D4 framing

See SF.


dual-attached concentrator. FDDI or CDDI concentrator capable of attaching to both rings of an FDDI or CDDI network. It can also be dual-homed from the master ports of other FDDI or CDDI concentrators.


Defense Advanced Research Projects Agency. U.S. government agency that funded research for and experimentation with the Internet. Evolved from ARPA, and then, in 1994, back to ARPA. See also ARPA.

DARPA Internet

Obsolete term referring to the Internet. See Internet.


dual attachment station. 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. Also known as a Class A station. Compare with SAS.

database object

1. In general, a piece of information that is stored in a database.
2. Chassis, card, or port defined in the configuration database of a LightStream 2020 ATM switch. Database objects have associated attributes that describe them.

data bus connector

See DB connector.

data channel

See D channel.

data circuit-terminating equipment

See DCE.

data communications equipment

See DCE.

Data Country Code

See DCC.

Data Encryption Standard

See DES.

Data Exchange Interface

See DXI.

data flow control layer

Layer 5 of the SNA architectural model. This layer determines and manages interactions between session partners, particularly data flow. Corresponds to the session layer of the OSI model. See also data link control layer, path control layer, physical control layer, presentation services layer, transaction services layer, and transmission control layer.


Logical grouping of information sent as a network layer unit over a transmission medium without prior establishment of a virtual circuit. IP datagrams are the primary information units in the Internet. The terms frame, message, packet, and segment are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.

Datagram Delivery Protocol

See DDP.

data-link connection identifier


data link control layer

Layer 2 in the SNA architectural model. Responsible for the transmission of data over a particular physical link. Corresponds roughly to the data link layer of the OSI model. See also data flow control layer, path control layer, physical control layer, presentation services layer, transaction services layer, and transmission control layer.

data link layer

Layer 2 of the OSI reference model. This layer 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. The IEEE has divided this layer into two sublayers: the MAC sublayer and the LLC sublayer. Sometimes simply called link layer. Roughly corresponds to the data link control layer of the SNA model. See also application layer, LLC, MAC, network layer, physical layer, presentation layer, session layer, and transport layer.

data-link switching

See DLSw.

Data Movement Processor

See DMP.

Data Network Identification Code


data set ready

See DSR.

data service unit

See DSU.

data sink

Network equipment that accepts data transmissions.

data stream

All data transmitted through a communications line in a single read or write operation.

data terminal equipment

See DTE.

data terminal ready

See DTR.



DB connector

data bus connector. Type of connector used to connect serial and parallel cables to a data bus. DB connector names are of the format 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 are defined by various EIA/TIA standards.


direct current. Electrical current that travels in only one direction. Direct current is generally used in electronic circuits. See DC."


Defense Communications Agency. U.S. government organization responsible for DDN networks such as MILNET. Now called DISA. See DISA.


Data Country Code. One of two ATM address formats developed by the ATM Forum for use by private networks. Adapted from the subnetwork model of addressing in which the ATM layer is responsible for mapping network layer addresses to ATM addresses. See also ICD.


data communications equipment (EIA expansion) or data circuit-terminating equipment (ITU-T expansion). The devices and connections of a communications network that comprise the network end of the user-to-network interface. 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. Compare with DTE.

D channel

1. data channel. Full-duplex, 16-kbps (BRI) or 64-kbps (PRI) ISDN channel. Compare to B channel, E channel, and H channel.
2. In SNA, a device that connects a processor and main storage with peripherals.


Distributed Data Management. Software in an IBM SNA environment that provides peer-to-peer communication and file sharing. One of three SNA transaction services. See also DIA and SNADS.


Defense Data Network. U.S. military network composed of an unclassified network (MILNET) and various secret and top-secret networks. DDN is operated and maintained by DISA. See also DISA and MILNET.


Datagram Delivery Protocol. Apple Computer network layer protocol that is responsible for the socket-to-socket delivery of datagrams over an AppleTalk internetwork.


dial-on-demand routing. Technique whereby a Cisco router can automatically initiate and close a circuit-switched session as transmitting stations demand. The router spoofs keepalives so that end stations treat the session as active. DDR permits routing over ISDN or telephone lines using an external ISDN terminal adaptor or modem.


discard eligible. See tagged traffic.


1. Unresolved contention for the use of a resource.
2. In APPN, when two elements of a process each wait for action by or a response from the other before they resume the process.


Abbreviated dB.


Group of communications products (including a protocol suite) developed and supported by Digital Equipment Corporation. DECnet/OSI (also called DECnet Phase V) is the most recent iteration and supports both OSI protocols and proprietary Digital protocols. Phase IV Prime supports inherent MAC addresses that allow DECnet nodes to coexist with systems running other protocols that have MAC address restrictions. See also DNA.

DECnet routing

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).

decorative raceway

Type of wall-mounted channel with removable cover used to support horizontal cabling. Decorative raceway is big enough to hold two cables.


