HCS Glossary of Terms

HCS Glossary of Terms


A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Z





That portion of fiber optic attenuation resulting of conversion of optical power to heat.

Abrasion Resistance

Ability of a wire, cable or material to resist surface wear.


Alternating current.

Accelerated Aging

A test that simulates long time environmental conditions in a relatively short time.


The difference between attenuation and crosstalk, measured in dB, at a given frequency (acronym for

Attenuation Crosstalk Ratio).

Important characteristic in networking transmission to assure that signal sent down a twisted pair is stronger at the receiving end of the cable than are any interference signals imposed on that same pair by crosstalk from other pairs.

Acknowledgement (ACK)

A character that is used as a reply in communications protocols.   An ACK is sent from the receiving device in response to the successful transmission of another character.

Active/Passive Device

In current loop applications, a device capable of supplying the current for the loop (active) and a device that must draw its current from connected equipment (passive).

Active Terminator

In SCSI, a terminator that can compensate for variations in the terminator power supplied by the host adapter through means of a built-in voltage regulator. A type of terminator containing a sophisticated circuit that can compensate for variations in the power supplied by the host adapter, as well as variations in bus impedance of complex SCSI systems. Active terminators works to control the impedance using a voltage regulator. Fast & Wide SCSI and beyond can do better with active termination on both ends of the bus. It cost a little more but will provide more stability and less problems on your part.


The device that connects a piece of equipment to the network and controls the electrical protocol for communication with that network; also called network interface card, or NIC.

Adaptive Technology

An Intel technology (supported in adapters and switches) that automatically and dynamically customizes product performance to match network operating conditions, thus helping to optimize network performance.

Advanced Paketized Voice (APV)

Low overhead voice digitizing technique.  Will produce acceptable voice signals while only requiring a bandwidth of 9.6 Kbps.


Audio frequency.

Aggregate Input Rate

Aggregate input rates measure the total data rate of all terminals connected to a multiplexer.  Burst aggregate input rate refers to the maximum data rate accepted by the multiplexer at any given instant.

Air Core

Cables that are not gel filled.

Air-Gap Dielectric

A coaxial design in which a monofilament of plastic holds the center conductor in place allowing the remainder of the dielectric to be air. Typical velocities of up to 84% can be achieved in this design.


A combination of two or more different polymers/metals. Usually combined to make use of different properties of each polymer metal.

Alternating Current (AC)

Electric Current that alternates or reverses polarity continuously. The number of alternations per second are described as cycles, (hertz or Hz).


Amplitude modulation. A means of signal transmission whereby transmitter (light source) signal intensity is varied in relation to the amplitude of the input signal.


Conditions existing at a test or operating location prior to energizing equipment (e.g.: ambient temperature).


Current handling capability. The maximum current a conductor can carry without being heated beyond a safe limit.


A standard unit of current. Defined as the amount of current that flows when one volt of emf is applied across one ohm of resistance. An ampere of current is produced by one coulomb of charge passing a point in one second.


The maximum value of a varying wave form.


Representation of data by continuously variable quantities.

Analog Signal

An electrical signal which varies continuously, not having discrete values. Analog signals are copies or representations of other waves in nature. An analog audio signal, for instance, is a representation of the pressure waves which make up audible sound.


To soften and relieve strains in any solid material, such as metal or glass, by heating to just below its melting point and then slowly cooling it. Annealing generally lowers the tensile strength of the material, while improving its flex life and flexibility.


American National Standards Institute.

Aramid Yarn

Strength element used in Siecor cable to provide support and additional protection of the fiber bundles. Kevlar is a particular brand of aramid yarn.

Asynchronous (async) Transmission

A transmission method in which time intervals between transmitted characters may be of unequal length. Transmission is controlled by start and stop bits on each character, rather than by clocking as in synchronous transmission.


The American Society for Testing and Materials, a standards organization which suggests test methods, definitions and practices


Asynchronous Transfer Mode. The SONET standard for a packet switching technique which uses packets of a fixed length.

American Standard Code for Information Interchange (ASCII)

(Pronounced “ASK key”} A 7-bit binary data code used in communications with most minicomputers and personal computers to achieve compatibility between data services.



The decrease in magnitude of a signal as it travels through any transmitting medium, such as a cable or circuitry. Attenuation is measured as the logarithm of a ratio. It is expressed in decibels or dB.


A passive optical component that intentionally reduces the optical power propagating in a fiber.


A term used to describe sounds within the range of human hearing. Also used to describe devices which are designed to operate within this range (20 Hz to 20 kHz).

Audio Frequency

Frequencies within the range of human hearing: approximately 20 to 20,000 Hz.

AutoBaud Rate Detection (ABR)

With autobaud rate detection a receiving device can determine the data rate, code level, and stop bits of incoming data by examining the first character received, usually a pre-selected sign-on character.  ABR was designed to allow a receiving device to accept transmissions from multiple sending devices all at different speeds without having to be configured for each speed in advance.

Automatic Dial Backup (ADB)

A feature of many modern modems that allows them to automatically switch from lease lines to dial lines when impairments on lease lines reach critical levels. The modems will also return to the use of the lease lines when conditions improve.

Automatic Request for Retransmission (ARQ)

A form of data transmission error correction.  A receiving device will inform the transmitting device which blocks of data were received correctly and the transmitting device will retransmit any that were not received correctly.

Average Power

The average over time of a modulated signal.

Attachment User Interface (AUI)

Most commonly used with reference to the 15 pin AD type connector and the cables used to connect single and multiple channel equipment to an Ethernet transceiver. The interface between a transceiver and a NIC or other network node in a 10 BASE5 Ethernet network.


American Wire Gage. A wire diameter specification. The smaller the AWG number, the larger the wire diameter.


Appliance Wiring Material (UL Term).



The cable used to connect all systems of a multi-level distributed system to an intermediate system.

Back Reflection,
Return Loss

Light reflected from the cleaved or polished end of a fiber caused by the difference of refractive indices of air and glass. Typically 4% of the incident light. Expressed in dB relative to incident power.


The scattering of light in a fiber back toward the source, used to make OTDR measurements. Bandwidth: The range of signal frequencies or bit rate within which a fiber optic component, link or network will operate.

Balanced Line

A cable having two identical conductors which carry voltages opposite in polarity and equal in magnitude with respect to ground, suitable for differential signal transmission.


A device for matching an unbalanced coaxial transmission line to a balanced two-wire system. Can also provide impedance transformation, as 300 ohm balanced to 75 ohm unbalanced.


The difference between the upper and lower limits of a given band of frequencies. Expressed in Hertz.


Unit of data transmission speed meaning bits per second (500 baud=500 bits per second).


A network transmission technique that uses voltage to represent data; similar to turning a light switch on and off.


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 bps. As speed increases, each event represents more than one bit, and baud rate does not truly equal bps. But, in common usage, baud rate and bps are often used interchangeably. Unit of signaling speed. The speed in baud is the number of discrete conditions or events per second. If each event represents only one bit condition , baud rate equals BPS. When each event represents more than one bit (e.g. dibit), baud rate does not equal BPD.

Bayonet Coupling

A quick coupling device for plug and receptacle connectors. Pins projecting from the outside of the cylindrical receptacle engage with corresponding cam slots in the bayonet plug.

Bayonet-Neil-Councelman (BNC)

A bayonet-locking connector for miniature coax. Contrast with TNC


A unit that represents the logarithm of the ratio of two levels. See dB.


Bit Error Rate. The number of errors occurring in a system per second. The lower the better.

Bend Loss

A form of increased attenuation caused by (a) having an optical fiber curved around a restrictive radius of curvature or (b) microbends caused by minute distortions in the fiber imposed by externally induced perturbations.

Bend Radius

Radius of curvature that a flat, round, fiber optic or metallic cable can bend without any adverse effects.


A tape or thread used for holding assembled cable components in place.


One binary digit.


Abbreviation for “Bayonet Neil Councelman”. A coaxial cable connector used extensively in video and R. F. applications and named for its inventor.


The method used to produce good electrical contact between metallic parts of any device. Used extensively in automobiles and aircraft to prevent static buildup. Also refers to the connectors and straps used to bond equipment.


Bits Per Second. The number of binary bits that can be transmitted per second - I.e. Mbps (Mega - millions), Gbps (Giga - billions).


A group of textile or metallic filaments interwoven to form a tubular flexible structure which may be applied over one or more wires, or flattened to form a strap.

Braid Angle

The angle between a strand of wire in a braid shield and the axis of the cable it is wound around.

Breakdown Voltage

The voltage at which the insulation between two conductors will fail and allow electricity to conduct or ‘arc’.


The point at which a conductor or conductors are separated from a multi-conductor cable to complete circuits at various points along the main cable.


A device that connects LANs running the same protocols and cabling. Compare router, gateway.


The transmission of data over a network for general reception rather than a specific terminal.


The technique used to multiplex multiple networks on a single cable without interfering with each other.


A protective coating over an optical fiber.


A term used to define a mounting style of connectors. Bulkhead connectors are designed to inserted into a panel cutout from the rear (component side) of the panel.

Bunch Strand

Conductors twisted together with the same lay and direction without regard to geometric pattern.


A collection of wires in a cable (or copper traces on a circuit board) used to transmit data, status, and control signals. ISA, EISA, VL-Bus, and PCI are examples of PC buses. SCSI is also a bus. Also, a LAN topology in which all workstations are connected to a single cable. On a bus network, all workstations hear all transmissions on the cable. Each workstation then selects those transmissions addressed to it based upon address information contained in the transmission. 1) A transmission path of channel; an electrical connection, with one or more conductors, by which all attached devices receive all transmissions at the same time. 2) A linear LAN topology, used by Ethernet, in which every network node listens to all transmissions, selecting certain ones based on address identification. 3) In computer architecture, a data path shared by devices within a system; for example, a computer’s input/output bus. A data path shared by many devices (e.g. multiprint line) with one or more conductors for transmission.

Bus Topology

The physical layout of a network in which all systems connect to a main cable; also known as linear bus.


A group of adjacent binary digits (8 bits).



Symbol designation for capacitance, and Celsius.


Either a standard conductor, with or without insulation and other coverings, or a combination of conductors insulated from each other.

Cable Assembly

A completed cable and its associated hardware.

Call Inhibit

A configuration option that does not allow a voice/fax channel to call another voice/fax channel.


The ability of a dielectric material between conductors to store energy when a difference of potential exists between the conductors. The unit of measurement is the farad. Cable capacitance is usually measured in picofarads (pF).

Capacitive Crosstalk

Cable crosstalk or interference resulting from the coupling of the electrostatic field of one conductor upon one or more others.

Capacitive Reactance

The opposition to alternating current due to the capacitance of a capacitor, cable, or circuit. It is measured in ohms and is equal to 1/6.28fC where f is the frequency in Hz and C is the capacitance in farads.


Two conducting surfaces separated by a dielectric material. The capacitance is determined by the area of the surfaces, type of dielectric, and spacing between the conducting surfaces.


