Telephony Terminology

Notification of the appearance of an incoming call by ringing on a POTS network or in a protocol message on an ISDN network.

Analog transmission (such as POTS signals) consists of sound traveling over lines as variations in an electrical current. Analog signals are very vulnerable to interference and noise on the line. They are also limited to the bandwidth of amplifiers, analog-to-digital converters, and other network equipment.

A standard phone line. Signals on an analog line use a set of standard in-band tones for call progress and DTMF signaling.

An application calls a function in the DLL, which then completes and returns to the application. If it completes immediately, this completion is known as synchronous, but if it is sent off to another system entity and the application goes on to other activities before the function completes (and the system later sends a message to the application announcing the function's completion), that completion is known as asynchronous.

If an application invokes multiple functions that complete asynchronously, their completion (reply) messages will not necessarily be received in the same sequence in which the functions were invoked. (This is the actual asynchronous nature of the function.)

A 64 KBPS channel on an ISDN line that can carry voice or data.

The range of frequencies that a circuit can handle. With POTS, for example, the bandwidth is very narrow. The broader the range of frequencies, the more information the line can handle. The typical POTS circuit has a bandwidth of 3,100 Hz centered between 300 Hz and 3,400 Hz.

The type of coding, or compression, which the telephone network is permitted to perform on the bit stream carried on the bearer channel. In POTS, the bearer mode will always be 3.1 kHz voice. The "speech" bearer mode is the most compressible, "voice" less so, and so on. A data bearer mode implies that the data stream will not be compressed by the network (the connection is "clear channel").

Services designed to transfer information from point A to point B.

A CCITT-defined "Basic-Rate Interface" ISDN connection consisting of two B channels of 64 KBPS each for voice or data, and one D channel of 16 KBPS for control (2B+D). See also PRI-ISDN.

Two or more parties exchanging information (or attempting to exchange information) using telephony equipment. Many of the functions in TAPI and in the Telephony SPI operate on calls.

Setting up a telephone call goes through several phases. Taking the phone offhook returns dial tone to indicate that a number can be dialed. Hearing the dial tone, the user dials the desired number. When the call reaches the destination phone, the caller will either receive a busy indication, indicating the called number is busy, or a ring back indication, indicating the dialed party is being alerted. Call progress is the process of monitoring the progress of a call through the various stages.

In analog networks, audible tones generated by the network provide the call progress indications to the user. Different tones allow the human ear to interpret the progress of the call, and telephony hardware for use on POTS networks is often designed to detect these tones as well. On digital networks (such as PBX or ISDN), the network may send indication messages (normally through a D-channel protocol) to the phone to indicate the status of the call, and the phone may generate most tones locally, driven by those messages.

A callback is a mechanism through which an application is notified of events outside the application. The callback function is a function in the application that is called by Windows to inform the application that a message has arrived for it.

NOTE: A developer who creates a function can choose to make it a callback function. Functions defined in an API have already been defined to be (or not be) callback functions, and must be used accordingly.

An identification (number, name) of the party being called. This identification is of interest when you transfer or forward a call. For example, when an unanswered call is forwarded to a voice messaging system, the called-ID of the original call is used to locate the mailbox of the called party.

An identification (number, name) of the party initiating a call, as displayed to the called party prior to answering the call. A caller-ID may also be either unknown (due to telephone switch limitations), blocked (concealed by the caller), or not yet known, but received later.

To Telephony, a calling card is not just a means for billing calls, it represents a distinct dialing procedure. For example, an applications can use calling-card functions to present a menu of pre-configured calling cards to the user. The user's response determines the proper dialing sequence for the call, which tells TAPI whether to make the call with the default carrier, override the default carrier and use a given calling code, or use another specific dialing sequence.

A public switch, directly connected to a number of telephones in a given geographical area.

A service provided by central offices that provides a virtual PBX to a set of extensions. It offers features such as transfer, conference, and forward within that set of extensions.

The host environment for which the Windows Telephony API is defined. This is the computer running the Windows operating system version 3.1 or higher.

The former name of ITU-T.

central office

A channel on an ISDN line that can carry signaling information and low-speed packet data.

The logical pairing of a user's computer and telephone.

A connection to the switch that allows attempts at establishing calls to be made.

A digital station line on a PBX or digital-key system. Signaling on a digital line uses vendor-specific (proprietary) protocols to exchange messages between the switch and the phone. A digital line typically requires a "matched" phone set.

Dual Tone Multiple Frequency. Pressing a button on the keypad of a touch-tone phone generates a pair of tones of specified frequency and duration. The network or the equipment at the other end of the connection (such as a remote control for a phone answering machine) detects and interprets these tones.

In analog networks, audible tones generated by the network provide the call progress indications to the user. Different tones allow the human ear to interpret the progress of the call. On digital networks (such as PBX or ISDN), the network may send indication messages to the phone to indicate the status of the call, and the phone may generate most tones locally, driven by those messages.

The switch that connects or disconnects the device from the phone line. On a telephone, for example, this is the switch that is automatically activated when a user lifts the receiver from the cradle to get a new dial tone.

A telephone whose connection is configured so that no dialing is required. As soon as the hot phone goes offhook, a destination phone (at a predetermined address) automatically begins to ring.

