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
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)