When surveying, remember that there are only three nonoverlapping channels
for 802.11b
. In order to
maximize the data rate, use these channels. The use of nonoverlapping channels
ensures that the APs will not interfere with each other.
When designing
the WLAN, survey using the channel on which the AP is intended to operate. A
test for interference is required as part of the survey. If every AP is
surveyed using the same channel, and not the actual channel the AP will be
using, there is no certainty that no interference exists on the channel that
the AP will actually be using. After determining the minimum data rate the
customer will be using, survey at that data rate
.
The chosen data rate will drastically affect the results of the site survey.
In the example in Figure
, the
same warehouse is surveyed at two different data rates. If six APs are required
to cover the facility at 2 Mbps, twelve APs might be required at 5.5 Mbps.
It is important to know what the customer needs. If a survey is completed at
the wrong data rate and the customer installs the WLAN, the connection may only
work in certain areas, or it may not work at all.
If there is too much overlap, the wireless engineer may encounter a
situation where an additional AP would provide too much coverage, but the
current number of APs provides too little coverage
. The engineer
can solve this in a number of ways. The first option is to use a different
antenna to obtain more coverage from the APs. The second option is to use
smaller antennas and add more APs. Another possibility is to change the power
levels on one or more of the APs to change the size of the coverage cells. The
final option is to use a combination of these options to obtain the necessary
coverage.
This process is one of trial and error. Therefore, experience
plays a vital role. Site surveys are often like puzzles. Sometimes individuals
are so sure that they have discovered the best solution that they fail to see
other possible solutions. Whenever possible, ask other wireless engineers about
their surveys. Creative solutions may be found that can be implemented in
future designs.
Sometimes an engineer will survey a few APs, only to
discover that the proposed plan will not work. Instead of starting the process
over, an engineer may try a series of options to find the last piece of the
puzzle.
Sometimes site survey problems are due to frustration or
laziness. An engineer might become focused on one solution to avoid having to
start the survey over again. In this situation, it is best to take a break from
work. This is a good time to go for a cup of coffee, check voicemail, or take a
lunch break. The best solution will often present itself after this type of a
break. If not, it may be necessary to start over. There may be trouble spots
that had been overlooked the first time. When completing the survey the second
time, these trouble spots will be evident and can be factored in when planning
the new layout of the APs. It is always better to start the survey over and
design the WLAN properly than to try and force a solution or use a solution
that may not provide the best coverage.