Browse this online glossary of terms and have industry terms explained. Most of these definitions have been extracted (with editing for space restrictions) from Newton's Telcom Dictionary (15th Edition), published by Telecom Library, Inc. To purchase a copy, visit your favorite local bookstore.


ATM Asynchronous Transfer Mode. ATM is a high bandwidth, low delay, packet-like switching and multiplexing technique. Usable capacity is segmented into fixed-size cells, consisting of header and information fields.

Attenuation The decrease in the power of a signal, light beam, or light wave. Measured in decibels. Opposite of gain.

AWG American Wire Gauge. Standard measuring gauge for non-ferrous conductors (i.e., non-iron and non-steel). Gauge is a measure of the diameter of the conductor (the thickness of the cable).



Backbone Wiring The physical/electrical interconnections between telecommunications closets and equipment rooms. Cross-connect hardware and cabling in the Main and Intermediate Cross-Connects are considered part of the backbone wiring.

Bandwidth The difference between the highest and the lowest frequencies of a transmission channel (path for information transmission). Identifies the amount of data that can be sent through a given channel. Measured in Hertz (Hz); higher bandwidth numbers mean higher data capacity.

BICSI (Building Industry Consulting Service International) BICIS is a non-profit industry association, concerned with promoting correct methods for all aspects of the installation of communications wiring.

Bus A network topology in which nodes are connected to a single cable with terminations at each end.



CO Central Office. Telephone company facility where subscribers’ lines are joined to switching equipment for connection to each other, locally and long distance. Sometimes the same as the overseas term "public exchange".

Coaxial Cable A cable composed of an insulated central conducting wire wrapped in another cylindrical conductor (the shield). The whole thing is usually wrapped in another insulating layer and an outer protective layer. A coaxial cable has great capacity to carry vast quantities of information. It is typically used in high-speed data and CATV applications.

Compliance A wiring device that meets all characteristics of a standard is said to be in compliance with that standard. Example: a data jack that meets all of the physical, electrical and transmission standards for TIA-568 Category 5 is compliant with that standard. See also Device and Performance.

Conductor Any substance, usually a wire or cable, that can carry (i.e., offer a relatively small opposition to the passage of) an electrical current.

Connecting Block Also called a terminal block, a punch-down block, a quick-connect block, a crossconnect block. A plastic block containing metal wiring terminals to establish connections from one group of wires to another. Usually each wire can be connected to several other wires in a bus or common arrangement. There are several types of connecting blocks: 66 clip, BIX, Krone, 110, etc. A connecting block has insulation displacement connections (IDCs), which means you don’t have to remove insulation from around the wire conductor before you "punch it down" (terminate it).

Connector A device that connects wires or fibers in cable to equipment or other wires or fibers. Wire and optical connectors most often join transmission media to equipment or cross connects. A connector at the end of a telephone cable or wire is used to join that cable to another cable with a mating connector or to some other telecommunications device.

Crossconnect Distribution system equipment used to terminate and administer communication circuits. In a wire crossconnect, jumper wires or patch cords are used to make circuit connections. In an optical crossconnect fiber patch cords are used. The crossconnect is located in an equipment room, riser closet, or satellite closet.

Crosstalk See Near-End Crosstalk.



Daisy Chain In telecommunications, a wiring method where each telephone jack in a building is wired in series from the previous jack. Daisy chain is NOT the preferred wiring method, since a break in the wiring would disable all jacks "downstream" from the break. See also Home Run.

dB (Decibel). A dB is a unit of measure of signal strength, usually the relation between a transmitted signal and a standard signal source. Every 3 dB = 50% of signal strength, so therefore a 6 dB loss = a loss of 75% of total signal strength.

Demarcation Point The point of interconnection between telephone company facilities and your building wiring. The demarcation point ("demarc") shall be located on the subscriber’s side of the telephone company’s protector, or the equivalent thereof in cases where a protector is not required.

Device As distinguished from equipment. In telecommunications, a "device" is the physical interconnection outlet. Equipment (a computer, phone, fax machine, etc.) then plugs into the device. See also Equipment and Plug.