A continuous signal that is coupled with a second, information-carrying signal.

Carrier Detect (CD)

A control signal that indicates that a local modem is receiving a signal from a remote modem.

Carrier Sense Multiple Access with Collision Detection


A LAN transmission technique implemented in Layer 2 of the OSI model and employed by 10Mbps Ethernet and Fast Ethernet.


To connect a multiple-port device to an identical device, increasing the total number of ports available.


Rating of a cable established by TIA/EIA to indicate the level of transmission performance.


Community Antenna Television.


A metallic enclosure in some types of tubes and circuits within which resonant fields may be excited at the microwave frequency to which the cavity is tuned. Usually referred to as resonant cavity. A defined hole in the connector insert or housing into which the contact must fit.

Comité Consultatif Internationale de Télégraphique et Téléphonique (CCITT)

An international consultative committee that set worldwide communications standards such as V.32, V.34, and X.25. Replaced by the ITU-TSS.


Closed-circuit television.

Cellular Polyethylene

Expanded or "foam" polyethylene, consists of individual closed cells of inert gas suspended in a polyethylene medium, resulting in a desirable reduction of the dielectric constant.

Central Office

The building where common carriers terminate customer circuits and the switching equipment that connects those circuits is located.

Central Processing Unit (CPU)

The main processing chip or chips of a computer system. Often refers to the computer system in its entirety.

Centronics Parallel

A de-facto standard 36-pin interface for connecting parallel printers to PCs.


The horizontal cable including the workstation outlet and patch panel in the telecommunications closet plus a patch cord at each end, with maximum total length of 100 meter.

Channel Loopback

A diagnostic test that performs the loop at a multiplexer's channel interface.

Channel Service Unit (CSU)

A digital DCE used to terminate digital circuits (such as DDS or T1 lines) at the customer site. It conditions the line, ensures network compliance with FCC rules, and responds to loopback commands from the central office.

Characteristic Impedance

In a transmission line of infinite length, the ratio of the applied voltage to the resultant current at the point the voltage is applied, or the impedance which makes a transmission cable seem infinitely long, when connected across the cable's output terminals.

Chromatic Dispersion

The temporal spreading of a pulse in an optical waveguide caused by the wavelength dependence of the velocities of light.


A system of conducting media designed to pass an electric current.


A low refractive index material that surrounds the core of an optical fiber causing the transmitted light to travel down the core and protects against surface contaminant scattering. A layer of metal applied over another. Cladding is often chosen to improve conductivity or to resist corrosion.


Timing signal used in synchronous transmission.  Also the source of timing signals.

Closed Entry Contact

A female contact designed to prevent the entry of a pin or probing device having a cross-sectional dimension (diameter) greater then the mating pin.

Coaxial Cable

A cylindrical transmission line comprised of a conductor centered inside a metallic tube or shield, separated by a dielectric material, and usually covered by an insulating jacket.

Coil Effect

The inductive effect exhibited by a spiral-wrapped shield, especially above audio frequencies.

Color Code

A system of different colors or stripes used to identify components of cables such as individual conductors or groups of conductors.


1) In LAN technology, two stations attempting to use the same transmission medium at the same time. 2) In a half-duplex system, the result of both ends trying to transmit at the same time.

Collision Domain

The maximum length of the wiring media that allows collision detection. For example, the collision domain in Fast Ethernet using 100BASE-TX is 205 meters

Communications Protocol

The means of communications amongst items on a data link to ensure orderly exchange of information.


The signal of a multiplexer on the line side that includes all data for the multiplexer including synchronous, asynchronous, and voice.

Composite Loopback

A diagnostic test that performs the loop at the line side of the multiplexer.


A method for reducing bandwidth requirements by reducing the number of data and voice bits across a channel.


A device used to divide a data into two or more channels of lower average speed, dynamically allocating channel space according to demand in order to maximize throughput.

Concentric Stranding

A group of uninsulated wires twisted together and containing a center core with subsequent layers spirally wrapped around the core with alternating lay directions to form a single conductor.


The ability of a material to allow electrons to flow, measured by the current per unit of voltage applied. It is the reciprocal of resistivity.


A substance, usually metal, used to transfer electrical energy from point to point.


A tube of metal or plastic through which wire or cable can be run. Used to protect the wire or cable and, in the case of metal conduit, make it fireproof.


An addition of equipment to improve the quality of a leased voice-grade line so that it will reach the specifications for data transmission.


The components and technology that enable devices to exchange data across electronic links.


A data communications path, the process of establishing this path, or the point of attachment for this path.


A device designed to allow electrical flow from one wire or cable to a device on another cable. A connector will allow interruption of the circuit or the transfer to another circuit without any cutting of wire or cable or other preparation.

Connector Assembly

Includes housing and contact plus additional components such as hardware used to hold the assembly together and/or make the assembly a functional connector.


An electrically conductive component designed for use in a multi-circuit connector.

Contact Cavity

A defined hole in the connector insert or housing into which the contact must fit. A metallic enclosure in some types of tubes and circuits within which resonant fields may be excited at the microwave frequency to which the cavity is tuned. Usually referred to as resonant cavity.

Contact Durability

The number of insertion and withdrawal cycles that a connector must be capable of withstanding while remaining within the performance levels of the applicable specification.

Contact Engaging and Separating Force

Force required to either engage or separate contacts. Values are generally established for maximum and minimum and forces.

Contact Inspection Hole

A hole, perpendicular to the cylindrical rear portion of screw machined contacts, used to check the depth to which wire has been inserted into the barrel.

Contact Resistance

Measurement of electrical resistance of mated contacts when assembled in a connector under typical service use. Electrical resistance is determined by measuring from the rear of the electrical area of one contact to the rear of the contact area of the mating contact (excluding both crimps) while carrying a specified test current.


A first-come, first-served method of access used in public telecommunication or PBX systems in which multiple devices must access a limited number of communication ports.

Control Character

A character used to start, stop, or modify a function.

Copper Distributed

Another name for ANSI X3T9.5 Committee's proposed 100 Mbps over UTP standard, TP-PMD Data Interface


(Twisted Pair Physical Media Dependent). CCDI is a trademark of Crescendo Communications/CISCO.


A flexible insulated cable or a terminated cable.


The light conducting central portion of an optical fiber with a refractive index higher than that of the cladding. The center of a cable construction. Most often applies to a coaxial cable, where the core is the center conductor and the dielectric material applied to it.


The ionization of gasses about a conductor that results when the potential gradient reaches a certain value.


An optical device that splits or combines light from more than one fiber.


The transfer of energy (without direct electrical contact) between two or more cables or components of a circuit.


How well a metal shield covers the underlying surface. Measured in percent.


Abbreviation for cycles per second or Hertz.


Central Processing Unit.


The final configuration of a terminal barrel after the necessary compression forces have been applied to cause a functional union between the terminal barrel and the wire.

Crimp Height

A top to bottom measurement of the crimped barrel, using a crimp height comparator in the prescribed manner.

Crimping Dies

A term used to identify the shaping tools that, when moved toward each other, produce a certain desirable shape to the barrel of the terminal or contact that has been placed between them. Crimping dies are often referred to as die sets or as die inserts.

Crimping Head

Tooling containing jaws and linkage for use in pneumatic or hydraulic powered units to crimp loose-piece contacts/terminals that may be too large for hand tool applications.

Crimping Tool

A term commonly used to identify a hand held mechanical device that is used to crimp a contact, terminal or splice.

Cross-pinned Cable, Crossover Cable

A cable configured to allow two DTE devices or two DCE devices to communicate. Also called a "null-modern cable” or “modern-eliminator cable.”

Crossed Pinning

Configuration that allows two DTE devices or two DCE devices to communicate.


Conductor which runs through the cable and connects to a different pin number at each end.


Interference from one pair being coupled into adjacent pairs.


Canadian Standards Association, the Canadian version of the Underwriters Laboratories.

Current Rating

The maximum continuous electrical flow of current recommended for a given situation. It is expressed in amperes.

Cutback Method

A technique for measuring the loss of bare fiber by measuring the optical power transmitted through a long length then cutting back to the source and measuring the initial coupled power.

Cutoff Wavelength

The wavelength beyond which singlemode fiber only supports one mode of propagation.


One complete sequence of values of an alternating quantity, including a rise to maximum in one direction and return to zero; a rise to maximum in the opposite direction and return to zero. The number of cycles occurring in one second is called the frequency.

Cyclic Redundancy Check (CRC)

A form of error correction in which the block check character is the remainder after dividing the serialized bits in a transmission block by a predetermined binary number.



An insulating medium when used in a signal-carrying design.

Dielectric Constant

That property of a dielectric which determines the amount of electrostatic energy that can be stored by the material when a given voltage is applied to it. Actually, the ratio of the capacitance of a capacitor using the dielectric to the capacitance of an identical capacitor using a vacuum (which has a Dielectric Constant of 1) as a dielectric. A number which indicates the quality of a material to resist holding an electrical charge when placed between two conductors.

Dielectric Loss

The power dissipated in a dielectric as the result of the friction produced by molecular motion when an alternating electric field is applied.

Dielectric Strength

The voltage an insulation can withstand before it breaks down. Usually expressed as volts per mm.

Digital Signal

An electrical signal which possesses two distinct states (on/off, positive/negative).

Digital Loopback

A diagnostic test that performs a loop at a modem's DTE interface.

Digital Signal Processor (DSP)

A microprocessor used in voice compression and fax demodulation.


The Deutsches Institut fur Normung (German Institute for Standardization). 2) A type of rounded connector standardized by this organization.

Direct In Termination (DIT)

A service that allows incoming calls to a PBX to be routed directly to a selected telephone or set of telephones without operator intervention.

Direct Inward Dialing (DID)

A service that allows an outside caller to dial an internal extension without passing through an operator.

Direct Inward System Access (DISA)

A service that allows incoming calls into a PBX to have dialing access within the private network. This access is often restricted by a dialed password.

Disconnect Supervision

A protocol that indicates to the local user that the remote end has been disconnected.


Rated interconnection; a broken connection (open circuit) or the loss of a specified connection characteristic. Transient phenomena: Short term (temporary) interruption or unacceptable variation in current or voltage.

Diskless Workstation

A system on the network that boots up its operating system from a remote boot server, rather than locally.


Unusable or lost energy, such as the production of unused heat in a circuit.


The cause of bandwidth limitations in an optical fiber. Dispersion causes a broadening of input pulses along the length of the fiber. Two major types are (a) mode dispersion caused by differential optical path lengths in a multimode fiber, and (b) material dispersion caused by a differential delay of various wavelengths of light in a wave guide material.

Dispersion-Shifted Fiber

A single-mode optical fiber that has its minimum-dispersion wavelength shifted, by the addition of dopants, toward its minimum-loss wavelength. Synonym: EIA Class IVb fiber.


Any undesired change in a wave form or signal.

