10BASE2 An Ethernet standard
that uses a thin coaxial cable. Also called Thin Ethernet,
ThinWire, and ThinNet. A 10-Mbps baseband signal with
a maximum (repeaterless) distance of 185 meters.
10BASE5 The original Ethernet standard that
uses a thick coaxial cable. Also called Thick Ethernet,
ThickWire, and Thicknet. A 10-Mbps baseband signal with
a maximum (repeaterless) distance of 500 meters.
10BASE-FL The portion of the 10BASE-F standard
that defines a fiber optic link between a concentrator
10BASE-T The most common 10-Mbps Ethernet standard.
It uses twisted-pair wires and RJ-45 connectors.
100BASE-T A high-speed version of Ethernet (IEEE
802.3). Also called Fast Ethernet, 100BASE-T transmits
at 100 Mbps.
1000BASE-T Another high-speed version of the
IEEE 802.3 Ethernet standard. Often called Gigabit Ethernet.
It transmits at 1000 Mbps, or 1 Gbps, and its commonly
used for Ethernet backbone connections.
2B+D IDSNs basic service is called Basic
Rate Interface, or BRI. BRI is made up of two 64-kbps
B channels and one 16-kbps D channel (2B+D).
Active/Passive Devices In Current
Loop applications, a device capable of supplying the current
for the loop is active and a device that must draw its
current from connected equipment is passive.
ADSL (Asymmetric Digital Subscriber Line service)
ADSL (ANSI standard T1.413) is commonly used for downstream
transmissions like Internet access in homes or businesses.
Its downstream/upstream transmission rates range from
9 Mbps/640 kbps over short distances to 1.544 Mbps/16
kbps over longer distances.
Analog A transmission mode in which data is represented
by a continuously varying electrical signal. Compare with
ANSI (American National Standards Institute)
The principal standards-development body in the U.S. ANSI
is a nonprofit, nongovernmental body supported by trade
organizations, professional societies, and industry. Its
the U.S.s member body to the ISO (International
ASCII (American Standard Code for Information Interchange)
Pronounced askee. A binary data code consisting
of 7 data bits plus 1 bit for parity or special symbols;
established by ANSI for compatibility between data services.
Asynchronous Transmission Transmission in which
time intervals between transmitted characters may be of
unequal length. Transmission is controlled by start and
stop bits at the beginning and end of each character.
Compare with synchronous transmission.
ATM (Asynchronous Transfer Mode) A high-speed
cell-switching network technology that handles data and
real-time voice and video. ATM is defined in the Broadband
ISDN (BISDN) standard and provides bandwidth-on-demand
by charging customers for the amount of data they send.
Data rates are scalable, starting as low as 1.5 Mbps with
intermediate speeds of 25, 51, and 100 Mbps, to high speeds
of 155 or 622 Mbps and up in the OC (Optical Carrier Network).
AUI (Attachment Unit Interface) The network interface
used with standard Ethernet (10BASE5); its a 15-pin
Autosensing Automatically adjusts to different
operating conditions. For example, an autosensing power
supply will provide the correct power level whether its
plugged into 115- or 230-volt power.
Backplane The rear of a device
enclosure, where connectors are located, cables are attached,
and components are inserted.
Balun (BALanced UNbalanced) A device that connects
a balanced line, such as twisted pair, to an unbalanced
line, such as coaxial cable.
Bandwidth The range of frequencies available
for signaling; the difference between the highest and
lowest frequencies of a band, measured in Hertz.
Baud A unit of signaling speed. The speed in
baud is the number of line changes (in frequency, amplitude,
etc.) or events per second. At low speeds each event represents
only one bit condition, and baud rate equals bits per
second (bps). As speed increases, each event represents
more than one bit, and baud rate does not truly equal
BERT/BLERT (Bit Error Rate Test/Block Error Rate Test)
Tests that measure data-transmission quality by
comparing received data with an established data pattern
and then counting the number of mismatches (errors). Measurements
are made of either bits or block errors.
Bisynchronous Transmission (BSC) A byte- or character-oriented
IBM® communication protocol that has become an industry
standard. It uses a defined set of control characters
for synchronized transmission of binary-coded data between
stations in a data-communication system.
Bit (Binary Digit) The smallest unit of information
in a binary system; a one (1) or zero (0) condition.
BRI (Basic Rate Interface) An ISDN service referred
to as 2B+D, BRI provides two 64-kbps, bearer digital channels
plus a 16-kbps delta channel. ISDN terminal adapters replace
modems as the customer-premise connection to this service
for direct connections of data and voice transmissions.
