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WinLab Reviews
Head To Head: Terminal Adapters
Speedy ISDN Made Easy

-- by Joel T. Patz

When it comes to high-speed cruising on the infobahn for smaller businesses and home users, ISDN still leads the way. Unfortunately, ISDN also makes installation and use tougher than they need to be. Now, two companies not widely known for modems step in to show us how it ought to be done.

The 3ComImpact IQ and the Adtran Express XRT have much in common. Both offer two analog jacks in the back for plugging in telephones and fax machines. Since you can't plug standard analog devices into an ISDN connection, these come in handy when you want to use ISDN's second channel for non-data tasks.

Each modem includes a built-in NT-1 interface to convert the two-wire U interface (from the phone network) to the four-wire ST interface required for a single ISDN line. Each supports current ISDN standards, including National ISDN-1 and AT&T 5ESS, as well as the AT command set for placing calls manually.

Both the IQ and the Express XRT connect to standard ISDN Basic Rate Interface services and can deliver up to 230.4Kbps throughput by combining data compression (Stacker LZS) with Multilink PPP. The 3ComImpact IQ uses its own QuickSelect and Async-Sync PPP features, so the modem will automatically detect, select and be converted to the appropriate protocol-PPP or V.120-for connecting to the Internet or your corporate intranet. Adtran's technology, called Fallback, does the same thing.

While the modems can send and receive at rates as high as 230.4Kbps, you probably won't achieve this performance. A 16550 serial port, like the one on my test system, limits modems to connections at 115.2Kbps. You'll need an enhanced-speed serial port to achieve the 230.4Kpbs, and you'll have to configure the modem to this faster speed. (The Express XRT has a switch on the back; the IQ is controlled through software.) The IQ supports ISDN Caller ID; the Express XRT supports that plus your phone company's custom calling features, including Call Waiting and Call Forwarding.

3ComImpact IQ's setup software makes installation the easiest I've seen. Once you enter your ISDN phone numbers, the SPID Wizard and the installation program's Automatic Switch Detect feature work together to determine the line parameters (switch type and SPIDs, for example), configure the device and even place a test call (from one line to the other, then in reverse). In fact, 3Com is so sure you'll be up and running in less than 15 minutes that it offers a 30-day money-back guarantee on the product. The company also has a toll-free telephone number to help you negotiate the process of obtaining an ISDN line.

The package includes a serial cable, U interface cord (for connecting the modem to your wall plug) and an excellent user guide. You'll also find several CD-ROM-based programs with the attractive, lightweight unit, including Qualcomm's Eudora Light electronic mail program, Netscape Navigator and subscription software for popular Internet service providers.

The 3ComImpact IQ uses flash memory for future, user-installable firmware updates. Eight LEDs on the front of the unit indicate power, selftest status, D channel status, B1 and B2 channel activity, and the three familiar indicators: Send Data, Receive Data and Data Terminal Ready.

The Adtran Express XRT is a great choice if you want both analog and digital communications but don't want to expend another IRQ and COM port. On the unit's back is the port that connects to your PC, as well as an additional port to hook up to your existing analog modem. To switch between analog and ISDN modes for an outgoing call, click on the icon stowed in the Windows 95 system tray. The XRT automatically determines if an incoming call is from an analog or ISDN modem and adapts accordingly.

The Express XRT is almost as easy to set up as the IQ, thanks to the Express Configuration Wizard, which asks only for your ISDN phone numbers. Four DIP switches on the rear panel control device settings. There are fewer LEDs than on the IQ: five that flash or are constant amber or green to indicate power, B1 and B2 channel traffic, transmit and receive.

Analog ports on both units worked well. When connected to my Internet service provider both B channels were available, but when the phone rang or I needed to place a call, the system ceded one of the channels to the phone without disconnecting my Web browser. When the phone call ended, both modems automatically reestablished the online connection with both B channels.

If you use your existing analog modem along with the ISDN modem on the same COM port, you'll want to consider the Adtran. If you need an ISDN and analog modem in a single unit, the U.S. Robotics Courier I-Modem on our Recommended List remains the best way to go. But most of us just need an easy-to-use ISDN terminal adapter, and for that the 3ComImpact IQ is definitely the device of choice. It easily earns our Recommended seal.

W Info File

3ComImpact IQ
Price: $349
Pros: Ease of installation
Cons: No analog support
Platforms: 3x, 95, NT
3Com Corp.
800-NET-3Com, 408-764-5000
Circle #620 or visit Winfo Online
WinMag Box Score: 4.5

W Info File

Adtran Express XRT
Price: $469
Pros: External analog modem support
Cons: More complex installation
Platforms: 3x, 95, NT
800-9ADTRAN, 205-971-8000
Circle #621 or visit Winfo Online
WinMag Box Score: 4.0

Copyright (c) 1997 CMP Media Inc.

Windows Magazine, March 1997, page 126.

[ Go to March 1997 Table of Contents ]