Distribution Cables

In a CATV system, the transmission cable between the distribution amplifier and the drop cable.

Disturbed Conductor

A conductor that receives energy generated by the field of another conductor or an external source. e.g. the quiet line.

Downline Loading

Sending configuration parameters or operating software from a controlling device to another device.

Drain Wire

A non-insulated wire in contact with parts of a cable, usually the shield, and used in the termination to that shield and as a ground connection.


A software module that, under control of the processor, manages peripheral I/0.


Single channel attachment to the horizontal wiring grid (wall plate, coupling, MOD-MOD adapter).

Drop Cable

In a CATV system, the transmission cable from the distribution cable to a dwelling.

Dual Inline Package (DIP)

A switch for opening and closing leads between two points. Often used to configure devices.

Dual Tone Multifrequency (DTMF)

Used for call addressing on pushbutton telephones.  Also known as multifrequency Pushbutton (MFPB) in Europe.

Dumb Terminal

Both hard-copy and VDT-type ASCII asynchronous terminals that do not use a data transmission protocol and usually send data one character at a time. A monitor and keyboard that displays information only (as opposed to the processing capability of a PC); usually connected to a mainframe.

Dummy Load

A dissipative device used at the end of a transmission line or waveguide to convert transmitted energy into heat, so essentially no energy is radiated outward or reflected back to its source.

Dust Cap

A device attached to a connector to provide protection against dust and foreign debris.



A European standard for digital transmission at 2.048 Mbps. It has 31 64-KB channels available for traffic.


When transmitted data is returned to its origin.


Electronic Industries Association.  standards organization in the U.S. specializing in the electrical and functional characteristics of interface equipment. It used to designate its recommended standards with the "RS"-prefix ("RS-232, " "RS-485," etc.); now it designates them with the "FIA-" or "EIA/TIA-" prefix.


One of two similar but noninteroperable ways to attach the wires in twisted-pair cable to RJ45 connectors, specified in the EIA/TIA-568 standard. Used for ISDN, 568A assigns Pair 2 to Pins 3 and 6 and Pair 3 to Pins 1 and 2. Not as popular as its counterpart EIA/TIA-568B. EIA 568A- This is the newest of the sequence options as published in the EIA Commercial Building Cabling Specification Draft 9.0 as the preferred sequence for termination of UTP data cabling (this is the international ISDN standard). This is similar to the 568B sequence except that pairs #2 and #3 are transposed. This provides backward compatibility to the USOC sequence for the two pairs instead of the single pair of 568B. EIA/TIA 568A-uses pins 1 and 2, 3 and 6, in an RJ (Registered Jack)- plug/receptacle in both EIA/TIA 568A and 568B.


One of two similar but noninteroperable ways to attach the wires in twisted-pair cable to RJ-45 connectors, specified in the EIA/TIA-568 standard. In EIA/TIA 568A and 568B, pins 1 and 2 are paired, pins 3 and 6 are paired, pins 4 and 5 are paired, and pins 7 and 8 are paired. Used for 10BASE-T, 568B assigns Pair 2 to Pins 1 and 2 and Pair 3 to Pins 3 and 6. More popular than its counterpart EIA/TIA-568A. EIA 568B-This has become the most widely specified sequence worldwide for new data installations. It is also a subset specified by the IEEE 802.3 10Base-T Ethernet over twisted pair standard. This sequence is only applicable to eight wire polarizations (WE8W and WE8K). In the 568B sequence, pair #1, and pair #3 correspond to pair #1 and pair #2 of the USOC sequence, providing backward compatibility with 2-pair systems (such as analog voice). EIA/TIA 568B-uses pins 1 and 2, 3 and 6. In EIA/TIA 568A and 568B, pins 1 and 2 are paired, pins 3 and 6 are paired, pins 4 and 5 are paired, and pins 7 and 8 are

Environmentally Sealed

A unit is provided with gaskets, seals, grommets, potting or other means to keep out moisture, dust,, air or dirt which might reduce or impair its performance.


Any material that will return to its original dimensions after being stretched or distorted.


Referring to the combined electric and magnetic fields caused by electron motion through conductors.

Electromagnetic Coupling

The transfer of energy by means of a varying magnetic field. Inductive coupling.


Equal level Far End Crosstalk (dB) - A subtraction of attenuation from FEXT. (ACR at the far end)


The increase in length of a wire or cable cause by longitudinal tension.

Electromagnetic Compatibility (EMC)

The ability of an electronic device to operate in its intended environment without its performance being affected by EMI and without generating EMI that will affect other equipment.


Electromotive force (voltage). The term most often used to designate electrical pressure that exists between two points and is capable of producing a flow of current when a closed circuit is connected between the two points. Voltage is measured in volts, millivolts, microvolts and kilovolts. The terms electromotive force (emf), potential, potential difference and voltage drop are often referred to as voltage.


Electromagnetic interference. Filtering protection from "background noise" that could alter or destroy data transmission. Energy generated by outside sources, such as lighting systems and electric motors, which is received by copper data/voice cable and interfere with transmission.


A device used by modems to compensate for distortions caused by telephone line conditions.


A network standard first developed by Xerox, refined by DEC and Intel, and codified as the IEEE 802.3 standard. It interconnects up to 1,024 personal computers in a bus topology on each network. In its original form, it supports a 10-Mbps data rate.


Abbreviation for a copper refining process called Electrolytic Tough Pitch. This process produces a conductor that is 99.95% pure copper resulting in high conductivity.

Extended Binary Coded Decimal Code (EBCDIC)

(Pronounced "EBB-see-dick") An 8-bit character code used primarily in IBM equipment; the code provides Interchange for 256 different bit patterns. Compare with ASCII.

Extended Capabilities Port (ECP)

EPP with daisy chain capability. ECP utilizes a new high-speed signaling method.

Expanded Polyethylene

Expanded or "foam" polyethylene, consists of individual closed cells of inert gas suspended in a polyethylene medium, resulting in a desirable reduction of the dielectric constant.





A unit of capacity that will store one coulomb of electrical charge when one volt of electrical pressure is applied.

Fast Ethernet

Any 100-Mbps Ethernet-based networking scheme.

Fast Packet Multiplexing

A technique that dynamically combines signals from voice, fax, async, sync, video, and LAN into one communications channel with a 95% or better efficiency on a remote network.

Fax Demodulation

A technique that detects a Group 3 fax signal on a voice line and reduces its speed from 64 Kbps to 9.6 Kbps for transmission across the data link.  On the other end, the fax will be reorganized and transmitted to the appropriate device.

Fax Sharing Device

A device that allows a fax machine to be shared between the Public Switched Telephone Network (PSTN) and a multiserver network. 

Federal Communications Commission (FCC)

The branch of the United States government that has the power to regulate all interstate communications systems within the United States as well as all international communications systems that originat or terminate in the United States.

Feeder Cable

In a CATV system, the transmission cable from the head end (signal pickup) to the trunk amplifier. Also called a trunk cable.


Fluorinated ethylene-propylene. A thermo-plastic material with good electrical insulating properties and chemical and heat resistance.


Composed of and/or containing iron. A ferrous metal exhibits magnetic characteristics.


A precision tube which holds a fiber for alignment for interconnection or termination. A ferrule may be part of a connector or mechanical splice.


Far End Crosstalk (dB) - Crosstalk induced on the pairs, measured at the far end of the cable.

Fiber Channel

A scalable, high speed, serial data transfer interface standard (ANSI X3TII)

Fiber Distributed Data Interface (FDDI)

An ANSI standard for fiberoptic links with data rates up to 100 Mbps. A standard for a 100 megabit-per-second local area network.

Fiber Optics

Light transmission through optical fibers for communication and signaling.


Non-conducting components cabled with the insulated conductors or optical fibers to impart roundness, flexibility, tensile strength, or a combination of all three, to the cable.

File Transfer Protocol (FTP)

An upper-level TCP/IP service that allows copying of files across a network.


In LAN technology, discarding packets that do not meet the criteria for forwarding.


Operating instructions stored for the long term in a device's read-only memory.


IEEE 1394 (also known as "FireWire") is a new technology. FireWire, with its transfer rate ranging from 100MBps to 400MBps, will target applications that require high-bandwidth devices like digital camcorders, cameras, and networks with video conferencing. It supports up to 63 devices and allows peer-to-peer device communication without using system memory or CPU time. It supports "true" Plug & Play, "Tree-Like" structure for daisy chaining, and devices can be connected and disconnected without interrupting system operation.

Flame Resistance

The ability of a material not to fuel a flame once the source of heat is removed.


A projection extending from or around the periphery of a connector and having holes that provide for mounting the connector to a panel or to a mating connector.

Flow Control

A method of regulating the flow of data between two devices; prevents the loss of data once a device's buffer has reached its capacity.


Frequency modulation.

Foam Polyethylene

Expanded or "foam" polyethylene, consists of individual closed cells of inert gas suspended in a polyethylene medium, resulting in a desirable reduction of the dielectric constant.

Forced Connection

A dedicated connection between two network channels.

Forced Perfect Terminator (FPT)

A type of terminator containing a sophisticated circuit that can compensate for variations in the power supplied by the host adapter as well as variations in bus impedance of complex SCSI systems. Forced Perfect Terminators are most recommended specially for high speed system. It alters its impedance to compensate for variations among many different cables, by means of diode switching and biasing. Plus, it has LEDs for troubleshooting.

Form Feed (FF)

A control character that tells a terminal device to go to the top of the next page.

Fox Message

THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG. 1234567890.  A test message that is produced in the fox message generator within a multiplexer unit.

Frame Relay

A high-speed, low-latency packet switching technology, based on a switched virtual network topology, used for WANs; popular for LAN-to-LAN connections. A packet network service, relying on the data integrity inherent in digital transmissions to speed up transmission. Unlike old X.25 networks, Frame Relay "assumes" the data is correct and starts error checking as soon as it received the header. Frame Relay services are offered with T1 and DDS connections.


The number of times a periodic action occurs in one second. Measured in Hertz.

Frequency Modulation (FM)

A scheme for modulating a carrier frequency in which the amplitude remains constant but the carrier frequency is displaced in frequency proportionally to the amplitude of the modulating signal. An fm broadcast is practically immune to atmospheric and man-made interference.

Frequency Response

The characteristic of a device denoting the range of frequencies over which it may be used effectively.

Fresnel Reflection

Back reflection, optical return loss: Light reflected from the cleaved or polished end of a fiber caused by the difference of refractive indices of air and glass. Typically 4% of the incident light.

Front Mounted

A connector is said to be front mounted when it is attached to the outside of the mating side of a panel. A front mounted connector can only be installed or removed from the outside of the equipment.

Front-End processor (FEP)

A dedicated computer linked to one or more host computers or multi-user minicomputers; performs data-communications and network-processing functions for the attached computers; in IBM SNA networks, an IBM 3704, 3725, or 3745 communications controller.

