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Clarification required on Wireless Interference.


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#1 sandy2810

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Posted 07 January 2011 - 12:16 AM

Hi guys,

I am posting after ages..dont quite remember my last post..anyways we were having a debate related to wireless interference. Suppose there are 2 APs, one with SSID "Linksys" and the other with "Cisco". Both these APs are operating on the same channel. My opinion as to this scenario was...interference. As per my understanding if there are 2 AP with different SSID but operating on the same channel interference will coexist. My colleague had other opinion. According to him interference will occur only if the SSID's are also the same. Can someone throw some light on this please.

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#2 chrcel

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Posted 07 January 2011 - 12:46 AM

I'd love to see wireless experts opinion. But IMO SSID is sitting above radio so it is independent. no doubt any 2 APs transmitting on the same channel would interfere. Saying this means they will coexist, but both would share the same radio spectrum so lots of retransmission, low RSSI (as the other APs transmitions are received as an interference) and thus low performance.
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#3 Sunfish

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Posted 07 January 2011 - 03:50 AM

Yes, if 2 AP's use the same channel then they are subject to interference causing low performance, connection problems, etc. Interference happens because they use the same frequency, it does not matter what kind of information (SSID) you are sending on that frequency. Therefore, any other devices transmitting on the same frequency, e.g. bluetooth devices, old microwave ovens, cordless phones, etc., also cause interference. Those troublemakers surely do not have the same SSID. ;)

Therefore, if you use 802.11b/g, you should use only channels 1, 6 and 11 in order to avoid interference. Only Japan has a fourth channel available that does not interfere with the others, i.e. channel 14.

Edited by Sunfish, 07 January 2011 - 03:55 AM.

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#4 chrcel

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Posted 07 January 2011 - 04:00 AM

Only Japan has a fourth channel available that does not interfere with the others, i.e. channel 14.

actually in CZ channel 14 is also available, or is it not? what ever for compatibility you should still use 1,6,11
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#5 Sunfish

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Posted 07 January 2011 - 07:51 AM

To the best of my knowledge channel 14 is limited to Japan only, while almost all European countries only allow channels 1 - 13. My Cisco AP's are also all limited to those channels (European models).

Just checked for some descriptive illustration and came across Wikipedia's List of WLAN channels which also shows nicely why typically channels 1, 6 and 11 are the ones to be used.

Anyway, I moved to 802.11a quite some time ago, as there are at least 8 alien SSID's in my neighborhood. No more WLAN problems and excellent performance. ;)
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#6 sandy2810

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Posted 09 January 2011 - 03:23 PM

Yes, if 2 AP's use the same channel then they are subject to interference causing low performance, connection problems, etc. Interference happens because they use the same frequency, it does not matter what kind of information (SSID) you are sending on that frequency. Therefore, any other devices transmitting on the same frequency, e.g. bluetooth devices, old microwave ovens, cordless phones, etc., also cause interference. Those troublemakers surely do not have the same SSID. ;)

Therefore, if you use 802.11b/g, you should use only channels 1, 6 and 11 in order to avoid interference. Only Japan has a fourth channel available that does not interfere with the others, i.e. channel 14.


The thing is, there are lot of AP's in my area and you will find atleast one AP each operating on 1,6 and 11 channels. So changing channel on the affected AP also will do no good as some other AP will cause interference.
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#7 chrcel

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Posted 09 January 2011 - 06:44 PM

that is when wireless survey and coordination comes to play. if those other APs are not under your control you'll have no other option then just transmit on the channel of the weakest AP.
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#8 sandy2810

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Posted 09 January 2011 - 08:00 PM

that is when wireless survey and coordination comes to play. if those other APs are not under your control you'll have no other option then just transmit on the channel of the weakest AP.


Thanks for the input, I had the same opinion as yours.

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#9 Sunfish

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Posted 09 January 2011 - 09:11 PM

In fact most of today's AP's are able to dynamically select the best channel automagically. However, they only "see" their own situation and it might be totally different where the users are. Hence you still need to perform a site survey to really find the best solution.

The situation becomes even more complicated if the other SSID's (the ones that are not under your control) are moving to other channels from time to time because they use this auto-discovery of the best channel. So just a few days or weeks after your site survey the performance of your own WLAN might be totally degraded again. In my case I have up to 8 other SSID's in the neighborhood and had to use a fixed channel as the solid armored concrete ceilings required the use of a repeater to cover the necessary spots.

