Internal ISDN TAs plug into an ISA or PCI slot. Their accompanying driver software suites usually install the devices to appear as "modems" available to the PC operating system's dial-up networking accessory. You can set up phonebook entries that contain the telephone number(s) assigned by your ISP or network Point of Presence (POP) so that placing an ISDN call is just like placing an analog modem call. Once connected, the TA device appears like an Ethernet NIC to applications software. (Under Windows95 or NT, if you want to see the IP address dynamically assigned to your device, open an MS-DOS command window and type WINIPCFG.) Because data is transferred on the PC's I/O bus, transmission speed is limited only by the capacity of the telecommunications circuits. Most of these TA s use BRI service up to 64 Kbps per channel, achieving totals of 128 Kbps or more with compression.

External devices don't take up precious PC slots. Motorola's BitSURFR Pro EZ remains a best-seller, perhaps because it is often bundled in ISDN promotion packages offered by ISPs or telephone companies. The external devices connect to one of the serial ports or, for laptops use PC Card serial communications. But there may lie a bottleneck. Typical PCs equipped with 16550 UART transfer data on a serial port no faster than 115.2 Kbps. You'll need a 16650 UART (230.4 Kbps) or a special high-speed serial card like one that MultiTech Systems offers with its IWayHopper (460.8 Kbps), or Computone offers with its PowerSurfer 921.6 Kbps ISDN Adapter, to be able to drive the ISDN service to its fullest.

A valuable feature on many TAs is the configuration of one or two RJ-11 jacks that permit plugging in a POTS phone or fax m achine. P ick up the telephone handset and one channel is taken out of the ISDN connection. You'll hear a dial tone, and will be able to place a voice call while continuing the network connection at reduced speed over the remaining B channel. This works for outgoing calls only--unless you have your ISDN service appropriately provisioned, and your TA can recognize an incoming call. This is another reason to consider an external TA; it will continue handling incoming and outgoing analog calls even when your PC is turned off.

Modern TA devices that accommodate U (two-wire) connections, which plug directly into the ISDN service, don't require an NT-1--the ISDN termination box that can cost $200 extra. If your ISDN service has to be shared with another application, such as videoconferencing, and you don't want to be swapping plugs frequently, you'll want S/T (four wire) termination on the devices and an NT-1 with sufficient S/T jacks.

Blended Personalities In the early days of dial-up communications, analog m odems talked to analog modems, and ISDN TAs understood only ISDN. An ISDN TA-originated call that connected to an analog modem would report an error condition and disconnect. Many of the newer devices listed in our charts exhibit more intelligence and versatility. For example, 3Com offers the Courier I-Modem with ISDN/ V.Everything. It adapts to almost every device it might encounter. If it fails to establish an ISDN connection, it takes on an analog personality and tries all the known V.xx protocols.

Teles Corp.'s TELES iSTAR is not quite as versatile. An ISDN call to a non-ISDN device will fail and disconnect. However, the driver software that Teles sets up in the Windows95 Dial Up Networking accessory lists two other phantom devices: a V.17 FaxModem, and a V.34 analog modem. If one of these is selected and dialed, driver software takes over and emulates a modem call to a compatible device at the other end.

Making the (Personal) Connection It can be quite confusing to pick your way through t he thicket of pricing, ordering, and getting your ISDN service installed.

The telephone industry has its own alphabet soup for Service Profile Identifiers (SPIDs), provisioning codes and switch configurations. This situation has improved greatly since Bellcore, the regional Bell operating companies and communications equipment manufacturers have developed nationwide ISDN ordering codes: easy-to-communicate letter designations that describe specific sets of desired ISDN features or services. "S," for example, identifies "alternate voice/data" on both B channels--a setup you'll need for a TA that can handle incoming and outgoing voice calls on an analog POTS handset. Most manufacturers identify these configuration codes in their product literature and give ISDN ordering tips on their Web sites.