Full Duplex (FDX)

Simultaneous, two-way, independent transmission in both directions. Transmission in either direction, but not both simultaneously.

Fusion Splicer

An instrument that splices fibers by fusing or welding them, typically by electrical arc.



The general standard for interfacing to digital high-speed circuits. It now includes specifications for both 1.544 Mbps and 2.048 Mbps data rates; however, G.703 is normally referred to for 2.048-Mbps applications.


A hardware-software combination that connects two LANs (or a LAN and a host computer) that run different protocols-for example, a TCP/IP LAN and an SNA mainframe. The gateway provides the protocol conversion. Compare with bridge, router.

Gigabit Ethernet Gigahertz (GHz)

High speed network data transfer protocol standard (IEEE 802.3z). A unit of frequency equal to one billion hertz.

Graded-Index Fiber

A type of multi-mode optical fiber in which the refractive index of the core is in the form of a parabolic curve, decreasing toward the cladding. This type of fiber provides high bandwidth capabilities.


An electrical connection between a circuit and the earth. Also refers to a conductor connected to earth. In some instances, can refer to a central metallic point designated as having "zero" potential.

Ground Conductor

A conductor in a transmission cable or line that is grounded.

Ground Loop

A completed circuit between shielded pairs of a multiple pair created by random contact between shields. An undesirable circuit condition in which interference is created by ground currents when grounds are connected at more than one point.

Ground Potential

The potential of the earth. A circuit, terminal, or chassis is said to be at ground potential when it is used as a reference point for other potentials in the system.


Half-Duplex (HDX)

Transmission in either direction, but not both simultaneously. Allows packets to be either transmitted or received, but not both at the same time. Compare with full duplex.


Exchange of predetermined signals between two devices, establishing a connection or providing flow control. Usually part of a communications protocol.

Hardware Reset

Resetting the date and time without losing the current configuration.


A device that is attached to the end of the connectorized feeder cable that converts the 25-pair into individual 4, 6, or 8-wire modular channels.


Pins on a circuit board that allows for cable or jumper attachment.


A type of plastic material that has been cross-linked. A term describing tubes, sleeves, caps, boots, films or other forms of plastic which shrink to encapsulate, protect or insulate connections, splices, terminations and other configurations.


Airtight, impervious to external influence, as in a hermetic package. Often used to describe metal-to-metal solder or weld-sealed packages.

Hermetic Seal

Hermetically sealed connectors are usually multiple contact connectors where the contacts are bonded to the connector by glass or other materials and permits maximum leakage rate of gas through the connector of 1.0 micro ft./hr, at one atmosphere pressure for special applications.


A practical unit of inductance that will produce a voltage drop of one volt when the current changes at the rate of one ampere per second (abbreviated H).

Hertz (Hz)

The number of changes in polarity which a signal makes in one second. An indication of frequency. Replaces cycles-per-second.

High-Level Data-Link Control (HDLC)

An international standard communication protocol defined by the ISO.

High Speed Serial Data Connector (HSSDC)

High speed Serial Data Connector and cable assemblies are a fully shielded, controlled impedance interconnect system approved for Fiber Channel and SSA applications and under consideration for other standards.


High frequency. The band from 3 to 30 MHz in the radio spectrum, as designated by the Federal Communications Commission.

Horizontal Cable

Cable used to go between the workstation outlet and the telecommunications closet.

Home page

The main page of a Web site and the first screen that a visitor sees displayed when connecting to that site; usually has links to other pages, both within that site and to other sites.

Home Run

A cable run usually consisting of two, three, or four pair cable from a wall plate in a fixed wall office to a termination point at the distribution frame.

Host Computer

The central computer (or one of a collection of computers) in a data-communications system. It handles the system's primary data-processing functions such as computing, mediating database access, and running special system-specific programs; often shortened to "host".


The core of a star-topology network or cabling system.


A term used to describe the 60- or 120 cycle per second noise present in the sound of some communications equipment. Usually hum is the result of undesired coupling to a 60 cycle source or to the defective filtering of 120 cycle ripple output of a rectifier.

Hybrid Cable

A fiber optic cable containing two or more different types of fiber, such as 62.5 µm multimode and single-mode.

Hypertext Markup Language (HTML)

The standard set of codes that allows documents on the World Wide Web to be read by any system.

Hypertext Transfer Protocol (HTTP)

The protocol used to transfer documents on the World Wide Web.



Symbol used to designate current.

I/O Interconnection

Input/Output interface to the "outside world."


Insulated Cable Engineers Association.


Abbreviation for Insulation Displacement Contact.

International Electrotechnical Commission (IEC)

An organization that cooperated with the ISO in setting technology standards.


Institute of Electrical and Electronic Engineers.

IEEE 1284

The Standard Signaling Method for a Bi-directional Enhanced Parallel Interface for Personal Computers provides high-speed bi-directional communication between a PC and an external peripheral that transmits 10 to 50 times faster than the original parallel port. ECP-(Extended Capabilities Port)-EPP with daisy chain capability. ECP utilizes a new high-speed signaling method. EPP-(Enhanced Parallel Port)-is used by a new generation of fast bi-directional printers (HP LaserJet IV & V, Tektronix Phasers, Lexmark Optra, IBM 4029 or 4039, Texas Instruments MicroLaser Pro 600) & peripherals (CD-ROMs, tape & hard drives).

IEEE 1394

IEEE 1394 (also known as "FireWire") is a new technology. FireWire, with its transfer rate ranging from 100MBps to 400MBps, will target applications that require high-bandwidth devices like digital camcorders, cameras, and networks with video conferencing. It supports up to 63 devices and allows peer-to-peer device communication without using system memory or CPU time. It supports "true" Plug & Play, "Tree-Like" structure for daisy chaining, and devices can be connected and disconnected without interrupting system operation.

IEEE 802.3

The IEEE standard for Ethernet; a physical-layer standard that uses the CSMA/CD access method on a bus-topology LAN. A physical layer standard for 10 Base T, 100 Base T, Ethernet, and Starlan.

IEEE 802.5

A physical layer standard for Token Ring; a physical-layer standard that uses the token-passing access method on a ring-topology LAN.

IEEE 802.12

A physical layer standard for 100 VG.


The total opposition that a circuit offers to the flow of alternating current or any other varying current at a particular frequency. It indicates the ideal transfer of signal from one piece of equipment to another. It is measured in ohms.

Impedance Match

A condition whereby the impedance of a particular circuit cable or component is the same as the impedance of the circuit, cable, or device to which it is connected.

Impedance Matching Transformer

A transformer designed to match the impedance of one circuit to that of another.


The property of wire which stores electrical current in a magnetic field around the wire. By coiling wire, the effect can be intensified. It is measured in Henrys.


The phenomenon of a voltage, magnetic field, or electrostatic charge being produced in an object by lines of force from the source of such fields.

Inductive Crosstalk

Crosstalk resulting from the coupling of the electromagnetic field of one conductor upon another.

Input Impedance

The impedance that exists between the input terminals of an amplifier or transmission line when the source is disconnected. The circuit, signal level and frequency must be specified.

Input Level

The level of relative analog signal strength obtained from attached telephone equipment.

Insertion Loss

A measure of the attenuation of a cable or component by determining the output of a system before and after the device is inserted into the system.


A material having good dielectric properties which is used to separate close electrical components, such as cable conductors and circuit components.

Insulation Crimp

The area of a terminal splice or contact that has been formed around the insulation of a wire.

Insulation Displacement (IDC)

A mass termination connector for flat cable with contacts that displace the conductor insulation Connector to complete termination.

Insulation Resistance

The electrical resistance between two conductors separated by an insulating material.


The region where two systems or a major and a minor system meet and interact with each other.

Interface Converter

A device that allows communication between two systems with incompatible electrical signals, connectors, and/or handshaking.


Disturbances of an electrical or electromagnetic nature that introduce undesirable responses into other electronic equipment.

Intermediate Frequency

A frequency to which a signal is converted for ease of handling. Receives its name from the fact that it is an intermediate step between the initial and final conversion or detection stages.

Integrated Services Digital Networking (ISDN)

A CCITT defined standard for a public-switched service that allows the digital transmission of voice, data, and video over one network. Being touted as "the next big thing" in voice, data, and video integration. A CCITT standard for a network that accommodates a variety of mixed digital-transmission services at 144 Kbps and 1.544 Mbps. A telecommunications standard for sending digitized voice, video and data signals over the existing public switched telephone network.

Integrated Service Unit (ISU)

A device that combines both the functions of a CSU and DSU.


International Standards Organization. A body that promotes computer standards and developed the OSI's model for network communication. The international "master organization" responsible for developing and maintaining worldwide standards for computers, data communications, and many other fields.

International Telecommunications Union (ITU)

The United Nations organization responsible for setting telecommunications standards. Replaced the CCITT.


1) Any large network made up of several smaller networks. 2) Capitalized, the international network of networks that connects educational, scientific, and commercial institutions.

Internet Service Provider (ISP)

A company or organization that provides dialup Internet access.

ISO 9000

An umbrella group of international standards (including ISO-9001, -9002, -9004, etc.) for quality assurance in business practices, ratified by the ISO beginning in 1987. Certification of ISO 9000 compliance is rapidly becoming a prerequisite for selling many types of goods and services (including data-communications equipment and services), especially to government bodies. Black Box Corporation and many of its subsidiaries are ISO-9001 certified.



A connecting device into which a plug can be inserted to make circuit connections. The jack may also have contacts which open or close to perform switching functions when the plug is inserted or removed. See also: receptacle.


The outside covering of a cable. Not part of the fiber or the fiber buffer.


A short length of conductor or flat cable used to make a connection between terminals or around a break in a circuit, or between circuit boards.



Kilobits per second.  One thousand bits per second.


See Aramid Yarn

Key Telephone System (KTS)

A telephone system that allows its users to select outgoing or incoming calls with pushbuttons without having to dial an access number such as 9.


1000 electron volts.


Kilovolt (1000 volts).



Symbol for inductance.


Local Area Network. A data network connecting any number of users, intended to serve a small area.


A coherent source of light with a narrow beam and a narrow spectral bandwidth (about 2nm).

Laser Diode, ILD

A semiconductor device that emits high powered, coherent light when stimulated by an electrical current. Used in transmitters for singlemode fiber links.


The length measured along the axis of a wire or cable required for a single strand (in stranded wire) or conductor (in cable) to make one complete turn about the axis of the conductor or cable. In a twisted pair cable, the lay length is the distance it takes for the two wires to completely twist around each other.

Lay Direction

The direction of the progressing spiral twist in a cable while looking along the axis of the cable away from the observer. The lay direction can be either "left" or "right".


Longitudinal conversion loss = Unbalance attenuation at the near end: Power fed at the near end into the common mode and coupled power measured at the near end in the differential mode.


Longitudinal conversion transfer loss = Unbalance attenuation at the far end: Power fed at the near end gnto the common mode and coupled power measured at the far end in the differential mode.

