Glossary of Telecommunications Terms

a-law A method of z, commonly used in Europe. Compare with mu-law.

AC signaling Any in-band signaling system that uses AC current. The most common AC signaling system is DTMF. Contrast with DC signaling.

adaptive differential pulse code modulation (ADPCM) A method of sampling and converting analog signals to digital signals. Similar to DPCM except that when a wide difference occurs between two successive samples of a signal, it uses a sophisticated algorithm to code the difference.

address A telephone number.

amplifier A device use in analog networks to boost the strength of a signal and extend its transmitted range. Equivalent to a repeater for digital networks.

amplitude The maximum value of a varying quantity, such as a peak in a waveform.

analog signal An electrical signal that varies continuously in amplitude and frequency.

answer supervision Signals sent through the network to indicate the status of a called phone, such as ringback and busy signal.

asynchronous Method of timing for transmission of multiplexed digital signals. The signal is in a framed format so the receiving end can identify the boundaries of the multiplexed channels.

attenuation Reduction of signal magnitude or signal loss, usually expressed in decibels. This occurs both from the distance a signal is transmitted and from the electrical resistance (or optical absorption in the case of fiber) of the medium.

automatic call distribution (ACD) ACD directs incoming calls to designated stations in a specified order of priority.

backward compatible The ability of a new technology or product to work with existing equipment.

bandwidth Information-carrying capacity of a communication channel. Analog bandwidth is the range of signal frequencies that can be transmitted by a communication channel or network. Digital bandwidth is the rate at which bits can be transmitted.

basic rate interface (BRI) User interface to the public ISDN network (usually through Centrex). Contains two B channels at 64 Kbps and a D channel that operates at 16 Kbps. The BRI line is the transmission path between the user's phone and the CO, where it is then switched by the CO to its eventual destination.

bit One binary digit; a pulse of data.

bits per second (bps) The number of bits passing a point per second. The transmission rate for digital information.

busy hour A designated one-hour period considered the hour of greatest phone use. It is used to study traffic volume for dimensioning purposes.

bypass A circuit between two points that would normally use the facilities and services of the local telephone company.

calling address Same as address: a telephone number.

carrier frequency A single frequency transmission, used to transport signal frequencies through some form of modulation. See modulation.

carrier system A transmission system for transporting multiple channels of multiplexed calls.

cell site Antennas used in cellular telephone systems to send and receive calls to and from mobile telephones. Cell sites are scattered around the area the cellular carrier serves to provide complete coverage. Cells sites communicate with mobile telephones and with Mobile Telephone Switching Offices (MTSOs).

cellular telephone network A telephone network of antennas (cell sites) connected to switches (MTSOs) designed to allow communication between portable radio telephones with no fixed connection to the network.

central office (CO) Local telco installation for LEC switching and other services.

Centrex Central Office based service provided to business customers that allows direct outward dialing from the organization as well as direct inward dialing to phone extensions. Provides most, if not all, PBX type features.

channel The smallest subdivision of a circuit that provides a type of communication service; usually a path with only one direction.

circuit A communication path or network; usually a pair of channels providing bidirectional communication.

circuit switching Basic switching process whereby a circuit between two users is opened on demand and maintained for their exclusive use for the duration of the transmission.

coaxial Cabling, usually comprised of a single copper cable surrounded by cladding and a sheath. Most commonly used in cable television, broad band networks, and some base band networks.

codec (COder/DECoder) Device that converts analog signals to digital signals for transmission on digital lines. It also reconverts (digital to analog).

common channel signaling (CCS) A signaling method in which one channel is reserved solely for network management.

companding Compression or expansion of encoding levels to increase signal quality when digitizing an analog signal.

control logic The computer in a switching system (either time-division, space-division, or multistage) that monitors and controls switching to properly route calls.

core In an optical fiber, the center region of high refractive index through which light passes.

cross-bar switch A type of CO switch. Incoming and outgoing lines are connected in a matrix of crosspoints. Electromechanical relays, or in newer cross-bar switches, transistors, open and close the crosspoints, making and breaking connections.

cross-connect A frame that supports terminals or terminal blocks and the connections between these terminals.

crosstalk Undesirable effect on one circuit or channel due to signals in another circuit or channel.

DC signaling Any in-band signaling system that uses DC voltage. There are several common DC signaling systems. Contrast with AC signaling.

delta modulation A method of sampling and converting analog signals to digital signals. It is based on encoding the direction of signal change, indicating the direction of signal change with a single bit.

demarcation point The location of the separation of network equipment from subscriber equipment. Usually occurs at a connecting device. Also called point of demarcation.

digital signal An electrical signal that varies in discrete steps.

digital loop carrier (DLC) Equipment outside the CO that concentrates analog local loop lines, and digitizes and multiplexes the calls for transport to the CO.

differential pulse code modulation (DPCM) A method of sampling and converting analog signals to digital signals. DPCM encodes the difference between successive sampled signals. It uses a 4-bit code for each sample.

dimensioning Determining the existing and likely use patterns of a specific area route and the specific network needs (switches, trunks, etc.) required by the area.

Dual-tone multifrequency (DTMF) Method of signaling using a specific combination of two tones. Used by push-button telephones for dialing and in network signaling.

DTMF dialing Tone dialing; using DTMF tones to send the address (phone number) of the called phone to the local CO.

DTMF signaling A method of interswitch signaling that uses dual-tone multifrequency (DTMF) tones.

