The Cable Center

This glossary of cable and telecommunications terms is intended to enhance your understanding of this dynamic industry. We are able to present this information with the generous help of CableLabs - consult their website for the most up-to-date information about cable technology.

0 - 9 | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

D Channel
An ISDN Data channel is a data connection between the telephone company’s switching equipment and its customer’s ISDN equipment. It carries telephone connection information—signals to tell your equipment that a call is coming in and carry dialing and call handling information to the phone company’s equipment—and can also be used to carry multiple low-speed data connections using the X.25 packet-switching protocol.

Dark fiber
Unused fiber-optic cable that has been deployed but does not have the proper electronic and optical equipment to carry optical (light) signals. Generally considered to be extra fiber that will support future demand for communications capacity.

Data Communication
The movement of encoded information by means of electrical transmission systems. The transmission of data from one point to another over communication channels.

Data Compression
A technique that saves storage space by eliminating gaps, empty fields, redundancies, or unnecessary data to shorten the length of records or blocks.

Data Link Layer
Layer 2 in the Open System Interconnection (OSI) architecture; the layer that provides services to transfer data over the transmission link between open systems.

Data Over Cable Service Interface Specifications (DOCSIS)
Specification for transmission of data over a cable network that has been approved by the ITU as an international standard. DOCSIS was developed by CableLabs and a consortium of North American multi-system cable operators.

Decibel (dB)
A unit to measure the relative levels of current, voltage or power. An increase of 3 dB indicates a doubling of power, an increase of 10 dB indicates a 10x increase in power, and an increase of 20 dB indicates a 100x increase in power.

Decipherment
A procedure applied to ciphertext to translate it into plaintext.

Declarative Data Essence (DDE-1)
A standard by SMPTE (Society of Motion Picture and Television Engineers) formalizing ATVEF.

Decryption
A procedure applied to ciphertext to translate it into plaintext.

Decryption key
The key in the cryptographic algorithm to translate the ciphertext to plaintext.
Delay The elapsed time between the instant when user information is submitted to the network and when it is received by the user at the other end.

Delay Distortion
Distortion resulting from non-uniform speed of transmission of the various frequency components of a signal; i.e., the various frequency components of the signal have different times of travel (delay) between the input and the output of a circuit. Also known as Group Delay Distortion.

Delay spread
An effect of multi-path for a digital interface in which multiple reflections of the same signal arrive at the receiver at different times, creating a noticeable degradation in signal quality.

Demographics
Breakdown of television viewers by such factors as age, sex, income levels, education and race.

Demodulation
The extraction of the modulation or information from a radio-frequency current.

Dense Wavelength Division Multiplexing (DWDM)
A technology that provides in fiber the equivalent of frequency division multiplexing, in metallic wire. Separate parallel channels are transmitted on a single fiber, with one wavelength for each channel. Current products enable 16 channels of 2.5Gb each for a total of 50Gb per fiber. DWDM can operate over existing single-mode fiber, and therefore reduce upgrade costs.

Descrambler
An electronic circuit that restores a scrambled video signal to its standard form.

Detector
The photodiode in optical receivers.

Dial-up
Data communication established by a circuit-switched connection over a telephone network. Generally associated with less than broadband speeds (56 kbps or less).

Differential Gain
In color TV, the change in gain, expressed in dB, for the 3.58-MHz color sub-carrier as the level of the luminance signal is varied from blanking to white.

Differential Phase
Variation in phase of the color sub-carrier of a TV signal as the level of the luminance signal is varied from blanking to white.

Differentiated Services (DiffServ, or DS)
A protocol for specifying and controlling network traffic by class so that certain types of traffic get precedence – for example, voice traffic, which requires a relatively uninterrupted flow of data, might get precedence over other kinds of traffic. Differentiated Services is the most advanced method for managing traffic in terms of what is called Class of Service (CoS). Unlike the earlier mechanisms of 802.1p tagging and Type of Service (ToS), Differentiated Services avoids simple priority tagging and depends on more complex policy or rule statements to determine how to forward a given network packet. An analogy is made to travel services, in which a person can choose among different modes of travel – train, bus, airplane – degree of comfort, the number of stops on the route, standby status, the time of day or period of year for the trip, and so forth. For a given set of packet travel rules, a packet is given one of 64 possible forwarding behaviors – known as per hop behaviors (PHBs). A six-bit field, known as the Differentiated Services Code Point (DSCP), in the Internet Protocol (IP) header specifies the per hop behavior for a given flow of packets.

Digital
(1) In communications and computer technology, digital refers to a method of encoding information using a binary system made up of zeroes and ones. In communications technology this takes the form of two very different electrical voltages, several volts positive and negative, to represent the two values. This substantial difference in voltages for each state makes it unlikely that minor fluctuations in voltage due to electro-magnetic interference will change the way a signal is interpreted when received. (2) Information that is encoded into bits and bytes, or packets (0s and 1s, computer binary language). Generally perceived to be an advanced communication form offering clearer signals and increased transmission capacity.

