Overlapping

[jorge.s]

Hi,

how do I dettect, only accessing the Access Points, if a certain access point is having interferences due to the use of another channel by another access point, but which is overlapping the used one?

Jorge

[robert.wright · Accepted solution]

Jorge! Long time no talk ;) Come to ohio my friend so we can chat about wireless over a few beers ;) Maybe get rob to come down as well =p

Anyways i am not fully understanding your question(s). I am going to treat it as if you asked a two part question.

Q. How do you tell if your having interference?

A. Issue the following command, "show interfaces dot11Radio 0 statistics". Take note of "CRC errors".

Likly you have never cleared these stats so they can be very mis-leading... so issue "clear dot11 statistics dot11Radio 0" to reset the stats. Wait give or take 10-15 minutes and issue the show command again. CRC Errors is a good guide for any and all interference. This is going include anything from the lunch room microwave, rogue ap, to a damaged wireless nic.

Q. How do i detect what AP or channel is overlapping with my AP and causing problems.

A. Theres two ways to do this.. One is to obtain the *nix utility Kismet or the windows utility NetStumbler. Both are free. This will show the available APs in one given spot. You should compile a list of your APs MAC addresses prior to this to make it more helpful.

The other way, and likly more fruitful is to obtain a sprectrum analyzer such as Cognio's Spectrum Expert.

[jorge.s]

Hi Robert,

in fact I would not mind to join you in Ohio, I'm leaving in Portugal, but working accross Europe, and the problem I'm having is in this case, in Belgium.

Sometimes de Wireless Devices just get de-associated from the WLAN, and we cannot connect to them anymore (in fact, Linksys Wireless Bridges).

I've done like you described, and yesterday I cleared the counters, today when I look at it I find that there are some CRC errors. Do you think that this ammount is abnormal?

I will ask my boss for a travel there, probably we are having some interferences.

STR0115#sh interfaces dot11Radio 0 statistics

DOT11 Statistics (Cumulative Total/Last 5 Seconds):

RECEIVER TRANSMITTER

Host Rx Bytes: 22093373 /8622 Host Tx Bytes: 16997467 /4482

Unicasts Rx: 295418 / 127 Unicasts Tx: 174725 / 46

Unicasts to host: 295418 / 127 Unicasts by host: 174268 / 46

Broadcasts Rx: 1009 / 0 Broadcasts Tx: 469461 / 48

Beacons Rx: 315 / 0 Beacons Tx: 468814 / 48

Broadcasts to host: 1009 / 0 Broadcasts by host: 647 / 0

Multicasts Rx: 0 / 0 Multicasts Tx: 24910 / 3

Multicasts to host: 0 / 0 Multicasts by host: 24909 / 3

Mgmt Packets Rx: 0 / 0 Mgmt Packets Tx: 454 / 0

RTS received: 0 / 0 RTS transmitted: 2144 / 0

Duplicate frames: 470 / 0 CTS not received: 1458 / 0

CRC errors: 1876 / 0 Unicast Fragments Tx: 174399 / 46

WEP errors: 0 / 0 Retries: 4050 / 0

Buffer full: 0 / 0 Packets one retry: 837 / 0

Host buffer full: 0 / 0 Packets > 1 retry: 348 / 0

Header CRC errors: 143512 / 0 Protocol defers: 220 / 0

Invalid header: 71 / 0 Energy detect defers: 1234 / 0

Length invalid: 0 / 0 Jammer detected: 0 / 0

Incomplete fragments: 0 / 0 Packets aged: 0 / 0

Rx Concats: 0 / 0 Tx Concats: 0 / 0

RATE 1.0 Mbps

Rx Packets: 1080 / 0 Tx Packets: 126 / 0

Rx Bytes: 62820 / 0 Tx Bytes: 8661 / 0

RTS Retries: 698 / 0 Data Retries: 1550 / 0

[scottmac]

I think I'd start looking at the clients.

Obviously, some clients are getting through with good traffic, others, I believe, are sending incompatible or self-mutilating traffic. Self-mutilating would be a client carrying something like Bluetooth or two wireless NICs concurrently.

There are also some bad drivers. Earlier versions of te Intel BG2200 drivers would send (what was interpreted as) MIC errors and cause the AP to shut down (per spec).

If possible, try a representitive sample of clients, one at a time while monitoring the AP stats.

If it were channel overlap, I believe you'd see CRCs or bad trafic equally between the headers and the body of the frame.

As previously mentioned, something like Netstumbler (www.netstumbler.com) would give you an idea of the activity and interference (though Kismet is a bit more specific).

If you are using Symbol handheld devices, they also have a history of not working or playing well with some flavors of AP/Firmware. Try changing the AP IOS/firmware and / or the Symbol firmware and see if the problem persists.

If you believe it is an interference issue, decreasing your RTS value should improve performance / throughput. Pick an arbitrary value ... maybe sart at half and monitor the affect.

Check it out and let us know.

(Having grown up on the edge of the MSU campus, I'm curious .... they got electricity in Ohio now? *AND* technology like wireless? ... Golly!! ;-} )

Scott

[johnruffing]

Ekahau Site Survey tool can measure the RF from the access points in question and plot the areas on the floorplan where the interference is ocurring.

http://www.ekahau.com

This allows you to adjust your power / channel settings and/or add another AP on a different channel in the area where the interference is being shown on the floorlan.

[robert.wright]

Your CRC errors look fine, but your header crc's appear to be through the roof.

Now CRC errors i know theres the direct relationship with radio interference. The header CRCs are related to some sort of layer 1 problem. But sadly thats all i know... I am going to have to ask Rob or one of the other fellas...

This is purely my personal troubleshooting of what i would attempt to do. This is by no means a 'how you fix the issue'...

I would try and obtain some sort of spectrum anaylzer... even if its teh $99 wi-spy. I would also look at swapping out the antenna and seeing if that has any sort of impact. If there are presently the low loss antenna cables installed i would look at swapping those out after ruling the AP out.

{Waves hands in the air like a mad man to get the attention of Rob}

[Rob Huffman]

Hey Guys,

I would love to join you for a beer (Bengals or Browns?). I appreciate the nice vote on this, but I am an idiot when it comes to trying to break down these type of errors (THINK EYES GLAZED OVER LOOK). I think that they really just indicate some sort of RF interferance,antenna alignment,or channel overlap issues. Have a look at his guide that relates to problems with Cisco Bridges;

Reasons for Intermittent Connectivity Issues in Wireless Bridges

Here are the common reasons for intermittent connectivity issues in wireless bridges:

Radio Frequency Interference

Sub-optimal/Incorrect Data Rate Settings on the Wireless Bridges

Fresnel Zone and Line of Sight Issues

Problems with Antenna Alignment

Clear Channel Assessment Parameter (CCA)

Other Issues that Degrade the Performance of Wireless Bridges

From these excellent Cisco docs;

http://www.cisco.com/en/US/products/hw/wireless/ps5279/products_tech_note09186a0080508551.shtml#rfi

http://www.cisco.com/en/US/products/hw/wireless/ps5279/products_tech_note09186a0080508551.shtml#crcplcp

Cheers!

Rob

[robert.wright]

Rob, im in columbus so the only team is the Buckeyes.

[Rob Huffman]

Hi Robert,

OSU Number 1 this year?? Maybe we could go see the Blue Jackets take on my team the ***Calgary Flames***

Take care!

Rob