D


D (delta) channel

The channel used on ISDN to communicate signaling and other related information. Use of the D channel leaves the B channels free for data communication.

See also [B channel]


D-shell connector

A connector that is shaped like a letter D and uses pins and sockets to establish connections between peripheral devices, using serial or parallel ports. The number that follows DB in the name of a D connector is the number of pins used for connectivity; for example, a DB-9 connector has 9 pins, and a DB-25 connector has 25 pins.



daemon

A service or process that runs on a UNIX or Linux server.



DAS (dual attached station)

A device on an FDDI network that is connected to both rings. Compare with SAS.



DAT (digital audio tape)

A tape recording technology that uses the helical scan recording method. This technology has been used in videotape recorders and VCRs since the 1950s.



Data field

In a frame, the field or section that contains the data.



datagram

An information grouping that is transmitted as a unit at the network layer.

See also [packet]


data-link layer

Layer 2 of the OSI model, which is above the physical layer. Data comes off the cable and goes into the data-link layer. The data-link layer has two distinct sublayers: MAC and LLC.



DB-9

A 9-pin connector that is used for serial port or parallel port connection between PCs and peripheral devices.



DB-25

A 25-pin connector that is used for serial port or parallel port connection between PCs and peripheral devices.



DDNS (dynamic DNS)

A form of DNS that allows systems to be registered and deregistered with the DNS system dynamically. DDNS is facilitated by DHCP, which passes IP address assignments to the DNS server for entry into the DNS server records. This is in contrast with the conventional DNS system, in which entries must be made manually.



DDS (digital data storage)

A format for storing computer data on a DAT. DDS-formatted tapes can be read by either a DDS or DAT drive. The original DDS standard specified a 4mm tape cartridge with a capacity of 1.3GB. Subsequent implementations of DDS have taken the capacity to 40GB with compression.



dedicated line

A dedicated circuit that is used in WANs to provide a constant connection between two points.



default gateway

Normally a router or a multihomed computer to which packets are sent when they are destined for a host on a different network.



Delete or Erase

A right that is given to users, which allows them to delete a file or files in a directory or to delete a directory.



demarcation point

The point at which communication lines enter a customer's premises. Sometimes shortened to simply "demarc."



destination address

The network address to which the frame is being sent.



DHCP (Dynamic Host Configuration Protocol)

A protocol that provides dynamic IP addressing to workstations on the network.



dial-up networking

Refers to the connection of a remote node to a network using POTS.



differential backup

A backup of only the data that has been created or changed since the previous full backup. In a differential backup, the state of the archive bits is not altered.



directory services

A system that allows network resources to be viewed as objects which are stored in a database. This database can then be divided up and distributed among different servers on the network.Examples of directory services systems include Novell Directory Services and Microsoft Active Directory.



disaster recovery plan

A plan for implementing duplicate computer services in the event of a natural disaster, a human-made disaster, or another catastrophe. A disaster recovery plan includes offsite backups and procedures to activate information systems in alternative locations.



disk duplexing

A fault-tolerant standard that is based on RAID 1 that uses disk mirroring with dual disk controllers.



disk mirroring

A fault-tolerant standard that is defined as RAID 1 and mirrors data between two disks to create an exact copy.



disk striping

An implementation of RAID in which data is distributed across multiple disks in a stripe. Some striping implementations provide performance improvements (RAID 0), whereas others provide fault tolerance (RAID 5).



distance-vector routing

A type of routing in which a router uses broadcasts to inform neighboring routers on the network of the routes it knows about. Compare with link-state routing.



DLT (digital linear tape)

A high-performance and high-capacity tape backup system that offers capacities up to 220GB with compression.



DMA (direct memory access)

The process of transferring data directly into memory at high speeds, bypassing the CPU and incurring no processor overhead.



DNS (Domain Name Service)

A system that is used to translate domain names, such as www.quepublishing.com, into IP addresses, such as 165.193.123.44. DNS uses a hierarchical namespace that enables the database of hostname-to-IP address mappings to be distributed across multiple servers.



domain

A logical group of computers in a Windows NT/2000 network. Also, a section of the DNS namespace.



domain name server

A server that runs application software that enables the server to perform a role associated with the DNS service.



DoS (Denial of Service) attack

A type of hacking attack in which the target system is overwhelmed with requests for service, resulting in it not being capable of servicing any requestslegitimate or otherwise.



downtime

A period of time during which a computer system or network is unavailable. This may be because of scheduled maintenance or due to hardware or software failure.



drive mapping

A process through which an alias makes a network path appear as if it were a local drive.



DSL (Digital Subscriber Line)

A public network technology that delivers high bandwidth over conventional copper wiring over limited distances.



DSU (data service unit)

A network communications device that formats and controls data for transmission over digital lines. A DSU is used in conjunction with a CSU.



DTE (data terminal equipment)

A device used at the user end of a user network interface that serves as a data source, a destination, or both. DTE devices include computers, protocol translators, and multiplexers.



dumb terminal

A keyboard/monitor combination that allows access to a multiuser system but provides no processing or storage at the local level.



duplexing

In RAID, a RAID 1 mirror set in which each drive is connected to a separate controller to eliminate the single point of failure that the controller created.



dynamic routing

A routing system that enables routing information to be communicated between devices automatically and can recognize changes in the network topology and update routing tables accordingly. Compare with static routing.



dynamic window

A flow control mechanism that prevents the sender of data from overwhelming the receiver. The amount of data that can be buffered in a dynamic window varies in size, hence its name.





    Network+ Exam Cram 2
    Network+ Exam Cram 2
    ISBN: 078974905X
    EAN: N/A
    Year: 2003
    Pages: 194

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