Telephony Glossary

Alerting

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

Analog

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.

Analog Line

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

Asynchronous completion

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.)

B channel

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

Bandwidth

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.

Bearer mode

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").

Bearer services

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

BRI-ISDN

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.

Call

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.

Call progress

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.

Callback function

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.

Called-ID

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.

Caller-ID

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.

Calling card

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.

Central office

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

Client computer

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.

CCITT

The former name of ITU-T.

CO

Central office

D channel

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

Desktop

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

Dial tone

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

Digital line

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.

DTMF

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.

Hookswitch

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.

Hot phone

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.

Inband

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.

ISDN

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.

Key system

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.

Line

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.

Media

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

Media mode

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.

Out of band

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.

PBX

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.

Phone

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.

POTS

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.

PRI-ISDN

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).

PSTN

Public Switched Telephone Network.

Ringback

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

"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.

Station

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.

Switch

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.

Synchronous completion

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.

TAPI DLL

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.

Voice

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.

Windows Telephony API

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.

Windows Telephony application

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.

Windows Telephony service provider

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.

Windows Telephony SPI

The Windows Telephony SPI (Service Provider Interface) is the interface that a service provider must implement to make its telephony services available to applications through the API. The SPI is a collection of C-language function definitions, message definitions, type and data-structure definitions, along with descriptions of their meanings in English.


Fax Glossary

Coding Methods

Refers to the manner in which fax images are compressed for transmission. The more effective the compression, the smaller the file size-and the faster the transmission. Four types (in ascending order of performance) are: Modified Huffman (MH), Modified Read (MR), Modified Modified Read (MMR), and Joint Binary Image Group (JBIG). To work at its highest level of compression, a machine must communicate with one that uses the same method, otherwise it steps down to a lower level (e.g., a JBIG machine works with MMR level when communicating with an MMR device that doesn't have JBIG).

Confidential Reception

Allows faxes to be received in memory so that they can be printed only when you enter a PIN code.

Delayed Send

Very common feature that enables you to program a machine to transmit a document at a specific time. Allows you to take advantage of cheap phone rates in the middle of the night.

Department Codes

Feature that allows you to allocate outgoing fax calls to different cost centers.

Dual-Access

Productivity-enhancing feature that allows you to do two things on your fax at once. For example, you could be scanning a document into memory while the machine is transmitting or receiving something else.

Fast Scan

With dual-access, machines are typically set up so that all documents are scanned into memory before dialing begins (as opposed to each page being pulled from the document feeder only when its turn has come to transmitted). Scanning all the pages before dialing means that the sender can walk away from the machine quickly, with the document in hand-it saves waiting for the transmission to go through or having to return later to pick up the originals.

Fax/Tel Switches

A large number of plain-paper fax machines-especially toward the lower end of the price spectrum-are designed so that they can share a phone line with regular voice traffic, instead of having one of their own. These machines have a "fax/tel switch," a device whose function is to distinguish automatically between incoming voice and fax calls.

Handshakes

The exchange of information that occurs at the start of each fax transmission, between each page, and at the end. Characterized by a series of high-pitched squeals if you're listening in on the line (lasting for about 10-11 seconds for the opening handshake in the case of 14,400bps machines and about three seconds for the between-page and closing ones).Handshake duration is never included in venders' quoted fax speeds -hence, if a vendor claims six-seconds-per-page speeds (based on the Slerexe page), this excludes all the handshakes that occur during billable phone time. Keep in mind that on shorter transmissions, the handshakes can take longer than the actual pages!

Image Smoothing

A feature found on a large number of laser-class faxes that enables a receiving machine to enhance the defects of incoming transmissions by smoothing lines and edges and removing some of the fuzziness.

Ink Jet

Imaging technology found on some low-end machines-mostly multifunctional ones aimed at small-office/home-office buyers. The main drawback with ink jet machines is that black-and-white PC print quality is not as good as laser at its best. v

ITU Test Chart

Industry-standard page used for the purposes of quoting transmission times and memory storage (both of which are image-density-sensitive). Known formally as the "ITU Test Chart #1" and also referred to as the "Slerexe page" (because the page is actually a short letter printed on the stationery of a fictitious entity called The Slerexe Company).

