A Manager’s Guide to IT Terminology (D-H)

D (data) channel: used for common channel signaling by both the telephone company switch and the customer

equipment; provides the call signals that set up B channel connections

DACS (digital access and cross-connect system): a piece of telecommunications equipment used for

routing T1 lines; can cross-connect any T1 line in the system with any other T1 line in the system

Data: information manipulated inside the computer in the form of bits and bytes

Datagram: data packet that is sent over an IP network; associated with the network layer when communication

protocol is connectionless

DCE (data communications equipment OR data circuit-terminating equipment): a device that

communicates with a data terminal equipment (DTE) device in a particular standard

DDP (datagram delivery protocol): a member of the AppleTalk networking protocol suite, mainly responsible

for socket-to-socket delivery of datagrams over an AppleTalk network

DE (discard eligibility): signal used to identify less important data traffic that can be dropped during periods

of congestion on the system

DLCI (data link connection identifier): a channel number that tells the network how to route the data

DMA (direct memory access): a feature that allows certain hardware subsystems in a computer to

access system memory for reading and/or writing independently of the CPU; can include disk drive controllers,

graphics cards, network cards, and sound cards

DOS (disk operating system): a family of closely related operating systems (COS) that ran on IBM PCtype

hardware.

DNA (digital network architecture): a set of specifications or protocols created by Digital Equipment

Corporation (DECnet) that evolved into one of the first peer-to-peer network architectures

DNS (domain name system): service that connects a domain name to an IP address

DRAM (dynamic random access memory): primary choice for holding large amounts of information

due to its inexpensive cost; must be refreshed or rewritten frequently (about every 386 milliseconds)

DS0 (digital signal, level 0): basic digital signaling rate of 64 kbit/s, corresponding to the capacity of

one voice-frequency-equivalent channel

DS1 (digital signal, level 1): also known as T1; widely used to transmit voice and data between devices

DSL (digital subscriber line): technology that delivers digital data transmission over the wires of a local

telephone network

DSU (data service unit): transforms digital signal from a unipolar DTE to a bipolar digital network signal

on the transmitting side and vice versa on the receiving side

DTE (data terminal equipment): a device that performs functions at the network end of a communications

line; see DCE (data communications equipment).

DVD (digital versatile disc): can hold over seven times as much information as CDs; drives are backward-

compatible with CD-ROM drives

DWDM (dense wavelength-division multiplexing): an optical technology used to increase bandwidth

over existing fiber optic backbones (see building backbone, campus backbone)

EBCDIC (extended binary coded decimal interchange code): 8-bit character encoding table used

by ISM mainframes

EGP (exterior gateway protocol): a protocol commonly used between hosts on the Internet to

exchange routing table information

EMI (electromagnetic interference): radiation that causes unwanted signals (interference or noise) to

be induced in other circuits; also called radio frequency interference or RFI

Enterprise network: connects many types of networks

Ethernet: most commonly used protocol designed to change the packets into electrical signals that can be

sent out over the wire

Exterior protocols: routing protocol used between autonomous systems

FAT (file allocation table): table that the operating system uses to locate files on a disk; because a file may

be divided into many sections that are scattered around the disk, the FAT keeps track of all the pieces

FDDI (fiber distributed data interface): a set of ANSI protocols for sending digital data over fiber optic

cable (see ANSI)

FDM (frequency-division multiplexing): permits a range of input signals to be carried over a communication

line that uses separate carrier frequencies for each signal channel; mostly used for analog information

but can carry digital

File management system: way to store and retrieve information from disk drives; controls how files can be

created, accessed, retrieved, and deleted

Firewall: a barrier between a network and the Internet through which only authorized users can pass; set of

security policies to screen incoming and outgoing messages; also used to isolate one part of a network from

another

Floppy drive: early versions were actually floppy; today, they use hard 3.5 inch disk; also referred to as

removable drive

FM (frequency modulation): blending data into a carrier signal; a modem modulates data by converting it

to audible tones that can be transmitted on a telephone wire, and demodulates received signals to get the data

Frame: data structure that collectively represents the transmission stream (headers, data, and the trailer) and

provides the information necessary for the correct delivery of the data

Frame relay: service with standards and specifications designed to transmit data; some users have had success

at transmitting voice

FRAD (frame relay access device): software that frames the customer’s payload with the Frame Relay

overhead information, including the first DLCI (data link connection identifier) address, to prepare it for delivery

to the network

Frequency: number of times a wave repeats a cycle in a one-second period; measured in cycles per second,

or hertz

FTP (file transfer protocol): application used to transfer a copy of a file from one computer to another

computer with one acting as client and the other as server; a login with a user name and password is typically

required

Full-duplex link: enables both sides to simultaneously send and receive data; could require two separate

cables, one in each direction or a single multiplexed cable

Gateways: a node on a network that translates (converts protocol) from one operating system environment

to another

Gateway routers: used to implement exterior protocols and interconnect autonomous systems

