Interactive Buyer's Guide: ISDN

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ISDN

How to Buy ...
An ISDN Terminal Adapter

Service

ISDN is a new and unfamiliar world for many users, so a number of modem companies have set up prepurchase support lines or Internet sites with the information you need to get started. These support services provide help and advice throughout the installation process and may even hook you up with the correct phone service provider.

Price

ISDN terminal adapters are becoming more affordable every day. Expect to pay between $200 and $400 for a basic ISDN TA with a single analog port. Products with enhanced features, such as automatic switching between ISDN and analog connections, can add as much as $300.

Internal or External

Internal devices, which hook into a free ISA slot, allow you to avoid the speed limitations of your communications ports. External units, which hook into the serial port, cost about $50 more and sometimes include extras such as diagnostic LEDs. You may prefer an external terminal adapter if you will be switching between computers.

Drivers

Most terminal adapters provide Windows 95 drivers. If you need Windows 3.x or NT drivers, make sure the model you're considering offers them.

When people talk about buying an "ISDN modem," what they really mean is an ISDN terminal adapter. Why go ISDN? One reason is to take advantage of its speed for dialing into the Web. ISDN allows your uncompressed digital data to run over twisted-pair copper wires (standard phone lines) at up to 128Kb per second (two 64Kbps B channels combined).

One difference among ISDN TAs is the way they handle received signals. For instance, some terminal adapters will add or drop one of the two B channels as needed, allowing you to cut your bandwidth usage and lower your service charges--a feature known as dynamic bandwidth allocation. Multilink PPP support means that the product can combine both B channels for faster data-transfer rates. If you need simultaneous voice and data, make sure the terminal adapter you are purchasing supports that feature. You can also buy an ISDN terminal adapter with a built-in analog modem combined with it. A unit of this type will automatically "fall back" when it encounters an analog line.

An ISDN terminal adapter will only work if you can get an ISDN line installed at your site. See the next page to find out some things you need to know when you talk to your local phone company.

Get Connected

ISDN service is available in 85 to 90 percent of the United States; areas that don't get ISDN are mostly rural. Standard BRI ISDN service (also called C, Generic Data M, S or Standard) gives you two B channels for data and one D channel to control the initiation and disconnection of calls (called 2B+D), each capable of voice or data transmission.

You will definitely pay more for ISDN than for your standard phone line. How much more depends on your location and your phone service provider. Some plans provide flat-rate, unlimited service for $45 to $80 per month, while others give a specified number of hours for a designated rate. Sometimes flat-rate services are for home users only, so that during business hours you have to pay a per- minute charge as well. ISDN calls are often billed from the moment of call initiation rather than the moment of connection. Charges per minute can range from 2 to 8 cents per channel (an important caveat since ISDN is a multiple-channel technology). You should also figure in installation and activation charges, anywhere from $100 to $250.

Sites to Be Seen