There are so many acronyms for telephone services and links between data communications and telecommunications that it's hard to keep track of what's being offered. And with the many services available, it's that much harder to make knowledgeable and sensible recommendations to your customers. Let's take a look at some of the most popular links and services, with emphasis on the acronym and its definition.

ADSL (Asymmetric Digital Subscriber Line): A method of carrying high-speed traffic over existing copper twisted-pair wires. ADSL offers three channels: a high-speed (between 1.5 Mbps and 6.1 Mbps) down link from the carrier to the customer; a full-duplex data channel at 576Kbps; and a plain old telephone service (POTS) channel. A key feature of ADSL is that POTS is available even if the extra ADSL services fail.

ANI (Automatic Number Identification): A system that identifies the telephone number of the calling party for the call recipient. Most consumers know it as "Caller ID." When using a T-1 line, the ANI information also includes the geographic coordinates of the originating call's central office.

ATM (Asynchronous Transfer Mode). Using the same 53-kB packets as SMDS, ATM uses virtual circuits to transfer data at speeds of 34 Mbps to multiple gigabits per second.

DS-0 (A Digital Signal Level Zero): One of the 64,000-bps circuits in a T-1 or E-1 line. It consists of 8-bit frames transmitted at 8000 frames per second. The usable bit rate is often only 56,000 bps. DS-1, or Digital Signal Level One, is often used as a synonym for T-1, but it more precisely refers to the signaling and framing specifications of a T-1 line. See T-1.

DTMF (Dual-Tone Multi-Frequency): A description of the audio bleeps you hear when you dial a touch-tone telephone. Each row and each column of keys is assigned a separate frequency. A combination of row and column frequencies produces each key's frequency. For example, the second column's assigned fre quency is 1336 Hz, and the third row is 852 Hz. Pressing the number "8" generates both those tones. By decoding the two frequencies, the telephone company's central office, your PBX, or an interactive voice response system can detect which button was pressed. ("Touch-Tone" is AT&T's trademark for DTMF.)

Frame relay: This refers to a shared-bandwidth wide area network (WAN) based on a subset of High-level Data-link Control (HDLC) called LAP-D (link access procedure-D channel). It's designed to be carried over high-speed, high-accuracy links such as T-1 or the still emerging T-3. A 56-kbps line is the most common implementation. Individual frames can vary in size, but they're usually 4096 bytes. Users reserve a specific data rate called the CIR (committed information rate), but they can attempt to burst data at higher rates. Extra frames are discarded if the carrier's network doesn't have sufficient capacity. HDSL (High Bit-Rate Digital Subscriber Line): This circuit offers a full-duplex 784-kbps connection over two twisted pairs. HDSL can carry either a full T-1 connection (over the two twisted pairs) or a fractional T-1 connection over a single twisted pair of wires.

ISDN (Integrated Services Digital Network): A telephone system service that provides access to both the public-switched telephone network and packet-switched services (such as X.25 and frame relay). ISDN offers two types of channels: B (bearer), which are 64-kbps voice channels, and D (delta), which are channels for setup, coordination, and control. Telephone companies offer ISDN in two main varieties: basic rate interface (BRI), which contains two B channels and an 8-kbps D channel, and primary rate interface (PRI), which has 23 B channels and a 64-kbps D channel.

IVR (Interactive Voice Response): The basic voice-mail system that can decode DTMF signals, which is how your call can be routed without the aid of a real human being.

PBX (Private Branch Exchange): A telephone switch used within a business or other enterprise, as opposed to the switches used at a public telecommunication service provider's central office (CO). PBXs might offer basic telephone service, some level of computer-telephony integration (CTI), voice mail, or other features. When you dial within your company, the PBX provides the dial tone; when you dial 8 or 9 for an outside line, the CO provides the dial tone.

SMDS (Switched Multi-megabit Data Services): Using a connectionless networking plan, each SMDS packet has its own address and does not require a virtual circuit. Proposed speeds are from 1.5 Mbps to 45 Mbps using a fixed-length packet of 53 KB. Many regional carriers are beginning to offer this service for local traffic.

Switched 56: A service that creates a virtual network over existing public phone lines with a 56-kbps data rate. It is slow but cheap, and it is ideal for intermittent data swapping between wide area networks (WANs). Unlike ISDN, most carriers already offer this service.

T-1: A North American standard for point-to-point digital circuits over two twisted pairs. A T-1 line carries twenty-four 64,000-bps channels (also known as DS-0) for a total usable bit rate of 1,536,000 bps. (If you include extra bits used to synchronize the frames, the actual bandwidth is 1,544,000 bps.) Customers may lease a fractional T-1, using only some of the 24 T-1 slots. A T-1C contains two T-1 lines; T-2 supports four T-1 circuits. A T-3 communications circuit supports 28 T-1 circuits, and a T-4 consists of 168 T-1 circuits. E-1 through E-5 are similar standards used in Europe and Japan, but offer different numbers of channels. See DS-0.

TAPI (Telephony Application Programming Interface): A method promoted by Microsoft and Intel for letting PCs control telephones. TAPI applications can dial a telephone from within software and check caller ID, as well as perform other functions. TAPI links PC workstations and individual telephones, as opposed to TSAPI (see below), which links the PC server to a PBX (Private Branch Exchange).

TSAPI (Telephony Services Application Programming Interface): A system promoted by Novell and AT&T to integrate PCs and PBX servers. TSAPI allows computer control of most aspects of the local telephone system. Contrast this to TAPI, which links the PC and the local telephone sitting on the user's desk. X.25 A standard for packet-based wide area networks (WANs). For both leased lines (such as T-1s) and public-switched connections (like ISDN), you pay by the minute or month. However, an X.25 connection has the advantage of being measured and billed by the number of packets or bytes actually sent or received.