Handbook of Local Area Networks, 1998 Edition:Advanced LAN Issues and Solutions Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next Radio-Wave Point-to-Point Techniques Most radio wave point-to-point wireless products use spread spectrum over ISM frequency bands, but they operate over a highly directional antenna. The directionality increases the gain of the antenna, offering much greater line-of-site distances (i.e., as high as 30 miles). In addition, most of the vendors sell a remote network bridge combined with the wireless transceiver. The bridge segments data traffic by filtering each packet in terms of its destination address, ensuring packets only traverse parts of the network where they are intended to go. This increases system performance by making better use of network bandwidth. One example of a point-to-point wireless product is Cylink Corp.'s AirLink Bridge. Based on spread spectrum radio technology, AirLink Bridge guarantees useful throughput up to its fully rated speed over 20 miles at 1,872 64-byte packets-per-second. The AirLink Bridge filters packets at full, wired Ethernet speed and forwards only the appropriate packets across the wireless link. In addition, the link is full-duplex, so there are no end-to-end collisions that degrade throughput and cause retransmission. The radio link synchronization is maintained at all times, minimizing radio overhead and wasteful transmission time. The main advantage of radio wave point-to-point products is that they are easy to install and relatively inexpensive compared to other methods. The radio wave types, however, are susceptible to interference with other devices that emit electromagnetic energy. At $2,000 to $5,000 a pair (including the remote bridges), these products are fairly expensive. The cost of expensive wireline connections (e.g., T1 or 56K bps lines), however, is avoided. Infrared-Based Point-to-Point Techniques Some point-to-point wireless modems use infrared laser light as a medium to carry data between networks. Laser modems produce highly directional light with a wavelength of 820 nm, which is invisible to the naked eye. Under safe operating power, the laser modems will operate over distances as far as 4,000 feet. The main advantage of using laser modems is that it has an abundance of bandwidth, offering very high data rates. These modems are also not susceptible to interference from the wide array of radio-based systems. This makes the laser modem ideal in very large cities where the airways are full of interfering radio waves. Standards for infrared point-to-point links have not matured to the point of those of wireless LANs. The Infrared Data Association (IrDA), however, has made some headway. The IrDA's stated purpose is “to create an interoperable, low-cost, low-power, half-duplex serial data interconnection standard that supports a point-to-point user model that is adaptable to a wide range of appliances and devices.” The IrDA plans to release standards within the next couple of years. There are not many companies that produce laser-based network devices, however, Laser Communications, Inc., has a variety of laser-based modems (see Exhibit 2-6-9 for a listing of other point-to-point wireless products). One product is the LOO-28, which is compatible with Ethernet and operates at 10M bps. The LOO-28 has both fiber-optic and attachment unit interfaces (AUI). Another product, the LOO-59, is compatible with Token Ring and operates at either 4M bps or 16M bps. These products both can create links that cover a distance of 3,300 feet. Exhibit 2-6-9.  Point-to-Point Wireless Products Wireless WANs If an organization needs to provide mobile access to the network from outside the facility, a wireless WAN may be the solution. To implement wireless WAN connectivity for portable computers, a wireless modem and a service provider that offers the wireless WAN infrastructure is needed. The following sections describe service providers that offer economical wireless WAN interconnectivity at data rates as high as 20K bps. Ardis Ardis provides wireless WAN coverage using packet switching technology in 410 of the largest metropolitan areas in the U.S., Puerto Rico and the U.S. Virgin Islands. These areas cover more than 80% of the population and 90% of business activity. Packet switching is a carrier sense protocol that works very much like the radio-based wireless LAN protocols, except over a much wider area. Many companies, including Pitney Bowes, IBM, AVIS, Wilson Sporting Goods, and Otis Elevator, are currently using Ardis's wireless data communications services to allow salespeople to communicate with the home office. This provides faster and more efficient service to their clients. Ardis offers a packaged wireless solution called RadioMail, developed by RadioMail Corp., which enables users to exchange E-mail with other RadioMail users or anyone who has a public E-mail or Internet address. RadioMail software also includes faxing and paging in addition to E-mail. With RadioMail, users can access other wireless networks, use automatic roaming, exchange messages in real time, or store and forward messages. Motorola's Personal Messenger 100D PCMCIA Type II wireless modem card can be used to interface with Ardis. 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