There are so many acronyms for telephone services and links between data communications and telecommunications, it's hard to keep track of what's available. And with so many services offered, it's that much harder to make knowledgeable and sensible recommendations to your customers.

Acronyms and techie jargon are some of the worst things about the communications business. Some came from telephone companies, some from the computer world, and some started out as advertising trade names. Keeping track of them all is extremely difficult.

Here are some of the most popular links and services, with an emphasis on the acronym and its definition. Hopefully, you will remember some of them, but don't be afraid to ask people to identify the terms they are using if you don't know them. It's better to admit your ignorance and learn what they are talking about than to present a false "know-it-all" front.

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 1.5Mbps and 6.1Mbps) downlink 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: 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 this as "Caller ID." When using a T1 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 34Mbps to multiple gigabits per second.

DS-0 (A Digital Signal Level Zero): One of the 64,000-bps circuits in a T1 or E1 line. It consists of 8-bit frames transmitted at 8000 frames per second. The usable bit rate is often only 56,000bps. DS1, or Digital Signal Level One, is often used as a synonym for T1, but it more precisely refers to the signaling and framing specifications of a T1 line.

DTMF (Dual-Tone Multi-Frequency): A description of the audio bleeps you hear when you dial a touch-tone telephone. Each row and column of keys has a separate frequency. A combination of row and column frequencies produces each key's frequency. For example, the second column's assigned frequency is 1336 Hz, and the third row is 852 Hz. Pressing the number "7" generates both tones. By decoding the two frequencies, the telephone company's central office, your PBX, or an interactive voice response system can detect which button you 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). A frame relay is designed to travel over high-speed, high-accuracy links such as T1 or the still-emerging T3 line. A 56-kbps line is the most common implementation. Individual frames can vary in size, but they're usually 4096 bytes. Additionally, users reserve a specific data rate called the CIR (committed information rate), but they can attempt to burst data at higher rates. If the carrier's network doesn't have sufficient capacity, the system discards extra frames.

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 T1 connection (over the two twisted pairs) or a fractional T1 connection over a single twisted pair of wires.

ISDN (Integrated Services Digital Network): A telephone system service providing 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), containing two B channels and an 8-kbps D channel; and primary rate interface (PRI), having 23 B channels and a 64-kbps D channel.

IVR (Interactive Voice Response): The basic voice-mail system that can decode DTMF signals. This is how the system routes your call without the aid of a 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 1.5Mbps to 45Mbps using a fixed-length packet of 53KB. 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's slow but cheap, and is ideal for intermittent data swapping between WANs. Unlike ISDN, most carriers already offer this service.

T1: A North American standard for point-to-point digital circuits over two twisted pairs. A T1 line carries twenty-four 64,000-bps channels (also known as DS0) 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 1,544,000bps.)

Customers may lease a fractional T1, using only some of the 24 T1 slots. A T1C contains two T1 lines; T2 supports four T1 circuits. A T3 communications circuit supports 28 T1 circuits, and a T4 consists of 168 T1 circuits. E1 through E5 are similar standards used in Europe and Japan, but offer different numbers of channels.

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 definition 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 local telephone sitting on the user's desk.

X.25: A standard for packet-based WANs. For both leased lines (such as T1s) 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.