D Channel: Data control Channel; usedin various ISDNimplementations, providing; control, signaling and user information. Alsoused within B-ISDNusing ATMproviding the same function. Data channel. Full-duplex, 16-kbps (BRI)or 64-kbps (PRI) ISDN channel.
DA: Distribution Area; The area served by aremote terminal.
DAB: Digital Audio Broadcast; digital radionetworks operating in the L-Band (1452 to1492 MHz).
DAC: Digital to Analog Converter; An analogintegrated circuit that takes in a group of binary digits and generatesan analog signal of corresponding magnitude.
DACS: digital access and cross-connect system;In communications systems, a digital system in which (a) access is performedby T-1 hardware architecture in private and public networks with centralizedswitching and (b) cross-connection is performed by D3/D4 framing for switchingdigital-signal-0 (DS-0) channels to other DS-0channels. Note: Modern digital access and cross-connect systems are notlimited to the T-carrier system, and may accommodate high data rates suchas those of SONET.
DAN: Desk Area Network
DAP: Data Access Protocol, (DEC)
DARPA: Defense Advanced Research ProjectAgency, (DOD)
DARS: Digital Audio Radio Service, see AMRC.
DAT: Digital Audio Tape
Data Bus: (or "data path", "datapath")The connections between the CPU, memory and peripheralsused to carry data. The width of the data bus is one of the main factorsdetermining the processing power of a computer. Most current processordesigns use a 32 bit bus, meaning that 32 bitsof data can be transferred at once.
data element: An item of data as representedbefore encoding and after decoding.
data link layer: Layer 2 of theOSI reference model. This layer provides reliable transitof data across a physical link. The data link layer is concerned with physicaladdressing, network topology, line discipline, error notification, ordereddelivery of frames, and flow control. The IEEE hasdivided this layer into two sublayers: the MAC sublayerand the LLC sublayer. Sometimes simply called link layer.Roughly corresponds to the data link control layer of the SNAmodel. See also application layer, LLC, MAC,network layer, physical layer, presentation layer, session layer, and transportlayer.
Datapath: see DataBus.
DAVIC: Digital Audio-VIsual Council, (www.davic.org)
DBS: Digital Broadcast System
DBS: Direct Broadcast Satellite
DCE: Data Communication Equipment
DCE: Distributed Computing Environment, (OSF)
DC: Direct Current
DC: Discrete Cosine
DC: District of Colombia
DCT: Discrete Cosine Transform; A mathematicaltransform that can be perfectly undone and which is useful in image compression.
DD: Dolby Digital
DDE: Dynamic Data Exchange
DDIF: Digital Document Interchange Format,(DEC)
DDM: Distributed Data Management
DDM: Data Driven Machine, (IEEE1978)
DDS: The class of service offered by telecommunicationscompanies for transmitting digital data as opposed to voice.
DDS: The first private-line digital serviceoffered by AT&T, with data rates typically at 2.4,4.8, 9.6 and 56 kilobits per second. DDS is now part of AT&T'sAccunet family of services. Most LEC (local exchangecarriers) and IXC (IntereXchange Carriers) offer similarservices
DEC: Digital Equipment Corporation; DEC'sHome Page
decoded stream: The decoded reconstructionof a compressed bit stream.
decoder: An embodiment of a decoding process.
decoding (process): The process definedin the Digital Television Standard, (see the Advanced Television StandardsCommittee; ATSC) that readsan input coded bit stream and outputs decoded pictures or audio samples.
DECT: Digital European Cordless Telecommunications
demultiplexing: The separating ofmultiple input streams that have been multiplexed into a common physicalsignal back into multiple output streams. See also multiplexing.
DES: Data Encryption Standard
DFP: Data Facility Product, (IBM)
D-frame: Frame coded according to an MPEG-1mode which uses DC coefficients only.
DFS: Distributed File System, (OSF);based on the Andrews File System (AFS) -- a distributedcell architecture.
DHCP: Dynamic Host Configuration Protocol;
DIA: Document Interchange Architecture, (IBM)
DIN: Device Identification Number
DIS: Draft International Standard, (ISO/IEC)
distance vector routing algorithm:Class of routing algorithms that iterate on the number of hops in a routeto find a shortest-path spanning tree. Distance vector routing algorithmscall for each router to send its entire routing table in each update, butonly to its neighbors. Distance vector routing algorithms can be proneto routing loops, but are computationally simpler than link state routingalgorithms. Also called Bellman-Ford routing algorithm. See also link-staterouting algorithm and SPF.
DLC: Digital Loop Carrier
DME: Distributed Management Architecture
DMT: Discrete Multi-Tone; An xDSLmodulation methodology, (see also CAP,QAM)
DNS: Domain Name Service; A general-purposedistributed, replicated, data query service chiefly used on Internet fortranslating hostnames into Internet addresses. Also, the style of hostnameused on the Internet (though such a name is properly called a fully qualifieddomain name).
DNS-MX: Domain Name Service & Mail eXchange;DNS resourcerecords which handle email for a particular domain.
