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Wireless Networking Handbook
(Publisher: Macmillan Computer Publishing)
Author(s): Jim Geier
ISBN: 156205631x
Publication Date: 09/01/96

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CHAPTER 2
Wireless Local Area Networks (LANs)

Do you need wire-free interconnectivity between users’ computers, servers, and printers within a room or building? A wireless LAN might be a feasible alternative to traditional ethernet and token-ring networks to satisfy needs for user mobility in offices, warehouses, retail stores, and hospitals.

This chapter describes several approaches to wireless networking within a local environment. These methods include the following:

  Radio waves
  Infrared light
  Carrier currents

Wireless LAN Applications

Figure 2.1 illustrates the concept of a wireless LAN. Most wireless LANs operate over unlicensed frequencies at near-ethernet speeds (10 Mbps) using carrier sense protocols to share a radio wave or infrared light medium. The majority of these devices are capable of transmitting information up to 1,000 feet between computers within an open environment, and their costs per user range from $150–$800. In addition, most wireless LAN products offer Simple Network Management Protocol (SNMP) to support network management through the use of SNMP-based management platforms and applications.


Figure 2.1  A wireless local area network.

Wireless LANs can save money when installing networks in difficult-to-wire environments, such as old facilities containing asbestos, historical buildings, and jails. The removal of asbestos can cost thousands of dollars to facilitate the installation of wiring. Historical societies forbid changes, such as holes and wallplates, in certain building structures. Jails consist of large amounts of concrete and steel to keep prisoners from escaping. As a result, justice departments do not allow the drilling of holes for cable runs because it might weaken security. In these cases, as well as others where it’s expensive or time consuming to install wire, consider using a wireless LAN.

Radio-based Wireless LANs

The most widely sold wireless LAN products use radio waves as a medium between computers and peripherals. An advantage of radio waves over other forms of wireless connectivity is that they propagate through walls and other obstructions with fairly little attenuation. Even though several walls might separate the user from the server or wireless bridge, users can maintain connections to the network—supporting true mobility. With radio-LAN products, a user with a portable computer can move freely through the facility while accessing data from a server or running an application.

A disadvantage of using radio waves, however, is that an organization must manage the radio waves along with other electromagnetic propagation. Medical equipment and industrial components utilize the same radio frequencies as wireless LANs, which could cause interference. As mentioned in Chapter 1, “Introduction to Wireless Networking,” an organization must determine whether potential interference is present before installing a radio-based LAN. Because radio waves penetrate walls, security may also be a problem. Unauthorized people from outside the controlled areas could receive sensitive information. As mentioned in Chapter 1, however, vendors often scramble the data signal to protect the information from being understood by inappropriate people.

This section discusses the following radio-based wireless LAN topics:

  ISM bands
  Narrow band wireless LANs
  Spread spectrum wireless LANs

ISM Bands

In 1985, as an attempt to stimulate the production and use of wireless network products, the FCC modified Part 15 of the radio spectrum regulation, which governs unlicensed devices. The modification authorized wireless network products to operate in the Industrial, Scientific, and Medical (ISM) bands. The ISM frequencies are shown in figure 2.2. The FCC allows users to operate wireless products without obtaining FCC licenses if the products meet certain requirements, such as operation under 1 watt transmitter output power. This deregulation of the frequency spectrum eliminates the need for user organizations to perform costly and time-consuming frequency planning to coordinate radio installations that will avoid interference with existing radio systems. This is even more advantageous if you plan to move your equipment frequently because you can avoid the paperwork involved in relicensing the product at the new location. As you can see, more bandwidth is available within the higher frequency bands, which will support higher data rates.


Figure 2.2  The Industrial, Scientific, and Medical (ISM) frequency bands.

ISM Band Availability

The ISM band frequencies are not available in all parts of the world, limiting the capability to operate wireless products sold in the United States. Figure 2.3 identifies those countries that allow wireless LAN operation in the 902 MHz and 2.4 GHz ISM bands. The 2.4 GHz is the only unlicensed band available worldwide. This band was approved in North and South America in the mid-1980s and was accepted in Europe and Asia in 1995. Companies first began developing products in the 902 MHz band because manufacturing costs in this band were cheaper. The lack of availability of this band in some areas and the need for greater bandwidth, however, drove these companies to migrate many of their products to the 2.4 GHz band.


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