A data transmission format comprised of 12 frames of 192 bits each. A single 193rd bit is used for link control and error checking. D-4, also known as SF, has been superseded by the ESF format as the industry standard. However, ESF is not backward compatible and there is a large base of channel banks and DS-1 multiplexers that are D-4 based. D-4 is still the default private line formatting technique.

A multiplexer used to convert 24 voice grade analogue or data channels into a DS-1 This is the most common type of channel bank. The D-4 is not equipped for software-controlled provisioning or remote circuit testing.

Data Access Agreement.

Data communications equipment furnished or approved by a common carrier that permits attachment of privately owned data terminal and communications equipment to the common carrier's network.

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.

Digital Access and Cross-connect System

Used to interconnect both low and high-speed digital transmission circuits, while providing access to individual voice and data channels embedded within these circuits.

An inactive fibre-optic strand without electronics or optronics. Dark fibre is not connected to transmitters, receivers and regenerators.

Defence Advanced Research Projects Agency.

Dynamically Assigned Socket

Socket that is dynamically assigned by DDP upon request by a client. In an AppleTalk network, the sockets numbered 128 to 254 are allocated as DASs.

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.

A message based signalling system following the ISO based model developed by BT to provide multi-line IDA interconnection to the BT network.

A method of reducing the amount of data to be transmitted by applying an algorithm to the basic data at source. A decompression algorithm expands the data back to its original state at the other end of the link. Digital video requires up to 45 Mbps for reasonable performances but with compression can achieve the same in one to three Mbps.

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

Dynamic Buffer Management

Frame Relay and ATM service modules are equipped with large buffers and the patented Dynamic Buffer Management scheme 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.

Data Carrier Detect

A process that tells the computer whether or not the modem is connected to another modem.

Data Communications Equipment (EIA expansion)

Data Circuit-Terminating Equipment (ITU-T expansion)

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.

Distributed Component Object Model

Protocol that enables software components to communicate directly over a network. Developed by Microsoft and previously called Network OLE, DCOM is designed for use across multiple network transports, including Internet protocols such as HTTP.

Digital Cross-Connect System

A digital switching facility interconnection device allowing reconfiguration of lower levels of capacity within a circuit without the need for manual changes in the interconnections.

Discrete Cosine Transform

Distributed Data Management

Software in an IBM SNA environment that provides peer-to-peer communication and file sharing. One of three SNA transaction services.

Dial-on-Demand Routing

Technique whereby a router can automatically initiate and close a circuit-switched session as transmitting stations demand. The router spoofs keep-alives 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.

Digital Data Service

A private line channel at 56 or 64Kbps

Digital European Cordless Telephone

Digital Enhanced Cordless Telecommunications

A wireless communications standard used in Europe, the Mid-East and Africa.

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

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

Data Encryption Key

Used for the encryption of message text and for the computation of message integrity checks (signatures).

Process of returning a modulated signal to its original form. Modems perform demodulation by taking an analogue signal and returning it to its original (digital) form.

Separating of multiple input streams that were multiplexed into a common physical signal back into multiple output streams.

Document Enabled Networking

Destination End Station

Data Encryption Standard

Standard cryptographic algorithm developed by the U.S. National Bureau of Standards.

Distributed Feed-Back

The European Commission's directorate general on external relations and trade negotiations.

The European Commission's directorate general on competition.

The European Commission's directorate general for telecommunications, information industries and innovation.

Dynamic Host Configuration Protocol

Provides a mechanism for allocating IP addresses dynamically so that addresses can be reused when hosts no longer need them.

Dynamic Hypertext Mark-up Language

An extension to HTML that allows you to have objects that interact with each other.

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.

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

The sequence of numbers sent to a device, such as a modem, that can dial a telephone number.

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

An electronic switching system, found in telephone central offices, that switches groups of signals from one route to another, without the need to demultiplex them. An optical cross connect is an optical switch that routes wavelength channels from one fibre route to another without first converting them into electricity and back.

Deutsche Industrie Norm

German national standards organization.

Deutsche Industrie Norm Connector

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

A route that can reach a destination without going through any intervening routers.

Defence Information Systems Agency

Formerly DCA. U.S. military organization responsible for implementing and operating military information systems, including the DDN.

Problem with a communication signal resulting from non-uniform transmission speeds of the components of a signal through a transmission medium.

Downlink

Data/RF propagation away from the BTR; transmission direction from the central facility to the customer.

Digital Loop Carrier

Equipment that concentrates analogue loop lines, digitizing and multiplexing calls for transmission to the central office.

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).

Dynamic Link Library

A Windows library that can be shared by multiple applications.

Data Link Switching

Involves using routers at field offices and headquarters to encapsulate SNA traffic in TCP/IP, and then transmit that traffic over a leased-line or frame relay network.

Direct Memory Access

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.

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. This coding scheme is used by IEEE 802.5 and Token Ring networks.

Digital Multiplexer System

Discrete Multitone

A line coding protocol.

Digital Network Architecture

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

Domain Name System

System used in the Internet for translating names of network nodes into addresses.

Department of Defence Intelligence Information System Network Security for Information Exchange

Collection of security requirements for networking defined by the U.S. Defence Intelligence Agency.

(1) In IS, a logical set of networks.
(2) In SNA, an SSCP and the resources it controls.
(3) In the Internet, a portion of the naming hierarchy tree that refers to general groupings of networks based on organization-type or geography.
(4) Networking system developed by Apollo Computer (now part of HP) for use in its engineering workstations.

Downlink, data/RF propagation away from the BTR; transmission direction from the central facility to the customer.

Digital Private Network Signalling System

A British protocol for PBX-to-PBX communications.

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 is based on DQDB.

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.

Digital Signal Level 0

Framing specification used in transmitting digital signals over a single channel at 64 Kbps on a T1 facility.

Digital Signal Level

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).

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.

Destination Service Access Point

SAP of the network node designated in the Destination field of a packet.

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.

Digital Subscriber Line Access Multiplexer

The device placed in the CO that accepts all the DSL lines.

North America PDH transmission speed

Digital Signal Processor

A DSP analyzes and processes analogue signals, converting them to a digital format.

Data Set Ready

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

Direct-Sequencing Spread-Spectrum

A spread-spectrum techniques that operate over the radio airwaves in the unlicensed ISM band (industrial, scientific, and medical). DSSS uses a radio transmitter to spread data packets over a fixed range of the frequency band. WECA’s focus is on the use of DSSS for 11 Mbps high rate wireless LAN communications.

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.

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.

Direct-To-Home

Dual Tone Multi-Frequency

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

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.

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 neighbours.

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 neighbours. Distance vector routing algorithms can be prone to routing loops, but are computationally simpler than link state routing algorithms.

Dense Wavelength Division Multiplexing

Using optical multiplexers and optical amplifiers, DWDM combines multiple optical signals so they can be amplified as a group and transported over a single fibre to increase capacity.

An SMDS interface between the customer and the network used at low speeds, typically 64 Kbps. The data passes through the interface in full frames. The carrier's equipment converts the messages to small packets.

A path to another network that the router learns by means of dynamic updates from other routers, rather than by means of a static specification in a configured profile. Routers that use Routing Information Protocol (RIP) broadcast their entire routing tables every 30 seconds, updating other routers about which routes are usable. Hosts that run Internet Control Message Protocol (ICMP) can also send ICMP redirects to offer a better path to a destination network. Open Shortest Path First (OSPF) routers propagate link-state changes as they occur in order to update their routing tables.

A routing technique that enables a message's route to change as the message proceeds along the network.