I got quickly tired of having to adjust the channels every 2 - 3 weeks or so and finally moved to the 5 GHz band, i.e. 802.11a (which unfortunately required new hardware). However, never ran into any problems since then. :)
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#10 chrcel

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Posted 09 January 2011 - 10:02 PM

I got quickly tired of having to adjust the channels every 2 - 3 weeks or so and finally moved to the 5 GHz band, i.e. 802.11a (which unfortunately required new hardware). However, never ran into any problems since then. :)

exactly what I did :) well the radio flapped once, but that was a firmware problem...
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#11 sirkozz

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Posted 10 January 2011 - 01:48 AM

Here’s the thing, what your guys are talking about is not interference; interference is a microwave, or some other 2.4GHz transmitter. The biggest issue with 802.11 is that no matter what every STA is always listening to the media in case it has to transmit a packet, and it listens for any 802.11 packet regardless of SSID. 802.11 uses CSMA/CA to prevent collisions which can be supported with RTS/CTS in either dense “rouge AP” situations, or to resolve hidden-node issues. The major drawback of using RTS/CTS is that for every data frame a RTS/CTS exchange has to happen, and while not bandwidth intensive, from an RF utilization perspective it might double, time being the limiting factor. The truth is that a modern AP (Cisco 1242) is usually able to compensate in these types of environments, think an office building in Manhattan. The best way to proceed is document what other sources of 802.11 are around you and then make changes to your WLAN to compensate for it, changing channels, changing transmit power, even adding AP’s, sounds much easier than it is but it can be done. I’ve designed WLAN’s with 200% redundancy, 2 AP’s for every AP surveyed, and both on all the time, it can be done.
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#12 srinpathy

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Posted 12 January 2011 - 04:01 AM

Interference can be caused if two ap's are transmitting on the same channel irrespective of the SSID.
SSID as said is just the network identifier.It can also be caused by Cordless phones on the same freq range or by FHSS bluettoth devices if is 802.11b alone and not on 802.11g.

802.11a is less prone to interference but will fall short on coverage.

Also in SOHO environments we have liberty to choose whatever channels we want and we need not stick on to 1,6 or 11.
But when we consider Enterprise networks with wireless deploymnet i.e Aruba,Meru,Alcatel&Cisco they do dynamic channel shifting which is implemented through ARM(adaptive radio management) where in the calculation is done on a centralised controller which can change channels w.r.t SNR,Frame retries and interference index.
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#13 sirkozz

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Posted 12 January 2011 - 09:10 AM

Correct me if I’m wrong but technically speaking if 2 AP’s regardless of SSID are operating on the same channel while called interference isn’t really frame collisions?
SOHO not using 1,6, &11 is the reason I never ever want to go and do a trouble-shoot in NYC, come on, idiot guys running all their AP’s on the same channel, or 1 on 1, 2 on 2, ad naseum, you’d still be amazed at how many companies still use 40bit WEP!
Regarding ARM, controllers implementing both TPC(transmit power control) and ACS( auto-channel selection) can work ok in some environments. In my experience if a proper site survey was done prior to install/integration then usually only minor tweaking is required as part of the integration process to ensure optimal WLAN performance. Just because ARM can work doesn’t mean it’s the optimal way to integrate a WLAN. Cisco like all other vendors are in the business of selling gear, but even they have backtracked on RSSI for VoWLAN’s, why because customers started actually looking at what these controllers were saying and saw that if all there AP’s were operating @1mW, 0.01 RF utilization, and most had only 1 or 2 STA’s than wow, way too many AP’s!!! As to the “interference index” about 2 years ago started doing some work with Meru gear and at the time AirMagnet didn’t support single channel architecture, so I had to learn Ekahau. This spring had some major time to compare both products on fully operational WLAN’s and found something that surprised the hell out of me; while AirMagnet might say that 45% of coverage area had some measure of co-channel interference, Ekahau areas of co-channel interference, 10% of the coverage area corresponded directly to areas that the customers were saying that there were connection issues! The reason I relate this to you is that Cisco and most other vendors use AirMagnet’s algorithms for their controllers, Meru uses Ekahau, and Moto uses LAN planer and nobody in their right mind uses anything but static radio configuration using Moto or Meru gear which like I stated previously require a competent site survey. Imho all this auto stuff is just plain garbage, if you’re going to use it you might as well put a map on a dart board and put the AP’s where they land, but being serious for a moment guys go ahead and keep doing it, it helps keep me employed, tia!!!

Edited by sirkozz, 12 January 2011 - 09:12 AM.

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