E&M lead signaling Method of signaling using dual connection ports ("E" and "M"). When a signal reaches a switch, it enters through the E lead. Switches send signals out through the M lead.

electromagnetic interference (EMI) Disturbance or malfunction due to electromagnetic energy.

end office (EO) Another term for Central Office.

end-to-end digital connectivity Signals are in digital form during their entire path through the network.

exchange A switching center. Used synonymously with CO. A PBX (private branch exchange) is a switching center used in a private network.

fiber optics Optical technology that deals with the transmission of light through fibers made up of transparent materials such as glass or plastic.

4-wire Two twisted pairs. Used for multiplexed circuits.

frame A group of bits organized in fields of information. A field that has been filled with data and is ready to transmit onto the network is a packet.

framing Method for distinguishing digital channels that have been time-division multiplexed together.

frequency The number of cycles of periodic activity that occur in a discrete amount of time.

frequency division multiplexing (FDM) Most common method of multiplexing analog signals.

ground start Method of supervisory signaling, primarily for analog in-band networks. CPE transmits offhook condition by creating a zero voltage condition. See loop start and wink start for other similar techniques.

Hertz (Hz) Cycles per second.

hybrid A device that converts a 4-wire voice communication circuit into a 2-wire circuit.

in-band signaling Signaling that takes place over the same circuits as the call.

integrated services digital network (ISDN) Method for carrying many different services over the same digital transmission and switching facilities.

interexchange carrier (IEC) Provider licensed to carry transmissions between LATAs interstate; long-distance carrier.

interLATA Between local access and transport areas, i.e., long-distance service.

interswitch signaling Signaling of call set up and tear down information between switches. Usually initiated by the subscriber signaling process.

intraLATA Within local access and transport areas, i.e., local service.

ISDN-capable Equipment that operates according to ISDN standards.

kilobit One thousand bits.

key system CPE that provides line sharing capabilities to the user. Typical key systems share a few lines among many phones.

laser A device that produces highly coherent light using light amplification by stimulated emission of radiation; lasers are used in fiber optic communication systems as light sources.

light-emitting diode (LED) Semiconductor device that emits light; often used an indicator light or as the light source in fiber optic systems.

loading coil Equipment used to increase resistance to electrical current and balance attenuation of high and low frequency signals in the voiceband.

local access and transport area (LATA) Area served by a single telephone company.

local exchange carrier (LEC) A Bell or independent operating company offering interLATA telecommunications services over the public switched network.

loop start Method of supervisory signaling. CPE signals offhook condition by closing a relay at the CO. Contrast ground start and wink start.

megabit One million bits.

message-oriented Using complex codes transmitted as binary data (messages) for signaling.

metropolitan statistical area (MSA) A term used by government agencies and other organizations. The MSA divides the United States into 306 areas according to population density. Cellular contracts are awarded by MSA.

microwave Electromagnetic waves with wavelengths in the range 1 to 10 centimeters.

modem Device that converts digital signals to analog signals for transmission on analog lines.

modulate To superimpose characteristics of a signal on a carrier frequency for transmission of the signal.

mobile telephone switching offices (MTSO) A cellular CO, for use only within the cellular network. Interacts with cell sites.

modulation To superimpose characteristics of a signal on a carrier wave for transmission of the signal.

mu-law A method of companding, commonly used in Europe. Compare with a-law.

multiplexer A device for combining several channels to be carried by one line or fiber.

multiplexing Method of combining several channels to be carried by one line or fiber.

noise Caused by random electrical signals, which act to corrupt the bits being transmitted. Noise can come from outside electrical sources, including lightning or motors, or from crossed cables.

offhook Signaling state. A request from the telephone station set for access to the network. A set goes offhook when a caller picks up the handset.

onhook An inactive phone ready to send or receive calls. Represented by having the phone on the cradle or the "hang up" position.

out-of-band signaling Using a separate signaling channel for controlling a call.

point of demarcation The location of the separation of network equipment from subscriber equipment. Usually occurs at a connecting device.

point of presence (POP) Point of connection between an interexchange carrier and a local carriers to pass customer's calls into the IEC's network.

primary rate interface (PRI) A user interface to ISDN network with 23 B channels and one D channel, all at 64 Kbps. Primarily used to link PBXs and to connect a PBX to the PSTN.

private of contact is termed the tip. Although the manually switched line is obsolete, the terms are still used for line assignment purposes.

semiconductor A solid material with resistance between an insulator and conductor, such as silicon or germanium.

sideband The frequencies the modulated carrier wave occupies. Upper sideband transmissions occupy the relevant frequencies above the carrier wave. Lower sideband occupies the frequencies below the carrier wave.

sidetone The transmission of the speaker's input into the microphone section of the headset into the speaker's receiver.

signal transfer point (STP) Node in the interoffice (CCS7) network that communicates with central offices to assist in routing.

signaling Method of communication between network components to provide control management and performance monitoring.

signaling point (SP) Processor designed for handling the signaling function of a switch in a common channel signaling network.

signaling system 7 (SS7) ISDN signaling standard. Most often used between COs for network control purposes. Also used between a CO and the IEC POPs.

space-division switching (SDS) Similar in concept to cross-bar switching, but under computer control. Analog or digital transmissions are routed on a trunk-by-trunk basis through a matrix of incoming and outgoing trunks.

station message detail recording (SMDR) A feature found in Centrex and PBX offerings. Records and stores detailed calling information including length, destination and other designated data.

station set Another name for telephone.

step-by-step switch Also called a Strowger switch. One of the original electromechanical CO switch designs. Each stage of the switch switches one digit of the dialed number. Stages work independently of each other, making the switch slow and inefficient.

stored program control switch A computer-controlled switch.

Strowger switch A step-by-step CO switch.

subscriber Telephone customer.

subscriber loop Channel connecting the subscriber to the central office. Also termed the "local loop".

subscriber signaling CPE signaling to switch. Primary role is call set up and tear down.

supervisory signaling Signaling that informs