C
CDPD
A wireless standard that provides two-way, 19.2 kbps packet data transmission over existing cellular telephone channels. A method proposed (1993) and developed by IBM and McCaw Cellular Communications, Inc. (now owned by AT&T) to more efficiently carry data on existing analog (AMPS) cellular radio systems. 138-byte packets of data are sent at 19,200 bits/s during gaps in conversations or on unused (no voice conversation established at that time) channels, using the full 30-kHz bandwidth of the channel. Voice always has priority. Actual air traffic consists of blocks of 63 (47 are information, 16 are forward error correction information) six-bit symbols, resulting in a user data rate of about 9,000 to 14,400 bits/s. The forward error correction can correct up to eight six-bit symbol errors. Advantages over Ardis and Mobitex include the following: use of the existing cellular radio infrastructure (CDPD overlays it), resulting in lower usage charges; built-in encryption and authentication; the land-line interface is TCP/IP; security, since the data for a conversation are carried over many cellular radio channels (according to whichever has spare capacity), so it would be difficult to monitor the communication; V.42bis data compression; multicasting (to subsets of users); and a full-duplex option. Will be an open specification that will compete with the proprietary systems from Ardis and Mobitex (RAM). Is a packet-oriented service, so the call setup time is fast (much faster than circuit-switched), charging is by the kilobyte of traffic carried, and it is best-suited to smaller transactions (up to 5 Kbytes of data--larger transfers are better handled by circuit-switched methods, such as analog cellular with modems). Promoted by five of the seven U.S. RBOCs and Motorola, Microcom, and some cable TV companies.