two AP's with same SSID for coverage

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Having seen several threads about this type of WiFi coverage issue,
I was wondering about having 2 AP's at ends of the house
to provide better coverage....

If we have 2 AP's with the same SSID,
both connected to the same IP segment,
and the DHCP coming from one router,
then what are the issues involved ??

Not really a roaming issue, but it can be...

SO - we have one AP upstairs in the middle of the upstairs bedroom area,
and another down in the family room area.
Will you "associate" with the strongest signal ?
Is that always true ?  In the middle, can you ping-pong between them ?

For the most part - you are either upstairs or downstairs.
What happens if you are downstairs, and then walk up to your room
to do homework - will you re-associate with the closer & stronger AP ?

Just wondering how this could/would/should work with multiple AP's
all on the same channel & SSID ?

--
----------------------------------
"If everything seems to be going well,
you have obviously overlooked something." - Steven Wright



Re: two AP's with same SSID for coverage

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  The multiple APs should be set to DIFFERENT channels and the same SSID.
This is the standard configuration for multiple APs and wireless roaming.
It generally works well, though is best with identical APs and the same
protocol (e.g. both 11g).  Yes, you will re-associate with stronger signal
AP.   You will not generally "ping pong" between them is signal strengths
are similar.   It's generally no problem to move from one coverage area to
the other though if you have an active download (for example) going on it
could be interrupted by the momentary delay in the changeover.


Re: two AP's with same SSID for coverage
wrote:

~ Having seen several threads about this type of WiFi coverage issue,
~ I was wondering about having 2 AP's at ends of the house
~ to provide better coverage....
~
~ If we have 2 AP's with the same SSID,
~ both connected to the same IP segment,
~ and the DHCP coming from one router,
~ then what are the issues involved ??

Should work.  This is the best way to provide WiFi coverage to a house
that can't be covered by one AP.

~ Not really a roaming issue, but it can be...
~
~ SO - we have one AP upstairs in the middle of the upstairs bedroom area,
~ and another down in the family room area.
~ Will you "associate" with the strongest signal ?

Hopefully.

~ Is that always true ?  In the middle, can you ping-pong between them ?

Could happen.  Wireless clients have gotten smarter about roaming
over the last few years.

~
~ For the most part - you are either upstairs or downstairs.
~ What happens if you are downstairs, and then walk up to your room
~ to do homework - will you re-associate with the closer & stronger AP ?

You probably will.  Some clients may remain associated with first AP till
they absolutely can't communicate any longer, and only then will scan for
a new AP.

~ Just wondering how this could/would/should work with multiple AP's
~ all on the same channel & SSID ?

In the general case, this can be quite complicated, but in your
case I wouldn't worry about it too much.  However, I will note that
our APs should probably NOT be on the SAME channel, but on different
non-overlapping channels.  I.e., if using 2.4GHz in the US, you would
likely want to use two of {1, 6, 11}.  Otherwise the two APs will
interfere with each other and you will get reduced performance.

Aaron

Re: two AP's with same SSID for coverage
ps56k wrote:
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why the same? To eliminate the prob I have different SSID's (don't roam,
just associate with whichever gets the best signal, PPinUSA and PPinUSB)...
As to your q, depends on your client software, no standards if it hangs on
to the last one, or trys to reassociate with a stronger one).... Actually
have a 3rd now (PPinUSN, mimo/pre-n, so devices that can do it's n can see
that too)



Re: two AP's with same SSID for coverage

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Specifically to allow roaming.

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That works, but it ensures that clients will see a disconnect when
jumping from one to another, as well as requires additional
configuration upfront.  Maybe that's a problem, maybe not.

Re: two AP's with same SSID for coverage
On 23/01/2009 02:09, ps56k wrote:

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Meru Networks have been doing this as an enterprise solution for a while.
<http://www.merunetworks.com/pdf/whitepapers/WP_ATC_0807.pdf

Re: two AP's with same SSID for coverage
LR wrote:
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just looked at the setup screen thing for my linksys pre-n, and you CAN't
set a channel!
manual at
http://downloads.linksysbycisco.com/downloads/WRT160N_ug.pdf

(guess there are no more channels, but you can change the ssid)



Re: two AP's with same SSID for coverage
On Sat, 24 Jan 2009 01:03:49 -0500, "Peter Pan"

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If you want lots of speed, the spatial diversity flavor of 802.11n
needs two 40MHz wide channels crammed into 83.5MHz of available
spectrum.  The two channels are always the same.  In effect, this mode
uses the entire legally allowed bandwidth.  Therefore, you have no
choice.

If you set the access point for 20Mhz channel mode, which is the same
channel bandwidth used by ordinary 802.11g, you can select from the
usual assortment of 11 channels.

Incidentally, even if you decide to use two 40MHz channels (wideband
mode), all it takes is one local 802.11g radio, and the system
switches to CTS (flow control) mode to prevent collisions and
interference.  The only way it works fast is if everything is using
802.11n.  There are some comments on this and other issues at:
<http://en.wikipedia.org/wiki/802.11n

--
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

Re: two AP's with same SSID for coverage
On 24/01/2009 08:08, Jeff Liebermann wrote:
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Could you clarify the "use 2 40MHz channels" as I was under the
impression that 802.11n could use 2 20MHz channels to produce one 40MHz
channel and that they haven't as yet tried to bond 2 40MHz "channels".