Leased Line

A telephone line reserved for the exclusive use of a leasing customer without interexchange switching arrangements. A leased line may be point-to-point or multipoint. (Also called a "private line.")


Low frequency. A band of frequencies extending form 30 to 300 kHz in the radio spectrum, designated by the Federal Communications Commission.

Light Emitting Diode (LED Source)

A semiconductor device that emits incoherent light formed by the P-N junction. Light intensity is roughly proportional to electrical current flow.

Line Cord

The connecting cord between the terminal device and the drop.

Line Driver

A DCE device that amplifies a data signal for transmission over lengths of cable beyond the RS-232 limit of 50 feet (15.2 m)-even up to several miles. It also conditions the signal by reshaping distorted pulses. Also called a "limited-distance modem (LDM)" or "short-haul modem (SHM)." A signal converter that conditions a digital signal to ensure reliable transmission over an extended distance.

Line Feed (LF)

A control character used to tell the terminal to go the next line.

Line Impedance

Impedance as measured across the terminals of a transmission line; frequently the characteristic impedance of the line.


The horizontal cable including the workstation outlet and patch panel in the telecommunications closet plus two meters of cable at each end for testing.

Local Area Network (LAN)

A data communications system confined to a limited geographic area (up to six miles or about 10 kilometers) with moderate to high data rates (100 Kbps to 155 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.

Local Talk

Apple Computer's proprietary LAN, based on the Apple Talk architecture.


Type of diagnostic test in which the transmitted signal is returned to the sending device after passing through all, or a portion of, a data communications link or network. A loopback test permits the comparison of a returned signal with the transmitted signal. A test messages is sent to a device being tested. The message is then sent back to the originator and compared with the original transmission. Loopback testing may be performed with a locally attached device or conducted remotely over a communications circuit.

Loose Tube

A protective tube loosely surrounding a cabled fiber, often filled with a water blocking gel. 

Loose Tube Cable

Type of cable design, primarily for outdoor use, where one or more fibers are enclosed in hard plastic tubes. Fibers are usually buffered to 250 microns. 

Loss, Optical

The amount of optical power lost as light is transmitted through fiber, splices, couplers, etc.

Loss Budget

The amount of power lost in the link. Often used in terms of the maximum amount of loss that can be tolerated by a given link.

Low Voltage Differential Signals (LVDS)

Low Voltage Differential Signals reduce on-chip power consumption.



milliampere (one-thousandth of an ampere).

MAC Address

Unique address assigned to each active infrastructure end station (including adapters, LAN on motherboard, switch ports and router ports) The IEEE specification for the lower sublayer of the OSI Data Link layer; CSMA/CD and Token Ring are types of MACs.


Macroscopic axial deviations of a fiber from a straight line, in contrast to microbending


A large-scale computer system that can house comprehensive software and several peripherals.


Metropolitan Area Network. A data network intended to serve the area of a city or an area of similar size.

Management Information Base (MIB)

A database of objects that stores information used by SNMP-enabled management devices.


Abbreviation for Master Antenna Television.


A binary 1 state which signifies that there is no traffic for asynchronous transmission.

Matched Impedance

The coupling of two circuits in such a way that the impedance of one circuit equals the impedance of the other.


To join two connectors in a normal engaging mode.


Moisture Barrier. Aluminum or steel tape bonded to itself and to the outer polyethylene jacket, providing a vapor-tight moisture barrier.


Mega bits per second - the number of bits, in millions, transmitted per second.

Mechanical Splicing 

Joining two fibers together by mechanical means to enable a continuous signal. Elastomeric splicing is one example of mechanical splicing. 

Medium Dependent Interface (MDI)

The mechanical and electrical interface between the segment and the MAU.

Megahertz (MHz)

Unit of frequency equal to one million hertz (one million hertz per second).


Millionth of a meter.


That portion of the electromagnetic spectrum lying between the far infrared and conventional radio frequency range. The microwave frequency range extends from 1 GHz to 300 GHz. Microwaves are usually used in point to-point communications because they are easily concentrated into a beam.


Curvatures of the fiber which involve axial displacements of a few micrometers and spatial wavelengths of a few millimeters. Microbends cause loss of light and consequently increase the attenuation of the fiber. 

Military Specification

Military requirements. The demand imposed upon a system to meet a military operational need.

Millisecond (ms)

One-thousandth of a second.


A small type of DIN connector most often used for keyboard and mouse connections.


A single electromagnetic wave traveling in an optical fiber.


Device that converts signals in one form to another form compatible with another kind of equipment.

Modal Dispersion 

Pulse spreading due to multiple light rays traveling different distances and speeds through an optical fiber. 

Mode Field Diameter (MFD) 

The diameter of optical energy in a singlemode fiber. Because the MFD is greater than the core diameter, MFD replaces core diameter as a practical parameter. 

Mode Mixing 

The numerous modes of a multi?mode fiber differ in their propagation velocities. As long as they propagate independently of each other, the fiber bandwidth varies inversely with the fiber length due to multi?mode distortion. As a result of inhomogenejties of the fjber geometry and of the index profile, a gradual energy exchange occurs between modes with differing velocities. Due to this mode mixing, the bandwidth of long multimode fibers is greater than the value obtained by linear extrapolation from measurements on shod fibers. 

Modem Eliminator

A device used to connect a pair of DTEs in lieu of the pair of modems that would otherwise be necessary. See SME.

Modified Modular Jack (MMJ)

A six-wire modular jack with the locking tab shifted off to the right side. Used in the DEC wiring systems.


Altering the characteristics of a carrier wave to convey information. Modulation techniques include amplitude frequency, phase, plus many other forms of on-off digital coding.

Mono Filament

A single strand filament as opposed to a braided or twisted filament.

Mono-Mode Fiber

See Single-mode fiber


Consisting of a single wavelength. In practice, radiation is never perfectly monochromatic but, at best, displays a narrow band of wavelengths.


The MT-RJ fiber optic connector has been developed by the MT-RJ Alliance (AMP, Siecor, HP, Fujikura, US Conec).


A device that emits or a fiber that carries multiple modes of light.


A technique for putting two or more signals into a single channel.

Multiplexor (MUX)

A device that uses any of several methods to transmit and receive multiple signals across a single communication channel at the same time. Often shortened to "MUX." A device used for division of a transmission facility into two or more sub-channels, either by splitting the frequency band into narrower bands (frequency division) or by allotting a common channel to several different transmitting devices one at at a time (time division).


Describes communications lines or circuits that connect more than two stations, thus supporting terminals in several different locations. Compare with point-to-point.

Multi Station Access Unit (MAU)

A wiring concentrator used to form a star-wired ring configuration. A wiring concentrator used in Token Ring LANs.

Mutual Capacitance

Capacitance between two conductors when all other conductors are connected together and grounded.


Millivolt (one-thousandth of a volt).


N Connector

A large radio frequency connector covered by Military Specification. It has an impedance of 50 ohms and is designed to operate in the 0 to 11 GHz, frequency range. It has a threaded coupling and is physically larger than a TNC connector.

Nanometer (nm)

One billionth of a meter.


One billionth of a second.

Narrow Band

EMI generated from a device operating at a specific and limited range of frequencies. See also: electromagnetic interference (EMI).

National Electrical Code (NEC)

A publication of the National Fire Protection Association (NFPA) which outlines requirements for electrical wiring and building construction.


National Electrical Code. Defines building flammatory requirements for indoor cables.


National Electrical Manufacturers Association.


A network is a method of data communications between computers.

Network Interface

The physical point where the building or equipment wiring interconnects with the local exchange carriers.

Network Interface Card (NIC)

The circuit board within a terminal that connects it to the network.

Network Operating System (NOS)

Master software installed on a network to control all network functions, such as connecting devices, sharing resources, and transferring files.


Near end Crosstalk (dB) - Crosstalk induced on the pairs, measured at the near end.


National Fire Protection Association.


One half byte (4 bits).


A point of interconnection to a network. Normally, a point at which a number of terminals or tail circuits attach to the network. On a network, a terminal point at which data is transmitted, received, or repeated. Usually corresponds to an attached device such as a computer, network modem, or router.

Node Reset

Resets a node to either the current configuration (warm start) or factory defaults (cold start).


In a cable or circuit, any extraneous signal which tends to interfere with the signal normally present in or passing through the system.


Area that a cable can be installed in a building that is not used for air return.


A description of memory storage that doesn't lose its contents when power is lost.


The removal of the web section between conductors of a flat cable to aid in stripping, slitting, and termination.

Null Character

A character that is used to give a printer time for its mechanical actions to take place so that it will be ready for the next piece of data.   Also called idle character.

Null Modem

A device that connects two DTE devices directly by emulating the physical connections of a DCE device. The Null Modem cable is used to fool the computer that there is actually a modem attached to its serial port and it is used to connect 2 computers through their serial ports. On the pin out for DB9/DB25, pins 2 and 34 are reversed.

Numerical Aperture (NA)

A measure of the angular acceptance for a fiber. It is approximately the sine of the half-angle of the acceptance cone.


"O" Crimp

An insulation support crimp for open barrel terminals and contacts. In its crimped form it resembles an "O" and conforms to the shape of the round wire insulation. "O" crimp is also used to describe the  crimps used on COAXICON females.


A device that is attached to the end of a connectorized feeder cable that converts the 25-pair to individual 2-, 4-, 6-, and 8-wire channels.


Optical Time Domain Reflectometer A method for characterizing a fiber wherein an optical pulse is transmitted through the fiber and the resulting backscatter and reflections to the input are measured as a function of time. Useful in estimating attenuation coefficient as a function of distance and identifying defects and other localized losses. 


Taking a telephone off its cradle when attached to an active dial-up line.

Off-Premises Extension (OPX)

An off-premise telephone extension.


Abbreviation for oxygen-free, high conductivity copper. It has 99.95% minimum copper content and an average annealed conductivity of 101% compared to standard copper.


The unit of electrical resistance. The value of resistance through which a potential difference of one volt will maintain a current of one ampere.

Ohm's Law

Stated E=IR, I=E/R or R=E/I, the current I in a circuit is directly proportional to the voltage E, and inversely proportional to the resistance R.


Leaving the handset resting in its cradle when attached to an active dial-up line.

Optical Fiber 

Thin filament of glass. An optical waveguide consisting of a core and a cladding which is capable of carrying information in the form of light. 

Optical Waveguide

Dielectric waveguide with a core consisting of optically transparent material of low attenuation (usually silica glass) and with cladding consisting of optically transparent material of lower refractive index than that of the core. It is used for the transmission of signals with lightwaves and is frequently referred to as fiber. In addition, there are planar dielectric waveguide structures in some optical components, such as laser diodes, which are also referred to as optical waveguides. 


Open System interconnection, a LAN communication model developed by ISO.



A 68-wire cable used for 16-bit SCSI-3 buses, P-cables can be used with Q-cables for 32-bit SCSI-3 buses.


