Point-to-Multipoint Links
Setting up a
point-to-multipoint
configuration is much like setting up a point-to-point link with an
access point. The difference is that multiple clients connect to a
single AP. You can use any sort of antenna, but you should generally
choose one with the narrowest beam width that will cover the area
you're interested in. That helps reject noise from
all other directions, and minimizes the noise that your network will
cause for other people.
You should pay particular attention to the "hidden
node" problem when dealing with long-distance
point-to-multipoint links. On a simple point-to-point shot, this
isn't a problem, because both nodes can hear each
other by definition. But suppose you put an access point on a high
point and attach a high-gain omnidirectional antenna to it, thus
allowing multiple clients to connect. It is very likely that some
clients may not be able to hear the traffic of others, so
transmission collisions will occur. An increase in traffic passing
through your AP can bring down throughput considerably.
One common method for dealing with long-distance point-to-multipoint
collisions is to sectorize your AP: that is, add
multiple radios attached to tight beam sector (or other) antennas to
your AP site. Setting each antenna to a different channel and making
intelligent use of polarization tricks can help reduce the number of
clients associated with each AP, which reduces the problem of
collisions.
If your AP supports it, you can also use RTS/CTS (Request
To Send/Clear to Send). Using RTS/CTS guarantees that clients do not
transmit simultaneously—each frame must first be approved for
transmission, then acknowledged by the access point. Unfortunately,
there is considerable overhead involved with RTS/CTS, so it is
usually left off by default. If your AP is in a prominent place and
shows many receive errors, consider trying RTS/CTS. Run a throughput
test while the network is under typical load without using RTS/CTS,
then turn it on and try again. The error rate should go down
considerably on your AP, but your throughput may suffer. Fine-tuning
RTS/CTS can be difficult—since 802.11b gear has become cheaper,
it is frequently more effective to simply add more equipment to
accommodate more clients.
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