3com WAP - public/secure and 'N' vs B/G

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I was at our local library today,
and it appears they have several WAP's installed,
but I had problems getting connected...

There are two SSID's -
one is public and one is private/secure.

Using WiFi Hopper it displayed a couple of 3Com access points
with what appeared to be "related" MAC addresses...
    00:22:57:00:13:40  - public
    00:22:57:00:13:42 - secure

Anyway - when I got home & sent an email to their "tech" person
and mentioned that I was having a problem,
here was the jist of the reply...
Wrong tech language makes me not hopeful :)

Since I don't have anything running B/G/N -
could they have something configured to support N,
that accidently doesn't allow B/G to connect - and they don't know it :)


I have familiarity with our
wireless access points, and also have familiarity with wireless
setups.   I'm sorry you had some connection problems when you were at
the library.  Every so often, our wireless connection weakens, but
overall, the signal is usually pretty strong.   However, I have a few
ideas that you might want to try:

If you were sitting at a table near the magazines, you would be near
one of the wireless hubs, which should have boosted the
signal.   There are 4 hubs throughout the library (2 downstairs, one
in the meeting room and one in Youth Services upstairs) and are for
anyone to use.  You indicated that your laptop showed access points
of LLD-Public and LLD and if you select one of these, your laptop
should make the connection with no problem.

The LLD-Public connection is using the newest technology for 802.11 N.

In addition, some of the newer laptops, like ones from Dell, need a
utility enabled to allow complete connection.   We ran into this
problem with the new laptops for our computer classes, and I know
another patron who also had this problem.    If you are using a Dell,
you might want to check that you have enabled the Dell wireless
utility.  You can go to the Dell website if you need to download it
--- it is called Dell Wireless WLAN Card Utility.    If you are using
a laptop made by another manufacturer, there is most likely a similar
utility for your wireless connections.

Another thing to double-check are any firewall settings you might
have enabled to protect your laptop when accessing wireless
points.  Our wireless network is unsecured (no password required).

Re: 3com WAP - public/secure and 'N' vs B/G

Quoted text here. Click to load it
here's some info I found -
but it discusses the client side, not the AP side -
802.11n APs that use the ISM band will indeed compete for air space with
existing 802.11g APs. That's especially troublesome for 802.11n APs that use
40 MHz channels, since the ISM band is only wide enough to fit 3
non-overlapping 20 MHz channels (1, 6, and 11). For example, suppose an
802.11n AP uses Channels 6 and 7 to create a 40 MHz channel -- that will
generate co-channel interference for every other 802.11g AP in the vicinity
except those using Channel 1.

However, if you deploy old and new clients in the same frequency band,
you'll need to enable coexistence mechanisms. Specifically, there are two
modes of coexistence specified by 802.11n: legacy mode and mixed mode.

In legacy mode, 802.11n clients behave just like 802.11g clients -- this
amounts to using new hardware in the same old way as before, with little
performance improvement. In mixed mode, 802.11n clients will send both the
old-fashioned 802.11g preamble and the new 802.11n preamble before they
start transmitting data.

Mixed mode lets 802.11n clients take advantage of performance improvements
like space-time block coding, short guard intervals, and frame aggregation,
while giving 802.11g clients a "heads up" to avoid collisions. The extra
preamble does add overhead, preventing 802.11n clients from achieving
optimum throughput. But this is the price you must pay to ensure peaceful
coexistence between old and new clients sharing the same band.

Re: 3com WAP - public/secure and 'N' vs B/G

On 05/03/2009 03:23, ps56k wrote:
Quoted text here. Click to load it

If they are running an 802.11n network they don't have to allow 11b or
11g to connect, the draft 11n only says that the equipment must be
capable of connecting to legacy devices so the user can have the
capability to run a mixed network.
Look at the set of 11n mode options that are available on this emulator

Re: 3com WAP - public/secure and 'N' vs B/G

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If the access point is setup for 802.11n - and only 802.11n -
would I still see the SSID beacon on my b/g card via NetStumbler, Hopper,
Windows ?

Is the beacon packet seen by all technologies at the lowest common "speed &
and only goes into 802.11n mode after the actual connection ?

