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Circuit Digital Packet Data (CDPD)

Circuit Digital Packet Data (CDPD) is the data communications technique most commonly trialed and deployed in U.S. cellular networks, although it has not taken hold outside U.S. borders. Based on TCP/IP, CDPD operates over existing AMPS networks in the 800 MHz band, although substantial additional carrier investment is required to implement the technology. CDPD is intended to take advantage of the natural idleness in cellular networks in between disconnections and connections, and during the break and make process—even heavily used cellular networks are said to be idle approximately 20% to 30% of the time. CDPD uses these periods of idleness to transmit packetized data at rates up to 19.2 Kbps, using connectionless protocols including either the Internet Protocol (IP) or OSI Connectionless Network Protocol (CLNP) [12-21]and [12-22]. WAN connectivity is through the traditional AMPS cellular networks, as well as through the separate CDPD internetworks.

CDPD modems are frequency-agile, searching for available channels over which to send encrypted 128-byte packets, which contain a destination address, source address, sequence number and payload [12-3]. PCMCIA modems are in development to replace the external modems currently required. The contention method is DSMA/CD (Digital Sense Multiple Access/Collision Detect), which is much like CSMA/CD used in Ethernet LANs. CDPD operates at Layers 1 and 2 of the OSI Reference Model, providing access to Layer 3 protocols IP or ISO Connectionless Network Protocol (CLNP). As a result of its IP-based design, applications such as Netscape and Mosaic can run on top of CDPD in order to provide access to the Internet and World Wide Web.

CDPD was formalized in July 1993 by the Cellular Digital Packet Data consortium, which comprises Ameritech Cellular, Bell Atlantic Mobile (BAM), CONTEL Cellular (now GTE Mobilnet), GTE Mobilnet, McCaw Cellular Communications, NYNEX Mobile Communications, PacTel Cellular (now AirTouch), and Southwestern Bell Mobile Systems. The CDPD Forum, established in June 1994, intends to promote the commercialization of products and services. CDPD service was available in most of the 50 U.S. markets by year end 1995. CDPD is being trialed by all of the companies in the CDPD consortium, although interconnection is not provided (read no roaming).

Third-party application developers have recently announced the availability of a number of CDPD applications, including LAN access, host access, call management and voice processing, credit card verification, check verification, and vehicle tracking and dispatch [12-23]. In a cellular radio application, costs for CDPD include activation charges, which can be in the range of $45; monthly charges in the range of $15 to $125; and usage charges currently based on some combination of minutes of usage, packets and kilobytes, whether calls are incoming or outgoing [12-24], [12-25], and [12-26].

Wireless Data Networks: Packet Radio

Data-specific wireless networks have been deployed in the United States during the last several years, with coverage of virtually all major metropolitan areas using the 800 to 900 MHz radio bands [12-3]. Such networks offer data communications to data-intensive users using proprietary packet protocols; integrated messaging service is supported, as well. Applications include sales agents, maintenance and truck fleets. Providers include RAM Mobile Data, Ardis, and Metricom—these companies face competition from Destineer Corp., Nextel Communications and Geotek Communications. Charges generally include a setup charge and a cost per packet, which can vary by packet size. Package plans are also available.

RAM Mobile Data (NY) was formed in 1988 by RAM Broadcasting Corporation; BellSouth acquired a 49% stake in 1992. RAM operates its nationwide Mobitex service using 200 channels in the 900 MHz band, each channel offering throughput of 8 Kbps. At year end 1994, RAM claimed to have a presence in 216 metropolitan areas and 7,500 U.S. cities, covering 90% of the population [12-27]. RAM cities are interconnected through the RAM packet data backbone, with planned WAN access through X.25 and other options. Major users include Kodak and Unisys [12-3].

ARDIS (IL), formerly known as Advanced Radio Data Information Services, was formed in 1990 as a wholly-owned subsidiary of Motorola, merging the IBM Network and Motorola Data Radio Network. ARDIS builds on the foundation of a network originally designed for IBM in 1983. ARDISnet operates over 600 channels in the 800 MHz band and offers throughput from 4.8 Kbps (nationwide) to 19.2 Kbps in selected cities. While the 4.8 Kbps service makes use of a proprietary protocol (Motorola Data Protocol or MDP), 19.2 Kbps service is based on RD-LAP (Radio Data-Link Access Protocol). ARDIS claims to have reached over 10,700 cities in 400 U.S. metropolitan areas as of year end 1994 [12-27]. WAN access is provided through PDNs (Public Data Networks), as well as the ARDIS packet data backbone network. CSX Transportation, a railroad operator, is a major user [12-3].

Metricom offers Ricochet, which provides data transmission at 77 Kbps in the unlicensed 902 to 928 MHz band using spread spectrum technology. Ricochet uses a mesh of low-power, microcellular radio sites in the San Francisco Bay Area, Seattle, Boston, Houston and other major cities. WAN connections are provided through asynchronous modems or via The Internet.


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