The reverse application of an encryption algorithm to encrypted data, thereby restoring that data to its original, unencrypted state. See also encryption.

dedicated LAN

Network segment allocated to a single device. Used in LAN switched network topologies.

dedicated line

Communications line that is indefinitely reserved for transmissions, rather than switched as transmission is required. See also leased line.

de facto standard

Standard that exists by nature of its widespread use. Compare with de jure standard. See also standard.

default route

Routing table entry that is used to direct frames for which a next hop is not explicitly listed in the routing table.

Defense Advanced Research Projects Agency


Defense Communications Agency

See DCA.

Defense Data Network

See DDN.

Defense Information Systems Agency


Defense Intelligence Agency

See DIA.

de jure standard

Standard that exists because of its approval by an official standards body. Compare with de facto standard. See also standard.


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

demand priority

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. See also 100VG-AnyLAN.


Demarcation point between carrier equipment and CPE.


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. See also modulation.


The separating of multiple input streams that have been multiplexed into a common physical signal back into multiple output streams. See also multiplexing.

dense mode PIM

See PIM dense mode.

Department of Defense

See DoD.

Department of Defense Intelligence Information System Network Security for Information Exchange


Dependent LU

See DLU.

Dependent LU Requester


Dependent LU Server



Data Encryption Standard. Standard cryptographic algorithm developed by the U.S. NBS.

designated bridge

The bridge that incurs the lowest path cost when forwarding a frame from a segment to the route bridge.

designated router

OSPF router 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

Address of a network device that is receiving data. See also source address.

destination MAC


destination service access point


deterministic load distribution

Technique for distributing traffic between two bridges across a circuit group. 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.

Deutsche Industrie Norm

See DIN.

Deutsche Industrie Norm connector

See DIN connector.


See node.


Document Interchange Architecture. Defines the protocols and data formats needed for the transparent interchange of documents in an SNA network. One of three SNA transaction services. See also DDM and SNADS.

dial backup

Feature supported by Cisco routers that provides protection against WAN downtime by allowing the network administrator to configure a backup serial line through a circuit-switched connection.

dial-on-demand routing

See DDR.

dial-up line

Communications circuit that is established by a switched-circuit connection using the telephone company network.

differential encoding

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

differential Manchester encoding

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. The coding scheme used by IEEE 802.5 and Token Ring networks.

Diffusing Update Algorithm


Digital Network Architecture

See DNA.

digital signal

Language of computers comprising only two states, on and off which are indicated by a series of voltage pulses.

digital signal level 0

See DS-0.

digital signal level 1

See DS-1.

digital signal level 3

See DS-3.

Dijkstra's algorithm

See SPF.


Deutsche Industrie Norm. German national standards organization.

DIN connector

Deutsche Industrie Norm connector. Multipin connector used in some Macintosh and IBM PC-compatible computers, and on some network processor panels.

directed search

Search request sent to a specific node known to contain a resource. A directed search is used to determine the continued existence of the resource and to obtain routing information specific to the node. See also broadcast search.

direct memory access

See DMA.

directory services

Services that help network devices locate service providers.


Defense Information Systems Agency. U.S. military organization responsible for implementing and operating military information systems, including the DDN. See also DDN.

discard eligible

See DE.

discovery architecture

APPN software that enables a machine configured as an APPN EN to automatically find primary and backup NNs when the machine is brought onto an APPN network.

discovery mode

Method by which an AppleTalk interface acquires information about an attached network from an operational node and then uses this information to configure itself. Also called dynamic configuration.

disk assembly

The combination of a hard disk drive, a floppy disk drive, and a disk power supply on a LightStream 2020 ATM switch. Each NP card in a LightStream 2020 chassis has its own disk assembly.

Distance Vector Multicast Routing Protocol


distance vector routing algorithm

Class of routing algorithms 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. See also link state routing algorithm and SPF.

distortion delay

Problem with a communication signal resulting from nonuniform transmission speeds of the components of a signal through a transmission medium. Also called group delay.

distributed computing (processing)

See client-server computing.

Distributed Data Management

See DDM.

Distributed Queue Dual Bus



data-link connection identifier. 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). See also LMI.


data-link switching. Interoperability standard, described in RFC 1434, that provides a method for forwarding SNA and NetBIOS traffic over TCP/IP networks using data link layer switching and encapsulation. DLSw uses SSP (Switch-to-Switch Protocol) instead of SRB, eliminating the major limitations of SRB, including hop-count limits, broadcast and unnecessary traffic, timeouts, lack of flow control, and lack of prioritization schemes. See also DLSw+, SRB, and SSP (Switch-to-Switch Protocol).


Data Link Switching Plus. 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. See also DLSw.


Dependent LU. An LU that depends on the SSCP to provide services for establishing sessions with other LUs. See also LU and SSCP.


Dependent LU Requester. The client half of the Dependent LU Requestor/Server enhancement to APPN. The DLUR component resides in APPN ENs and NNs that support adjacent DLUs by securing services from the DLUS. See also APPN, DLU, and DLUS.

DLUR node

In APPN networks, an EN or NN that implements the DLUR component. See also DLUR.


Dependent LU Server. The server half of the Dependent LU Requestor/Server enhancement to APPN. The DLUS component provides SSCP services to DLUR nodes over an APPN network. See also APPN, DLU, and DLUR.