Would I see the SSID, but just not be able to establish a connection ?

Re: 3com WAP - public/secure and 'N' vs B/G

On 05/03/2009 15:33, ps56k wrote:
Quoted text here. Click to load it
Without actually trying it I am not sure exactly what you would get.
I have been doing a bit of reading about N recently and am not sure how
some of these "legacy" connections are supposed to work.
"During the definition of the 802.11g standard, it was realized that,
since legacy 802.11b devices would not be able to decode the newer
802.11g frames, there ought to be a separate mechanism to help legacy
  devices set their NAV correctly and therefore to reduce the percentage
  of collisions on air. The 11g standard made use of existing ‘protection
  mechanisms’ – RTS and CTS – to help legacy stations set their NAV.
  A similar situation arose during the definition of the 802.11n
standard. Legacy 802.11a/b/g devices would not be able to decode the
802.11n headers – and therefore a protection mechanism becomes
necessary. One of these is the transmission of legacy preamble and
header that enable the 802.11a/g/ device to detect the 802.11n packet
and to decode the information in its signal field, from which the
correct packet duration can be determined."

My reading of that is you should see something.

There was also this
"Backward compatibility with legacy devices also may be enabled by
forcing devices that are compliant with a newer version of the standard
to transmit special frames using modes or data rates that are employed
by legacy devices. For example, the newer devices may transmit Clear to
Send/Ready to Send exchange frames or Clear to Send to self frames as
may be employed in standard 11g. These special frames contain
information that sets the network allocation vector of legacy devices
such that these devices know when the wireless channel is in use by
newer stations."

I have heard mixed reports about even using an 11n card with netstumbler
with some people not getting it to work and one person who got it work
only having it report speed at 54Mbs.

Re: 3com WAP - public/secure and 'N' vs B/G

I recently borrowed a wireless router that accepts cell phone 3G wireless
cards (this one was Sprint) and rebroadcasts the signal as standard b/g/n (I
don't recall the router brand)... I wanted to see if this setup had enough
bandwidth to allow three b/g equipped laptops to surf the net without
obvious slowdowns (it did). The default was set to broadcast in b/g/n modes,
so at one point I selected "n only" and after the router restarted all three
laptops detected the network but could not connect. I then connected to the
router with a network cable to change the setting back...

Re: 3com WAP - public/secure and 'N' vs B/G

On Thu, 5 Mar 2009 09:33:30 -0600, "ps56k"

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Which N mode?  Beam forming or spatial diversity?  In beam forming
(Ruckus Wireless), it's basically the same as 802.11g and you will see
beacons.  In spatial diversity (Airgo), I think (not sure) that the
spec demands beacons.  However, I'm fairly sure that the beacons are
at 6Mbits/sec as in 802.11g.  If you enable 802.11b compatibility,
beam forming still works and it will belch beacons at 1Mbits/sec.
However, with spatial diversity, 802.11b and MIMO speeds are mutually
exclusive and I really doubt that you'll see 1Mbit/sec 802.11b
beacons.  No clue what happens with the new chips that do both beam
forming and spatial diversity.  I gotta read the specs to be sure, but
I think that's the way it works.

What you're basically asking is how does 802.11n operate in
"greenfield" mode, where there are no constraints imposed by prior
technology and legacy junk compatibility.  Greenfield mode is very
unlikely to be seen in a public library.  My guess(tm) is what you're
seeing is your wireless client software annoucing the highest level of
acronym compliance.  Just because you can't connect doesn't mean the
lower standards are not available or functional.  It would be nice if
Netstumbler and others annouced all the various supported 802.11
acronyms, but they don't.  

Incidentally, se have a somewhat similar derrangement at the local
hospital.  You can connect, but can't do anything without a WPA
password *AND* and a VPN client.  Everyone complains that they can't
connect, until they finally read the splash page.  Anyway, don't
assume that it's 802.11n "only" just because you can't connect.

Quoted text here. Click to load it

I don't know if this really helps, but the section on backwards
gives a few clues on how much of 802.11g technology (beacons) is
compatible with 802.11n.  I'm to busy/lazy right now to find a better

Gone to waste the whole weekend:

Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

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