is often called triple play.

DWDM: Dense Wavelength Division Multiplexing
Dense Wavelength Division Multiplexing(DWDM) is an optical technology used to increase bandwidth over existing fiber-optic backbones. DWDM works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber. In effect, one fiber is transformed into multiple virtual fibers. A key advantage of DWDM is that it's protocol and bit-rate independent. DWDM-based networks can transmit data in IP, ATM, SONET/SDH, and Ethernet, and handle bit-rates between 100 Mb/s and 2.5 Gbps.

DXI: Data Exchange Interface
Data Exchange Interface (DXI) is an ATM Forum specification described in RFC 1483. It defines how a network device such as a bridge, router, or hub can effectively act as an FEP to an ATM network by interfacing with a special DSU that performs packet segmentation and reassembly.

Dynami Routing
Dynami Routing, also called adaptive routing, is a type of routing that adjusts automatically to network topology or traffic changes.

Dynamic Adaptive Routing
Dynamic Adaptive Routing refers to the automatic rerouting of traffic based on a sensing and analysis of current actual network conditions, not including cases of routing decisions taken on predefined information.

Dynamic Address Resolution
Dynamic Address Resolution is the process of using an address resolution protocol to determine and store address information on demand.

Dynamic Buffer Management
Dynamic Buffer Management is a scheme used Frame Relay and ATM service modules for allocating and scaling traffic entering or leaving a node on a per-VC basis. The WAN switch dynamically assigns buffers to individual virtual circuits based upon the amount of traffic present and service-level agreements. This deep pool of available buffers readily accommodates large bursts of traffic into the node.

Dynamic Ports
Dynamic Ports, also known as Private Ports, refer to one of the classes of the TCP/UDP ports, defined by IETF IANA. The other two classes are the Registered Ports and the Well Known Ports. The Private/Dynamic are those with a port number from 49152 through 65535, which could be used by any company for their private applications without any IANA administration.

Dynamic Routing
Dynamic Routing, also called adaptive routing, is a type of routing that adjusts automatically to network topology or traffic changes.

Dynamic Routing Protocol
Dynamic Routing Protocols allow network devices to learn routes. RIP and EIGRP are dynamic routing protocols which enable dynamic routing when routers talk to adjacent routers, informing each other of what networks each router is currently connected to. The routers must communicate using a routing protocol.

Dynamic Switched Call
Dynamic Switched Call is a type of VOIP call dynamically established across a packet data network based on a dialed telephone number.