A sequence of data, with associated control information, that is switched and transmitted as a whole; refers mainly to the field structure and format defined with the CCITT X.25 recommendation. A chunk of data bits and associated information, including source address and destination address, formatted for transmitting from one node to another.

Packet Analyzer

A network diagnostic tool that hooks into a LAN and analyzes its traffic; capable of capturing a packet, examining it and breaking it down into its component parts of destination, origin, protocol, data, etc.

Packet-Switched network

A data-communications network such as X.25 that takes packets from different sources, routes them according to addresses, interleaves them, and sends them to their destinations.

Pad Character

A character inserted to fill a blank time slot in synchronous transmissions or to fulfill a character count requirement in fixed block length transmissions.  Leading pad characters help establish synchronization and trailing pad characters prevent the dropping of RLSD/RTS signal too soon after the end of the frame.

Panel Mount

A method of fixing a connector to a board, panel or frame. The mounted connector is usually the receptacle or female connector. The plug or male connector is usually the removable portion.

Parallel Transmission

Transmitting a number of bits of data simultaneously over separate lines (for example, eight bits over eight lines); usually unidirectional. Compare with serial transmission.

Parity Bit

A bit added to a character to make sure that the total number of 1's in a group is either even, for even parity, or odd, for odd parity, allowing for error detection.

Part 15, FCC

A part of the FCC regulations that defines the amount of electro-magnetic emissions an electronic device can have.

Part 68, FCC

A part of the FCC regulations that defines the technical requirements that a device must meet to be connected to a telephone network.

Passive Terminator

Passive terminators are comprised only of resistors and are susceptible to variations in the power supplied by the host adapter. Passive Terminators sit on the bus to minimize reflections at the end of the cable and simply provides an impedance that's close to the impedance of the cable. Reflected signals interfere with accurate data transmission. Maximum reach is 20ft. For SCSI chain less than 16ft and with less than four SCSI peripherals, passive terminator can probably do the job.


A flexible piece of cable terminated at both ends with plugs.

Patch Panel

A centralized location for cross-connecting, monitoring and testing telecommunications cabling.

PC Card

A credit-card-sized device that can be easily plugged into and removed from portable or laptop computers to provide temporary memory enhancements, modem or LAN capability, or even disk storage. Originally called "PCMCIA cards."


Pulse Coded Modulation. 


Plastic Clad Silica


(Chemical)-The passage or diffusion (or rate of passage) of a gas, vapor, liquid or solid through a barrier without physically or chemically affecting it.


(Magnetic)-The measure of how much better a material is than air as a path for magnetic lines of force. Air is assumed to have a permeability of 1.

Personal Computer Memory Card International Association (PCMCIA)

A credit-card-sized device that can be easily plugged into and removed from portable or laptop computers to provide temporary memory enhancements, modem or LAN capability, or even disk storage. Now called PC Cards.


An angular relationship between waves.

Phase Shift

A change in the phase relationship between two alternating quantities.

Physical Layer

The actual portion of a network that is used to physically connect computers of a network and over which the data is transmitted - the cable.

Picofarad (pF)

One billionth of a farad. A micro-microfarad.


Fiber optic cable that has connectors installed on one end. See also Cable Assembly. 


(Picture element)-The smallest indivisible part of a video image.


A compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system.

Plastic Optical Fiber (POF)

Because plastic optical fiber is less expensive than glass optical fiber, it is being designed to support fiber to the desktop.


A male housing with male or female contacts.

Point-to-Point Wiring

Wiring that consists of continuous conductors terminated at each end to circuit destination.

Polarization Stability

The variation in insertion loss as the polarization state of the input light is varied. 

Polyethylene (PE)

A thermoplastic material having excellent electrical properties. Low dielectric constant, a stable dielectric constant over all frequencies, very high insulation resistance. In terms of flexibility, polyethylene can be rated stiff to very hard, depending on molecular weight and density - low density being the most flexible and the high-density, high molecular weight formulation being very hard. Moisture resistance is rated excellent.


A substance made of many repeating chemical units or molecules. The term polymer is often used in place of plastic, rubber, or elastomer.

Polyolefin (PO)

Any of the polymers and copolymers of the ethylene family of hydrocarbons, such as polyethylene and polypropylene.

Polypropylene (PP)

A thermoplastic similar to polyethylene but stiffer and having a higher softening point (temperature). This material is primarily used as an insulation material. Typically, it is harder than polyethylene. This makes it suitable for thin wall insulations. The dielectric constant is 2.25 for solid and 1.55 for cellular designs.

Polyurethane (PU, PUR)

Broad class of polymers noted for good abrasion and solvent resistance. Can be in solid or cellular form. This thermoplastic material is used primarily as a cable jacket material. It has excellent oxidation, oil, and ozone resistance. Some formulations also have good flame resistance. It is a hard material with excellent abrasion resistance. It has outstanding "memory" properties, making it an ideal jacket material for retractile cords.

Polyvinyl chloride (PVC)

A general purpose thermoplastic compound used for wire and cable insulation and jackets. Has inherent flame retardancy. Emits toxic fumes (HCl) during fire.


A physical connector on the back of an electronic device.

Post, Telephone, and Telegraph Authority (PTT)

A government agency that acts as a common carrier in many areas of the world.


A glass structure from which an optical fiber waveguide may be drawn. 

Primary Coating 

The plastic coating applied directly to the cladding surface of the fiber during manufacture to preserve the integrity of the surface. 

Printed Circuit Board (PCB)

An insulating board serving as a base for a printed circuit. When the printing process is completed, the board may include printed components, as well as printed wiring.

Private Branch Exchange (PBX)

A privately owned telephone system installed within the organization that allows users to call within the organization as well as the outside world.  The only difference between a PBX and a key telephone system, is that PBX users have to dial a 9 to reach an outside line.

Propagation Delay

Time required for a signal to pass from the input to the output of a device.


The procedures used to control the orderly exchange of information between stations on a data link or on a data communications network or system. Any defined set of procedures, conventions or methods that, when adhered to, allow two devices to interoperate; used to implement LAN services.

Protocol Converter

A device that translates from one communications protocol into another, such as IBM SNA/SDLC to ASCII; compare with gateway.


A model suitable for use in the complete evaluation of form, design and performance.

Pseudo Random NRZ

A wave form of binary signals that may be used in a computer system. It is called NRZ, Non-Return to Zero, because the voltage does not return to zero.


A current or voltage which changes abruptly from one value to another and back to the original value in a finite length of time. Used to describe one particular variation in a series of wave motions.

Pulse Width

The length of time that the pulse voltage is at the transient level. Electronic pulse widths are usually in the millisecond, microsecond, or nanosecond range.


Poly-Vinylidene Fluoride.



A four conductor cable. Also called "star quad".

Quadrature Amplitude

A modulation technique that combines phase and amplitude modulation  in order to increase

Modulation (QAM)

number of bits per baud in a transmission.



Symbol for resistance.


(Redundant Array of Inexpensive Disks)-A method of storing data on multiple hard-disk drives, for faster access, greater reliability, or both. There are six officially defined "levels," each designed for a specific kind of application.

Random Access Memory (RAM)

Type of data storage that is usually volatile.

Rapid Relay Technology

Data compression, speech compression, fast-packet multiplexing, and automatic fax demodulation combined and used in multiserver products.

Read Only Memory (ROM)

Nonvolatile type of data storage that is manufactured with predefined contents.

Received Data (RD)

RS-232 signal sent from a DCE to a DTE.

Receive Inhibit

A configuration option that prevents the voice/fax channel from taking calls.

Rated Temperature

The temperature range at which an electric component can operate for extended periods without loss of its basic properties.

Rated Voltage

The maximum voltage at which an electric component can operate for extended periods without undue degradation or safety hazard.


A measure of the combined effects of capacitance and inductance on an alternating current. The amount of such opposition varies with the frequency of the current. The reactance of a capacitor decreases with an increase in frequency; the opposite occurs with an inductance.


An electronic package that converts light energy to electrical energy in a fiber optic system.


Jack. A female housing with male or female contacts. 


The change in direction (or return) of waves striking a surface. For example, electromagnetic energy reflections can occur at an impedance mismatch in a transmission line, causing standing waves.

Reflection Loss

The part of a signal which is lost due to reflection of power at a line discontinuity.

Refractive Index

The ratio of light velocity in a vacuum to its velocity in the transmitting medium.

Remote Analog Loopback

A diagnostic test that forms a loop at the analog side (output) of the remote modem.

Remote Channel Loopback

A diagnostic test that forms a loop at the channel side (input) of the remote multiplexer.

Remote Composite Loopback

A diagnostic test that forms a loop at the composite side (output) of the remote multiplexer.

Remote Digital Loopback

A diagnostic test that forms a loop at the the digital side (input) of the remote modem.


In a switching mode, reorders are a response to most unsuccessful call attempts.


A receiver and transmitter combination used to regenerate an attenuated signal.

Request to Send (RTS)

An RS-232 modem interface signal (sent from the DTE to the modem on pin 4) which indicates that the DTE has data to transmit.


In dc circuits, the opposition a material offers to current flow, measured in ohms. In ac circuits, resistance is the real component of impedance, and may be higher than the value measured at dc.


An ac circuit condition in which inductive and capacitive reactance interact to cause a minimum or maximum circuit impedance.

Response Time

The elapsed time between the generation of the last character of a message at a terminal and the receipt of the first character of the reply. It includes terminal delay and network delay.

Return Loss (RL)

The ratio between the outgoing signal and the reflected signal strength, expressed in dB. It is an indication of the "roughness" of the input impedance and the proximity of the characteristic impedance to nominal impedance.


Radio-frequency. Usually considered to be frequencies ranging from 1 MHz to 3GHz.


Ratio Frequency Interference.


"RG" is the abbreviation for "radio guide," a military designation for a coaxial cable, and "U" stands for "utility."


Abbreviation for the three parts of color video signal: red, green and blue, and also refers to multi-coaxial cables carrying these signals.

Ribbon Cable

A flat cable made with parallel round conductors in the same plane. Also referred to as planar and/or flat cable. Any cable with two or more parallel conductors in the same plane encapsulated by insulating material.


A Local Area Network topology in which data is sent from workstations via a loop or ring. One conductor of a pair (vs. tip).

Ring Indicator (RI)

RS-232 interface signal, sent from the DCE to DTE which indicates that a call is coming in.

Rise Time

The time required for a component or logic circuit to change from the quiescent to the transient state when an input is applied. (i.e., elapsed time between application of input and attainment of full output level).


Registered jack.


Wiring with 4- or 6-wire modular connectors; commonly used for standard telephone lines. Uses 1 pair of pins: 1 and 2.


Uses 2 pairs of pins: 1 and 4, and 2 and 3.


Uses 3 pairs of pins: 1 and 6, 2 and 5, and 3 and 4.


Modular telecommunications connector (IEC 60603-7).


Has four voice circuits, T1 voice and/or data 2-pair connection, frame relay voice and/or data 2-pair connection and switched 56 (2-pair) I.e. to ATM machines.


