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Nathan J. Muller
In the wireless data market, major telephone companies are implementing CDPD service to meet the needs of a mobile work force. CDPD is an appealing method of transporting data over cellular voice networks because it is flexible, fast, available internationally, compatible with a vast installed base of computers, and has security features not found in other cellular systems.
CDPD is a data-over-cellular standard for providing LAN-like service over cellular voice networks. CDPD employs digital modulation and signal processing techniques, but it is still an analog transmission. The CDPD infrastructure employs existing cellular systems to access a backbone router network that uses the IP to transport user data. Personal digital assistants, palmtops, and laptops running applications that use IP can connect to the CDPD service and gain access to other mobile computer users or to corporate computing resources that rely on wireline connections.
Because CDPD leverages the existing $20 billion investment in the cellular infrastructure, carriers can economically support data applications and avoid the cost of implementing a completely new network, as most competing technologies would require. CDPD also offers a transmission rate that is four times faster than most competing wide area wireless services, which are limited to 4.8K b/s or lower.
Unlike circuit-switched schemes, which use dialup modems to access the cellular network, CDPD is a packet-switched technology that relies on wireless modems to send data at a raw speed of 19.2K b/s. 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 CDPDs faster and more reliable data transmission.
CDPD is in fact a blend of digital data transmission, radio technology, packetization, channel hopping, and packet switching. This technology lets the cellular network carry the 1s and 0s of binary digital code more reliably than is usually possible over cellular voice networks.
Digital Transmission Technology. Digital transmission technology is reliable and more resistant to radio interference than analog transmission technology. The digital signals are broken down into a finite set of bits, rather than transmitted in a continuous waveform. When signal corruption occurs, error-detection logic at the receiving end can reconstruct the corrupted digital signal using error correction algorithms. Digital technology also enables processing techniques that compensate for signal fades without requiring any increase in power.
Digital Cellular Radio Technology. DCRT is used for transmitting data between the users mobile unit and the carriers base station.
Packetization. Packetization divides the data into discrete packets of information before transmission. This approach is commonly used in wide area and local computer networks. In addition to addressing information, each packet includes information that allows the data to be reassembled in the proper order at the receiving end and corrected if necessary.
Channel Hopping. 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 may also dedicate voice channels for CDPD traffic.
Packet Switching. Packet switching, using the IP, accepts data packets from multiple users at many different cell sites and routes them to the next appropriate router on the network.
The wireless-industry consortium that funded the development of the CDPD specification includes Ameritech Cellular, Bell Atlantic Mobile, Contel Cellular Inc., GTE Mobilnet, Inc., McCaw Cellular Communications, Inc., NYNEX Mobile Communications, AirTouch (formerly PacTel Cellular), and Southwestern Bell Mobile Systems. Three principles guided their efforts: that emerging CDPD recommendations could be deployed rapidly, economically, and in conjunction with technology already available in the marketplace.
More specifically, the consortiums stated objectives include:
CDPD allows traditional wireline networks to reach a new class of remote user: the roaming mobile client. With the establishment of a wireless link to the cellular carriers CDPD network, remote users can operate their terminals as if they were located on the desktop in a branch office. Mobile workers, for example, can regain much of the productivity they lose while away being from their main offices by using CDPD to send and receive E-mail from computers or personal digital assistants.
Another application example is a debit card. Commuters could purchase a debit card to run through a card-reading device on a bus or another transit system and the fare would be deducted automatically from the cards total. That fare information could be transmitted to a central processing center in less than a second for just a few cents. CDPD could also be used by service providers to monitor and control devices such as traffic lights, alarm systems, kiosks, vending machines, and automated teller machines.
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