Cellular digital packet data
Cellular Digital Packet Data (CDPD) is an obsolete wide-area mobile data service which used unused bandwidth normally used by Advanced Mobile Phone System (AMPS) mobile phones between 800 and 900 MHz to transfer data. Speeds up to 19.2 kbit/s were possible, though real world speeds seldom reached higher than 9.6 kbit/s.
Source: Wikipedia — Cellular digital packet data (CC BY-SA 4.0)