Voice-Data Integration | Voice Communications

As much as cellular telephones are a useful tool for mobile voice communications, they are also becoming indispensable for those requiring portable data communications capabilities. Cellular networks are used for the transmission of fax traffic, electronic mail, remote order entry and inquiries, file transfers, and most data communications applications for which the wired telephone network is used. Remote metering locations for pipelines, electrical substations, and other unattended locations that may be far from the nearest telephone lines rely on cellular equipment to provide a connection.

Cellular phones and the networks through which they communicate were not originally designed for data communications purposes, and, until recently, adapters were the only means by which a conventional data modem could interconnect with the cellular network. Even then, not all cellular phones were capable of connecting to a modem. To provide cellular connectivity, a cellular phone must have an outlet into which the user can insert a cable that connects the phone to a modem, the other end of which is plugged into a portable computer. The phone must also support special signaling features that allow it to communicate with the modem. In a properly designed cellular modem, the modem automatically reconfigures itself for cellular operation, enabling the user to send e-mail and faxes, or access on-line service providers like CompuServe or America Online, or "surf the Internet" with the portable computer.

Fortunately, a great number of telephones now incorporate data communications interface capabilities as part of the cell phone unit. Cellular phones do not present the typical dial tone and electrical characteristics to a modem as does a standard telephone line. In addition, while the process of a network hand-off from one cell site to another can be quite acceptable for carrying voice traffic, it can effectively terminate any data communications session in progress. Adapter units compensate for this when used in conjunction with telephones that are not inherently data capable, and data-ready phones do not require these adapters. The adapter units or special cellular-capable modems require that the remote end of the link between the phone and the MTSO also has a device that can communicate using the same cellular-capable protocol. These devices can be provided by the carrier in a pooled configuration within the MTSO available to all users, or the user can ensure that a proper unit is installed at the remote computer location to which the cellular phone is attempting to communicate.

Modems are available that allow the remote user to utilize them for both cellular and "landlines" (wired phone lines). The popular units for users of current generation laptop computers are the PCMCIA card modems that take far less space than a conventional modem or than early versions of cellular modems and also allow the computer to utilize standard phone lines when they are available. The cellular network is not as capable of carrying high-speed data communications as is the wired network, but speeds of near 9.6 Kbps and 14.4 Kbps are possible for data and fax traffic, respectively. Cellular digital packet data, or CDPD, transmission, which is currently being implemented in a number of areas, promises to provide more reliable data communications via existing cellular networks, and at slightly higher throughput rates (to 19.2 Kbps), although not at speeds equivalent to that of landlines. CDPD is appropriate for most applications that might also use conventional packet networks, such as for routine short duration use by individuals and bursty transaction processing.

Several manufacturers have included a data communications capability in their cellular phones. They not only support e-mail, pages, and fax alerts, but provide access to the Internet as well. AT&T Wireless Services, for example, offers an integrated mobile device that operates as a fully functional cellular phone and Internet appliance. The AT&T PocketNet Phone contains both a cellular circuit-switched modem and a CDPD modem to provide users with fast and convenient access to Internet information and two-way wireless messaging services.

At the heart of the AT&T PocketNet Phone is a specialized Web browser that is specifically tuned to send and retrieve only text-based information on the Internet, not burdensome multimedia and graphics which are bandwidth-intensive. With this approach, the browser optimizes the cellular phone's compact display size, memory footprint and wireless connectivity for information services. Web developers can program the PocketNet Phone for remote, wireless information access to corporate intranets and two-way messaging applications that effectively transform the device into a mobile e-mail terminal. Special "@.com" keys facilitate e-mail communication over the Internet.

Cellular data calls are subject to the same challenges as cellular voice calls; specifically, multipath distortion, signal fading, fluctuating power levels, poor frequency response, and external noise. A variety of factors, such as tall buildings, electronic equipment and street traffic, affect the quality of the connection. Although cellular modems contain advanced, cellular-specific error correction protocols (i.e., MNP-10, MNP-10EC, HST, ETC, EC2, and TX-CEL) to compensate for the external factors that impact cellular transmission, data calls should occur from a stationary position, away from power lines or electrical equipment, to ensure the highest transmission speed.

With more cellular phones supporting data communications, we will see a new breed of cellular phone that provides connectivity to PC desktop and databases via infrared or RS-232 connections.

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