Table of Contents
Advertisement
Chapter 2: Preparing for a WAN Deployment
Now that you have a sense of how Wi-Fi performance changes over distance and with noise, look at some ways to perform
channel assignment. If two access points are on the same channel right next to each other, they are forced to share the same
spectrum. This means that they share the 54-Mbps speeds available in 802.11a/g or the 300-Mbps speeds in 80211n rather than
each being capable of 54- or 300-Mbps speeds independently. This essentially halves the bandwidth for each access point. To
manage this situation, make sure that neighboring APs are on different channels and that their power is adjusted so that it does
not overlap that of other APs with the same channel.
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.
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.
Page 24
3-to-1 Layout Pattern
1
6
11
1
6
11
1
6
11
1
6
11
1
6
11
1
6
11
1
6
11
11
1
11
1
6
6
11
1
6
Figure 2-6 Channel layout patterns.
724-746-5500 | blackbox.com
7-to-1 Layout Pattern
36
52
36
52
60
64
40
60
64
40
44
56
36
52
44
56
36
52
60
64
60
64
40
44
56
36
52
44
56
36
52
60
64
40
60
64
40
44
56
36
52
44
56
60
64
40
44
56
40
Advertisement
Chapters
Table of Contents
