One of the oldest services for sending data over a cellular communications network is known as “Cellular Digital Packet Data” (CDPD), which provides a way of passing Internet Protocol (IP) data packets over analog cellular voice networks at speeds of up to 19.2 kbps. Although CDPD employs digital modulation and signal processing techniques, the underlying service is still analog.
The medium used to transport data consists of the idle radio channels typically used for Advanced Mobile Phone System (AMPS) cellular service. Channel hopping automatically searches out idle channel times between cellular voice calls. Packets of data select available cellular channels and go out in short bursts without interfering with voice communications.
Alternatively, cellular carriers also may dedicate voice channels for CDPD traffic to meet high traffic demand. This situation is common in dense urban environments where cellular traffic is heaviest. Once the user logs onto the network, the connection stays in place to send or receive data. In accordance with the IP, the data are packaged into discrete packets of information for transmission over the CDPD network, which consists of routers and digital radios installed in current cell sites.
In addition to addressing information, each IP packet includes information that allows the data to be reassembled in the proper order via the Transmission Control Protocol (TCP) at the receiving end. The transmissions are encrypted over the air link for security purposes. Although CDPD piggybacks on top of the cellular voice infrastructure, it does not suffer from the 3-kHz limit on voice transmissions.
Instead, it uses the entire 30-kHz RF channel during idle times between voice calls. Using the entire channel contributes to CDPD’s faster data transmission rate. Forward error correction ensures a high level of wireless communications accuracy. With encryption and authentication procedures built into the specification, CDPD offers more robust security than any other native wireless data transmission method.
As with wireline networks, CDPD users also can customize their own end-toend security. To take advantage of CDPD, the user must have an integrated mobile device that operates as a fully functional cellular phone and Internet appliance. For example, the AT&T PocketNet Phone contains both a circuit-switched cellular modem and a CDPD modem to provide users with fast and convenient access to two-way wireless messaging services and Internet information.
GTE provides a similar service through its Wireless Data Services. Both companies have negotiated intercarrier agreements that enable their customers to enjoy seamless CDPD service in virtually all markets across the country. AT&T’s Wireless IP service, for example, is available in 3000 cities in the United States.
Among the applications for CDPD are access to the Internet for e-mail and to retrieve certain Web-based content. AT&T PocketNet Phone users, for example, have access to two-way messaging, airline flight information, financial information, show times, restaurant reviews, and door-todoor travel directions. AT&T provides unlimited access to featured sites on the wireless Internet, which means that there are no per-minute charges for surfing wireless Web sites.
Companies also can use CDPD to monitor alarms remotely, send/receive faxes, verify credit cards, and dispatch vehicles. Although CDPD services might prove too expensive for heavy database access, the use of intelligent agents can cut costs by minimizing connection time. Intelligent agents gather requested information and report back only the results the next time the user logs onto the network.
Wireless IP is an appealing method of transporting data over cellular voice networks because it is flexible, fast, widely available, and compatible with a vast installed base of computers and has security features not offered with other wireless data services. One caveat: The carrier’s wireless data network is different from its wireless voice network. Therefore, users of AT&T Digital PocketNet service, for example, will not be able to access that service everywhere voice calls can be made. It is important to look at coverage maps and compare service plans before subscribing to this type of service.
