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To access the contents, click the chapter and section titles.
Wireless Networking Handbook
SkyTels 2-Way Paging and Wireless Messaging ServicesSkyTel, in conjunction with their partners, has developed a variety of two-way messaging products based on their two-way paging system. SkyTels 2-Way Pager is pocket-sized, weighs only 5.5 ounces, has 100K of memory, and features a 4-line LCD display. The pager runs on a single AAA battery for up to several weeks. SkyTels 2-Way paging service is available in 1,300 cities, including the top fifty U.S. metropolitan areas, and includes the following features:
One-Way Paging ServicesToday, there is a mixed bag of one-way paging services that provide wireless WAN functionality. CompuServe, for example, offers wireless pager capability, which enables subscribers to send e-mail messages from their PCs to other subscribers pagers. CompuServe contracts with RPA, Inc., which acts as a hub and interconnects with every paging company in the United States. CompuServes service enables subscribers to send e-mail messages directly to numeric and alphanumeric pagers. The maximum message length depends on the specific paging vendor, but most pagers support messages up to 80 characters long; some permit up to 240 characters. Another service enables users to receive notification via their pager when their CompuServe mailbox receives e-mail. Other online services such as Microsoft Network, Prodigy, and America Online are deploying similar services, too. Socket Wireless Messaging Services (SWiMS)Socket Wireless Messaging Services (SWiMS) is a suite of services administered by the National Dispatch Center and handled by Socket Communications. SWiMS receives messages or data sent to a personal 800 number over conventional phone lines and then routes messages to your pager. If someone wants to send you a message, he calls your personal 800 number using a phone, fax machine, or modem-equipped personal computer. For voice calls, a dispatch operator will answer and transcribe the message into text and send the message immediately to your pager. If someone sends you a voice mail or fax, a notice is sent to your pager and you can call into your account to listen to the voice mail or forward the fax to a particular fax machine. Also, someone can send e-mail to your pagers Internet address that includes your 800 number as part of the address (such as 8005551212@pagecard.com). SWiMS will convert the first 600 characters of the e-mail message into a page and transmit it to your pager. SWiMS gives you the following coverage options:
Satellite CommunicationsMost wireless WAN services, such as analog cellular, packet radio, CDPD, and paging networks, have fairly good coverage, but offer relatively low data rates. If you are looking for high speed transmission with complete worldwide coverage for your portable users, then satellite communications might be a good alternative. The main issues with satellite systems are high costs and limited support for mobile users. The monthly service costs are not too bad, but initial equipment costs are very high. The portability aspect of satellite, mainly because of the antenna sizes and point-to-point uplink, requires users to setup the antenna dish and align it with the satellite before sending data. Satellite System ComponentsSatellite systems today support transmission of video, voice, and data for a variety of companies that require global coverage. The components that make this possible are shown in figure 4.8. The satellite is a platform that hosts a series of transponders acting as signal repeaters. The transponders receive directed signals on the uplink from earth stations and broadcast the signals back to earth on a downlink frequency where users over a very wide area are able to receive the signal. A satellite in geostationary orbit has a 24 hour period at an altitude of 22,300 miles over the equator. The satellite takes one full day to orbit the Earth, which makes the satellite appear to look stationary from the Earths surface. This enables the earth station antennas to remain fixed, not having to track the satellite. The geostationary orbit puts the satellite directly over the Earths equator; therefore, earth stations in the Northern Hemisphere point their antennas toward the southern horizon, and stations near the equator, such as Bogoeé, Columbia, point their antennas straight up into the sky. Some satellite systems, though, work at lower altitudes to lessen the amount of propagation delay. These lower altitude satellites are not geostationary and will appear to move across the sky, requiring earth stations to have tracking antennas.
To utilize a satellite system, you need a satellite station that consists of an antenna, satellite transceiver, and an interface to your computer. There are several companies that sell these components. For example, California Microwave has a product called LYNXX Transportable Inmarsat-B Earth Station that provides 64 Kbps throughput for voice, video, fax, and data transmissions. LYNXX can serve as a global wide area gateway into the Internet, as well as corporate networks. The system has a 50 foot interface cable, enabling you to mount the antenna outside and operate the computer from within a building.
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