DOCIS: Data Over Cable Interface Specification;A 'data over cable system' adds to a cable television system, a high- speeddata communications path that is transparent to the Internet Protocol (IP),between subscriber locations and the cable operator's headend. (see http://www.cablemodem.com/mcnsfaq.htm#2)
DOCSS: Data Over Cable Security Specification
DoD: Department Of Defense
DoE: Department Of Energy
DOS: Disk Operating System
DPA: Demand Protocol Architecture
DPMS: DOSProtected Mode Service
DQDB: Distributed Queue Dual Bus; Datalink layer communication protocol, specified in the IEEE802.6 standard, designed for use in MANs. DQDB,which permits multiple systems to interconnect using two unidirectionallogical buses, is an open standard that is designed for compatibility withcarrier transmission standards, and is aligned with emerging standardsfor B-ISDN. SIP (SMDS InterfaceProtocol) is based on DQDB. (see also IEEE 802.6 MANarchitecture)
DRAM: Dynamic Random Access Memory
DS0: Digital Signal 0; 64 Kb/s
DS1: Digital Signal 1; 1.544 Mb/s; In the USa DS1 "frame" is composed of 24 eight-bit bytes (commonly from 24 DS0sources, originally digitised voice-grade telephone signals), plus oneframing bit (193 bits). 8000 bytes per second come from each source, andthus 8000 frames per second are transported by the DS1 signal. The resultis 193*8000 = 1,544,000 bits per second.
DS3: Digital Signal 3; 1. A digital signalrate of 44.736 Mb/s, corresponding to the North American T3 designator.2. A digital signaling rate of 32.064 Mb/s, corresponding to the JapaneseT3 designator.
DS4: digital signal 4; 1. A digital signalrate of 274.176 Mb/s, corresponding to the North American T4 designator.2. A digital signaling rate of 97.728 Mb/s, corresponding to the JapaneseT4 designator.
DS Level: Digital Signalor Data Service level; Originally an AT&T classificationof transmitting one or more voice
conversations in one digital data stream. The best known DS levelsare DS0 (a single conversation), DS1(24 conversations
multiplexed), DS1C (two DS1scombined), DS2, and DS3.
By extension, the DS level can refer to the raw data rate necessaryfor transmission:
DS0 64 Kb/s
DS1 1.544 Mb/s
DS1C 3.15 Mb/s
DS2 6.31 Mb/s
DS3 44.736 Mb/s
DS4 274.1 Mb/s
(where K and M signify multiplication by 1000 and 1000000, rather thanpowers of two). In this sense it can be used to
measure of data service rates classifying the user access rates forvarious point-to-point WAN technologies or standards (e.g.
X.25, SMDS, ISDN,ATM, PDH).
DSL: Digital Subscriber Line / Loop
DSM: Digital Storage Medium; A digital storageor transmission device or system. (MPEG)
DSM-CC: Digital Storage Media Command andControl (MPEG)
DSP: Digital Signal Processor/Processing
DSS: Digital Satellite System
DSS: Digital Signature System
DSSSL: Document Style Semantics & SpecificationLanguage
DSU: Data Service Unit; Converts the DTE-orienteddata signals into pipolar digital signals. The DSU also performs clocking,signal regeneration, and equalization of the channel.
DTE: Data Terminal Equipment
DTH: Direct To Home
DTS: Decoding Time-Stamp; A field that maybe present in a PES packet header that indicates thetime that an access unit is decoded in the system target decoder.
DV: Digital Video
DVB: 1. Digital Video Broadcasting. 2.Digital Video Broadcasting, (Project); Towards the end of 1991, broadcasters,consumer electronics manufacturers and regulatory bodies came togetherto discuss the formation of a group that would oversee the developmentof digital television in Europe - the European Launching Group (ELG).The ELG expanded to include the major European mediainterest groups, both public and private, the consumer electronics manufacturers,common carriers and regulators. It drafted a Memorandum of Understanding(MoU) establishing the rules by which this new and challenginggame of collective action would be played. The concept of the MoUwas a departure into unexplored territory and meant that commercial competitorsneeded to appreciate their common requirements and agendas. Trust and mutualrespect had to be established. The MoU was signed byall ELG participants in September 1993, and the LaunchingGroup became DVB (Digital Video Broadcasting). By 1997 the development of the DVB Project had successfully followed theinitial plans, and the
project had entered its next phase, promoting its open standards globally,and making digital television a reality.
DVB-C: Digitial Video Broadcasting - CableSystem; The DVB-C cable system is based on DVB-S,but the modulation scheme is used is Quadrature Amplitude Modulation (QAM)instead of QPSK. For cable networks, no inner-codeforward error-correction (FEC) is needed. The systemis centred on 64-QAM, but lower-level systems, suchas 16-QAM and 32-QAM, and higherlevel systems such as 128-QAM and 256-QAMcan also be used. In each case, the data capacity of the system is tradedagainst robustness of the data. In terms of capacity, an 8 MHzchannel can accommodate a payload capacity of 38.5 Mbit/s if 64-QAMis used, without spill-over into adjacent channels. Higher-levelsystems, such as 128-QAM and 256-QAMare also possible, but their use depends on the capacity of the cable networkto cope with the reduced decoding margin.