Gbps (gigabits per second; billions of bits per second): a data transfer speed measurement for highspeed

networks

GUI (graphical user interface): easy way of accessing applications with the use of a pointing device, such

as a mouse; pronounced “gooey”

Half-duplex link: enables one side to transmit and receive, but not simultaneously; information only flows in

one direction at a time using a control procedure to mediate

Hard drive: large data storage devices permanently mounted in the computer's case

Hardware: physical devices located on a desk or in a server-room rack

Host-to-host layer: part of the TCP/IP model that performs the same function as the transport layer in the

OSI model

Host address: part of an IP address that is uniquely assigned by an administrator

HTTP (hypertext transfer protocol): protocol users interact with (by means of a browser) to access Web

pages over an internet or intranet

Hubs: bring the users of the network into contact with one another

Hz (hertz): unit of frequency; one hertz simply means one cycle per second, applied to any periodic event

(e.g., one tick of a clock is 1 Hz; the human heart beats at 1.2 Hz)

A Manager’s Guide to IT Terminology (A - C)

10Base5 (10 Mbps, baseband, 500 meter): one of several physical media specified by 802.3 for use in an Ethernet local area network LAN); consists of Thickwire coaxial cable with a maximum segment length of 500 meters

10Base2 (10 Mbps, baseband, 185 meter): one of several physical media specified by IEEE 802.3 for use in an Ethernet local area network LAN); consists of Thickwire coaxial cable with a maximum segment length of 185 meters

10BaseT (10 Mbps, baseband, unshielded twisted-pair): one of several physical media specified by IEEE 802.3 for use in an Ethernet local area network (LAN); is ordinary telephone twisted pair wire

100BaseT (100 Mbps, baseband, unshielded twisted-pair): one of several physical media specified by IEEE 802.3 for use in an Ethernet local area network (LAN); referred to as Fast Ethernet because of its higher transmission speed

AAL (ATM adaptation layer): adapts PDUs passed down from higher layers onto ATM cells

Active hub: allows multiple devices to be wired to a central location to share the same media and regenerate the signal; also referred to as multiport repeaters

ADSL (asymmetric digital subscriber line): typical form of xDSL telephone companies offer to residences

AGP (accelerated or advanced graphics port): a high-speed, point-to-point channel for attaching a graphics card to a computer’s motherboard, primarily to assist in the acceleration of 3D computer graphics

ALU (arithmetic logic unit): a digital circuit that calculates an arithmetic operation (e.g., addition, subtraction) and logic operations between two numbers; the fundamental building block of the Central Processing Unit (CPU) or a computer

AM (amplitude modulation): a technique used for transmitting information via a radio carrier wave.

Amplitude: height of the wave at any point in the wave

ANSI (American National Standards Institute): a voluntary organization that coordinates the development and use of consensus standards in the United States and represents the needs and views of U.S. stakeholders in standardization forums around the globe

API (application programming interface): gives programmers a formal set of routines to call on to use underlying network services

Application layer: performs the functions of file transfer, e-mail, etc. (see OSI Model)

Architecture: how a system is designed; includes how the components are connected to and operate with each other

ARP (address resolution protocol): network layer protocol provided with TCP/IP; used to map an IP (internet protocol) address to a MAC (media access card) address

ASCII (American Standard Code for Information Interchange): relates a number from 0 to 255 in the binary (base 2) form to keyboard characters

ASIC (application-specific integrated circuit): an integrated circuit designed for a particular use (e.g., a chip designed solely to run a cell phone)

Asynchronous communication: describes when devices, such as computers, rely on their own internal clocks; it provides connectivity to printers, modems, fax machines, etc.

