[Cu-wireless] C-U Wireless Status Update
dyoung at pobox.com
Fri Oct 24 18:52:24 CDT 2003
An update on the wireless project is long overdue.
C-U Wireless won a $4500 grant to buy tools and rooftop routers.
We have put together a kit for installing rooftop routers, and we
have also bought the parts to construct six routers. Each router
resembles the ammo can router we use at IMC: it consists of a Soekris
single-board computer, a weatherproof enclosure, 8dBi antenna, an
802.11 radio, CompactFlash card, 50 feet of outdoors-proof ethernet,
and a PoE injector. We will use these nodes to spread the network
northeast from Zach Miller's house, just as soon as our order for
CompactFlash cards is (re-)fulfilled with the right part.
We have submitted a grant proposal to George Soros' Open Society
Institute. I think our chances of getting this grant are pretty slim,
however, the time spent writing the grant was well spent because
I think we have strongly articulated the project philosophy and
goals. Also, the grant contains a comprehensive technical development
plan, time and cost projections, etc. Expect to see excerpts from
the text on the mailing list and on the Web.
Here and there I have made an improvement to the software
distribution. It fits in just 32MB, now, so we can fit an
"experimental" and "last good" copy onto one 64MB CF card. Trouble
is, we cannot revert to the "last good" copy if the "experimental"
version fails. If somebody wants to port the FreeBSD bootloader
or else to hack the NetBSD bootloader to give us a "boot once"
capability, it would be an ENORMOUS help. I just don't have the time
to do all this stuff.
I have ported from FreeBSD to NetBSD the driver for 802.11a/b/g cards
based on the Atheros chipset. Great improvements to the 802.11 layer
accompanied that driver. (NetBSD & FreeBSD contain a *nice* 802.11
networking layer that makes it easy to provide uniform support for
lots of 802.11 gear, especially the new 802.11 ASICs. Because of the
802.11 layer, I think that you're going to see NetBSD leapfrog Linux
and OpenBSD in terms of open-source drivers for new 802.11 radios.)
I don't think that we can take good advantage of a/g modulation
(needs too-high S/N ratio), but Atheros-based 802.11b cards are a
LOT more versatile than Prism-based cards. I want to eliminate Prism
cards from the C-U Wireless network ASAP ... they just don't expose
provide enough knobs and dials for one to do interesting things on
I've programmed an uncomplicated rate adaptation algorithm that I
read about in someone's research paper. It is more sophisticated
than Lucent's algorithm for choosing the transmission rate (1, 2,
5.5, 11Mbps), which is what most people use. A decent rate adaptation
algorithm will improve performance on the C-U network.
I tried and failed to make the CUW software distribution run both a
DHCP client and server on each interface, so that any port can act
as a WAN or LAN port, so that you can build a true "plug & play"
network. I think that I will try again a little later.
New Directions for Rooftop Routing
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I've mentioned before that the MIT Roofnet's ETX routing metric
(or something like it) is something we must start using before
we have much greater density of nodes, or else the shortcomings
of shortest-path routing on wireless are going to show up with
My software development plan describes the architecture for a routing
daemon for rooftop networks that uses the ETX routing metric.
There is a Ph.D. thesis (written in 1980!) that has really influenced
the way I think about wireless networks. I mean, as if it wasn't
clear enough, routing in wireless networks is a LOT different than
routing in wire networks, mainly because of (self-)interference.
The Ph.D. thesis is concerned with a link-state routing protocol
called "Least Interference Routing." I have made two spiral-bound
copies of the thesis if you want to find out more.
If you are an experienced C programmer, I could use your help
to develop the next generation of routing software and also
a pretty slick name service, 802.11 innovations, etc., for C-U
Wireless. Network-programming experience is a plus; kernel programming
experience is a plus-plus. Let me know if you are interested, and
I will send you a copy of the development plan.
Research Guinea Pigs
-------- ------ ----
Professor Christian Sandvig in Communications is interested in
the history and potential social impacts of community wireless
networks. Students of his will be interviewing Sascha and I as part
of a research project on community wireless networking.
Major Interference Source Identified
----- ------------ ------ ----------
I have found out the operator of the powerful wireless interference
source. We are working together to solve the problem. It turns
out that the interference comes from channel 6 (we are on channel
11). It is possible that a defective radio (no filtering on our end)
or a software defect (their end) is to blame; it is also possible
that our systems are just way too close to each other to co-exist
on channels 6 and 11.
I don't think any outdoor 802.11 network can scale very big without
using both power control and "adaptive" or "smart" antennas---that is,
antennas that let you either choose propagation pattern presets or
form beams in the direction of network peers. Power control helps
you re-use the same frequency in a region (the analogy is that many
more people can communicate in one room by whispering one to another
than by shouting). Adaptive/smart antennas let you direct your
radio energy directly at your communication partner: it gives you
"spatial re-use" like power control, but at a finer level; it also
helps you protect against interference sources.
New radio chips (ADMtek, Atheros, Realtek) let you control power
levels. Atheros lets you control power for each packet---very nice!
I am trying to find out from Prof. Jennifer Bernhard if her 2.4GHz
adaptive antenna for space communications is suitable for rooftop
There are already some commercial projects with smart antennas, but
they are super-expensive. Somebody called me the other day to tell
me about their Vivato "wireless switch" which cost just more than
$10,000. If anybody is interested in seeing a demonstration of the
Vivato switch, the Ohio company that called me offers to demonstrate
for C-U Wireless next time they are in the area.
In my naivete, I do not see any reason that a computer-controllable,
2.4GHz adaptive antenna (less versatile and less complicated than
a "smart" antenna such as Vivato's) cannot be manufactured on
printed-circuit board for $150 or less.
Senior Project Team
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This year I am working with a second senior project group. The
project for last year (roaming in 802.11 networks) involved kernel
programming and other low-level stuff, so it was really too much
for time-strapped seniors to finish in one year. This year, four of
Ralph Johnson's students are working on a Java app for visualizing
802.11 packet traces, such as tcpdump and ethereal create. The reason
for programming a visualizer is that you can use it to teach about
the 802.11 MAC protocol, to verify 802.11 implementations, and to
identify/characterize interference. If you've seen the protocol
diagrams in the 802.11 protocol spec, then you know roughly what
the visualizer will produce.
David Young OJC Technologies
dyoung at ojctech.com Urbana, IL * (217) 278-3933
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