Digital Application Software Environment (DASE)
A standard by ATSC addressing interactive television.

Digital Audio Visual Council (DAVIC)
DAVIC is an international consortium working on the development of standards for interactive television.

Digital certificate
A binding between an entity’s public key and one or more attributes relating to its identity, also known as a public key certificate.

Digital Compression
An engineering technique for converting a cable TV signal into a digital format which may then be processed in a manner that requires a smaller portion of spectrum for its transmission. This compressed format allows many channels to be carried in the bandwidth normally required for one signal. This format can also be easily stored and manipulated.

Digital Enhanced Cordless Telecommunications (DECT)
A European standard governing panEuropean digital mobile telephony. Specified in DVB MHP as a type of return channel network interface for use in receiving and transmitting IP packets.

Digital Facility
A switching or transmission facility designed specifically to handle digital signals.

Digital Program Insertion (DPI)
The digital splicing of one Moving Pictures Experts Group (MPEG) program (typically a commercial) into another based on digital 'cues' within the MPEG transport stream.

Digital Set-Top Box
A device which accepts digital encoded television broadcasts and converts them to display on an analog television set.

Digital Signal
A signal that has a limited number of discrete states prior to transmission. This may be contrasted with an analog signal which varies in a continuous manner and may be said to have an infinite number of states.

Digital signature
A data value generated by a public-key algorithm based on the contents of a block of data and a private key, yielding an individualized cryptographic checksum.

Digital Storage Media-Command and Control (DSM-CC)
A syntax defined in the MPEG-2 standard, part 6, for VCR-like control over a bitstream. Playback commands include Still-Frame, Fast-Forward, Advance, Goto.

Digital subscriber line (DSL)
High-speed technology to transfer data over an existing twisted-pair copper telephone line. Asynchronous technology (ADSL) provides data transmission rates up to 7 Mbps in one direction, generally within approximately three miles from a telephone central office. See also HDSL and VDSL.

Digital Subscriber Line Access Multiplexer (DSLAM)
A DSLAM is an xDSL line-interface device located in a telephone company Central Office. One side of a DSLAM connects to customer premises network interface devices (NIDs) over the local loop. The other side interfaces with the PSTN and a wide area (Frame Relay or ATM) network system.

Digital Transmission
A mode of transmission in which all information to be transmitted is first converted to digital form and then transmitted as a serial stream of pulses. Any signal-voice, data, television-can be converted to digital form.

Digital Transmission Licensing Administration (DTLA)
Licensing authority for DTCP, HDCP and 5C certificates. See www.dtcp.com.

Digital Video Broadcasting (DVB)
Digital video broadcasting is a European standard for digital television.

Digital Video Interface (DVI)
A specification created by the Digital Display Working Group (DDWG) to accommodate analog and digital monitors with a single connector. There are three different DVI configurations: DVI-A, designed for analog signals, DVI-D, designed for digital signals, and DVI-I (integrated), designed for both analog and digital signals.

Digital Video Recorder (DVR)
A consumer device which uses a hard disk drive to record television programs. Also provides pause of live television feature. See PVR.

Digital Video Subcommittee (DVS)
An ANSI-sponsored standardization committee of the SCTE.

Direct Broadcast Satellite (DBS)
Satellite technology that transmits directly to the end user or customer. DBS describes a frequency allocation and wide spacing between satellites that generally permits higher-powered transmissions through the air to small 18- to 24-inch receiving facilities located on customer premises (“point-to-multipoint” transport). Also known as direct-to-home (DTH) satellite.

Directional Coupler
A high-quality tapping device providing isolation between tap and output terminals.

Directive Transmission
Directionally focused signal transmission from a base station to a remote user made possible by certain smart antenna systems with digital signal processing capabilities; these base stations use information obtained during reception to transmit signals selectively toward certain users and away from others.

Directory Number (DN)
The number you dial to reach someone by telephone; a telephone number.

DirectX
Software controls that allow a Windows program to manage video and sound hardware in a computer. It is usually used for multimedia in games or Web pages running under Windows 95/98 and Windows NT.

Dish Antenna
A high-gain antenna, shaped like a dish, that is used for the transmission and reception of ultra-high-frequency and microwave signals.

Disk Operating System (DOS)
The basic term for software which controls a computer’s operation and is loaded from some form of disk, usually a hard drive.

Distant Signal
Broadcast TV signals which originate at a point too far away to be picked up by ordinary home reception equipment; also signals defined by the FCC as outside a broadcaster’s license area. Cable systems are limited by FCC rules in the number of distant signals they can offer subscribers.

Distortion
The departure, during transmission and amplification, of the received signal wave form from that of the original transmitted wave form.

Distribution Hub
A location in a cable television network which performs the functions of a headend for customers in its immediate area, and which receives some or all of its television program material from a master headend in the same metropolitan or regional area.