ATM (asynchronous transfer mode): high bandwidth, cell-switching technology; designed to carry many different types of information, including voice, video, image, data, and graphics; another form of STDM (statistical time division multiplexing)

AUI (attachment unit interface): a 15-pin connection that provides a path between a node’s Ethernet interface and the medium attachment unit (MAU); also known as a transceiver

AS (Autonomous system): collection of IP networks under the control of a single entity

B (bearer) channel: carries voice, video, image, or data traffic, depending upon the equipment and applications available

Bandwidth: expressed in a range of frequencies using hertz as the unit of measurement; also called analog capacity

Base 2 System: binary number system, only two discrete values (0 and 1) are possible and all numbers are a combination of these two characters; digital signals are numbers sent in the Base 2 system

Base 10 System: the decimal system

Baseband: describes signals and systems whose range of frequency is measured from 0 to a maximum bandwidthor highest signal frequency; sometimes used as a noun for a band of frequencies starting at 0

BGP (border gateway protocol): an interautonomous system routing protocol; a network or group of networksunder a common administration and with common routing policies

Bit: contraction of the expression “binary digit”; smallest unit of data in a computer

BIOS (basic input/output system): the firmware code run by an IBM-compatible PC when first powered on, known as “booting up”; primary function is to prepare the machine so other software programs can load, execute, and assume control of the PC

Bluetooth: specification that allows mobile phones, computers, and PDAs to be connected wirelessly over short ranges

Bps: bits per second: common measure of data speed for computer modems and transmission carriers

BRI (basic rate interface): an integrated services digital network configuration, usually intended for the home and small enterprise (see also PRI)

Bricks and clicks: business that existed pre-Internet that now uses e-commerce technology to sell on the Internet

Bridge: interprets the LAN hardware adapter address contained in MAC and decide whether to filter or forward the frame; does not change the frame in any way

Browsers: client applications that access WWW servers

Building backbone: connects LANs within a building

Bus: electrical connection between any two components in a computer

Bus topology: system layout where electrical signals generated by a device connected anywhere on the bus are received by all other connected devices

Byte: the standard size of data in a computer; 8-bits

Cache: keeps data the processor is likely to need quickly close at hand; increases processor operation speed

CAD/CAM (computer-aided design/computer-aided manufacturing): software used to design products such as electronic circuit boards in computers

Campus backbone: connects building LANs together

CAP (competitive access provider): company that provides network links between the customer and the IntereXchange carrier or directly to the internet service provider (ISP); CAPs operate private networks independent of local exchange carriers

CD-R (compact disc - recordable): special type of CD-ROM that can be written onto by any computer with a recording drive; can only be written onto once

CD-ROM (compact disc read-only memory): optical storage device read by lasers; can hold up to 700 megabytes of data

CD-RW (compact disc - rewritable): special type of CD-ROM that can be written onto by any computer with a recording drive; can be written onto more than once

Checksum: a count of the number of bits in a transmission unit that is included with the unit so the receiver can check to see whether the same number of bits arrived

CIR (committed information rate): describes the user information transfer rate the network supports during normal network operations

CLEC (competitive local exchange carrier): In the US, a telecommunications provider company (also called a carrier) that competes with other, already established carriers (the local telephone company; e.g., GTE, Bell South).

CLI (command line interface): allows precise control of a function, but requires remembering all the correct commands and switches necessary for a task; used in DOS; a way of interacting with computers

Client/server architecture: network where some computers are dedicated clients (workstations) and some are dedicated servers; information is centralized on the server, and an administrator sets policies and manages it

CMOS (complementary metal-oxide semiconductor) RAM: requires very little power; maintains information even when the computer is off

Collision: occurs when several network users communicate at the same time and interfere (collide) with one another

Collision domain: logical network segment where data packets can "collide" with one another for being sent on a shared medium, in particular in the Ethernet networking protocol

Computer networking: a combination of hardware and software that lets the various computers in an organization communicate with one another

Computer operating system (OS): special computer program that provides an environment in which other programs can use the computer’s central processor and the attached input/output devices

Connectivity devices: bring users of the network into contact with one another

Constant bit rate (CBR): transmission that uses a set amount of network capacity on a continual basis; used when the arrival of the information is time-sensitive

Convergence: the merging and sometimes clashing of voice and data networks

CPE (customer premises equipment): generally refers to telephones, DSL or cable modems, or purchased set-top boxes for use with communication service providers’ services

CPS (cycles per second): measure of how frequently an alternating current changes direction; has been

replaced by the term hertz (Hz)

CPU (Central Processing Unit): the brain of the computer system where calculations and decisions are

made; also referred to as the processor

CPU Speed: how fast the CPU works

CSU (channel service unit): provides a loopback function for telephone company testing, and checks bipolar

signal generation

CRC (cyclic redundancy check): method of checking for errors in data that has been transmitted on a communications

link; a function used to produce a Checksum against a block of data

CS (convergence sublayer): particular protocols that are responsible for gathering and formatting higher

layer information so it can be processed by the lower layers

CSMA/CD (carrier sense multiple access/collision detect): set of rules for determining how network

devices response when two devices collide