DAMA (demand assigned multiple access)
A system for allocation of communication satellite time to earth stations as the need arises.
Fiber-optic cable that has been installed but is not lit?that is, there are no active light sources.
The computer-equipped, central location within an organization. The data center processes and converts information to a desired form such as reports or other types of management information records.
A communications facility that enables transmission of information in digital form.
The transmission and reception of data between computers and/or remote devices according to appropriate protocols.
data compression and coding
Techniques used to reduce bandwidth requirements for transmission of information over a particular communication link. May also be used in noncommunications applications, such as data storage and retrieval.
The use of data by more than one program or system.
data line privacy
Critical system extension lines, such devices as facsimile machines and computer terminals, are very sensitive to extraneous noise. Data privacy prohibits activities that would insert tones on the station line while the line is in use. Data lines can then be connected through the PBX without danger of losing data through interference.
(1) The equipment and rules (protocol) used for sending and receiving data. (2) Any serial data communication transmission path, generally between two adjacent nodes or devices and without any intermediate switching nodes.
OSI Layer 2, which defines how data is packetized and transmitted to and from each network device. It is divided into two sublayers: medium access control and logical link control.
Provision of access to data, monitoring or storage of data, and control of input/output devices.
A switch that enables a user on an attached circuit to select from other circuits, usually one at a time and on a contention basis, for the purpose of establishing a through connection. A data PBX is distinguished from a PBX in that data transmission, and not voice, is supported.
The speed at which a channel carries data, measured in bits per second (bps).
A digital service offered for data communications at subscriber locations.
An infrequently used term for modem.
The equipment installed at a single location to provide switching functions, such as circuit switching, message switching, and packet switching.
A system for the storage and retrieval of data, its transmission to terminals, and controls to provide adequate protection and ensure proper usage.
The movement of information from one location to another by means of some form of communication media.
A message of fixed maximum length, sent without network provided facilities for assuring its accuracy, delivery, or correct sequencing with respect to related messages, that carries the full destination address used for routing.
A unit for measuring relative strength of a signal parameter such as power or voltage. The number of decibels is 10 times the logarithm (base 10) of the ratio of the power of two signals, or ratio of the power of one signal to a reference level.
DBS (Direct Broadcast Satellite)
A satellite system that can transmit digital TV signals directly to individual homes.
DCE (data communications equipment)
Equipment that provides an interface between the DTE and the transmission channel (that is, between the carrier’s networks). It establishes, maintains, and terminates a connection between the DTE and the transmission channel and is responsible for ensuring that the signal that comes out of the DTE is compatible with the requirements of the transmission channel.
DCLEC (data-competitive local-exchange carrier)
A company that is specifically focused on supporting data services in the local loop (for example, providers that offer DSL services to end users).
DCS (digital cross-connect system)
A device that enables the reconfiguration of a digital network in response to congestion or failure in the network, as well as on-demand reconfiguration. DCSs add, drop, and/or switch payload as necessary across multiple links.
DDS (digital data service)
A digital transmission service that supports speeds up to 56Kbps/64Kbps.
An end-to-end communications line used exclusively by one organization. Also called a dedicated circuit.
The change in a signal from the transmitting end to the receiving end resulting from the tendency of some frequency components within a channel to take longer to be propagated than others.
A method of representing a speech wave form (or other analog signal) in which successive bits represent increments of the wave form. The increment size is not necessarily constant. Produces digitized voice at 56Kbps.
The process of recovering data from a modulated carrier wave. The reverse of modulation.
DEN (Directory Enabled Networking)
An industry group formed by Microsoft and Cisco to create a common data format for storing information about users, devices, servers, and applications in a common repository. DEN describes mechanisms that enable equipment, such as switches and routers, to access and use directory information to implement policy-based networking.
DES (Data Encryption Standard)
A cryptographic algorithm designed by the National Bureau of Standards (now the National Institute of Standards and Technology) to encipher and decipher data using a 56-bit key. As a secret-key, symmetric system, it requires the exchange of secret encryption keys between users.
Software routines or microcode used to check equipment malfunctions or to pinpoint faulty components.
A signal, generated by a service circuit within the local exchange or PBX, that is sent to an operator or user as an audible indication that the switch is ready to receive dialing digits.
The process of, or the equipment or facilities involved in, establishing a temporary connection via the switched telephone network.
DID (direct inward dialing)
Incoming calls from the exchange network can be completed to specific station lines without attendant assistance. Also called direct dialing in (DDI).
A public-key algorithm used mostly for exchanging keys; its security rests on the difficulty of computing discrete algorithms in a finite field, generated by a large prime number.
DiffServ (Differentiated Services)
An approach to providing QoS in networks that use a small, well-defined set of building blocks from which a variety of services can be built. DiffServ evolved from IETF’s IntServ. It is a prioritization model, with preferential allocation of resources based on traffic classification.
Communications procedures, techniques, and equipment whereby information is encoded as either binary 1 or 0; the representation of information in discrete binary form, discontinuous in time, as opposed to the analog representation of information in variable, but continuous, wave forms.
A method for registering user identities with a third party, a CA. A digital certificate binds a user to an electronic signature that can be trusted like a written signature and includes authentication, access rights, and verification information.
A technique for testing the digital processing circuitry of a communications device. It can be installed locally, or remotely, via a telecommunications circuit; the device being tested will echo back a received test message, after first decoding and then reencoding it, the results of which are compared with the original message.
A network that incorporates both digital switching and digital transmission.
A discrete or discontinuous signal, one whose various states are identified with discrete levels or values.
The process of establishing and maintaining a connection, under stored program control, where binary-encoded information is routed between an input and an output port. Generally, a virtual circuit is derived from a series of time slots (time-division multiplexing), which is more efficient than requiring dedicated physical circuits for the period of time that connections are set up.
