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© 2017 DIGIFON

From "The ISDN Studio" by Dave Immer
Audio Engineering Society 99th Convention
Oct. 8, 1995 , New York City

Glossary of Terms


Most basically: A set of processes in a computer program used to solve a problem with a given set of steps. In digital audio codecs: A set of instructions that specifies what data can be discarded, modified or reconstitued when certain conditions are met in order to reduce the bit rate of the audio.
Analog Transmission

A method of sending signals - voice, video, data - in which the transmitted signal is analogous to the original signal. Analog technology uses continuously changing electrical waves (instead of coded electrical pulses) to generate voice or low-speed data signals. Telephony used analog transmission exclusively until digital signal technology was introduced by AT&T in 1962.

Basic Rate Interface (BRI)

The ISDN standard governing how a customer's terminals and telephones can be connected to an ISDN switch. It specifies two B synchronous channels capable of simultaneous voice and data service, and one D channel that carries call set-up information and customer data.

B channel

The circuit-switched "bearer" channel that is a fundamental component of ISDN interfaces. It carries either voice or data at 64,000 bits per second in either direction.


The smallest unit of information that a computer recognizes. A bit (short for binary digit) is represented by the presence or absence of a pulse. Thus, a bit is either on or off. Bits are expressed by a small-case "b" as in kbs (kilo-bits per second).


An assemblage of 8 bits. Bytes are expressed by an upper-case "B" as in a 400 MB (Mega-Byte) hard drive.

Central Office (CO)

Telephone switching location where your twisted-pair copper telephone lines originate. It is usually here that an ISDN digital switch must be installed for service to local customers.


A physical transmission link between two telephones or terminals capable of carrying voice, data and image signals.

Circuit Switching

A technique of switching voice, data or image calls by dedicating an entire circuit (in analog switches) or a specific selection of time slots (in digital switches) to a given call, as opposed to packet switching. In ISDN, B channels are circuit switched. D channels are packet switched.

Clear Channel

A communications link in which the full 64 kbs capacity is used for transmission. In ISDN, B channels are clear channels, since all set-up and signaling information is carried on the D channel.


Acronym for coder-decoder. A circuit that reduces the bit stream of digital audio or video so that it will fit within the speed limitations of a network, and then restores the bit stream to a reasonable likeness of the original.

Customer Premises Equipment (CPE)

Literally, communications equipment located at the customer's, rather than the telephone company's premises.

D Channel

The "demand" or "data" channel of the ISDN interface, used to carry packet-switched control signals and customer call data. In the BRI, the D channel operates at 16 kbs. In the PRI, the D channel operates at 64 kbs.

Data Reduction

Process by which data that matches a certain mathematical model called an algorithm is discarded in order to create a more compact file or a slower bit-stream.

Dedicated Circuit

A transmission circuit leased be one customer for exclusive use around the clock. Also called a private line, or leased line.


The process by which we make all our telephone calls to anywhere in the world over public switched networks. A circuit established by the caller keying in the telephone number "address" of the recipient, initiating a sequence of switching and signaling which culminates in a "virtual private circuit" between the two ends.

Digital Transmission

A method of sending and receiving information coded with on-and-off pulses of electricity or light.

Inter Exchange Carrier (IEC)

An organization, such as AT&T, MCI or SPRINT, which provides long distance communication facilities and services between areas served by local telephone companies.


Integrated Services Digital Network. As officially defined by CCITT: A limited set of standard interfaces to a digital communications network.

Kbs (Kilobits (per) second)

Thousands of bits per second, a standard unit of transmission speed.

LAN (Local Area Network)

Private data network of PCs, peripherals and other equipment linked to a local packet switch, usually serving a cluster of geographically related offices or buildings.

Local Exchange Company (LEC)

A telephone company that provides customer access to the worldwide public switched network through one of its central offices.

Local Loop

In telephone networks, the lines that connect customer equipment to the switching system in the central office.

Mbs (Megabits (per) Second)

One million bits per second, a measurement of transmission speed.


Acronym for modulator-demodulator, equipment that connects a digital computer or device to the analog public switched network by converting the digital signals to analog tones, and vice versa.

Out-of-Band Signaling

In ISDN, the D channel that carries set-up and other call information through a separate network of transmission channels. The major advantage of out-of band signaling, compared to in-band signaling, is that it does not tie up circuits - B channels - attempting to complete calls to busy phones or terminals. It also produces much faster call set-up.


A unit of digital data with a set number of bits, including some bits that serve as an address code. the packet can be sent through a packet switching network by the best route, and reunited at its destination with the other message packets, regardless of the route each took, or the order of their arrival.

Primary Rate Interface (PRI)

In the United States, the PRI consists of 23 B or "bearer" channels operating at 64 kilobits per second, and one D or "demand" channel also functioning at 64 kilobits per second. The combined signal-carrying capacity is 1.544 megabits per second. In Europe, the PRI consists of 30 B channels and one D channel.


A set of rules which define procedures for the transfer of information in a computing or communications system. Data communications protocols deal with, among other things, call set-up procedures and formatting.

Public Switched Network

The combined transmission facilities of the world's telephone companies and administrations, including all those circuits available to subscribers on an unrestricted basis.


Equipment that facilitates transmissions between multiple, and often dissimilar, LANs or WANs.


Telecommunications equipment which controls the routing of transmission signals entering and leaving a central office or a toll office. On customer premises, a PBX (Private Branch Exchange) functions as a private switch that links various phone extensions to each other and to the outside world.

Synchronous transmission

Data transmission in which the transmitter and receiver are synchronized by periodically transmitted timing signals, or synchronization bits. Transmission of large files is much more efficient, since no start or stop bits are used, as in asynchronous transmission.


Name given to the most popular copper-wire transmission system and the rate at which it operates (about 1.544 megabits per second)


Acronym for Telephone Company.

Twisted Pair

Two insulated wires, usually copper, twisted together and often bound into a common sheath to form multi-pair cables. In ISDN, these cables are the basic path between a subscriber's terminal or telephone and the PBX or the central office.

WAN (Wide Area Network)

An integrated private voice/data network that links geographically dispersed local area networks (LANs).


The author wishes to thank Tom Ballister of Controlware, John Knapton of APT, the authors of the CCS CDQ2000 manual, and the authors of France Telecom's "ISDN: A Users Guide" for help and information.