Drop Wire Outside wire pair(s) from the telco plant (cable), to a house or building for connection to a protector.

DTMF Acronym for Dual Tone, Multi-Frequency. See Tone Dial.



Electromagnetic Interference (EMI) The interference in signal transmission or reception caused by the radiation of electrical and magnetic fields.

Equipment As distinguished from Device. Telecommunications equipment (computers, phones, faxes, etc.) plug into the telecommunications outlet or device. See also Device.

Ethernet A local area network used for connecting computers, printers, workstations, terminals, etc. within the same building. Ethernet operates over twisted pair wire and over coaxial cable at speeds up to 10 Mbps. Ethernet LANs are being promoted by DEC, Intel and Xerox. Compare with Token Ring.



FDDI Fiber Distributed Data Interface. FDDI is a 100 Mbps fiber optic LAN. It is an ANSI standard. It uses a "counter-rotated" Token ring topology. An FDDI LAN is typically known as a "backbone" LAN. It is used for joining file servers together and for joining other LANs together.



Headroom (also called Overhead or Margin) The number of decibels by which a system exceeds the minimum defined requirements. The benefit of headroom is that it reduces the bit-error rate (BER), and provides a performance 'safety net' to help ensure that current and future high speed applications will run at peak accuracy, efficiency and through-put.

Home Run Phone system wiring where the individual cables run from each phone directly back to the central switching equipment. Home run cabling can be thought of as "star" cabling. Every cable radiates out from the central equipment. All PBXs and virtually all key systems work on home run cabling. Some local area networks work on home run wiring. See also Star Wiring, Daisy Chain.

Hub The point on a network where a bunch of circuits are connected. Also, a switching node. In Local Area Networks, a hub is the core of a star as in ARCNET, StarLAN, Ethernet, and Token Ring. Hub hardware can be either active or passive. Wiring hubs are useful for their centralized management capabilities and for their ability to isolate nodes from disruption.

Hybrid Connector A connector containing both optical fiber and electrical conductors.



Insulation Displacement Connection (IDC) A type of wire termination in which the wire is "punched down" into a metal holder which cuts into the insulation wire and makes contact with the conductor, thus causing the electrical connection to be made.

IDF Intermediate Distribution Frame. A metal rack designed to connect cables and located in an equipment room or closet. Consists of components that provide the connection between inter-building cabling and the intra-building cabling, i.e. between the Main Distribution Frame (MDF) and individual phone wiring. There’s usually a permanent, large cable running between the MDF and IDF. The changes in wiring are done at the IDF. This saves confusion in wiring.

IEEE 802.3 IEEE stands for the Institute of Electrical and Electronic Engineers, a publishing and standards-making body responsible for many standards used in LANs, including the 802 series. Ethernet and StarLan both follow the 802.3 standard. Typically they transmit at 10 megabits per second. This is the most common local area network specification.

Impedance The total opposition (i.e. resistance and reactance) a circuit offers to the flow of alternating current. It is measured in ohms, and the lower the ohmic value, the better the quality of the conductor.
Interconnect 1
. A circuit administration point, other than a crossconnect or an information outlet, that provides capability for routing and rerouting circuits. It does not use patch cords or jumper wires, and typically is a jack-and-plug device used in smaller distribution arrangements or that connects circuits in large cables to those in smaller cables. 2. An Interconnect Company is one which sells, installs, and maintains telephone systems for end users, typically businesses.

ISDN Integrated Services Digital Network. According to AT&T, today’s public switched phone network has many limitations; ISDN’s vision is to overcome these deficiencies.



Jack A receptacle used in conjunction with a plug to make electrical contact between communication circuits. Jacks and their associated plugs are used in a variety for connecting hardware applications including cross connects, interconnects, information outlets, and equipment connections. Jacks are used to connect cords or lines to telephone systems. A jack is the female component of a plug/jack connector system, and may be standard, modified, or keyed. See also Plug and RJ.



LAN Local Area Network. A short distance network (typically within a building or campus) used to link together computers and peripheral devices (such as printers) under some form of standard control.

