[26239] in resnet
Re: Xirrus wireless
daemon@ATHENA.MIT.EDU (jim warner)
Fri Apr 15 12:03:12 2011
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Message-ID: <4DA86C17.8030508@ucsc.edu>
Date: Fri, 15 Apr 2011 09:02:31 -0700
Reply-To: Resnet Forum <RESNET-L@listserv.nd.edu>
From: jim warner <warner@ucsc.edu>
To: RESNET-L@listserv.nd.edu
Randy Ethridge <rlethridge@EIU.EDU> asked:
> Does anyone have experience/opinion on Xirrus wireless equipment?
I have no experience. I do have opinions.
It is technically very challenging to place radios operating in a single
band
in a package in a way that they don't end up walking over each other.
That they operate on separate WiFi channels withing the same band is
only a little comfort. It is as good as the filters that separate the
channels --
between 20 and 25 dB. For colocated radios, it is a 60 dB problem.
Xirrus claims that they have this all handled. But if you install in a
small
room, the feedback from wall bounce by itself will overload the receivers
in the unit that is adjacent to transmitters.
There is a technical solution to this problem that, unfortunately, is not
part of the WiFi specification. It calls for synchronizing the transmitters
so when one radio is listening, they all are listening and the radio front
ends don't get wiped out by a nearby transmitter. This technique is
used by the Motorola Canopy system.
It is very difficult for users to test for capacity. It takes lots of
clients well
spread out and generating lots of traffic. It is much easier to test for
coverage. But coverage is not why you would buy a multihead radio.
If there is an application which fits Xirrus best, it would be a very large
room were the distance from the Xirrus in the center to the closest wall
is great, and the room has a high density of active Wi-Fi users. At Santa
Cruz, we have a few large lecture halls that fit that description. But
the ceilings in these rooms are very high and our guys don't want to need
to call in the scissor lift to get to the wi-fi gizzmo. A few APs
mounted on
the walls at the edge of the room works, and is a lot easier than a
super-WAP
in the center of the space.
And remember: if you do a test in a large lecture hall and it comes up good,
that will tell you next to nothing about how well that solution will
work e.g.
in a dorm room environment with a maze of twisty little passages made of
lots of walls and doors.
A lot has been written about how to set up WiFi for large meeting rooms.
I think Aruba has a white paper. While it may be comforting to think you
can buy a package solution to this problem that attaches to the ceiling with
velcro and you're done, I'm not sure that's the path to the best solution.
If you're read this far, you should go back and read the very first line
I wrote.
-jim warner, UCSC network engineer
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