Glossary | Business Communications

Business Telephones Support


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Class 4 toll switch made by Lucent


End Office switch made by Lucent


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ATM Adaptation Layer. Protocol used on top of ATM to support high-level service requirements, converting non-ATM bit streams into ATM cells.


ATM Adaptation Layer 2. Used for carrying both CBR traffic and VBR traffic simultaneously, usually in support of voice over ATM.


ATM Adaptation Layer 5. AAL functions in support of VBR, delay-tolerant connection-oriented traffic requiring minimal sequencing or error detection support.

Access Tandem

A tandem switch that is used to interconnect between carriers for equal access. Typically, this used to interconnect ILECs with IXCs, but now also includes CLECs.

Adaptive Differential Pulse Code Modulation (ADPCM)

A ITU-TS standard technique for voice encoding and compression. It allows analogue traffic to be carried within a 32Kbps digital channel.

Advanced Intelligent Network (AIN)

An evolving, service-independent architecture that allows a carrier to quickly and economically create and modify telecommunication services for its customers.


In the SS7 world, an A-link is a signalling link that connects a STP to a SSP or SCP. A-links operate at a transmission speed of 56 Kbps.

American National Standards Institute (ANSI)

A non-government organization which develops and distributes standards for transmission codes, protocols and high-level languages for suggested use in the U.S.

Applications Programming Interface (API)

Software that an application program uses to request and carry out lower level services performed by the computers or a telephone systems operating system. For Windows, the API also helps applications manage windows, menus, icons and other GUI elements. In short, an API is a "hook' into software. An API is a set of standard software interrupts, calls and data formats that application programs use to initiate contact with network services, mainframe communications programs, telephone equipment or program-to program communications. For example, applications use APIs to call services that transport data across a network. Standardization of APIs at various layers of a communications protocol stack provides a uniform way to write applications. NetBIOS is an early example of a network API. Applications use APIs to call services that transport data across a network.

Asymmetric Digital Subscriber Line (ADSL)

Technology using digital filtering to remove noise from twisted-pair copper lines, enabling broadband transmission. There are several varieties of ADSL using varying hardware, modulation software and compression techniques. ADSL-2 can deliver up to four VCR-quality video signals but has limited upstream response. ADSL can only work over distances of less than 12,000 feet.

Asynchronous Transfer Mode (ATM)

An international standard for high-speed broadband packet-switched networks operating at broadband digital transmission speeds. The technology is based on fixed-length, 53-byte cells. ATM includes protocols that specify how diverse kinds of traffic are transformed into standardized packets whose transport can be managed uniformly within the network.

Automatic Location Identifier (ALI)

A feature of E911 systems that provides information such as name, phone number, address, nearest cross street, to agents answering E911 calls.

Automatic Message Accounting (AMA)

The network functionality that measures, collects, formats, and outputs subscriber network-usage data to upstream billing and other operating systems.

Automatic Number Identification (ANI)

More colloquially called Caller ID. A service provided by local exchange carriers in which the telephone number of a caller is sent to the called-party's telephone between the first and second ring. This is one of several CLASS services, all of which require SS7 interoffice signalling.


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B channel

Message-bearing 64 Kbps digital channel used for digital transmission of high speed data and video.

B-Links, D-Links and B/D Links

Links interconnecting two mated pairs of STPs are referred to as B-links, D-links or B/D links.


Part of a network used to connect smaller segments of networks together.


The relative range of frequencies that can be passed by a transmission medium. Greater bandwidths mean a higher information carrying capacity of the transmission circuit. Usually measured in Hertz, bandwidth is assessed as the number of bits that can be transferred per second.

Basic Rate Interface (BRI)

The ISDN interface standard for single-line ISDN service. This standard provides for two message-bearing 64 Kbps B channels for speech and data, plus a 16 Kbps D channel for network signalling and data.

Billed Number Screening (BNS)

When consumers decide who can and who cannot charge a call to their phone based on an agreement with their local telephone company to screen calls.

Bit Rate

The number of bits transmitted over a telephone line per second.


A term used to describe a channel with more bandwidth than a standards voice grade channel. Broadband channels are used to carry multiple high-speed voice and data transmissions on a common communications path.

Broadband Integrated Services Digital Network (B-ISDN)

An evolving standard for the second generation of integrated services digital networks. Broadband ISDN services employ packet switching to integrate voice and data services over a high-speed, packet-based infrastructure.

Broadband ISDN User’s Part (B-ISUP)

An SS7 protocol defining the signalling messages to control connections and services.


Data transmitted in short, uneven bursts with relatively long, silent intervals between.

Busy Hour

An uninterrupted 60-minute period during which the average volume of telecommunications traffic is at its maximum.

Busy Hour Call Attempts (BHCA)

A measure of dynamic traffic calls that can be attempted in an average Busy Hour.

Busy Hour Call Completion (BHCC)

A measure of dynamic traffic calls that can be completed in an average Busy Hour.

Busy Line Verification (BLV)

A feature that allows an attendant to verify the busy or idle state of telephone lines and to break into the conversation.


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Carrier Access Code (CAC)

The sequence that an end user dials in order to access the carrier's switch service. The codes are composed of 7 digits in the form of 101xxxx, where xxxx is the CAC.

Calling Card Validation (CCV)

Process of verifying that a calling card is valid and then processing a call to be billed to that account.

Call Detail Record (CDR)

A billing feature of a telephone system which allows the system to collect and record information on outgoing and incoming phone calls such as who sends/received them, where they went, what time, how long etc.

Called Party Number (CPN)

When a call is set up over an ISDN network, SS7 send an initial address message which contains the as part of the ISUP protocol.

Circuit Identification Code (CIC)

An SS7 term used to identify a particular circuit within a trunk group.

Circuit Switch

A switching system that establishes a dedicated physical connection between end points, in a network, for the duration of the communication session.

Class 4 Office

A switching centre for toll calls. A class 4 office switches toll traffic originating at class 5 offices to other class 4 offices, or offices of a higher class. In addition, a class 4 office relays toll traffic from class 4 toll offices to the class 5 end office serving the destination address.

Class 5 Office

The lowest level in a hierarchy of central offices. Class 5 offices serve as the network entry point for user access lines and are a switching centre for local calls.


Links that interconnect mated STPs. They are used to enhance the reliability of the signalling network in cases where one or multiple links are unavailable.

Competitive Local Exchange Carrier (CLEC)

These are new local carriers, typically formed after the US Telecommunications Act of 1996, to compete with the incumbent RBOCs.


Reducing the size of the data, image, voice or video file sent over a telephone line, lessening the bandwidth needed to transmit the file.

Computer Telephony Integration (CTI)

The combining of data with voice systems in order to enhance telephone service. Examples include the delivery of Caller ID information via a PC, and the ability to access mail via the PC.

Custom Local Area Signalling Services (CLASS)

Basic local exchange telephone service or enhanced telephony services that utilize the signalling system7 (SS7) channel to carry data about a call. CLASS provides subscribers with the ability to screen and selectively reject, forward, trace, and redial incoming call—Caller ID is one example.

Customer Premise Equipment (CPE)

Equipment which resides on the customer’s premise, such as a PBX or IAD.


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