Loop 1. Typically a complete electrical circuit. 2. The loop is also the pair of wires that winds its way from the central office to the telephone set or system at the customer’s office, home or factory (i.e., 'premises' in telephony terms).



Mbps MegaBits Per Second. One million bits per second.

MDF Main Distribution Frame. A wiring arrangement which connects the telephone lines coming from outside on one side and the internal lines on the other. A main distribution frame may also carry protective devices as well as function as a central testing point.

MHz MegaHertz. A unit of frequency denoting one million Hertz (i.e., 1,000,000 cycles per second).

MMJ Modified Modular Jack. A six-wire modular jack with the locking tab shifted off to the right hand side. Used in the DEC wiring system.

Modular Equipment is said to be modular when it is made of "plug-in units" which can be added together to make the system larger, improve the capabilities, or expand its size.



Near-End Crosstalk (NEXT) Electrical noise coupled from one pair of wires to another within a multi-pair cable.

Network A network ties things together. Computer networks connect all types of computers and computer-related things—terminals, printers, modems, door entry sensors, temperature monitors, etc. The networks we’re most familiar with are long distance ones, like phones and trains. Local Area Networks (LANs) connect computer equipment within a building or campus.



Off-Hook When the handset is lifted from its cradle it’s off-hook. The term originated when the early handsets were actually suspended from a metal hook on the phone. In modern phones, when the handset is removed from its hook or cradle, it completes the electrical loop, thus signaling the central office to provide dial tone.

On-Hook When the phone handset is resting in its cradle. The phone is not connected to any particular line. Only the bell is active—i.e., it will ring if a call comes in. Opposite of Off-Hook.

Open (Fault) Means that the circuit is not complete or the cable/fiber is broken.

Outlet A telecommunications outlet is a single-piece cable termination assembly (typically on the floor or in the wall), containing one or more modular telecom jacks. Such jacks might be RJs, coaxial terminators, fiber optic couplers, etc. See also Device and Equipment.


Part 68 Requirements
Specifications established by the FCC as the minimum acceptable protection communications equipment must provide the telephone network.


Patching A means of connecting circuits via cords and connectors that can be easily disconnected and reconnected at another point. May be accomplished by using modular cords connected between jack fields or by patch cord assemblies that plug onto connecting blocks.

PBX Private Branch Exchange. A small, privately-owned version of the phone company’s larger telephone central switching office.

Performance Compare with Compliance. A device can exhibit performance characteristics without being compliant to an industry standard.

Plug A male component of a plug/jack connector system. In premises wiring, a plug provides the means for a user to connect communications equipment to the communications outlet.

Polarity Which side of an electrical circuit is the positive? Which is the negative? Polarity is the term describing which is which.

POTS Plain Old Telephone Service. The basic service supplying standard single line telephones, telephone lines and access to the public switched network. Just receive and place calls. No added features like Call Waiting or Call Forwarding.

Power Sum A test method for four pair cable whereby the mathematical sum of pair-to-pair crosstalk from three pairs to one pair is measured.

Premises Telephony term for the space occupied by a customer or authorized/joint user in a building(s) on continuous or contiguous property (except railroad rights of way, etc.) not separated by a public road or highway.

Premises Wiring System The entire wiring system on the user’s premises, especially the supporting wiring that connects the communications outlets to the network interface jack.



RBOC Regional Bell Operating Company. Seven RBOCs exist, each of which owns two or more Bell Operating Companies (BOCs). The RBOCs were carved out of the old AT&T/Bell System during the divestiture of the Bell operating companies from AT&T in 1984.

RCDD The RCDD (Registered Communications Distribution Designer) title is a professional rating granted by BICSI (the Building Industry Consulting Service International). RCDDs have demonstrated a superior level of knowledge of the telecommunications wiring industry and associated disciplines.

Return Loss A measure of the similarity of the impedance of a transmission line and the impedance at its terminations. It is a ratio, expressed in decibels, of the power of the outgoing signal to the power of the signal reflected back.

Ring As in Tip and Ring. One of the two wires needed to set up a telephone connection. See Tip.