Bridge A device that connects two LAN segments
together, which may be of similar or dissimilar types,
such as Ethernet and Token Ring.
Buffer A temporary-storage device used to compensate
for a difference in data rate and data flow between two
devices (typically a computer and a printer); also called
Byte A unit of information, usually shorter
than a computer word. Eight-bit bytes are
most common. Also called a character.
CBT (Computer-Based Training) Programs that provide
interactive training sessions through some form of computer-based
CCITT (Comité Consultatif Internationale de Télégraphique
et Téléphonique) International association
that once set worldwide communication standards (such
as V.21, V.22, and X.25). Replaced by the ITU-TSS.
CE A certification products must attain in order
to be sold in the European Union (EU) that involves complying
with a number of different EU standards including strong
resistance to EMI/RFI as well as low EMI/RFI emissions.
Composite Link The line or circuit connecting
a pair of multiplexors or concentrators; the circuit carrying
Composite Video The video-only (no audio) part
of a TV signal that mixes red, green, blue, and sync signals.
Contention The facility provided by the dialup
network or a data PABX that enables multiple terminals
to compete on a first-come, first-served basis for a smaller
number of computer ports.
Crossed Pinning A cable configuration that enables
two DTE or two DCE devices to communicate.
Crossover A conductor that runs through the cable
and connects to a different pin number at each end.
CSU (Channel Service Unit) A digital DCE used
to terminate digital circuits (such as DDS or T1 lines)
at the customer site. It performs line coding, line conditioning,
and equalization functions and responds to loopback commands
sent from the central office.
Current Loop A method of interconnecting terminals
and transmitting signals in which a mark (binary 1) is
represented by current on the line, and a space (binary
0) is represented by the absence of current.
D Channel (Delta Channel) A 16-kbps
channel used to signal the telephone-company computer
to make calls, put them on hold, and activate features
such as Conference Calling and Call Forwarding. It also
receives information about incoming calls, as in Caller
DCE (Data Communications Equipment) Devices
providing the functions required to establish, maintain,
and terminate a data-transmission connectionfor
example, a modem.
DDS (Dataphone® Digital Service) A communications
service from AT&T in which data is transmitted in digital
rather than analog form.
Digital Transmission in which data is encoded
as either a binary one (1) or zero (0). Compare to analog.
DSL (Digital Subscriber Line) The root of all
xDSL services. It consists of two 64-kbps bearer (B) channels
and one 16-kbps (D) channel (2B+D). These channels are
bundled together into a 128-kbps pipeline for simultaneous
transmission of voice, data, fax, or video signals.
DSU (Digital Service Unit) The interface between
a users data terminal device (DTE) and a digital
data service, usually via a CSU. Converts an RS-232C or
other terminal interface to a DSX-1 interface.
DSX-1 Interface The CSU interface to which a
T1 line is attached. This can be either a DB15 female
or an RJ-48C female connector.
DTE (Data Terminal Equipment) Devices acting
as data source, data sink, or both.
DVI (Digital Visual Interface) A high-performance
interface between a computer and a display device that
uses transition-minimized differential signaling (TMDS)
for improved video signal accuracy. DVI-D is a digital-only
connector; DVI-I integrates both digital and analog connections.
E1 The European standard for
high-speed digital transmission at 2.048 Mbps, with 31
64-KB channels available for traffic. Also called 2-Meg,
European T1, or Conference European Post Telecom.
EIA (Electronic Industries Association) A standards
organization in the U.S. specializing in the electrical
and functional characteristics of interface equipment.
EMI/RFI (Electromagnetic Interference/Radio-Frequency
Interference) Filtering Protection from background
noise that could alter or destroy data transmission.
ESF (Extended SuperFrame) An enhanced T1 format
used to enable a line to be monitored during normal operation.
It uses 24 frames grouped together to provide room for
CRC bits and other diagnostic commands.
FCC (Federal Communications Commission) The regulatory
body for U.S. interstate telecommunications services as
well as international service originating in the U.S.
Firewall A security-oriented network node set
up as a boundary to prevent one segments traffic
from crossing over to another segment. Firewalls are often
used to protect LANs from hackers on the Internet.
Flow Control The procedure for regulating the
flow of data between two devices; it prevents the loss
of data once a devices buffer has reached its capacity.
FRAD (Frame Relay Assembler/Disassembler) A
communications device that formats outgoing data into
the format required by a Frame Relay network.
Frame Relay A packet-switched network similar
to X.25 but with end-to-end error checking and high-speed
FT-1 (Fractional T1) Digital service with data
rates between 56 kbps and 1.544 Mbps (full T1 speed).