DVB-IC: Digital Video Broadcasting - InteractiveCATV; a DVB Return Channel for CATVsystems. DVB-IC is an advanced specification allowing full use tobe made of Community Antenna TV and cable systems for interactiveservices. The DVB-IC specification is a practical implementation of theDAVIC 1.x cable modem allowing return path bitrates of up to 3.088 Mbit/s.
DVB-ID: Digital Video Broadcasting - InteractiveDECT; a DVB Return Channel for DECTT. The DVB Technical Module has completed work on a setof return channel specifications using wireless technology that could beused in a variety of systems. The new revised DECT(Digitally Enhanced Cordless Telephony) Return Channel specification containsa vital "Data Service Profile for Point-to-Point-Protocol (PPP)",which ties in seamlessly with DVB-NIP.
DVB-IM: Digital Video Broadcasting - InteractiveLMDS; a DVB Return Channel for LMDS. The new LMDS (Local Multipoint Distribution Systems)return channel specification is based on the DVB-RCCspecification, currently in the final stage of approval in ETSI.
DVB-IP: Digital Video Broadcasting - InteractivePSTN; a DVB Return Channel for PSTNand ISDN. The DVB-IP specification allows interactiveDVB systems to operate over existing PSTNand ISDN systems. Public-Switched Telephone Networks and IntegratedServices Digital Networks. DVB-IP users are able to select the interactivereturn channel that best suits their needs and budget.
DVB-NIP: Digital Video Broadcasting - NetworkIndependent Protocols; The DVB Network IndependentProtocols allow session control and protocol tunnelling in MPEG-2Transport Stream packets, the DVB's basic currency.MPEG-2 DSM-CC, Digital StorageMedia Command and Control is a complex specification adopted by DVBto ensure that DVB-I would be compatible with internationally agreed standardsand flexible enough to meet the challenges of future interactive systems.
DVB-S: Digital Video Broadcasting - Satellitesystem; The DVB-S system is designed to cope with the full range of satellitetransponder bandwidths. DVB-S is the oldest, most established ofthe DVB standards family, and arguably forms the coreof
the DVB's success. Services using DVB-S are on-airon 6 continents. DVB-S is a single-carrier system. One way of picturingit is to consider it as a kind of 'onion'. In the centre, the onion's core,is the payload, which is the useful bit rate. Surrounding this are a seriesof layers to make the signal less sensitive to errors, and to arrange thepayload in a form suitable for broadcasting.
The video, audio, and other data is inserted into fixed-length MPEGTransport Stream packets. The packetized data constitutes the payload. A number of stages of processing follow:
There are thus two variables for the service provider: the total sizeof the 'onion' and the thickness of the second error- correction outer'skin'. In each case, in the home, the receiver will discover the rightcombination to use by very rapid trial and error on the received signal.An appropriate combination of payload size and Inner Code can be chosento suit the service operator's environment.
One example of a parameter set would be for a 36 MHz(-1 dB) transponder to use a 3/4 Convolutional Code,in which case a useful bit rate of about 39 Mbit/s will be available asthe payload.
The 39 Mbit/s (or other bit rates allowed by parameter sets for a givensatellite transponder) can be used to carry any combination of MPEG-2video and audio. Thus, service providers are free to deliver anything frommultiple-channel SDTV, 16:9 WidescreenEDTV or single-channel HDTV, toMultimedia Data Broadcast Network services and Internet over the air.
DVB-T: The DVB-T system specification forterrestrial digital television was approved by ETSIin February 1997. DVB-T services both within and outside Europe areplanned for 1998. As with the other DVB standards,MPEG-2 sound and vision coding forms the basis ofDVB-T. Other elements of the DVB-T specification include:
DVCR: Digital Video Cassette Recorder
DVD: Digital Video Disk(s); single-sided willhold 4.7 GB or 7.5 GB; a double-sided,up to 17 GB.
DVD-ROM: Digital Video Disk - Read OnlyMemory; will replace CD-ROMs in all new computer systems, (see also DVD).
DVHT: Digital Video Home Terminal, (set-topbox)
DVI: Digital Video Interactive, (Intel)
DVMRP: Distance Vector Multicast RoutingProtocol
DVOD: Digital Video On Demand
DWDM: Dense Wavelength Division Multiplexing;DWDM works by combining multiple optical signals so that they can be
amplified as a group and transported over a single fiber to increasecapacity. For example, by multiplexing eight OC-48 signals, carrierscan increase new or existing fiber capacity from 2.5 to 20 Gb/s. A systemwith DWDM can achieve all this gracefully while maintaining or even exceedingexisting system performance and reliability. Already, 16 channelDWDM systems provide near 40 Gb/s aggregate bandwidth. And testingof 100 channel systems are already underway.
DWMT: Discrete Wavelet Multi-Tone
DXF: Drawing eXchange Format; CADfile format, (AutoDesk)