RJ Registered Jack. RJs are telephone and data jacks/applications registered with the FCC. Numbers, like RJ-11, RJ-45, etc. are widely misused in the telecommunications industry. A much more precise way to identify a jack is to specify the number of positions (width of opening) and number of conductors. Example: "8-position, 8-conductor jack" or "6-position, 4-conductor jack".



Series Wiring See Daisy Chain.

Service Loop When a device is terminated to the wire in the communications outlet, a fair amount of "slack" should be left on the wire and wound in the box to accommodate future trimming when devices are changed out.

Splice The joining of two or more cables together by connecting the conductors pair-to-pair.

Standards Agreed principles of protocol. Standards are set by committees working under various trade and international organizations.

Star Wiring See Home Run.



T1 A standard for digital transmission in North America. A digital transmission link with a capacity of 1.544 Mbps (1,544,000 bits per second.) T1 lines are used for connecting networks across remote distances. Bridges and routers are used to connect LANs over T1 networks.

Talk Battery The DC voltage supplied by the central office to the subscriber’s loop so as to allow you to have a voice conversation.

Telco An Americanism for TELephone COmpany.

Ten BASE-t See10Base-T at end of Glossary.

Terminate To connect a wire conductor to something, typically a piece of equipment.

Tip 1. The first wire in a pair of wires. (The second wire is called the "ring" wire.) 2. A conductor in a telephone cable pair which is usually connected to positive side of a battery at the telco. It is the phone industry’s equivalent of Ground in a normal electrical circuit. See Ring.

Tone Dial A push-button telephone dial that makes a different sound (in fact, a combination of two tones) for each number pushed. The technically correct name for tone dial is Dual Tone Multi Frequency, or DTMF.

Token Ring A ring topology for a local area network (LAN) in which a supervisory frame, or token, must be received by an attached terminal or workstation before that terminal or work-station can start transmitting. The workstation with the token then transmits and uses the entire bandwidth of whatever communications media the token ring network is using. A token ring can be wired as a circle or a star, with the workstations wired to a central wiring center, or to multiple wiring centers. The most common wiring scheme is called a star-wired ring. Whatever the wiring, a token ring LAN always works logically as a circle, with the token passing around the circle from one workstation to another. The advantage of token ring LANs is that media faults (broken cable) can be fixed easily, since it’s easy to isolate the faults. Token rings are typically installed in centralized closets, with loops snaking to served workstations.

Topology As in network topology. The geometric physical or electrical configuration describing a local communication net-work; the shape or arrangement of a system. The most common topologies are the bus, ring and star.

TP-PMD Twisted Pair - Physical Media Dependent. Technology under review by the ANSI X3T9.5 working group that allows 100 Mbps transmission over twisted-pair cable.

Twisted Pair Two insulated copper wires twisted around each other to reduce induction (thus interference) from one wire to the other. The twists, or lays, are varied in length to reduce the potential for signal interference between pairs. Several sets of twisted pair wires may be enclosed in a single cable. In cables greater than 25 pairs, the twisted pairs are grouped and bound together.



UL Underwriters Laboratories, a privately owned company that tests to make sure that products meet safety standards. UL also administers a program for the certification of Category-Rated Cable.

USOC Universal Service Order Code. An old Bell system term identifying a particular service or equipment offered under tariff.

UTP Unshielded Twisted Pair. See Twisted Pair.



Workstation The working area in a building required by one telecommunications user. Industry standards call for one voice drop and one data drop for each workstation. The voice drop is one 4-pair unshielded twisted pair (UTP). The data drop may be 100??4-pair UTP, 150??2-pair shielded twisted pair (STP), or optical fiber.



10BASE-T This is the IEEE standard that defines the requirement for sending information at 10 Mbps on unshielded twisted-pair cabling, and defines various aspects of running Ethernet on this cabling.

100BASE-T This is the IEEE standard that defines the requirement f or sending information at 100 Mbps on unshielded twisted-pair cabling, and defines various aspects of running baseband Ethernet on this cabling.

1000BASE-T This is the proposed IEEE standard that defines the requirement for sending information at 1000 Mbps on unshielded twisted-pair cabling, and defines various aspects of running baseband Ethernet on this cabling.