CDPD: Here and Now

Almost every cellular carrier plans to convert its networks to digital technology in the coming decade. Various digital cellular voice technologies -- time division multiple access (TDMA), code division multiple access (CDMA), Global System for Mobile Communications (GSM), and Personal Communications Services (PCS) -- are vying to replace Advanced Mobile Phone System (AMPS) as the U.S. standard for voice. Time will tell which one prevails. But for now, those who need digital wireless voice and data communications that are widespr ead, reasonably priced, and reliable can use the AMPS/CDPD (Cellular Digital Packet Data) combination.

The analog technology, CDPD, was standardized in 1993 and can transmit data at 19.2 Kbps. It uses the existing cellular infrastructure and works in the 800- to 900-MHz range of the radio spectrum. According to the CDPD Forum, CDPD is deployed in 151 domestic markets and over 30 international markets. It makes up over 50 percent of the U.S. market. Both the Mitsubishi and Samsung smart phones use CDPD 1.1 and overlay CDPD (used for data communications) onto AMPS (used for voice communications).

CDPD itself is a layered architecture that can be sandwiched between many different types of voice technologies, such as CDMA and TDMA. In fact, CDPD doesn't care which technology it is overlaid onto. Both CDMA and TDMA are currently being developed to work in conjunction with CDPD. However, specifications have only recently been set.

CDPD's advantages include the abi lity to use TCP/IP, which makes any CDPD modem a true IP node. The Mitsubishi and Samsung phones I tested have the ability to connect to a laptop (via a serial cable) and act as a packet modem (and in the Mitsubishi phone's case, a circuit-switched modem as well). There's also a broad range of development tools for CDPD. Unwired Planet's UP.Link Internet-access software uses an open language -- Handheld Device Markup Language (HDML) 2.0 -- to develop applications.