Repeaters
Unfortunately,
long-distance line of sight isn't
always possible. Sometimes, you will encounter an obstacle that you
simply can't go over (or through). Or you might need
to stretch a link to go farther than your available radios and
antennas permit. Maybe you are just on the edge of range of a good
AP, but need to provide access to a room full of people (and they
don't all have high-gain antennas). A repeater may
help in your application.
A radio repeater is a
piece of equipment with two complete radios in it. Any traffic heard
on the first radio is repeated to the second, and vice versa. If
directional antennas are used, a weak signal reaching one of the
transmitters is then rebroadcast over the other channel, as if it had
originated from that point. Figure 7-6 shows the
use of this technique to extend range or get around obstacles.
While a classic radio repeater might work fine with 802.11b, I
unfortunately don't have access to radio gear
capable of broadcasting a 25MHz wide signal at 2.4GHz. But I do have
the next best thing: a couple of 802.11b PCMCIA cards.
The following sections describe three repeater-like configurations
that are useful in many circumstances.
7.6.1 Two Cards in One PC
If you have a PC with two PCMCIA slots, you can configure Linux to
use both interfaces and pass packets between them. Insert two
wireless cards, and you have the hardware needed for a repeater
application. While many client cards specifically disable Ethernet
bridging, you can still use masquerading between the interfaces to
bring two networks together.
One intriguing portable device that works well as a repeater is the
Fujitsu Stylistic 1000. It is an old 486/100 tablet PC that comes
with a monochrome LCD screen, stylus, 200MB PCMCIA hard drive, no
keyboard, all of the usual PC ports, a lithium ion battery, and two
extra PCMCIA slots. You can pick them up through used parts suppliers
for around $100. (Thanks to the BAWUG crew for finding these nifty
little devices!)
Take a look at Chapter 5 for details on how to get
Linux installed and configured for masquerading. Once the software is
configured, the only remaining issue is: how do you squeeze one card
on top of the other? Most wireless cards have a slight protruding
bulge to make room for their internal antenna and
won't fit in a stacked PCMCIA bay.
There are a couple of ways around this problem. Obviously, if
you're using a card like the Cisco LMC35x or Senao
2511 for the bottom card, there is no bulge and therefore no problem.
If you're using a card like the D-Link
DWL-650, the bulge is small enough that you can just squeeze two
cards in at the same time. If you're using an
Orinoco card on the bottom, your only recourse is to pop the plastic
cover off and remove the two silver internal antenna tabs. This will
make the card more or less useless without an external antenna, but
can be worth it if you're pressed for time (or cash)
and have a card that you're willing to dedicate to
long-distance work. Remember to connect an external antenna to both
radios when using two in one machine, or else the transmitters will
be operating right next to each other, causing a tremendous amount of
interference.
This technique, shown in Figure 7-7, is very
inexpensive, but it isn't exactly ideal. Even
setting the radios to different channels doesn't
help eliminate all of the interference, as the transmitters are
simply too close together (it will work fine on lightly loaded
networks, but won't stand up to a large amount of
constant traffic). To be able to take full advantage of using two
radios in a repeater, you'll need to separate them
slightly, which is much easier to do in a desktop PC.
7.6.2 Two APs Back-to-Back
Many access points are capable
of bridging
the wireless network directly to the wire. What happens if you
connect two APs in bridging mode back-to-back over a crossover CAT5
cable? Naturally, you have a bridging repeater.
I have only tried this with two Apple
AirPorts, shown in
Figure 7-8, but theoretically any AP capable of
bridge mode should work fine. In this configuration, anyone within
range of access point A will have their traffic repeated verbatim to
access point B, and vice versa. As the Apple AirPorts actually use
Orinoco Silver radio cards, the necessary external antenna connectors
are already present inside the UFO. In fact, by removing the outer
shell, it is possible to mount both AirPorts in a single, small,
weatherproof box, with each connected to its own directional antenna.
Each AirPort can even be configured with its own channel and security
settings, if necessary. Performance won't be as
optimal as with a straight shot (because you have doubled your
chances of a data collision), but it can make a connection possible
where one might otherwise be impossible.
With the advent of client devices (such as the Linksys WET11), it is
also possible to extend your network by using such a device directly
connected to an AP in bridge mode. Set the client adapter to the
ESSID of the network you want to link to, then set the bridged AP to
a different ESSID, and away you go. Be sure to use the latest
firmware on the WET11 before trying this; early models had severe
problems (specifically, with MAC table overruns) that were fixed in
later revisions.
7.6.3 Advanced Host AP Modes
Wireless Distribution System (WDS) is part
of the 802.11b standard. It is designed to allow APs to talk to each
other over the air and to bridge two attached networks together.
Unfortunately, access points that implement interoperable WDS are
rare, and AP to AP bridges tend to be manufacturer- specific. The
Host
AP driver discussed in Chapter 5 has the
beginnings of an interesting WDS implementation that allows the host
to simultaneously serve as a WDS bridge and a standard AP.
Unfortunately, due to limitations in the Prism 2 firmware, true WDS
isn't yet achievable, but a WDS-like mode is still
possible through a clever protocol hack. This allows machines running
Host AP to talk to each other over the air and serve local clients at
the same time, potentially allowing for rapid expansion of wireless
networks.
Another interesting bit of code just appearing in the Host AP CVS
tree is the possibility of running as a BSS master and BSS client at
the same time—as long as both operate on the same channel. This
performs the same function as a working WDS implementation, but could
theoretically work with any access point (even APs that
don't support WDS). This code is very experimental
at this stage, but it looks promising. Keep an eye on the Host AP
development if you are interested in pushing the limits of what is
possible with access point technology.
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