Uses 4 pairs of pins: 1 and 8, 2 and 7, 3 and 6, and 4 and 5.


FRAME RELAY-uses 2 pairs of pins: 1 and 2, 7 and 8, on an RJ (Registered Jack) plug/receptacle in USOC.


(EIA/TIA 568A Compatible)-T1 uses 2 pair of pins: 1 and 2, and 4 and 5.


Root-mean-square. The effective value of an alternating current, corresponding to the direct current value that will produce the same heating effect.


A network device that examines the network addresses within a given protocol, determines the most efficient pathway to the destination, and routes the data accordingly. Compare with bridge, gateway.


The process of selecting the correct circuit path for a message.


The industry's most common serial interface standard; an EIA-recommended standard for the interface between computer devices. It is identical in function to the combined CCITT standards V.24 and V.28. Interface between data terminal equipment and data communication equipment employing serial binary data interchange.

RS-422, RS-423

EIA serial transmission standard that extends transmission speeds and distances beyond those of RS-232, RS-423 is an unbalanced system; RS-422 is a balanced system with a higher level of noise immunity. (RS-422)-Electrical characteristics of balanced-voltage digital interface circuits. (RS-423)-Electrical characteristics of unbalanced-voltage digital interface circuits.


EIA standard specifying the pinning for RS-422 and RS-423 when a DB37 or DB9 connector is used. General purpose 37-pin and 9-pin interface for data terminal equipment and data circuit-terminating equipment employing serial binary data interchange.


EIA serial interface standard for multipoint lines.


EIA standard specifying the pinning for RS-422 when a DB25 connector is used.



Society of Automotive Engineers.


A type of optical fiber connector. The SC utilizes the same 2.5mm ferrule as the ST, held in a housing that allows for "push-pull" insertion and removal of the connector form the adapter. Rapidly becoming the connector of choice for data networks.

Scalable Coherent Interface (SCI)

Scalable Coherent Interface is a point-to-point, unidirectional link for handling large amounts of data in scalable, massively parallel processors.

Screw Machine Contact

A contact which is machined from solid bar stock.


The characteristic of a material that extinguishes its own flame after the igniting flame is removed.

Self Test

A diagnostic feature that tests the voice/fax channel locally that does not include the link.


A cable containing a flexible inner core and a relatively inflexible sheathing.

Serial Storage Architecture (SSA)

Serial Storage Architecture is a serial data transfer standard (ANSI X3710.1).

Serial Transmission

Transmitting data one bit at a time. Contrast with parallel transmission. The most common transmission mode; in serial, information bits are sent sequentially on a single data channel.

Serve Shield

A metallic shield consisting of several strands of wire, helically wound around a cable core.


A network node that provides services to client PCs, for example, file access, print spooling or remote execution.


1) A connection between two stations that allows them to communicate. 2) The time period during which a user engages in a dialogue with an interactive computer. 3) In the IBM SNA, the logical connection between two network-addressable units.


The outer covering of a jacket over the insulated conductors to provide mechanical protection for the conductors. Also known as the external conduction surface of a shielded transmission line.


A tape, serve or braid (usually copper, aluminum, or other conductive material) placed around or between electric circuits or cables or their components, to prevent signal leakage or interference.

Shield Coverage

The optical percentage of a cable actually covered by shielding material.

Shield Effectiveness

The relative ability of a shield to screen out undesirable interference. Frequently confused with the term shield coverage.

Shield Percentage

The percentage of physical area of a circuit or cable actually covered by shielding material.

Shielded Twisted Pair

Twisted-pair cable that has a foil and/or braided shield to minimize interference. A thin-diameter network wire, wrapped with a metal sheath for extra protection against electrical interference.


(Mechanical)-(1) An abrupt impact applied to a stationary object. (2) An abrupt or nonperiodic change in position, characterized by suddenness, and by the development of substantial internal forces.

Short Hand Mode

A single converter which conditions a digital signal to ensure reliable transmission over DE continuous private line metallic circuits without interfering with adjacent pairs in the same telephone cable.

Short-Haul Modem

See line driver. (Line driver-A DCE device that amplifies a data signal for transmission over lengths of cable beyond the RS-232 limit of 50 feet (15.2m)-even up to several miles. It also conditions the signal by reshaping distorted pulses. Also called a "limited-distance modem (LDM)" or "short-haul modem (SHM)."


Handshaking used between telephone equipment.   Provides supervising (on/off hook status), alerting (ringing), and call addressing (dialing) for switched services.

Sign-On Character

The first character sent on an ABR circuit.  It is used to determine data rate.

Simple Network Management Protocol (SNMP)

A de facto standard for managing network devices, including adapters, switches, routers, servers and workstations; garners information from various agents.

Sine Wave

A wave which can be expressed as the sine of a linear function of time, space or both. A waveform, often viewed on an oscilloscope, of a pure alternating current or voltage.

Single-Ended Mode (SEM)

Single-Ended Mode is a method for defining and/or measuring impedance.

Single Mode Fiber

A fiber wave guide in which only one mode will propagate. The fiber has a very small core diameter of approximately 8 micro meters. It permits signal transmission at extremely high bandwidths and is generally used with laser diodes.


Varying in proportion to the sine of an angle or time function. Ordinary alternating current is sinusoidal.


A measurement of the difference in the electrical length of two conductors or pairs of conductors and generally measured in picoseconds.

Skin Effect

The tendency of alternating current to travel only on the surface of a conductor as its frequency increases.


The insulated or metallic covering over the barrel of a terminal.

Slow Busy Signal

In a switching mode, the response to a call attempt when the called extension is busy.  In a force-connect mode, the signal heard when a sync loss occurs, or when the link goes down.

Small Computer System Interface (SCSI)

An intelligent bus for transmitting data and commands between a variety of devices. There are many implementations (Pronounced "scuzzy")-of SCSI, including Fast SCSI, Wide SCSI, Fast Wide SCSI, Fast-20, and Fast 40. An interface that provides high-speed bus connections between small computers and intelligent peripherals such as hard disks, printers, and optical disks.


The second generation of SCSI; includes many improvements to SCSI-1, including Fast SCSI, Wide SCSI, and mandatory parity checking.


The third generation of SCSI; Introduces Fast-20 and Fast-40 as improvements to the parallel bus. The standard also includes a number of specifications for high-speed serial bus architectures such as SSA, Fiber Channel, and IEEE 1394. SCSI Terminators SCSI termination is required on all SCSI systems. There are two categories of terminators: Internal & External. External terminators have 4 different types: Passive (good), Active (better), FPT (best), and Differential. Is there a way to daisy chain an external SCSI device without installing a SCSI card? Yes, if you already have an internal hard drive SCSI controller. A SCSI port adaptor will allow you to add external SCSI port using the same internal hard drive SCSI controller. Its easy to install and inexpensive. The maximum number of devices for SCSI 1 chain is still seven.


Signal to Noise Ratio. Commonly used interchangeably with ACR - the difference between attenuation and crosstalk, measured in dB, at a given frequency (acronym for Attenuation Crosstalk Ratio). Important characteristic in networking transmission to assure that signal sent down a twisted pair is stronger at the receiving end of the cable than are any interference signals imposed on that same pair by crosstalk from other pairs.


A piece of equipment that separates two other pieces of equipment from eachother.

Spectral Bandwidth

The difference between wavelengths at which the radiant intensity of illumination is half its peak intensity.


Frequencies that exist in a continuous range and have a common characteristic. A spectrum may be inclusive of many spectrums (e.g., the electromagnetic radiation spectrum includes the light spectrum, radio spectrum, infrared spectrum, etc.).

Speed of Light (c)

2.998 x 10 (to the power of 8) meters per second.


A device that multiplies one input into a number of identical outputs.


(Simultaneous Peripheral Operation On Line)-A program or device, usually with a large buffer, that controls data going to an output device; also called "spooler".

Spread Spectrum

The process of modulating a signal over a significantly larger bandwidth than is necessary for the given data rate, in order to reduce the number of errors caused by interference.


A registered trademark of AT&T for their fiber optic connector. Originally, an acronym for "Straight Tip."

Standing Wave Ratio (SWR)

A ratio of the maximum amplitude to the minimum amplitude of a standing wave stated in current or voltage amplitudes.

Star Quad

Term given to 4-conductor microphone cables where the conductors are spiraled together. Which, when connected in an "x" configuration, greatly increases common mode noise rejection.

Star Topology

A network cabling configuration that uses a central connection point (called a hub), through which all communication must pass.

Step-Index Fiber

An optical fiber in which the core is of a uniform refractive index with a sharp decrease in the index of refraction at the core/cladding interface.


Shielded Twisted Pair(s).

Stranded Conductor

A conductor composed of groups of uninsulated wires.

Structural Return Loss (SRL)

The ratio between the outgoing signal and the reflected signal strength, expressed in dB. It is an indication of the "roughness" of the input impedance and its proximity to the characteristic impedance of the cable.

Super High Frequency (SHF)

The Federal Communications Commission designation for the band from 3,000 to 30,000 MHz in the radio spectrum.

Super VGA (SVGA)

Refinement of VGA that offers higher resolution, at least 800 x 600 pixels.


A temporary and relatively large increase in the voltage or current in an electric circuit or cable. Also called transient.


Testing the frequency response, or attenuation over frequency, of a cable by generating a voltage whose frequency is varied through a given frequency range and observing or graphing the results.


The process by which packets are received, stored and transmitted to the appropriate destination port.


Short for synchronous, synchronous transmission, or synchronization. In video, a means of synchronizing signals with timing pulses so that each step in a process occurs at exactly the right time.

Synchronous Idle (SYN)

A control character for synchronous transmission used to maintain synchronization and fill in time when data is absent.

Synchronous Transmission

Data transmission in which characters and bits are transmitted at a fixed rate, with the transmitter and receiver synchronized by a clock source. This eliminates the need for individual start bits and stop bits surrounding each byte, thus providing greater efficiency than asynchronous transmission.

Synchronous Modem

Modem that carries timing information with data.

Synchronous Modem Eliminator (SME)

A modem eliminator that operates synchronously rather that asynchronously; that is, it uses clocking rather than start and stop bits.

Synchronous Terminal

A data terminal that operates at a fixed rate with transmitter and receiver in synchronization.

Synchronous Transmission

Transmission in which data bits are sent at a fixed rate, with the transmitter and receiver synchronized.

Systems Network Architecture (SNA)

The total description of the logical structure, formats, protocols, and operating sequences for transmitting formation between IBM software and hardware devices.



Often referred to as DS1 (Digital Signals),  T1 is a term that is often applied to a digital carrier facility used to transmit a formatted digital signal at an aggregate data rate of 1.544 Mbps. A T1 carrier uses multiplexing to transmit large volumes of information across great distances at high speeds at a (potentially) lower cost than that provided by traditional analog service. It consists of one 4-wire circuit providing 24 separate 64-Kbps logical channels.