Transmitted within the data stream. Examples: POTS uses DTMF for inband dialing instructions and tones for inband notification that the remote station is busy or alerting.

Integrated Services Digital Network, a set of standards for a new class of telephone services. ISDN is an entirely digital telephone service that can be installed by the local telephone company to replace the old analog local loop (the connection to the telephone company's nearest central switching office, or CO) with a digital line. Because long distance lines are usually digital already, replacing the local loop with an ISDN line provides "end-to-end" digital service.

For Telephony, ISDN's major significance is its ability to provide multiple channels on a single line.

A switching system in which the phones have multiple buttons that permit the user to directly select incoming or outgoing lines. Key systems can typically support fewer users than PBXs, and their features are more limited.

One or more communication channels (accessed together as a unit) used by the application in performing telephony functions through TAPI and TSPI. A computer may provide its applications with access to multiple lines, and each line may provide different capabilities. It is the choice of the service provider for a line to decide how to model its resources.

Note that a line need not correspond to a physical connection from the switch to the computer. For example, the realization of a line may involve a LAN-based server and a shared control link to the switch.

In essence, a line is any device that implements the line behavior defined by TAPI and by the SPI as the set of functions and messages for lines.

The media is whatever takes place on a line, usually on a 3.1-kHz audio bearer channel.

A call's media mode describes what type of information the call is carrying, such as data or voice. An application can tell what media mode is indicated on an inbound call by examining a field in the call-info record. It can use this information, for example, to route the call to a more appropriate application, such as a data application for an incoming data call.

The information carried on a call -- that is, what actually is transmitted and r?ceived over the line, and can, with the necessary hardware, be read and written by a media stream API.

Transmitted over a separate signaling channel. For example, with the media stream on the B channel, ISDN uses protocol messages on the D channel to indicate call tates such as dialtone, ringback, and busy, and for signaling dialing instructions to the switch.

Private Branch Exchange. A digital switch on the customer's premises that provides switching (including a full set of switching features) for an office or campus. PBXs often use proprietary digital-line protocols, although some are analog based. The user features provided by the different PBX vendors are generally similar.

A device that behaves as a telephone set. This is usually, although not necessarily, the phone already on the user's desk located "next to" the computer.

Phones need not be physically connected to a computer, since other equipment, such as a LAN-based server with appropriate access to the switch may provide this logical connection. Note that TAPI and the Telephony SPI treat the phone and the line as separate devices that can be independently controlled by the application.

In essence, a phone is any device that implements the phone behavior defined by TAPI and by the SPI as the set of functions and messages for phones.

Plain Old Telephone Service. Basic single-line telephone service for the public switched telephone network (PSTN). With some exceptions, POTS only supports making and receiving calls, and POTS lines can handle only one conversation at a time. To use a conventional modem and a telephone at the same time on a POTS system, two lines are needed.

A "Primary-Rate Interface" ISDN connection, which in the U.S., Canada, and Japan consists of 23 64-KBPS B channels and one 64-KBPS D channel (23B+D). In Europe, PRI provides for 30 B channels and two D channels (30B+2D).  

Public Switched Telephone Network.

The tone heard by a calling party when, at the called-party's end, the telephone is ringing or the system is otherwise being alerted of the incoming call.

"Speech" is human speech, a specific type of "voice." Telephone networks treats speech and voice differently, because speech can be modeled and compressed more than voice. Voice is less likely to be compressed than speech (especially on international calls), because compression can interfere with some high-speed voice band data and fax transmissions. In contrast, speech can be understood when re-expanded even after being compressed to 1/18 its original bandwidth.

A peripheral device of the switch, a station is any piece of equipment connected to a switch over a phone line. Examples are telephone sets, fax machines, computers with add-in telephony cards, and answering machines.

Telephone switch. A piece of equipment capable of establishing telephone calls. Within the context of TAPI and TSPI, a switch can be a PBX, a key system, or a central office.

An application calls a functions in the DLL, which then completes and returns to the application. If it complete immediately, this completion is known as synchronous, but if it is sent off to another system entity and the application goes on to other activities before the function completes (and the system later sends a message to the application announcing the function's completion), that completion is known as asynchronous.

The client software module that interacts directly with the Telephony service provider by means of the Telephony SPI. TAPI DLL exports the Windows Telephony API to its clients. Its clients in turn are usually applications, which may be DLLs operating on behalf of applications. 

Anything that can be transferred on a POTS network, namely any signal that fits on a 3.1 kHz-bandwidth channel. Voice can consist of voiceband-modulated data or facsimile signals or human speech. 

The Windows Telephony API (TAPI) defines the interface that applications use to access telephony functions in Windows. The API is a collection of C language function definitions, message definitions, type and data-structure definitions, along with descriptions of their meanings in English.

Any software that uses the Windows Telephony API. The term "application" is used in its broadest sense possible; it need not necessarily be a user-level program, but can also be a dynamic link library (DLL) or system function that uses the API and provides higher level services by means of its interface.

The conglomerate of software code (DLLs, device drivers, firmware) and hardware (add-on hardware, server, phone set, switch, network) that jointly implement the Telephony SPI.