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Chapter 2: Preparing for a WAN Deployment

In the 2.4 GHz spectrum, there are 11 channels in the United States. However, a Wi-Fi signal consumes more than one channel.

Consequently, there are only 3 non-overlapping channels: 1, 6, and 11. To achieve optimal performance, you need to design a

channel layout pattern such as the one on the left in Figure 2-6.

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7-to-1 Layout Pattern

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111

6 111

3-to-1 Layout Pattern

Figure 2-13. Channel layout patterns.

NOTE: There are alternative 2.4-GHz channel layouts, such as one for four channels using 1, 4, 8 and 11 and another using

channels 1, 5, 9 to counter interference from microwaves, which tend to cause interference in the high end of the

spectrum. Black Box recommends alternative channel layouts only for the most challenging radio environments.

Designing a channel pattern is easier for the 5-GHz spectrum. Depending on the country and the device being used, there are

between 4 and 24 channels available for Wi-Fi use. However, in most countries there are at least eight 40-MHz-wide channels

with which to work. To simplify the layout of more than 3 channels, most use a 7-to-1 pattern, as is shown on the right in Figure

2-6. This channel layout is much more flexible than the 3-channel system and allows for much better capacity over all channels.

The last topic to cover is the concept of multipath. When a client receives a transmission from an access point (or vice versa), the

RF signal reaches the client first through a “direct path,” but then shortly thereafter by the “indirect paths” reflected off other

objects. The direct path combined with the indirect paths make up multipaths (see Figure 2-7). RF signals can bounce off almost

anything—walls, people, plants, and so on—but they bounce off metal most. As the RF signals bounce about while propagating,

one or more of the secondary paths can interfere with the primary path, causing the signal strength of the direct path to

diminish. In doing so, multipath can greatly decrease signal-to-noise ratio with legacy 802.11a/g radios. With 802.11n, a certain

amount of multipath is desirable and increases performance.

Primary Path

Secondary Path

Figure 2-14. Multipath radio waves.

NOTE: If you would like to learn more about how radio-frequency propagation works or the details of 802.11, Wikipedia provides

excellent background information under the entries “IEEE 802.11,” “radio propagation,” and “multipath.” Additionally,

spending a few hours with a site survey tool such as AirMagnet Surveyor or the Ekahau Site Survey (ESS) and a few test

APs can increase both your familiarity with Wi-Fi propagation and your confidence about how it behaves.

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Black Box LWN600CM-1 Guida Utente Pagina 33