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Hitting a Moving Target As a mobile user, you may be looking for more than just a high-performance modem. PC Card modems offer plenty of alternative communication methods not found in traditional modem s. For instance, you may want to dial in through a digital telephone line in your hotel room or office, or you may be in your car and wish to connect through your cellular phone.

Analog modems cannot be used over digital phone lines without a clunky digital-to-analog converter. Ositech Communications has developed the first analog modem with the ability to interface with digital phones. Ositech also has worked to increase usability of the modem by adding a user interface that provides on-screen step-by-step instructions for setting up the modem.

Keep an eye out for modems that offer protection from accidental connection to a digital phone line. Without a digital line guard you can accidentally fry your modem with the higher voltage that runs through digital loops.

Additionally, if you are interested in true mobile communication s, there are two cellular protocols that will help you hook up anywhere. Cellular connectivity faces the challenge of incredibly poor and unpredictable "line" conditions caused by multipath distortion, poor frequency response and signal fading, among other complications. Interfacing with the analog cellular network requires a few tricks to keep transfer rates high. Rockwell has taken the Microcom Networking Protocol (MNP) 4 error-correction protocol and altered it to work better with cellular communications by lowering the packet length dependent upon line conditions for cellular communications. Rockwell's new version is called MNP 10. To take advantage of V.34 modulation, Microcom and Rockwell have released an extension of MNP 10, called MNP 10EC. Paradyne's (formerly AT&T Paradyne) Enhanced Throughput Cellular (ETC) builds on top of V.32bis modulation to selectively reject 32-byte frames. Additionally, Paradyne has released ETC2 to take advantage of V.34 modulation. Unlike MNP 10EC, ETC2 does not require errored packets to be retransmitted in the original order. However, because MNP 10EC comes to us from Rockwell and Microcom, it is considered the de facto industry standard. Unfo rtunately, existing cellular protocols support only stationary usage (for several reasons, including signal distortion and the possibility that the user may change cell sites). In both cases you need to have a dial-in modem capable of handling the cellular protocol.

One of the more convenient on-the-road features we've seen in PC Card modems is voicemail. We've tested voicemail in the U.S. Robotics Megahertz XJ4336 and found the voicemail application to be extraordinarily easy to use. The XJ4336 is specifically designed to handle voicemail and must be used with a proprietary application, called Focal Point. To start up the answering service, you simply launch Focal Point; all of your messages are stored in .WAV format. The messages can be reviewed through the application or can be played separately through any .WAV player.

If you are wary of spending a large sum of cash just to use a faster modem in your laptop, several vendors, including Eiger Labs, are selling PC Card adapters that work in your desktop and are relatively inexpensive. We've seen adapters for ISA and PCI architectures that are supported in Windows95 in exactly the same way on your desktop as on your laptop. You can trade the same PC cards out of your laptop and into your desktop with the ease of plug-and-play capabilities.

Christopher Smith is a network consultant working at Syracuse University. He can be reached at chsmith@mailbox.syr.edu.