Distribution Plant
The hardware of a cable system—amplifiers, trunk cable and feeder lines, attached to utility poles or fed through underground conduits like telephone and electric wires.

Distribution System
The part of a cable television system used to carry signals from the headend to subscriber’s receivers. Often applied, more narrowly, to the part of a cable television system starting at the bridger amplifiers.

DMS-100
A telephone company central office switch manufactured by Northern Telecom that has ISDN and other digital telephony capabilities. See also 5ESS.

DOCSIS 1.0
The first version of the Data Over Cable Service Interface Specification. DOCSIS 1.0 defines requirements for cable modems and cable modem termination systems that enable broadband Internet access.

DOCSIS 1.1
The second version of the DOCSIS protocol specification. DOCSIS 1.1 includes an enhanced Media Access Control (MAC)-layer that contains quality of service (QoS) traffic management functions as well as cable modem authentication extensions.

DOCSIS 2.0
The third version of the DOCSIS protocol specification, developed to significantly increase the upstream data carrying capacity and the robustness to noise and interference. DOCSIS 2.0 includes advanced TDMA and S-CDMA physical layer technologies.

DOCSIS Radio Frequency Interface (RFI) Specification
CableLabs radio-frequency interface specifications for high-speed data-over-cable systems.

Document Object Model (DOM)
An API for parsing a document and providing a view of the document by constructing a ‘tree’ of objects in memory.

Document Type Definition (DTD)
A formal grammar to specify the structure and permissible values of XML documents.

Dolby AC-3
Dolby AC-3 refers to the audio encoding format adopted by the ATSC for its advanced television audio encoding. Also known as Dolby digital.

Domain of an application
The domain of an Xlet characterizes the space within which the Xlet is able to execute. This includes both the connection where the Xlet is delivered and other connections where an already executing Xlet is allowed ot continue executing. An application cannot run outside its domain. The maximum lifetime of an application extends from the moment the user navigates to its domain until the moment that the user navigates away from its domain. In the broadcast case, a connection corresponds to a DVB-service. Broadcast signalling indicates which services can load an application and which services allow an already active application to continue.

Domain Name Server (DNS)
A server that relates an Internet domain name (such as www.cablelabs.com) to an IP address.

Double-Click
Two rapid clicks of the primary (usually left) mouse button, usually performed to launch a program or open a file.

Down Conversion
The heterodyning of an input signal with the output of local oscillator, resulting in an intermediate frequency that is lower than the incoming signal frequency.

Downlink
Transmission of signals from a satellite to a dish or earth station.

Download
The transfer of data from a remote computer, or server, to a local computer. To the Internet user, downloading means to request a copy of a file from another computer and then receive that file.

DownloadInfoIndication (DII)
A message that signals the modules that are part of a DSM-CC object carousel.

Downstream
Flow of signals from the cable system control center through the distribution network to the customer. For communication purposes, associated with transmission (down) to the end-user. Or in cable television, the direction of transmission from the headend to the subscriber.

Drop
The cable and hardware from tap to subscriber is called the drop.

Drop Cable
Generally 330 feet or less, of coaxial cable, starting at a tap and continuing on to the subscribers connection. Or coaxial cable that connects to a residence or service location from a directional coupler (tap) on the nearest coaxial feeder cable.

Dual Cable
Two independent distribution systems operating side by side, providing double the channel capacity of a single cable.

Dual Tone Multi Frequency (DTMF)
Telephone “touch” tones. So-called because each keypad button pressed generates a unique combination of two audible tones or frequencies.

Duplex
In a communications channel, the ability to transmit in both directions.

DVB Network
A DVB-network is a collection of MPEG-2 Transport Stream multiplexes transmitted on a single delivery system. For example, all digital channels on a specific cable system make up a DVB network.

DVB-J
DVB-J referes to the Java platform as defined as part of the DVB-MHP 1.0.1 [9]. For the OCAP 1.0 implementation, DVB-J is part of the execution engine.

DVB-J API
DVB-J API refers to one of the Java APIs standardized as part of the DVB-MHP 1.0.1 [9]. For the OCAP 1.0 implementation, the DVB-J APIs are supported in the execution engine.

DVB-J application
A DVB-J application is a set of DBV-J classes that operate together and need to be signalled as a single instance to the application manager so that it is aware of its existence and can control its lifetime through a lifecycle interface. DVB-J applications as specified by the DVB-MHP 1.0.1 [9] are not directly supported by OCAP 1.0 without modifications pertaining to this specification.

Dynamic Host Configuration Protocol (DHCP)
An Internet protocol used for dynamically assigning network layer (Internet Protocol) addresses to IP hosts.

Dynamic Range
The ratio between the greatest signal power that can be transmitted over a multichannel analog transmission system without exceeding distortion or other performance limits, and the least signal power that can be utilized without exceeding noise, error rate or other performance limits.