Cellular Networking with CDPD
By Brian Jepson
Supporting a mobile work force using dialup typically involves pricey toll-free numbers, substandard hotel telephone lines, and employees who are online only part of the day. If you're fed up, and looking for an alternative to traditional modem connectivity for your organization's road warriors, Cellular Digital Packet Data (CDPD) might be a welcome solution.
CDPD is a wireless protocol that rides on top of existing cellular networks. It's inexpensive, but very slow. Yet, if you learn how to work around its idiosyncrasies, you can save money and keep your employees online around the clock.
Serial CDPD modems let any device or computer that supports PPP over a serial connection use the protocol. Modems are also available as PCMCIA cards for laptops, and some manufacturers are even producing models in the CompactFlash form factor, for use with handheld devices. The value of CDPD for handhelds is limited, however, as their underpowered processors aren't well equipped for compression software like Venturi (more on this later).
Roaming the Country
One drawback of CDPD is that data users can only access idle portions of the cellular network—those channels of bandwidth that aren't being used for voice calls. Each modem includes a built-in PPP server, however, so the user appears to experience uninterrupted (albeit slow) service, even when the modem is waiting for a free channel.
In my own experience, I've seldom lost out to roving bands of cell phone users; somehow my CDPD modem always finds a channel to use.
