Section 5 D Terms Flashcards
DAC
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.
Daemon
In the UNIX operating system, a computing process that, once started, is not under user control, but continues to run in the background. Daemons usually perform a particular purpose on demand, such as supplying information to another processor. An example in AppleTalk networking is the atalkad daemon, which supplies AppleTalk tunneling information to routers on request.
Daisy Chain
In LocalTalk, a daisy chain is made by linking LocalTalkconnectors together with patch cord. In telephony, a daisy chain refers to the method of linking a series of wall outlets together with twisted pair cabling rather than the normal practice of connecting the wall outlets to a central location (home run). In telephony, “daisy chaining” is equivalent to the “backbone” method of LocalTalk construction.
DAL
Data Access Language. A data base metalanguage designed byApple. A superset of SQL.
DARPA
Defense Advanced Research Projects Agency. US. government agency that funded research for and experimentation with the Internet. Evolved from ARPA, and then, from 1994, back to ARPA.
DARPA Internet
Obsolete term referring to the Internet. See Internet.
DAS
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.
DAT
Digital Audio Tape. A type of storage media used for the backup of computing data.
Data
Information represented in a format readable by a computer.
Data Base
A collection of data that can be selectively retrieved by a type of application knows as a Data Base Management System.
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.
Datagram
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 grouping at various layers of the OSI reference model and in various technology circles.
Data Link
The physical connection between two devices such as Ethernet,LocalTalk or Token Ring that is capable of carrying information in the service or networking protocols such as AppleTalk, TCP/IP or XNS.
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 Protocol
The protocol that controls the network signaling and receiving hardware, performing data integrity checks and formatting information according to the rules of the data link.
Data Sink
Network equipment that accepts data transmissions.
Data Stream
All data transmitted through a communications line in a single read or write operation.
dB
Decibels
DB Connector
Database 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.
DCA
Defense Communications Agency. US. government organization responsible for DDN networks such as MILNET. Now called DISA.
DCC
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.
DCE
Distributed Computing Environment.
D Channel
- 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.
DDM
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.
DDN
Defense Data Network. US. 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.
DDP
Datagram Delivery Protocol. Apple Computer network layer protocol that is responsible for the socket-to-socket delivery of datagrams over an AppleTalk internetwork.
DDR
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 adapter or modem.
DE
Discard eligible. See tagged traffic.
Deadlock
- 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.
Decay
A loss in the clarity or readability of an electronic signal caused by the interaction of the signal with its carrier and electrical environment.
Decibel
A measurement that refers to the ratio of the strength of one signal to another. Decibels are commonly used to express signal lossor the relationship of the signal strength to ambient noise.
DECnet
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 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).
Decryption
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.
De Jure Standard
Standard that exist because of its approval by an official standards body. Compare with de facto standard. See also standard.
Delay
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.
Demarc
Demarcation point between carrier equipment and CPE.
Demodulation
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.
Demultiplexing
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.
DES
Data Encryption Standard. Standard cryptographic algorithm developed by the US.
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 multi access network and has other special responsibilities in running OSPF. Each multi access 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 multi access network, which in turn reduces the amount or routing protocol traffic and the size of the topological database.
Desktop
In the Macintosh user interface, the background image of the Finder on which the icons for applications, directories and datafiles are displayed.
Destination Address
Address of a network device that is receiving data. See also source address.
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.
Device
See node.
Device Driver
Software that acts as an intermediary between a CPU and a peripheral device. The CPU sends a command to the device driver, which translates that command into a command meaningful to the peripheral device.
DFT
Distributed Function Terminal.
DIA
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.
Diagnostic
A test or the data from a test which indicates the condition of the state of a computer or network’s health.
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 IEEE802.5 and Token Ring networks.
Dijkstra’s algorithm
See SPF
DIN
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.
DISA
Defense Information Systems Agency. US. military organization responsible for implementing and operating military information systems, including the DDN. See also DDN.
Discovery Architecture
APPN software that enables a machine configured as an APPN EN to automatically fine 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.
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
A change in a electronic signal that occurs when different frequency components of the signal decay at different rates. In a signal made up of many frequency components (such as a square wave), the higher frequency components of a signal typically decay faster than the lower frequency components.
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.
DLCI
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.
DLL
Dynamically Linked Libraries. A component of Microsoft’s OLE.
DLSw
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 SRB, and SSP (Switch-to-Switch Protocol).
DLU
Dependent LU. An LU that depends on the SSCP to provide services for establishing session with other LUs. See also LU and SSCP.
DLUR
Dependent LU Requester. The client half of the Dependent LU Requester/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.
DLUS
Dependent LU Server. The server half of the Dependent LU Requester/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, an NN that implements the DLUS component. See also DLUS.
DMA
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.
DMAC
Destination MAC. The MAC address specified in the Destination Address field of a packet. Compare with SMAC. See also MAC address.
DNA
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.
DNIC
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.
DNS
Domain Name Service
DNSIX
Department of Defense Intelligence Information System Network Security for Information Exchange. Collection of security requirements for networking defined by the US. Defense Intelligence Agency.
DoD
Department of Defense. US. government organization that is responsible for national defense. The DoD has frequently funded communication protocol development.
domain
- In the Internet, a portion of the spanning 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.
Domain
Networking system developed by Apollo Computer (now part of Hewlett-Packard) for use in its engineering workstations.
DOS
The operating system of IBM-compatible personal computers.
Dot Address
Refers to the common notation for IP addresses in the form where each number n 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.
Download
The transfer of a file from a remote computer to a local computer.
Downsizing
The transfer of computing tasks previously performed by main frame or minicomputers to personal computers.
Downtime
- A temporary interruption in the usability of a computer system.2. A work stoppage caused by the temporary lack of usability of a computer system.
DQDB
Distributed Queue Dual Bus. Data link layer communication protocol, specified in the IEEE802.6 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.
DRAM
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.
Drive
A data storage device.
Drop
Point on a multi-point 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.
DS-0
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.
DS-1
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.
DS-1/DTI
DS-1 domestic trunk interface. Interface circuit used for DS-1 applications with 24 trunks.
DS-3
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.
DSAP
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).
DSP
Domain specific part. The part of a CLNS address that contains an area identifier, a station identifier, and a selector byte.
DSPU
Downstream physical unit. In SNA, a PU that is located downstream from the host.
DSPU Concentration
See DSPU and PU.
DSR
Data set ready. EIA/TIA-232 interface circuit that is activated when DCE is powered up and ready for use.
DSU
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 functions such as signal timing. Often referred to together with CSU, as CSU/DSU. See also CSU.
DSX-1
Cross-connection point for DS-1 signals.
DTE
Data Terminal Equipment. The X.25 term for an end node. 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. CTE includes devices such as computers, protocol translators, and multiplexers. Compare with DCE.
DTMF
Dual tone multi-frequency. Use of two simultaneous voice-band tones for dialing (such as touch tone).
DTR
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.
DTS
Distributed Time Service.
DUAL
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 Counter-rotation Ring
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.
DVMRP
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.
DXI
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 Addressing
A system of addressing in which the computer selects it s own address without the user’s intervention.
Dynamic Configuration
See discovery mode.
Dynamic Routing
Routing that adjusts automatically to network topology or traffic changes. Also called adaptive routing.
