Cellular Digital Packet Data (CDPD)

CDPD is a specification for overlaying digital data transmissions on the existing circuit-switched analog cellular phone service.

What is CDPD (Cellular Digital Packet Data)?

A specification for overlaying digital data transmissions on the existing circuit-switched analog cellular phone service. This phone service is called the Advanced Mobile Phone Service (AMPS). Cellular Digital Packet Data (CDPD) was developed by IBM (along with a consortium of Regional Bell Operating Companies) and other organizations to leverage the existing installed base of AMPS cellular equipment in the United States to provide low-cost, packet-switched data services. CDPD was first offered in 1994 by Bell Atlantic Mobile.

How CDPD Works

CDPD makes use of idle times between calls in cellular phone network channels for interleaving packets of digital data. In other words, CDPD makes use of the «bursty» nature of typical voice transmission on the AMPS cellular system. Voice communication has gaps or pauses where packet data can be inserted and transmitted without interfering with the communication taking place between customers.

Although CDPD supports data transmission rates of 19.2 Kbps and higher, actual data throughput is usually around 9.6 Kbps. This is because of the large overhead added by CDPD to each data block transmitted. This overhead is designed to ensure that communications are reliable and to maintain synchronization between the modems at each end of the transmission. In addition, a color code is added to every data block to detect interference resulting from transmissions on the same channel from neighboring cell sites.

CDPD uses the Reed-Solomon forward-error-correcting code to encode each block or burst of data sent, and includes built-in RC4 encryption to ensure security and privacy of the transmitted data. CDPD is also based on the industry standard Internet Protocol (IP), allowing data to be transmitted to and from the Internet.

A typical implementation of CDPD consists of three components:

  • Mobile-End System (M-ES):
    A user device such as a laptop equipped with a cellular modem. This system communicates in full-duplex mode with a Mobile Data Base Station (MDBS) using the Digital Sense Multiple Access protocol, which prevents collisions of data streams from multiple Mobile-End Systems.

     

  • Mobile Data Base Station (MDBS):
    A telco device for receiving and transmitting CDPD data.

     

  • Mobile Data Intermediate System (MDIS):
    Provides the central control for a CDPD network.

     

CDPD is typically used to provide wireless access to public packet-switched networks such as the Internet so that mobile users can access their e-mail and other services. Multiple users can share the same channel; the user’s modem determines which packets are destined for the user’s machine. CDPD also supports IP multicasting and is an open standard based on the Open Systems Interconnection (OSI) reference model for networking.