An algorithm that determines routes based on path length; it is used in OSPF.
A service that provides a white pages-like directory of the users and resources located on an enterprise network. Instead of having to know a device’s or user’s specific network address, a directory service provides an English-like listing for a user. The directory is being standardized collaboratively by the ITU (X.500 standards) and ISO.
Routing in which a router is only aware of routers that are directly connected to it. Each router sends its routing table to each of its neighbors; they in turn merge this routing table with their own.
The modification of the wave form or shape of a signal caused by an outside interference or by imperfections of the transmission system. Most forms of distortion are the result of the varying responses of the transmission system to the different frequency components of the transmission signal.
distributed computing environment
An architecture in which portions of the applications and the data are broken up and distributed among the server and client computers.
An application in which there are many clients as well as many servers. All databases at remote and local sites are treated as if they were one database. The data dictionary is crucial in mapping where all the data resides.
distributed data processing
Data processing in which some or all of the processing, storage and control functions, in addition to input/output functions, are situated in different places and connected by transmission facilities.
A corporate system that can function independently from the host to provide local processing capabilities that meet end-user requirements, yet can connect into the host network for file transfer, access to other applications, and host-specific functions.
A structure (typically wall-mounted) for terminating telephone wiring, usually the permanent wires from, or at, the telephone exchange, where cross- connections are readily made to extensions. Also called connector block, distribution block, MDF, or IDF.
DLC (digital loop carrier)
A type of concentrator, also called a remote concentrator or remote terminal. Traditional DLCs are not interoperable with some of the new DSL offerings, including ADSL and SDSL.
DLCI (data-link connection identifier)
An identifier in a Frame Relay header that specifies the Layer 2 virtual circuit.
DLI (data-line interface)
The point at which a data line is connected to a telephone system.
DMT (Discrete Multitone)
A multicarrier modulation scheme used in ADSL.
DNS (Domain Name System)
A set of protocols and databases that translates between Web site names and physical IP addresses in the Internet or in any TCP/IP based internet.
The portion of a satellite circuit extending from the satellite to the earth station.
The direction of transmission flow from the source toward the user.
The total time a system is out of service due to equipment failure.
DPNSS (Digital Private Network Signaling System)
The European standard for common channel signaling between PBXs.
DQDB (distributed queue dual bus)
The media access method of the IEEE 802.6 standard for metropolitan area networks.
A connection point between a communicating device and a communications network.
DS (Digital Signal) level
The increments of the PDH hierarchy (North American standard). DS-0 is a single channel with a capacity of 64Kbps; DS-1 is 24 DS-0 channels multiplexed into one 1.544Mbps T-1 digital trunk; DS-1C is a 3.152Mbps digital signal carried on a T-1 C facility; DS-2 is a 6.312Mbps digital signal carried over 96 DS-0 channels on a T-3 facility; DS-3 is a 44.736Mbps digital signal carried over 672 DS-0 channels on a T-3 facility; DS-4 is a 274.176Mbps digital signal carried over 4032 DS-0 channels on a T-4 facility.
DSI (digital speech interpolation)
A system of digitized speech in which the speech can be cut into slices such that no bits are transmitted when a person is silent. As soon as speech begins, bits flow again.
DSL (Digital Subscriber Line)
A family of broadband technologies that use sophisticated modulation schemes to pack data onto copper wires. They are sometimes referred to as last-mile technologies because they are used only for connections from a telephone switching station to a home or office, not between switching stations.
The process of linking together several DSL lines to configure bandwidth in between the T-1/T-3 and E-1/E-3 rates.
DSLAM (DSL access multiplexer)
A device at a phone company’s central location that links many customer DSL connections to a single high-speed ATM line.
DSSS (Direct Sequence Spread Spectrum)
A spread spectrum technique in which each data bit is converted to a series of 10 to 144 transmitted bits or chips.
DSU (digital or data service unit)
A synchronous serial data interface that buffers and controls the flow of data between a digital terminal and the CSU attached to a digital communications facility, converting between incompatible digital formats. DSUs can be considered as modem replacements in digital networks.
DTE (data-terminating equipment)
Equipment (including any type of computer terminal, including PCs, as well as printers, hosts, front-end processors, multiplexers, and LAN interconnection devices such as routers) that transmits data between two points without error. Its main responsibilities are to transmit and receive information and to perform error control. The DTE generally supports the end-user applications program, data files, and databases.
DTH (direct to home)
A satellite system that can transmit digital TV signals directly to individual homes.
DTMF (dual-tone multifrequency) signaling
The basis for operation of pushbutton telephone sets. A method of signaling in which a matrix combination of two frequencies, each from a group of four, is used to transmit numerical address information. The two groups of four frequencies are (a) 697Hz, 770Hz, 852Hz, and 941Hz, and (b) 1209Hz, 1336Hz, 1477Hz, and 1633Hz.
DTV (digital TV)
Television sent over a digital network. It is nearly immune to interference and degradation, and it can display a much better range of colors than can analog television.
Communications in which data can be transmitted between two stations in both directions at the same time, with the use of a four-wire circuit. Same as full-duplex.
A four-wire circuit used for transmission in both directions at the same time. It can be called full-duplex to distinguish it from half-duplex.
A signaling system that occupies the same cable pair as the voice path, yet does not require filters.
Simultaneous, two-way, independent transmission. Also called full-duplex transmission.
A procedure for separating the incoming and outgoing conversations.
DVB (Digital Video Broadcasting Group)
A European organization that has authored many specifications for satellite and cable broadcasting of digital signals.
DWDM (Dense Wavelength Division Multiplexing)
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.
Routing that automatically adjusts to network topology or traffic changes.