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To access the contents, click the chapter and section titles.
Wireless Networking Handbook
Analog Cellular WANsThe analog cellular telephone system has made it possible for millions of people to make phone calls using portable cell phones while away from the home or office. Many who start using cell phones cant imagine life without them. The general idea of an analog cellular WAN is to make use of the cell phones mobility and employ it as a means of transferring data, much like the use of traditional wire-based Plain Old Telephone Service (POTS). You can use the analog cellular phone system to dial-in to your corporate network to access applications and send e-mail, just like you can when using POTS. If you need to send large files, such as engineering drawings, this cellular phone approach might be the way to go. It provides data rates up to 28.8 Kbps, which is similar to conventional POTS telephone modems. The idea of this technology is to connect your computer to a cellular telephone via a modem and then with a remote system through a dialup connection. This provides a relatively easy way to obtain wireless data transfer wherever cellular telephone service exists, which covers most of the world. This cellular approach does have some drawbacks, though. For one, the usage costs are relatively high. The cost of sending data is based on the length of the call and the distance to the destination. You also pay more as you roam from one location to another. Roaming within three different regions in one day, for example, can cost approximately $10, not counting the standard air time. Another problem is the occasional transmission errors you receive that will cause retransmissions to occur. The cellular telephone system was built primarily to transmit voice, which can generally stand more transmission errors than data. Analog Cellular TechnologyThe analog cellular telephone system uses FM (Frequency Modulation) radio waves to transmit voice grade signals. To accommodate mobility, this cellular system switches your radio connection from one cell to another as you move between areas. Every cell within the network has a transmission tower that links mobile callers to a Mobile Telephone Switching Office (MTSO). The MTSO, which is owned and operated by the cellular carrier in your area, provides a connection to the public switched telephone network. Each cell covers several miles. Figure 4.5 illustrates the general topology of the cellular telephone system.
Most modems that operate over wireline telephone services will also interface and interoperate with cellular phones; however, modem software optimized to work with cellular phones minimize battery usage. MobileWare, for example, is software that enables you to communicate over cellular or regular wireline phones. MobileWare minimizes the cost of cellular service usage by making a connection only during actual information transmissions. MobileWare prepares the information for transmission and then transmits whenever possible. Thus, you can prepare correspondence such as e-mail messages and faxes while on an airplane, then MobileWare will send the information after you land. MobileWare also mobilizes the Lotus cc:Mail, meaning it enables you to effectively utilize cc:Mail on a wireless connection.
Cellular Digital Packet Data (CDPD) WANsTo establish a dedicated wireless data network for mobile users, a consortium of companies in the United States developed the Cellular Digital Packet Data (CDPD) standard. CDPD overlays the conventional analog cellular telephone system, using a channel-hopping technique to transmit data in short bursts during idle times in cellular channels. CDPD operates full duplex, meaning simultaneous transmission in both directions, in the 800 and 900 MHz frequency bands, offering data rates up to 19.2 Kbps. Whats the advantage of using CDPD versus analog cellular systems? Recall that the main advantage of the analog cellular system is widespread coverage. Since CDPD piggybacks on this system, it will also provide nearly worldwide coverage. The main plus with CDPD, though, is that it uses digital signals, making it possible to enhance the transmission of data. With digital signaling, its possible to encrypt the data stream and provide easier error control. CDPD is a robust protocol that is connectionless and utilizes Reed-Salomon forward error correction (FEC). FEC is an error control technique that corrects errors at the receiver without asking the source to retransmit the errored packet. Security with CDPD, which is accomplished using an encrypted key-passing technique, is very good. Also with CDPD, you only pay for the amount of data that is actually sent, which is less than the amount you would spend on an analog cellular call if sending the same data. CDPD is available today only in large cities within the United States, but it is spreading to other areas as well. However, many visualize CDPD as an interim solution until digital cellular telephone service becomes available. CDPD Design Goals and ObjectivesThe CDPD Forum is an industry association with over 90 companies interested in developing and promoting CDPD products and services. The Forums main mission is to develop a technical standard for CDPD, as well as develop the market place and promote the technology.
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