Re: two AP's with same SSID for coverage
LR wrote:
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my understanding is that instead of using numbers (and IMO there aren't 11
since some overlap, so you could really only use 3 anyway), now they use the
terms primary and secondary, each being twice as wide but not bonded...
think 3 1 inch hoses (labeled #1 #6 #11) versus 2 2 inch hoses (labeled
primary and secondary), which would you rather use on a fire? and when
bonding happens, one 4 inch hose



Re: two AP's with same SSID for coverage
On 24/01/2009 17:09, Peter Pan wrote:
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"Channel Bonding
The most straightforward way to increase the capacity of a network is to
increase the operating bandwidth. However, conventional wireless
technologies are limited to transmitting over one of several 20-MHz
channels. 802.11n networks employ a technique called channel bonding to
combine two adjacent 20-MHz channels into a single 40-MHz channel. The
technique more than doubles the channel bandwidth. Channel bonding is
most effective in the 5-GHz frequency given the far greater number of
available channels. The 2.4-GHz frequency has only 3 non-overlapping
20-MHz channels. Therefore, bonding two 20-MHz channels uses two thirds
of the total frequency capacity. Therefore, the IEEE has defined rules
on when a device can operate in 40MHz channels in the 2.4GHz space to
ensure optimal performance. Cisco expects that the greatest benefits of
channel bonding will be realized in the 5-GHz frequency."
<http://www.cisco.com/en/US/prod/collateral/wireless/ps5678/ps6973/ps8382/prod_brochure0900aecd806b8a92_ns767_Networking_Solutions_Brochure.html>

"The major changes this time around involved the implementation of the
40-MHz channel, which was tweaked to accommodate older 2.4-GHz band
devices that could be confused by the wider channel bandwidth.

Version 2.0 of the spec also calls for the use of two 20-MHz bands that
will allow the system to scan a given environment for legacy devices
that might not understand the wider bandwidth. If such a device is
found, the 802.11n device will defer and send data over only a single
20-GHz band. " I assume this is a misprint and should read 20MHz
<http://www.extremetech.com/article2/0,2845,2103646,00.asp

Re: two AP's with same SSID for coverage

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Sorry.  You're right.  It should be "use two 20MHz channels" which are
bonded to produce one 40MHz channel.

--
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

Re: two AP's with same SSID for coverage
Jeff Liebermann wrote:
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or plan B.... (using different ssid's one for normal and one for High Speed)
i use PPinUS for normal b/g pda/tablet/tivo's and PPinUSN for the two
laptops that can do n, and the router part connected to the powerline
(injectors? 1/2 a pair? what do you call one of two?) and use that (and an
ethernet cable) for even higher speed connection/stuff when i want to xfer
huge stuff.... at work i have two (different ssid's DH-Public and
DH-Private), now i have to admit it is nice having the same ssid at
airports/coffee shops/hotels/etc, but it's sort of neat to have different
ones for different purposes.... Wonder why people beat their heads against
the wall trying to get them the same, rather than celebrate diversity and
exploit the cool extra things you can do?



Re: two AP's with same SSID for coverage
On Sat, 24 Jan 2009 20:38:56 -0500, "Peter Pan"

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Won't work.  As soon as the 802.11n router hears *ANY* 802.11g signal,
regardless of SSID, it will switch on flow control to prevent
collisions.  In the 40Mhz mode, it can't decode the preamble or
beacons from an 802.11b/g AP or client.  So, it has to treat
everything as a potential co-channel user and give them some airtime.

Also, there are some AP's (i.e. DD-WRT) that can do multiple SSID's.
However, the only options offered for these multiple SSID's are
encryption and authentication methods.  The modulation and 802.11
modes are apparently identical for all SSID's.  It is therefore not
currently possible to set one SSID for 802.11g and the other to
802.11n.  I don't see any reason why an AP cannot do this, it's just
that I haven't seen any that have bothered.

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Huh?  Is this one router or two, each with it's own SSID?  I can see
how it would work with two routers, but not one.

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Well, 3 of a kind beats two pairs.... I dunno.

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Gigabit ethernet rules.  I've been slowly upgrading the office boxes
to gigabit.  It's sure noticable when cloning or backing up a machine
over the network.

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I kinda prefer to have some control over which access point I'm
connecting to.  Some wireless client managers will show multiple
SSID's differentiated by unique MAC addresses.  Most don't.

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Easy.  I once fired up WZC from the parking lot facing a big office
building complex in Los Gatos CA.  I didn't count the number of SSID's
but it was well over 100.  WZC was constantly refreshing and updating
as it scanned and picked up new SSID's.  The problem with a good thing
is that too much of it is considered pollution.


