Making an 802.11b Repeater
(This page is out of date. Instead I have a lot more information about day-to-day running a tiny wireless ISP at wispdiary.com.)
Looking at diagrams for large wireless networks, it's clear that
you need multiple access points, all connected to the wired network.
But what if you want to create a large wireless network
someplace where there's no existing wired network?
That's what I needed to do. I've just started a tiny WISP in a
rural area, and to improve coverage I wanted to put in a second access
point on some high ground that overlooks parts of the town that I
can't reach directly.
|
|
For the access point itself, I use the Linksys WAP11 with a sector
antenna from Superpass. Its ethernet connection goes into a cheap
10Base-T hub. Also connected to the hub is a Linksys WET11 with a
24dBi parabolic antenna. The WET11 connects to the primary access
point, and bridges traffic, so the WAP11 thinks it's connected to the
wired network. Voila! Instant repeater for:
| WET11 | 95 |
| WAP11 | 75 |
| hub | 20 |
| $190 |
|
The first time I plugged all the stuff together, I powered it up
and the WET11 immediately associated... with the WAP11 right next to it.
That wasn't what I wanted - it needed to associate with the primary
access point a mile away - so I set up the two access points to have
different SSIDs. The main access point has SSID 'acelere', the
repeater 'acelereB'. The repeater's WET11 is set up to associate with
'acelere', not 'acelereB'.
In a box
|
|
So much for the demonstration on a kitchen table. The stuff all
had to be put in an outside enclosure, with antennas, and had to be
powered somehow. For the enclosure, I used a sealable food container
bolted to a 1" aluminum pole. I put silicone sealant all over the
bolts and washers to keep water out. The two antennas - one for the
WET11, the other for the WAP11 - bolt on the aluminum pole. I drilled
holes in the the bottom of the food container for the antenna
connectors.
Standard 7" brackets fix the pole onto a couple of 2x4
cross members, which are nailed to the pole. It's actually kind of
fiddly getting the whole thing lined up, mostly because of the 24dBi
antenna. It helps to have at least one other person holding stuff
while you tighten the bolts.
|
|
Power
There's an excellent page on power over ethernet here. But one thing I've
found with running the WAP11 over more than a few feet of cat5 is that
the voltage drop is so great that it won't even power up properly, it
just cycles through its power up sequence repeatedly. So I now use a
higher voltage supply (24 or 48 volts) on the cable, and a DC-DC
converter inside to the food container to give exactly 5 volts that
the unit needs. The voltage drop on a long wire is proportional to
the current, so the appeal of 48v is that the current is kept pretty
low.
 |
Power over Ethernet |
|
Having experimented with a few approaches - and
rejected them because there's nothing worse than working with flaky
connectors - I made my own PoE extraction/injection connectors for a
about $1 each. It's a tiny single-sided PCB made in batches of
five. If anyone's interested, mail
me and I'll write some howto instructions. The ethernet
connectors are Jameco part number 167290. Not expensive.
|
This scheme had worked well for 50' cable runs, but the repeater
was rather more ambitious, being 600' from the nearest AC outlet. The
people in the house also needed Internet access, so there would be
data on the cable too. Having done the math I decided to use 48v
rather than 24v, and picked up the various bits and pieces from Halted.
The repeater uses about 10W, and the the WET11, WAP11 and hub all
run on 5v. Assuming that the DC-DC converter is 80% efficient, the
current in the cat5 cable will be about 0.25A, the 600' cable has a
resistance of about 10ohms, so the voltage drop over the run will be
10*0.25 = 2.5v. The DC-DC converter's minimum input is 36v, so that's
plenty of margin.
Digging
 |
500' to go... |
|
Running the cat5 from the house up to the repeater was a lot of work.
More work, in fact, than everything else in the endeavor combined.
The basic idea was to string steel cable from point to point, crimp it
to eye bolts, then cable tie the cat5 to it every 10 feet or so.
First the cable goes from the house to this small utility pole that we
put in. From there, up the fence to the repeater. |
 |
Thanks Chris! |
|
Arg!
| The repeater powered up fine - but the 600' cable run back to the
house was way too far for ethernet. The PC kept complaining the
network cable was unplugged, which suggested that the ethernet connection
was marginal. I guess one solution would have been to use a wireless
unit back at the house, and make it a client of the hillside AP. But
we had a cheaper idea. Judging a point about halfway along the
cable, we inserted another ethernet hub, powered by another 48v->5v
DC-DC converter. This used up 2 PoE units, one to draw power for the
hub, another to reinject 48v back into the second leg of the journey
up the hill. Total cost for this little box was about $30.
|
 |
A bridge too far |
|
Results
 |
Mmmm... subscribers |
|
I can see rooftops. The repeater is running now,
connecting at 11Mbs, and the signal looks good. The property owners
are happy with their broadband, and acelere.net can now reach about 3
times the number of homes. This took a lot of effort by quite a few
people - thanks everyone!
|
More notes are here.
James Bowman - acelere.net
(This page is out of date. Instead I have a lot more information about day-to-day running a tiny wireless ISP at wispdiary.com.)