Typically provided via 4-wire copper cable.
Full Duplex (FDX) Simultaneous, independent transmission
in both directions.
Ground Loop An unwanted continuous
ground current flowing between two devices that are at
different ground potentials.
Half Duplex (HDX) Transmission
in either direction but not both simultaneously.
Handshaking An exchange of predetermined signals
between two devices establishing a connection. Its
usually part of a communications protocol.
IEEE (Institute of Electrical and Electronic
Engineers) An international professional society
that issues its own standards and is a member of ANSI
Interface A shared boundary defined by common
physical-interconnection and signal characteristics, and
by the meanings of interchanged signals.
Internet 1) The worldwide computer network used
for reference, e-mail, and other services. 2) Any large
network made up of several smaller networks. 3) A group
of networks that are interconnected so they appear to
be one continuous large network and can be addressed seamlessly
at the OSI Model Network Layer through routers.
Intranet A network that connects a related set
of standard Internet protocols and files in HTML format
with employees using Internet browsers in an organizations
network and within corporate firewalls.
IP (Internet Protocol) The protocol used in gateways
to connect networks at the OSI Network Level (Layer 3)
and above. IP routes a message across networks.
IPX (Internet Packet Exchange) A communication
protocol in Novell® NetWare® that creates, maintains,
and terminates connections between network devices such
as workstations and servers.
ISDN (Integrated Services Digital Network) A
circuit made up of two 64-kbps bearer (B) channels and
one 16-kbps data (D) channel used for signaling. The B
channels can be combined to provide up to 128 kbps of
throughput via a dialup network.
ISO International Standards Organization.
ITU-TSS (International Telegraphic Union-Telecommunications
Standards Sector) The replacement organization
for the CCITT.
LATA (Local Access and Transport Area)
A U.S. geographical subdivision that defines local
(instead of long-distance) telephone service.
Leased Line A telephone line reserved for the
exclusive use of leasing customers without interexchange
Line Driver A DCE device that amplifies a data
signal for transmission over cable for distances beyond
the RS-232 limit of 50 feet, even up to several miles.
Also called limited-distance modem (LDM) or
short-haul modem (SHM).
Link A communications circuit or transmission
path connecting two points.
Local Area Network (LAN) A data communications
system confined to a limited geographic area (up to 6
miles or about 9.6 kilometers) with moderate to high data
rates (100 kbps to 1000 Mbps). The area served may consist
of a single building, a cluster of buildings, or a campus-type
arrangement. The network uses some type of switching technology
and does not use common carrier circuits (although it
may have gateways or bridges to other public or private
Loopback A diagnostic test in which the transmitted
signal is returned to the sending device after passing
through all or part of a data communications link or network.
A loopback test compares the returned signal with the
LPT1 In a PC, the logical name assigned to parallel port #1.
MMJ (Modular Molded Jack) A modular
connection used in DEC systems with six wires and
a locking tab on the side of the connector.
MNP® (Microcom Networking Protocol) A family
of communications protocols from Microcom, Inc., that
are de facto standards for error correction and data compression.
Modem (MOdulator-DEModulator) A device used to
convert serial digital data from a transmitting terminal
to an analog signal for transmission over a telephone
channel or to reconvert the transmitted analog signal
into serial digital data for acceptance by a receiving
Modem Eliminator A device used to connect a local
terminal and a computer port instead of the pair of modems
they would ordinarily need; enables DTE-to-DTE data and
control signal connections otherwise not easily achieved
by standard cables or connectors.
Multiplexor A device that divides a transmission
into two or more subchannels, either by splitting the
frequency band into narrower bands (frequency division)
or by allotting a common channel to several transmitting
devices, one at a time (time division).
Multipoint Line A single communications line
or circuit that interconnects several stations and usually
requires some kind of polling mechanism to address each
connected terminal with a unique address code.
Network Topology The physical
and logical relationship of nodes in a network, typically
a star, ring, tree, or bus topology, or some combination
NT1 (Network Terminator) A device that terminates
an ISDN line at the customers premises.
Null Modem A device that connects two DTEs directly
by emulating the physical connections of a DCE.
OC-1 (Optical Carrier Level 1)
The lowest optical-transmission rate in the SONET standard,
OSI (Open System Interconnection)
Packet A group of bits (including
data and call control signals) transmitted as a whole
on a packet-switching network. Usually smaller than a
PAD (Packet Assembler-Disassembler) An interface
between a terminal or computer and a packet-switching
Parity Check The addition of noninformation
bits to make up a transmission block that ensures the
total number of ones as always either even or odd; used
to detect transmission errors.