Tail Circuit

A channel to a network node, typically a leased line.


(Transmission Control Protocol/Internet Protocol)-A layered set of protocols that allows sharing of applications among PCs, hosts, or workstations in a high-speed communications environment; it governs data communication on the Internet.


Transverse conversion loss = Unbalance attenuation at the near end: Power fed at the near end into the differential mode and coupled power measured at the near end in the common mode.


Transverse conversion transfer loss = Unbalance attenuation at the far end: Power fed at the near end into the differential mode and coupled power measured at the far end in the common mode.

Telecommunication Industries Association

Sister organization to the EIA that collaborates with it in setting standards for data communication (EIA/TIA-568, for example).

Telephone Interface

The analog side of a voice or fax channel.

Telephone Interface Connector

Either a 6-pin RJ-11 for KTS and OPX type telephone interface equipment, which requires a modular cable, or a 8-pin terminal block for KTS, OPX, and E&M type telephone equipment, which requires a cable made of 8 Color Coded wires.


A virtual terminal service available through the TCP/IP protocol suite.

Tensile Strength

The pull stress required to break the tested element.


Any device capable of sending and receiving data over a data communications channel.


An electrical circuit attached to each end of a SCSI bus to minimize signal reflections and extraneous noise. SCSI defines passive, active, and forced-perfect termination schemes.


Placement of a connector on a cable.

Thermal Shock

The effect of heat or cold applied at such a rate that non-uniform thermal expansion or contraction occurs within a given material or combination materials. The effect can cause inserts and other insulation materials to pull away from metal parts.


Tetra-fluoro-ethylene. A thermoplastic material with good electrical insulating properties and chemical and heat resistance.


A material which will soften, flow, or distort appreciably when subjected to sufficient heat and pressure. Examples are polyvinyl chloride and polyethylene.


A material which will not soften, flow, or distort appreciably when subjected to heat and pressure. Examples are rubber and neoprene.

Thick Ethernet, Thicknet

See 10BASE5. (10BASW5-The original 10-Mbps Ethernet network standard defined by DEC, Xerox, and Intel, and implemented on thick, yellow-jacketed cable). Thick Ethernet-The original Ethernet cable specification, requiring an AUI connector; noise-resistant, but expensive and difficult to install. Thinnet: (Thin Ethernet)-A CSMA/CD network based on thin coaxial cable (also called thin Ethernet) that requires a BNC connector; based on the 10BASE-2 IEEE standard.

Thin Ethernet, ThinNet

See 10BASW2. (10BASE2-A 10-Mbps Ethernet network implemented on thin RG58 coaxial cable).


Telecommunications Industry Association.


A Circuit that delays the transmission of an impulse for a definite and desired period of time.


The expiration of a predefined interval which triggers an action.


(Threaded Neill-Concelman)-A threaded connector for miniature coax. Contrast with BNC.

TNC Connector

A radio frequency connector covered by Military Specification. It has an impedance of 50 ohms and is designed to operate in a 0 to 11 GHz frequency range. Reliability is assured by a threaded coupling that can be safely wired to prevent accidental disconnect.


A continuously repeating frame, transmitted onto the network by the controlling computer, the frame that polls for network transmissions. See Token Ring.

Token passing

A network transmission method that requires a node to have control of a "token" before it can send messages; typically fairer than CSMA/CD on busy networks, but more complicated to implement.

Token Ring

A LAN-access mechanism and topology, developed by IBM and standardized as IEEE 802.5, in which a supervisory frame or token is passed from station to station in sequential order. Stations wishing to gain access to the network must wait for the token to arrive before transmitting data. IBM's implementation of token passing, governed by the IEEE 802.5 standard; second most popular network topology after Ethernet. Token ring uses pins 3, 6, 4 and 5, in an RJ (Registered Jack) plug/receptacle in EIA/TIA 568A and 568SB and in USOC.


Twisted Pair-Physical Medium Dependent.


The architecture of a network, or the way circuits are connected to link the network nodes together.


A device for converting mechanical energy to electrical energy.


A device used in contention networks for sending data over the network and receiving data from the network. A device that links a node to a network cable, functioning as both a transmitter and receiver.


An abrupt change in voltage, or shut duration (e.g. a brief pulse caused by the operation of a switch).

Transmit (TX)

Abbreviation for transmitted or transmitting.

Transmitted Data (TD)

RS-232 signal where data is sent from the DTE to the DCE.

Transfer Impedance

For a specified cable length, transfer impedance relates to a current on one surface of a shield to the voltage drop generated by this current on the opposite surface of the shield. Transfer impedance is used to determine shield effectiveness against both ingress and egress of interfering signals. Cable shields are normally designed to reduce the transfer of interference - hence, shields with lower transfer impedance are more effective than shields with higher transfer impedance.

Transmission Line

An arrangement of two or more conductors, a coaxial cable, or a waveguide used to transfer signal energy from one location to another.


The electronic package that converts electrical energy to light energy in a fiber optic system.

Triaxial Cable

A cable construction having a conductor, and two isolated braid shields, all insulated from each other.

Trunk Cable

In a CATV system, the transmission cable from the head end (signal pickup) to the trunk amplifier. Also called a feeder cable.

Twinaxial cable

A cable that is similar to coaxial cable, but has two inner conductors instead of one. Used in IBM minicomputer and midrange systems such as Systems 34, 36, and 38, and AS/400.

Twisted Pair

Two lengths of insulated conductors twisted together.

Twisted-Pair cable

Cable made up of one or more twisted pairs.



Ultra high frequency, 300 to 3,000 MHz.


Abbreviation for Underwriters Laboratories, a nonprofit organization which tests and verifies construction and performance of electronic parts and equipment, including wire and cable.

Unbalanced Line

A transmission line in which voltages on the two conductors are unequal with respect to ground. A coaxial cable is a common type of unbalanced line.


(Uniform Resource Locator)-A standard Internet location address; for example, "http://www.mycompany.com."


(Universal Serial Bus)-Serial 4-wire bus architecture for peripheral I/O developed by Compaq, Intel, and Microsoft. It autosenses up to 128 peripherals and supports a maximum distance of 5m (16.4 ft.) and a maximum data rate of 12Mbps. USB was designed to collect all of PC's I/O port into one serial interface, and provide enough bandwidth (data transfer rate) to support multiple peripherals. With its 12MBps transfer rate, it is more than adequate to handle traditional PC peripherals such as keyboards, mice, joystick, scanners, printers, and advanced computer game devices. With USB hub, it can support up to 127 devices. USB supports "true" Plug & Play, "Tree-Like" structure for daisy chaining, and devices can be connected and disconnected without interrupting system operation. The series "A" connectors is a flattend rectangle. The series "B" connector is roughly square with beveled corners. The two connector series are different to prevent connections that violate the USB architecture topology.


Unshielded Twisted Pairs(s). Twisted pair cable without either individual or Overall Shielding. Twisted-pair cabling without EMI/RFI shielding. Compare STP. A thin-diameter network wire that is very popular in network cabling installations.





A CCITT interface recommendation defining interchange circuits; similar to and compatible with RS-232.B392+B392


A CCITT interface recommendation defining electrical characteristics for V.24 interchange circuits; similar to and compatible with RS-232.


A CCITT 9600-bps dial- or 2-wire leased-line modem recommendation.

V.32 bis

A CCITT 14.4-Kbps dial- or 2-wire leased-line modem recommendation.


An ITU-TSS modem standard for data transmission at 28.8 Kbps.

V.34 bis

(V.34+)-An ITU-TSS modem standard for data transmission at 33.6 Kbps.


A CCITT interface standard for high-speed communication. V.35 specifies a 34-pin connector and can transmit at speeds into the millions of bits per second. It can't connect, physically or electrically, to any other interface without the aid of a converter. CCITT Standard governing data transmission at 48 Kbps using 60-108 KHz group band circuits.


A CCITT standard for error correction by modems.

V.42 bis

A CCITT standard for data compression by modems.

Velocity of Propagation (VP)

The transmission speed of electrical energy in a length of cable compared to speed of light in free space. Usually expressed as a percentage.

Video Graphics Array (VGA)

A video standard for IBM PC and compatible computers. Standard VGA has a resolution of 640 x 480 and supports 16 colors.


Very high frequency, 30 to 300 HMz as designated by the Federal Communications Commission.

Virtual Terminal

A terminal emulation program that makes a workstation appear to be a dumb terminal connected to some remote system, such as a mainframe.

Virtual LANs

A switching technology that enables logical segmentation of switched networks, independent of physical grouping or collision domains.


Very low frequency, 10 to 30 kHz.

Voice-Frequency (VF)

Any frequency within that part of the radio frequency range essential to speech transmission of a commercial quality (I.e., 300 to 3400 Hz). Also referred to as telephone frequency.


A unit of electromotive force.


Electrical potential of electromotive force expressed in volts.

Voltage Drop

The voltage developed across a component or conductor by the current flow through the resistance or impedance of the component or conductor.

Voltage Standing Wave

The ratio of the transferring signal voltage as compared to reflected signal voltage measured along

Ratio (VSWR)

the length of a transmission line.


Voltage standing wave ratio.


A flammability rating established by Underwriters Laboratories 1581 for wires and cables that pass a specially designed vertical flame test, formerly designed FR-1.



(Wide-Area Network)-A network that serves an area of anywhere from several to thousands of miles, using common-carrier-provided lines; contrast with LAN.


The distance between positive peaks of a signal. As the frequency increases, and waves get closer together, the wavelength decreases.


(Also called segment)-A grouping of workstations, server(s) and any network devices dedicated to similar functions, using similar applications and/or sharing common resources, and serving as a subnetwork entity; members may have a common geography or function, e.g., engineering, marketing, manufacturing and administration.


For the purposes of this guide, a personal computer in a network; also called a client.



A WAN standard for protocols and message formats; used to access public packet-switching networks.


(Transmitter On/Transmitter Off)-Special characters used for flow control; they signal a device to start transmission (X-ON) and stop transmission (X-OFF). Control characters used for flow control, instructing a terminal to start transmission (X-ON and end transmission (X-OFF).


Crosslinked polyethylene is a thermoset and is crosslinked by radiation, thermally, or by moisture. 



Symbol for impedance

Zero-Dispersion Wavelength

1. In a single-mode optical fiber, the wavelength or wavelengths at which material dispersion and waveguide dispersion cancel one another. Note: In all silica-based optical fibers, minimum material dispersion occurs naturally at a wavelength of approximately 1.3 microns. Single-mode fibers may be made of silica-based glasses containing dopants that shift the material-dispersion wavelength, and thus, the zero-dispersion wavelength, toward the minimum-loss window at approximately 1.55 microns. The engineering tradeoff is a slight increase in the minimum attenuation coefficient. 2. Loosely, in a multimode optical fiber, the wavelength at which material dispersion is minimum, i.e., essentially zero. Synonym minimum-dispersion wavelength.