[Cu-wireless] the last meeting

Ralph Johnson johnson at cs.uiuc.edu
Thu May 16 16:32:42 CDT 2002

It was nice to finally meet some of you last Monday.
Those who were absent missed a good event.  I hesitate
to call it a "meeting" because meetings are rarely good!

Here is the design the way I understood it.

A few stations will have two radios, one to communicate
with neighbors, and one or more directional antennas to 
communicate with stations further away.  But most stations
will only have one radio, because radios are expensive, 
and they'll communicate only with neighbors.  In order
to talk to their neighbors, they'll have to use the same
channel.  Thus, there will be a set of stations using the
same channel that have the potential to communicate.  This
set of stations is called a "pod", and you can think of
a pod as a neighborhood.

In fact, it is posssible that two members of a pod will not
be able to communicate, but every member will be able to 
communicate with at least one other member, and all members
of the pod are connected.  If a pod has two halves, and 
stations in one half can't communicate with stations in the
other half, then it is actually two pods, not one.  Pods
must be connected.

Now, the traditional way to manage a pod is for one station
to be the center, and for all the others to  be able to
communicate with it.  The center station would probably have
an omnidirectional antenna and the others would have direcitonal
antennas.  That is not how we are going to do it.  Instead, all
the stations will have omnidirectional antennas, and each station
can talk to any other that is close enough.  This means that
there are lots of potential paths between nodes, and routing
becomes a bigger problem.  But it makes for a more reliable
network.  If a big truck parks between you and your usual
up-stream neighbor, you just have to send the packets to some
other neighbor and have them routed around the truck.

A station will have at least one radio and one ethernet connection.
If you want the computers at your house to be a part of CU wireless,
you'd set up a station and connect it to your home ethernet.  Some
stations will have two or three radios, but they will be less common,
because radios are expensive.

This design is the one Zach and David came up with.  However, it is
almost the same as mine, so I'd like to explain mine.

I live on Green Street in Champaign, near Prospect.  It is going
to be hard to put antennas on roofs because there are so many trees.
I thought that instead we could put antennas about 7-8 feet high on
the side of houses.  I've been looking at "patch" antennas, which
are sort-of-directional.  They are inconspicuous, which is important
to keep my wife from complaining about them.  (She would not want
Pringle cans on her house!)  They are also cheap.  I've got a pair
for $35 each, and you can get them for $25 in hundred quantity.

My idea is to put one on the east side of a house and one on the
west.  A block or two east, there will be a house on the other
side of the street that will have one pointing west, and it will
have one on its east side pointing east.  Suppose my station uses
channel 1 east adn channel 6 west.  Then the house to the east of
me uses channel 1 west and channel 6 east.  The houses to the block
or so east of me would just have one radio and use channel 1 to
talk either to my station or to the station to the east.

This would be identical to the official design except that my pods
would always have at least two stations with two radios.  This is
great because it means we can use the same software for either setup.
So, I plan to experiment with the setup this summer and see if it works.
If it does, we can look at the relative advantages and disadvantages of
each setup and decide what we want to do.

-Ralph Johnson

More information about the CU-Wireless mailing list