[OpenWrt-Devel] Multiple OpenWrt devices collectively managed?
david at lang.hm
Tue Mar 24 01:30:38 EDT 2015
On Mon, 23 Mar 2015, Janne Cederberg wrote:
> I was thinking the same thing David mentioned, namely, that
> determining channel change criterion is probably not trivial. Are you
> guys familiar with Cisco Meraki?
Thinking about the problem a bit more.
First off, I don't believe that this can be completely automated due to the
inability to check other channels while providing service, but I do think that
the initial setup of such a system can be greatly simplified.
first off, the situation that I'm talking about is setting up a multi-AP network
with a wired backbone in an area that has some other wifi activity. In my
particular focus, this is setting up a conference network where there is already
some wifi in place (and they don't turn it off), but if you are setting up a
business network and have another company in adjacent offices (or on an
adjoining floor) you have the same basic problems. The setup and calibration
will take place when things are generally fairly idle.
After the APs are deployed and live, scan standard channels for beacons (on 2.4
only bother with 1,6,11,14 the others shouldn't be considered). some of the
SSIDs are going to indicate networks that are giong to be busy, some are going
to be phones, printers, etc that may have bursts of activity, but are not going
to be ongoing major interference sources. It will take a person to categorize
these, although a SSID that is seen on multiple channels is probably one to be
concerned about. Interference sources on channels may or may not be significant.
It's going to be a judgement call.
Then put all the APs on the same channel and beacon frequencly for a bit to map
out what APs see each other at what strength.
Then try to figure out a channel plan that minimizes interference from outside
sources and other APs. Include estimates on how low the power can be turned down
to reduce AP <-> AP interference.
The admin needs the ability to insert modifications (i.e. "I know that a vendor
will be using channel X in this area heavily", insert an interference soruce
that wasn't detected)
Deploy the plan, then designate one or more mobile devices as targets and
monitor them closely as they move around the area. This requires an app on the
mobile device that can report the signal strength of the AP with each handhake
(fancy ping). If any dead areas are found, increase transmission power on the
closest APs to provide coverage. sometimes you will need to go back and
regenerate a plan.
This is a map coloring problem, but you don't know the shape of the vairous
areas (because of the shielding/absorbing/reflecting properties of structures),
the areas are 3-dimentional, and on 2.4GHz you don't have enough channels to do
a non-interference job in any case, so there is no one right answer, or even oen
clearly best answer. So different starting points in channel assignment and
different orders of walking through the APs to assign channels will result in
Since bodies will affect the range, either a fudge factor will need to be put
in, or a walkaround test will need to be doen once crowds form.
The entire test will need to be done independently on 2.4GHz vs 5GHz
Ideally the software that's doing this lets you take an image and place the APs
on that image to then produce a graphical representation of the expected
coverage (scale of coverage circles based on the signal strength that the APs
hear each other, assume that the picture and AP locations are fairly close to
scale accurate. Ideally the mobile app lets the people moving them around the
facility tag where they are easily to help show the effects of
If you have "extra" radios (either APs with multiple independent radios per
band, or APs that you aren't having users connect to), you may be able to do
some scanning while things are live to detect unexpected interference, but
generating and testing a plan is disruptive enough that you need to think hard
before you try changing things.
Since a lot of this is going to depend on where the APs are deployed, and what
antennas are put on them (and how the APs/Antennas are positioned), the planning
system is not going to be able to fix a bad layout, it can help make it less
painful, but there should always be the visualization of what's going on so that
an experienced person can look at what the computer decided to do and decide to
modify it or throw it out.
I've never had the time to go through all of these steps manually when deploying
to a location, but they are what I'm thinking of, and I make it a point to see
what sort of effect the different types of walls have on the signals when
planning for an event. I would love to be able to backstop my educated guesswork
with actual measurements to help me find problem spots.
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