DLUS node

In APPN networks, a NN that implements the DLUS component. See also DLUS.


direct memory access. 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.


destination MAC. The MAC address specified in the Destination Address field of a packet. Compare with SMAC. See also MAC address.


Data Movement Processor. Processor on the 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. See also Catalyst 5000.


Digital Network Architecture. Network architecture developed by Digital Equipment Corporation. The products that embody DNA (including communications protocols) are collectively referred to as DECnet. See also DECnet.


Data Network Identification Code. 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. See also X.121.


Domain Naming System. System used in the Internet for translating names of network nodes into addresses. See also authority zone.


Department of Defense Intelligence Information System Network Security for Information Exchange. Collection of security requirements for networking defined by the U.S. Defense Intelligence Agency.

Document Interchange Architecture

See DIA.


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


1. In the Internet, a portion of the naming hierarchy tree that refers to general groupings of networks based on organization-type or geography.
2. In SNA, an SSCP and the resources it controls.
3. In IS-IS, a logical set of networks.


Networking system developed by Apollo Computer (now part of Hewlett-Packard) for use in its engineering workstations.

Domain Naming System

See DNS.

domain specific part

See DSP.

dot address

Refers to the common notation for IP addresses in the form <a.b.c.d> where each number a represents, in decimal, 1 byte of the 4-byte IP address. Also called dotted notation or four-part dotted notation.

dotted notation

See dot address.

downlink station

See ground station.

downstream physical unit



Distributed Queue Dual Bus. Data link layer communication protocol, specified in the IEEE 802.6 standard, designed for use in MANs. DQDB, which permits multiple systems to interconnect using two unidirectional logical buses, is an open standard that is designed for compatibility with carrier transmission standards, and is aligned with emerging standards for BISDN. SIP (SMDS Interface Protocol) is based on DQDB. See also MAN.


dynamic random-access memory. RAM that stores information in capacitors that must be periodically refreshed. Delays can occur because DRAMs are inaccessible to the processor when refreshing their contents. However, DRAMs are less complex and have greater capacity than SRAMs. See also SRAM.


Point on a multipoint channel where a connection to a networked device is made.

drop cable

Generally, a cable that connects a network device (such as a computer) to a physical medium. A type of AUI. See also AUI.


digital signal level 0. Framing specification used in transmitting digital signals over a single channel at 64-kbps on a T1 facility. Compare with DS-1 and DS-3.


digital signal level 1. Framing specification used in transmitting digital signals at 1.544-Mbps on a T1 facility (in the United States) or at 2.108-Mbps on an E1 facility (in Europe). Compare with DS-0 and DS-3. See also E1 and T1.

DS-1 domestic trunk interface

See DS-1/DTI.


DS-1 domestic trunk interface. Interface circuit used for DS-1 applications with 24 trunks.


digital signal level 3. Framing specification used for transmitting digital signals at 44.736-Mbps on a T3 facility. Compare with DS-0 and DS-1. See also E3 and T3.


destination service access point. The SAP of the network node designated in the Destination field of a packet. Compare to SSAP. See also SAP (service access point).


domain specific part. The part of a CLNS address that contains an area identifier, a station identifier, and a selector byte.


1. downstream physical unit. In SNA, a PU that is located downstream from the host.
2. Cisco IOS software 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. This feature is sometimes referred to as DSPU concentration. See also PU and SNA.

DSPU concentration

See DSPU and PU.


data set ready. EIA/TIA-232 interface circuit that is activated when DCE is powered up and ready for use.


data service unit. 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 is also responsible for such functions as signal timing. Often referred to together with CSU, as CSU/DSU. See also CSU.


Cross-connection point for DS-1 signals.


data terminal equipment. Device at the user end of a user-network interface that serves as a data source, destination, or both. DTE connects to a data network through a DCE device (for example, a modem) and typically uses clocking signals generated by the DCE. DTE includes such devices as computers, protocol translators, and multiplexers. Compare with DCE.


dual tone multifrequency. Use of two simultaneous voice-band tones for dialing (such as touch tone).


data terminal ready. EIA/TIA-232 circuit that is activated to let the DCE know when the DTE is ready to send and receive data.


Diffusing Update Algorithm. 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. See also Enhanced IGRP.

dual-attached concentrator

See DAC.

dual attachment station

See DAS.

dual counter-rotating rings

Network topology in which two signal paths, whose directions are opposite one another, exist in a token-passing network. FDDI and CDDI are based on this concept.

dual-homed station

Device attached to multiple FDDI rings to provide redundancy.

dual homing

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 IS-IS

See Integrated IS-IS.

dual tone multifrequency



Distance Vector Multicast Routing Protocol. Internetwork gateway protocol, largely based on RIP, that implements a typical dense mode IP multicast scheme. DVMRP uses IGMP to exchange routing datagrams with its neighbors. See also IGMP.


Data Exchange Interface. ATM Forum specification, described in RFC 1483, that 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.

dynamic address resolution

Use of an address resolution protocol to determine and store address information on demand.

dynamic configuration

See discovery mode.

dynamic random-access memory


dynamic routing

Routing that adjusts automatically to network topology or traffic changes. Also called adaptive routing.