Laser

Premium imaging technology, but also the most widely used in the business world - the same as that used in laser printers.

LED

A variation of laser technology.

Memory

All plain-paper fax machines offer document memory. This is needed for functions such as dual-access, broadcasting, and out-of-paper reception. But the usefulness of most memory features is determined by the memory size. This is usually measured in the number of pages (the fax industry has come up with an agreed definition of how much is on a page to ensure that apples are compared to apples). The "official" fax industry page contains less actual image than is typically found on business documents. And because denser images take up more memory, the number of pages your fax machine will hold in reality will probably be somewhat less than the manufacturer-quoted figure.

Modem Speed

The speed at which fax machines can send and receive is determined by their modem. Industry standards restrict today's fax machines to one of three speeds - 9,600bps, 14,400bps, and 33,600bps.

Out-Of-Paper-Reception

If your machine runs out of paper and no one in your office notices, an incoming fax will go into memory. All laser-class machines have this feature, but the usefulness depends on the number of pages the memory will hold.

Paper Supplies/Sizes

With dual trays, you can keep letter-size paper in one tray, and legal paper in the other; the machine will automatically select from the paper source appropriate to the page size of the incoming fax. If you only have letter-size paper in your fax and you receive a document consisting of legal-size pages, one of two things can happen depending on the machine and how it's set up: either the incoming pages are automatically reduced to fit onto the smaller paper; or the bottom part may print onto a second letter-size sheet, with no reduction taking place.

Parallel Ports

Generally the best means of connecting a fax machine to a PC so that is can serve as a multifunctional device. A better choice than the serial-port options offer on some models.

Platen

The flatbed glass on some multifunctional machines on which you place originals for copying, faxing, or scanning. Without a platen, you can only work with originals that are suitable for going through the document feeder.

Print Engine Resolution

A factor only if you plan to use your fax machine as a PC printer. Only machines with "native" (or "true") resolutions of 600dpi match the specifications that are nowadays standard in the lower end of the single-function laser printer market. Machines with 200-400dpi resolutions are less than ideal.

Programmable Keys

Enable your fax to "memorize" certain complex instructions so that they can be executed by pressing just one button.

Reception Collation

Machines with this capability are able to receive entire fax transmissions to memory before printing them out in correct page order. This enables you to receive properly collated sets - even if the machine prints out pages face-up. Keep in mind that face-up printing without reception collation means that page one comes out first, page two lands on top, also face-up - and you end up with finished document with all the pages in reverse order.

Resolution

Electronic images - whether they are being captured, stored, or printed - are formed out of tiny dots. The more dots, the higher the resolution and the better the image quality.

Serial Ports

Enables you to connect a fax machine to a PC so that it can serve as a multifunctional device. However, you get better performance from machines that offer parallel-port connections.

Speed Dialing

Just like the feature you get on regular phones: it enables you to store frequently faxed numbers in memory, so that they can be dialed automatically at the press of one or two (or possibly more) keys. The best type is the one-touch variety, which gives each number its own dedicated key.

Thermal Transfer

A type of plain-paper fax technology - not to be confused with thermal paper machines. Uses thermal process to create images with heat-sensitive ribbons. Chiefly found only at the low end of the retail-channel market; main drawback is that image quality is not great unless you use special, smooth type of "plain" paper.

Transmission Confirmation Reports

All machines are capable of generating reports confirming the details of fax transmissions and whether the outcome was successful. The problem with many reports is quickly matching the detail in the print-out to the specific document that was sent. Unless you remember everyone's fax number and the time documents were faxes, the information can seem hard to relate to. A handy solution, offered on many laser-class machines, is a feature that prints a reduced-size image of the first page of every transmission the confirmation report itself (or a full-size reproduction of the top portion of the page).