CDPD works over the existing cellular network but adds a few twists. In a conventional cellular call, your modem connects to the nearest cellular antenna and stays connected to the network until you hang up. This scheme is known as a circuit-switched connection. CDPD, on the other hand, is a packet-switched technology. That means that you broadcast data in packets rather than owning a particular frequency at a given time.
Packet switching has two advantages.
Because you're not hogging a frequency, you're only paying for the packets you send and receive, rather than paying for a modem circuit that you're not using all the time. Costs vary by provider, but plans with a high monthly minimum get as low as 4 cents per kilobyte. Occasional users may opt for the low monthly rates but pay as much as 15 cents per kilobyte. Sending packets of data only when necessary also helps conserve battery life-a precious commodity in many mobile scenarios. Packet switching is only part of the CDPD picture.