--
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

Re: two AP's with same SSID for coverage
Jeff Liebermann wrote:
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actually 3, but i assume you only care about the first b/g and the third n
only... Tivo's and one of the PDA's and tablets will only do G, and one pda
and smartphone b only, so I tend to leave old working stuff working, and
just add new as I aquire it

oddly enuf, i've been getting them donated/collecting old ones (if nothing
else, now I have 7 I can play with/brick, and many spare/backup walwarts,
i'm retired, so now time to putter and fix instead of destroying :) , and
just change the starting ip address for both wired and wireless, so they
don't conflict, leaving the dhcp servers on etc, and plugging the router
parts/port to each other to daisy chain em



Re: two AP's with same SSID for coverage

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I thought there were perf probs when letting a B signal exist
within the same range as a G - and therefore probably N ?



Re: two AP's with same SSID for coverage
On Sat, 24 Jan 2009 23:56:04 -0600, "ps56k"

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There are mutual interference problems only if there is traffic moving
on BOTH access points.  If either access point is idle, a few
broadcasts and beacons aren't going to create much interence.  Of
course B and G can coexist if on different non-overlapping (1,6,11)
channels.

My rule-de-thumb is that if the receiving access point can decode the
packet, then the normal flow control mechanisms will only result in a
general slow down.  This is not the case with 40MHz 802.11n, where the
802.11b/g access point treat the modulation as if were noise or
garbage.

The problem between B and G was mostly historical.  When G first
appeared, it included an 802.11b compatibility mode.  If the G
receiver heard even one B packet, it would leave a big chunk of the
timing open to listen exclusively for slow B packets.  This was a huge
performance hit as initially implimented.  See:
<http://wireless.navas.us/wiki/Wi-Fi#Performance_and_Speed
802.11g only could get 24.4Mbits/sec thruput.  The same radio with
802.11b compatibility mode enabled would only get 14.4Mbits/sec or a
bit more than half.  (Remind me to update the chart one of these
daze).

However, the situation did not remain.  More intelligent algorithms
were invented to reduce the compatibility performance hit.  At this
time, the mere presence of an 802.11b access point, that's only
belching beacons and broadcasts, has almost no effect on 802.11g
thruput.  I've tried it with various access points and it works.
However, when 802.11b traffic is moving, even to a different system
with a differeent SSID, the 802.11g compatibility mode kicks in, and
things really slow down.

--
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

Re: two AP's with same SSID for coverage
ps56k wrote:
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beats me, I use wired connections for big stuff (like tivo recordings,
usually over 2 gig)... As for n, oddball things (and I have no clue why),
you know those silly wifi finders and some laptop clients that show what
wireless networks are around? the one set for n only, doesn't show up on
either! (microwave oven does on the wifi finder thing when I nuke my pizza
rolls, course my dog comes running and howls too, so I assume it (wifi
finder) looks for some sort of transmission on that frequency, but dang if I
know why the nuker shows but the n ap doesn't, nor why the dog comes running
((unless he likes snacks/junk food)))



Re: two AP's with same SSID for coverage

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So move to 5.8GHz.  There are so few 802.11a networks out there that
802.11n 5.8GHz is, at least for now, the holy grail of wireless
performance.

Re: two AP's with same SSID for coverage
On Sun, 25 Jan 2009 15:08:21 -0800, DevilsPGD

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Sure.  Depending on the customer, replacing everything with new
hardware will vary from a financial impossibility to an invitation to
find another consultant.  Nothing like a "change everything" solution
to a solving a minor problem.  Hmmm.... didn't we just elect a
president on that principle?  Oh-oh.

Besides, technology offers a possible solution.  If you can't coexist
with them, jam them with more RF:
<http://www.theruckusroom.net/2008/08/can-you-really.html
<http://www.merunetworks.com/technology/security/rfbarrier.php

Anyway, let's get real for just a moment.  There's no way you can get
a free lunch with wireless.  If you want speed, you're going to lose
range.  If you want reliability, it will cost you both speed and
range.  If you want backwards compatibility, you have to give it air
time which costs speed.  Free lunches only exist among congress
critters.  You have only to look at the product test results on
SmallNetBuilder.com to see the distinction between advertised claims
and reality.

Even MIMO doesn't offer a free lunch.  Since multiple streams depend
completely on the presence of reflections, if anything changes to
affect those reflections, the thruput falls apart.  I've seen this in
my "testing" (i.e. screwing around) with an Apple MacBook and Airport
Extreme (802.11n) base.  While running continuous Iperf or Jperf
benchmark tests, the thruput varied all over the place.  Thruput would
sometimes hit 60Mbits/sec followed by an immediate drop to perhaps
12Mbits/sec.  I only had to move the laptop slightly, and everything
would change.  Watching an H.264 QT video, while walking around the
room, was ummmm.... painful.  More:
<http://www.theruckusroom.net/2008/12/pest-control-80211ns-dirty-little-secret.html
With just the Apple stuff running in the room, performance was
respectable and methinks impressively high.  However, when I fired up
a wireless peer to peer network between two other laptops nearby,
thruput hit bottom at perhaps 3Mbits/sec.


--
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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