Information Technology Reference
In-Depth Information
You place the antennas where you want the signal. This means that you could
locate a bridge some distance from the antenna, and connect the two with coaxial
cable. Keep in mind that the dB losses increase rather quickly in proportion to the
length of the cable. So, you must use high-quality cable designed for operation at
the 2.4 GHz or higher frequencies to keep losses to a minimum. Most WLAN gain
antennas come with a 3- to 10-m segment of coax. Feel free to have a custom length
made, with appropriate test results to guarantee performance. Extremely low-loss
cable is available for a premium.
For that matter, you could make up your own cables, if you have the proper
tools and knowledge. After all, you are capable of doing your own twisted-pair
cabling. Just keep in mind that you are dealing with frequencies up to 20 times
higher than Ethernet networks, so you really have to construct these cables with
care. If you have access to a loss measuring device for 2.4 GHz, you can test the
resulting cable to be sure you get what you are paying yourself for.
A word about the care and feeding of coaxial cable is appropriate here. Just as with
our relatively fragile Category 5e and better cable, you must be careful with the handling
and routing of coaxial cable. As covered in Chapter 5, coax consists of an outer conduc-
tive shield and an inner center conductor, separated by an insulating “dielectric” layer.
Any distortion of the dielectric material, through bending or compression, will cause an
attenuation of the signal. Attenuation is sort of like a negative gain.
So don't crimp, bend, break, pull, or tie-wrap coaxial cables excessively. Take
care of your cable and it will take care of you.
Site Survey for Wireless LANs
In our simple building example, placing the wireless bridges was fairly easy. We had
a small number of offices to cover in a simple building structure. However, this is
rarely the case in practice. To create a much better estimate of the number of bridges
and their placement, you should conduct a site survey.
There are two ways to do a site survey, and as you might expect, the survey's
cost is the most apparent difference. However, the quality of the results is the real
defining factor. Be careful which you use, unless you have no problem adding addi-
tional APs and resituating existing ones to optimize coverage. If you have bid an
installation, with no caveats to the buyer, adding the additional bridge or two can
be very costly. On the other hand, if you are the user, and if you have a relatively
small location to cover, it may be much cheaper to use the “casual” site survey and
the advice of a knowledgeable installer to make a best-effort estimate of your
requirements.