PING (Packet InterNet Groper) A utility used
to determine which devices are available and responsive
on a network or at an Internet site.
POP (Point of Presence) The place where a line
from a long-distance carrier (IXC) connects to the line
of the local telephone company or to the user if the local
company is not involved.
POTS (Plain Old Telephone Service) The basic
analog service provided by the public telephone network,
without any added facilities.
Protocol A formal set of conventions governing
the formatting and relative timing of message exchange
between two communicating systems.
Public Switched Telephone Network (PSTN) Any
switching communications system, such as Telex, TWX, or
public telephone networks, that provides circuit switching
to many customers.
RIP (Routing Information Protocol)
A routing protocol in TCP/IP and NetWare that identifies
all attached networks as well as the number of router
hops required to reach them.
Router A computer system that stores and forwards
data packets by way of network addresses between Local
Area Networks (LANs) or Wide Area Networks (WANs).
RS-232 An EIA interface standard between DTE
and DCE that uses serial binary data interchange. Its
the industrys most common interface standard.
RS-422, RS-423 EIA interface standards operating
with RS-449 that specify electrical characteristics for
balanced circuits and extend transmission speeds and distances
beyond RS-232. RS-422 is a balanced-voltage system with
high noise immunity; RS-423 is the unbalanced version.
RS-449 An EIA general-purpose 37-pin and 9-pin
interface for DTE and DCE.
RS-485 A balanced interface similar to RS-422
but using tristate drivers for multidrop applications.
RS-530 Similar to RS-449 because it describes
a mechanical connector, RS-530 uses a DB25 connector and
supports RS-422, RS-423, RS-485, and V.35 electrical interfaces.
SDLC (Synchronous Data Link Control) The IBM®
standard protocol, superseding bisynchronous (BSC).
Short-Haul Modem See line driver.
S Interface The standard 4-wire interface between
an ISDN terminal adapter and the network channel termination.
SPID (Service Profile IDentifier) A number assigned
to an ISDN line by the ISDN service provider that identifies
certain characteristics of the line.
Statistical Multiplexing A multiplexing technique
in which bandwidth is dynamically allocated on the basis
Switched Line A communications link, such as
the public telephone network, for which the physical path
may vary with each usage.
Synchronous Transmission A transmission in which
data bits are sent at a fixed rate with the transmitter
and receiver synchronized. Synchronized transmission eliminates
the need for start and stop bits. Compare with asynchronous
T1 A digital carrier facility used to transmit
a DS-1 formatted digital signal at 1.544 Mbps, it can
be divided into 24 separate DS0 channels at either 56
or 64 kbps.
Time-Division Multiplexing (TDM) Transmitting
multiple channels on a single transmission line by connecting
terminals, one at a time, at regular intervals, interleaving
bits (Bit TDM) or characters (Character TDM) from each
U Interface A 2-wire ISDN local
loop of twisted-pair cables.
USB (Universal Serial Bus) Serial 4-wire bus
architecture for peripheral I/O ports that auto-senses
up to 127 peripherals at a distance of 5 meters (16.4
ft.). Version 1.1 supports 1.5- and 12-Mbps data rates.
Version 2.0 is backward compatible to Version 1.1 speeds
and also supports 480-Mbps data rates.
V.35 An ITU standard governing
data transmission at 48 kbps over 60- to 108-kHz group
VPN (Virtual Private Network) A means of segmenting
a network and prioritizing traffic based on a selected
set of users. VPNs can be built on the Internet with an
IP network like X.25, over Intranets, and on Ethernet
and Frame Relay networks. Tunneling encapsulates the data
on these networks; encryption protects it. VPNs can also
be built with hardware designed for tunneling or firewall
X.21 An ITU standard governing
the interface between DTE and DCE for synchronous operation
on public data networks.
X.25 An ITU standard governing the interface
between DTE and DCE for terminals operating in the packet
mode on public data networks.
X.25 PAD A device that permits communication
between non-X.25 devices and the devices in an X.25 network.
xDSL A term that encompasses a broad range of
digital subscriber line (DSL) services. These DSL-based
services, which operate over existing phone lines, provide
users with speeds significantly faster than those of 56-kbps
analog modems. A major draw for customers is that they
pay only for the bandwidth they use. Telephone companies
like DSL because they dont have to invest in costly
rewiring work to offer broadband services.
X-ON/X-OFF (Transmitter On/Transmitter Off) Control
characters used for flow control that instruct a terminal
to start transmission (X-ON) and end transmission (X-OFF).