CDPD 
Cellular Digital Packet Data 
A digital wireless data-transmission system that uses the available channels of an existing analog cellular network to deliver packet data. CDPD can send data at rates as high as 19.2Kbps over unused intervals in voice channels. If all the channels are in use, the data is stored and forwarded when a channel becomes available. CDPD modems are available as PC Cards for notebooks and PDAs.