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Problems with wireless networks

Posted on 2008-09-30
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Last Modified: 2012-05-05
I have 6 waps placed around the building. This is a school, and the wap's are used for the student computers.

I have each wap on separate channels. ie  1,3,5,7,9,11

On average I have about 20+ users on each wap. I am sure I am pushing the limit on these wap's, but money is low, and I kinda have to make due.

The problem is computers are losing there connection alot during the day. Or the routers just seem to die, then start working again. Some of the waps never have a problem, while the ones that are in each classroom. The classrooms are sit by side. These are having constant problems.

If anyone has a idea why. Or recommendations on different equipment to use. Or if I have these waps completely setup wrong.
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Question by:Con366
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by:R_Janssen
ID: 22606146
Most commonly if AP's are freezing up is due to the fact that their NAT table is full. The best 'workaround' for this is to set the network up so every AP is just bridged to the LAN and let one sole router (pref a server and not a AP) do the LAN/WAN interface.
 
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Author Comment

by:Con366
ID: 22606237
Can you elaborate more on what you said. I belive you may have hit it on the button
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by:Con366
ID: 22606256
So you believe the equipment is fine. And should be able to handle that many users, with no issues?
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by:Darr247
ID: 22607128
Separate the channels by 5. Any closer and the frequencies overlap, causing interference with each other.

Here's a list of the frequencies along with a chart to help visualize it better - http://www.moonblinkwifi.com/2point4freq.cfm

So use only channels 1, 6 and 11 if the APs are within 100 feet of each other.

e.g.

1                                       11                                       6






                  6                                         1                                               11


Channel 11 is the one most affected by microwave interference, ergo try to stagger them so 1 and 6 are used nearest areas where microwave ovens might be in use throughout the day. Wireless intercoms are another common source of interference.

Install netstumbler on a laptop and walk around with it checking signal levels (and noise levels if the card used provides that output to netstumbler - not all do).  That will also help find any rogue access points that may be running in the neighborhood... if you find a bunch using channel 6 (most manufacturers' default), then use only channels 1 and 11 on the APs nearest those.

For best horizontal coverage, both antennae should be pointing straight up, and higher is usually better (say, 10 or 11 feet up on the wall if ceiling height allows it).  Mounting them above drop ceilings usually doesn't block the signal too bad. If you need coverage directly below when they're mounted up high, position one antenna horizontal.  One antenna may work better than the other for that, so try it both ways if necessary.

What type of construction, btw?  Block wall?
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Author Comment

by:Con366
ID: 22607520
Yes block walls. Only one microwave in the building, but it is right besides the classrooms with the waps
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by:Darr247
ID: 22607971
Did you change the channels they're using so they're separated by 5?
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by:Con366
ID: 22610176
Will be.

But i have 6 wap's. So you are saying I should have them all on the same ssid, but make the channels 1,6,11. what i am not understanding is, if there are 6 wap's should the ones close to each other use the same channels?
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by:Darr247
ID: 22610427
> what i am not understanding is, if there are 6 wap's should the ones close to each other use the same channels?

Not if you want to maximize the number of clients they serve and the maximum bandwidth they provide.
But if the AP's close to each other ARE on the same channel they should share politely, according to the 802.11 specs, and not cause interference of the kind that makes everything appear to disconnect.

But, what are you considering ''close to each other'' ?

And what is their purpose... are they connecting to a local server, or providing internet access, or access between campuses across an internet link... or a combination of some/all of those?
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Author Comment

by:Con366
ID: 22610654
The computers will mainly be used for internet to do online classes, and research, and a few programs that write to a db on the server.  All the computers are on a domain

what i mean by close to each other

I have a 3 story building. I have 4 waps located on the first floor about 50 foot from each other. 2 waps on the 3rd floor that are about 50 foot from each other.

basically I have 3 classrooms on the first floor in rooms side by side. The 4th wap is located on the other side of the building. This is true also for the 3rd floor. each wap handles 2 of the labs.

each wap at the moment handles roughly 20 to 25 computers.
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by:Johnjces
Johnjces earned 600 total points
ID: 22610749
By having your APs "stacked" in a multi-floor building it will be impossible to properly "channelize" them, i.e. keeping them 5 channels apart, which is best practices for a lot of reasons.

