Chapter 6: Managing the Wireless Infrastructure

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6.1 Introduction

Of all the communications services available today, wireless is having the most dramatic impact on our personal and professional lives, enhancing personal productivity, mobility, and security. In particular, the impact of cellular phone services on our lives is well documented, but wireless fidelity (Wi-Fi) promises to also have a comparable dramatic effect in the near future. In fact, emerging broadband cellular phone and Wi-Fi services are not mutually exclusive, but complementary, so much so that a single PC card for notebooks and some personal digital assistants (PDAs) will soon support both services, switching between the two networks automatically as the user changes locations or applications.

Wi-Fi operates in unlicensed frequency bands and is based on a set of standards promulgated by the Institute of Electrical and Electronics Engineers (IEEE). The most popular standard, called the 802.11b, specifies requirements for connecting devices at the maximum throughput rate of 11 Mbps using the 2.4-GHz frequency band, while 802.11a specifies requirements for connecting devices at the maximum throughput rate of 54 Mbps using the 5-GHz frequency band. Proprietary extensions to each of these standards enable speed bursts of 22 and 72 Mbps, respectively. The 802.11g standard boosts the speed of 802.11b to 54 Mbps.

The innovation in Wi-Fi does not stop here. New standards will support video sessions and voice calls. For these and other reasons, Wi-Fi overshadows any other wireless LAN technology, including Bluetooth, which is limited in range to 30 feet and limited in bandwidth to less than 1 Mbps. Likewise, infrared once showed promise as a LAN technology, but this too has largely been abandoned for the more economical, scalable, and flexible Wi-Fi. Whereas Bluetooth and infrared will continue to address a few niche applications, Wi-Fi will eventually dominate as the technology of choice for wireless LANs.

Ultrawideband (UWB) was also supposed to challenge 802.11 for wireless networking. It promises a powerful combination of low power, high throughput, and better inherent security. Instead of sending its signal over a carrier wave, UWB sends its signals over the noise sections of the radio spectrum. Instead of getting drowned in the noise, UWB’s brief, sometimes nanosecond, signals get tremendous range with little transmission power. More importantly, UWB is inherently more secure than Wi-Fi—UWB transceivers are not only low powered and use what any ordinary radio frequency (RF) device will render as noise, but they can only communicate with each other if they are using the exact same signaling scheme. Because of this, it is much harder for someone to tap into UWB than it is for someone to listen in on an 802.11b network.

What will stop UWB from becoming widely deployed is the huge stride Wi-Fi has already made in the marketplace. With the cost of wireless equipment continually going down to the point of reaching parity with wired gear, Wi-Fi networks are now widely deployed in a number of settings, such as college campuses, business parks, office buildings, and even homes. Such networks are also being implemented by a number of service providers in public places such as airports, hotels, retail locations, and cafés to give users of notebook computers and handheld devices wireless access to the Internet for e-mail and Web browsing. In the corporate environment, Wi-Fi enables users to access LANs to search databases, share files, and print documents—all without requiring them to find an available port and set up a cable connection. And since many employees visit other locations in a building or campus throughout the day, wireless connections facilitate mobility without impeding productivity.

Aside from the low cost of equipment, the following are several other reasons for the growing popularity of Wi-Fi networks:

• They not only work, they work well, and they are undergoing continuous refinement, particularly in the area of security.

• Wireless connections are easy to set up, especially with Windows XP, which provides integral support for Wi-Fi, eliminating the need to manually install drivers. Some notebook computers even come with Wi-Fi antennas embedded into their lids, eliminating the need for a PC card for wireless LAN connections.

• There is nothing new to learn about using Wi-Fi; anyone who uses an Ethernet LAN at work or at home will readily appreciate the convenience and performance of Wi-Fi, which is also based on Ethernet.

• Connectivity is available from a growing number of service providers, so Wi-Fi can be used between the home and workplace at various “hot spots” such as airports, hotels, and cafes, which greatly extends its utility.

Bridges and access points are used to build the wireless infrastructure. The function of a bridge is to extend the range of a wireless link and make multiple LANs at different locations appear to users as a single network. Both functions are important, but in wireless networks, bridges are especially valued for their operating range and management features.

Wireless bridges come in a variety of configurations and price points. At the low end, there are workgroup bridges that extend the range of wireless connections within a building. At the mid-range are bridges that provide building-to-building coverage within a campus environment. At the high end are bridges that extend wireless links across town or even between towns.

While access points provide wireless clients with connections to wired LANs, wireless bridges connect access points to each other, expanding wireless coverage over greater distances. Bridges do not normally accept connections from clients, only from other bridges and access points (see Figure 6.1), but they can be modified to serve as access points. This has the advantage of greatly extending the range of an access point, allowing clients across town to access the wired LAN, while continuing to pass traffic with other bridges.

Figure 6.1: The role of a bridge in a wireless network.

Wireless bridges differ in their operating range. The actual range depends on many factors, including the data rate (bandwidth) desired, line of sight, antenna type, antenna cable length, and device receiving the transmission. In an optimal installation, range can be up to 25 miles. Ranges up to 50 miles have been reported using various combinations of off-the-shelf and custom-built components. But with greater distance there is greater susceptibility to signal interference. Police radar and heavy machinery, for example, are intermittent sources of interference.

Another important consideration is that the bridge’s antennas require not only visual line of sight but also radio line of sight, which includes an elliptical region around the visual line of sight called the Fresnel zone (see Figure 6.2). For optimal performance, the Fresnel zone must be clear of all obstructions, including trees, power lines, buildings, and topographic features, such as mountains.

Figure 6.2: Relationship of visual line of sight to the Fresnel zone.

The bridge comes with its own MAC address. The IP address for a bridge is usually-obtained via the DHCP. Alternatively, the network administrator can console in and set a static IP address. Bridges also support 40/64-bit and 128-bit wired equivalent privacy (WEP), which encrypt the payload of packets sent across a radio link. The WEP key is a user-defined string of characters used to encrypt and decrypt data.

Since wireless bridges are usually placed outdoors, commercial versions usually come in moisture-proof enclosures and feature built-in lightening protection. They support power over Ethernet (PoE) in which the unused pairs in the Category 5 cable carry electrical current from an Ethernet switch to power the unit. Bridges also can be configured to support the SNMP, making the units easy to monitor.

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