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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 it’s commonly used for Ethernet backbone connections.

2B+D — IDSN’s 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 digital.

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. It’s the U.S.’s member body to the ISO (International Standards Organization).

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); it’s a 15-pin socket.

Autosensing — Automatically adjusts to different operating conditions. For example, an autosensing power supply will provide the correct power level whether it’s 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 bps.

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 a spooler.

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 graphic presentation.

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 multiplexed data.

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 ID.

DCE (Data Communications Equipment) — Devices providing the functions required to establish, maintain, and terminate a data-transmission connection—for 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 user’s 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 segment’s 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 device’s 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 transmission rates.

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. It’s 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 and ISO.

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 organization’s 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 switching arrangements.

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 networks).

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 transmitted signal.

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 terminal.

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 of these.

NT1 (Network Terminator) — A device that terminates an ISDN line at the customer’s 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, 51.48 Mbps.

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 transmission block.

PAD (Packet Assembler-Disassembler) — An interface between a terminal or computer and a packet-switching network.

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. It’s the industry’s 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 of need.

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 transmission.


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 terminal.


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 band circuits.

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 applications.



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. See PAD.

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 don’t 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).





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