1). Make sure the SSID is identical on each access point.
2). Check and see if your APs will auto-configure thier channel settings usually through the web interface.

You "could" try setting each AP to the same channel. Now this is not necessarily a good practice but I have experimented at length with this and in my environment had few if any problems. See:

http://www.experts-exchange.com/Q_22963288.html

The link is quite a discussion I was involved with on channels. Now, each AP is different in many ways. I had no problems setting all of my APs on the same channel, but I ultimately allowed them to set their own channel and they all came out 5 channels apart. But I am not "stacked but spread out down long halls.

John
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Author Comment

by:Con366
ID: 22610974
John,

i was kinda thinking the same thing. I read over the link you provided. If I knew these things could handle 50 computers each, then I could easily do a 1, 6, 11.

Can anyone think of a better way to do the layout. In the future I will hard wiring everything, but this will be months away.
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by:Johnjces
ID: 22611050
I cannot. I am not "keen" on those linksys models as I believe them to be more for a home systems with just a few clients. There is some OpenSource firmware for certain models of the LinkSys APs...I think for access points... have to look it up as my brains are fried.

Like DD-WRT, OpenWRT and others... but I think it is for routers...

Here's a link for further info anyway, towards the bottom of the wiki page.

John
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Darr247 earned 1400 total points
ID: 22611062
> basically I have 3 classrooms on the first floor in rooms side by side.

First, disable the auto-20/40MHz and lock all six to Standard-20MHz.
For Standard channel put one of each of those three on channels 1, 6 and 11.


> The 4th wap is located on the other side of the building.

Put that one's Standard channel on Auto.


> for the 3rd floor. each wap handles 2 of the labs.

Put each of those at least 5 channels away from the one that's nearest to being directly below.
e.g.

3rd fl                     1                            11

2nd fl

1st fl                11                6                   1


There are many ways to arrange them... that's one.

I presume you have AP Isolation enabled on all of them.

What are you using for DHCP?

WPA2-AES+TKIP / 802.1x RADIUS for security?
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by:Johnjces
ID: 22611085
Woops... forgot the link

http://en.wikipedia.org/wiki/Linksys_WRT54G_series

And darr247's idea sounds like something to try... just make certain that physical distance and max obstacles are in between those APs on the same channel.

John
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Author Comment

by:Con366
ID: 22611114
Darr247:

AP Isolation is not enabled on them. That will be done in the morning. I like your idea, and will try it out tomorrow.

As to answer your question.

I have server 2003 handling the dhcp. A second DHCP server will be up tomorrow, cause I will be moving the student to a new subnet.

WEP is all that is going on the wap's. I know bad choice. I just hadn 't got to changing that yet. Till today I had alot of Win 2000 machines on the network.

do yall see a bandwidth problem coming from 20+ users on each wap.
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Expert Comment

by:Johnjces
ID: 22611138
If they are all "surfing" at the same time, yes. That pipe will get smaller as your AP has to service each PC in turn and that does take time.

If the page or data is fairly static or such as database information form a server in a client server database app, then the data comes in bursts and the latency will not be so great.

It would be interesting to ping something continuously ping -a google.com on one PC, then on another and so on and see what the trip time is as each PC is added... or ping your local 2003 server.

This would give you an idea of the latency.

John
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by:Johnjces
ID: 22611150
I AM losing it...

ping -t some_IP

will do it continuously
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by:Darr247
ID: 22611339
> do yall see a bandwidth problem coming from 20+ users on each wap.

~28Mbps effective throughput divided by 20 users = 1.4Mbps for each client. That's about as fast as a T1 connection on the wireless side, and plenty fast for streaming most video. I think the bottleneck would be your Internet connection[s]. You would need multiple T3 (aka DS3, about 45Mbps) lines, and maybe they'd be subsidized for a school, but each T3 is about $9000/month around here.  

Ahhh... a quick search online tells me the cost of a T3/DS3 line ranges from $2600 to $13000 per month.
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