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New Member

Interference

Dear friends,

what is interference actully in wireless system? what is best pracitce to decrease the interference in wireless system?

why the interference happen and how i can stop or decrease it?

many thanks in advance

13 REPLIES

Interference

The best way to describe interference is with a normal every day example.

Assume for a moment that you and I are on a phone call. Then all of a sudden 2 more folks get on the same call as us. These 2 folks are chatting and we are chatting. As you can see when you have 2 different conversations going on at the same time we may stop repeatable and say "huh, can you say that again?". Our conversations are interferering with each other.

Same holds true with WiFi. If you have 2 aps on channel 6 and they are wihin 20 db and they are both passing traffic to clients you will get this same type of senerio. One of the tell tail signs is the retire counters, if you open a wifi analyzer.

Does this help ?

__________________________________________________________________________________________ "Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin ___________________________________________________________
New Member

Interference

thanks george for your reply

george how i can calculate about that both of 2 aps are within 20db ?

i mean how long distance could be between each others in meters?

many thanks in advance

Cisco Employee

Interference

It depends if there are obstacles between them or not. The best way is to stand where one AP is standing and see if you get the signal from the other AP loud or not.

Interference can also come from other types of devices.

Some wireless security cameras are known to use the same RF band as wifi and kills most of your transmission. DECT phones, microwave ovens ... Bluetooth from mobile phones

Hall of Fame Super Gold

Interference

Xbox.  Please don't forget those infernal X-box.  I've got a G0d-forsaken Xbox that's blasting channels 1, 6, and 11 24-hours a day.  Y'all have no idea how much I'm itching to hit the "contain" option. 

Cisco Employee

Interference

As the 2.4 and 5 Ghz channels are open bands any device that operates in this frequency can interefere with your WiFi devices.

Examples : RFID's , Bluetooth, Microwaves etc

Other Ap's in nearby networks are alos sources of interference

Clear Channel Assessment is around -82 dbm for AP's on the same channel

Make sure you keep these interference away fromthe AP's deployed within your network

If you have a WLC you can check the interefrence profile per AP on the WLC under the Monitor tab

New Member

Interference

thank you so much friends for your good answers

actually i am going to estblish a WLAN in a hospital n i have 1262N series LAPS

what  about if i have a big lecture hall like 25x20 meters and inside there  140 users sit in there n all of them have laptops,iphones,and ipads and  also they want to use voice over wireless,

i this case if i calculate that i need 15 access points in there and each access points is 7 meters away from his neighbor AP

what you say there will be interference or not

if yes than how i can distribute them in this area with 0% interference?

any seggustion please

many thanks in advance

Hall of Fame Super Silver

Interference

If you are going to place 15 AP's in a wide open area (25x20), then you would need to manually configure the power on each AP to the minimum or at least one of the lower power levels. You will have more overlap on the 802.11b/g/n side for sure since you have 3 channel (at least in the USA).  The 5ghz you have more channels to play with.  The only other thing is if you deploy 802.11N and channel bond in the 5ghz, that will reduce tha number of channels.  Lowereing the tx level on the AP is prbably the best way to limit the overlap.

-Scott
*** Please rate helpful posts ***
New Member

Interference

thanks for reply scott fella

actually this is big hal not an open area it has 25x20 squer meter dia and its height is 7 meters from its ground but i have also more than three meters above the ceiling on the roof so i can install some of them hidden inside the seiling on the concrete roof and some of them down of cieling, but also there would be distance between aps under the ceiling like 7 meters afar to each other

so what is the best practice to distribue them in this hal in your mind??

also i have voice over wireless and big number of users in this hal so i should use this number of aps in this hal.

any seggustion please

many thanks in advanse

Hall of Fame Super Silver

Interference

There really isn't a best practice for this.  There are ways you can get it to work.  The main factor is how do you know if you have too many ap's and that causes issues.  The biggest factor is how the devices handle all the beacons from X amount of AP's it hears and how much bandwidth do you have left if you do have overlapping channels.  There is no one way of doing this and having to troubleshoot installs like this varies a lot.  Once you have decided on your AP placements and powered the AP's up, that is when you need to set he tx power level down on those AP's in the hall, or you can see how the WLC RRM feature does it.  Then you would have to analyze your channels and maybe turn off some of the 2.4ghz radios if you don't have enough separation between channels.  You might ahve to do this also on the 5ghz, but that is if you do see a lot of overlap.  What is not know, are all the different devices that will be usning this network and how these devices will work in a dense deployment.  Things like truning down the power and disabling data rates to limit the coverage are all tweaks you can do to make the coverage area smallers.

Placing AP's up above the ceiling is okay, but ifyou have a lot of piping, duct work, etc... you might not be able to utulize that AP.

-Scott
*** Please rate helpful posts ***
Hall of Fame Super Gold

Interference

so i can install some of them hidden inside the seiling on the concrete roof

That is no longer "best practice".

Shut down the 802.11 b radio and put warning signs that 802.11 b/g is not supported.  This will minimize the calls.

New Member

Interference

thank you so much for your reply leolaohoo

but i have voice over wireless too what about that than?

should that work fine ?

actually i would have laptops, iphones,ipads and wirless phones in that hal for enduser devices and would be approximately

more than 150 users and each of them has minimum of three devices want to connect with wlan.

in that case what are you advising to do?

and if i will shut down the 802.11 b radios and put warning sign on 802.11 b/g is not supported in that case inteference will not happen??

i have one question

this is hospital + university's wireless network that i am going to make on

and for sure there are big number of users with their multiple devices

if i configure my wireless user vlans with /24 subnets are not enough and i have to define too many ap groups for that too

what you say if i increase my subnet from /24 to /21 than i would have 2000 subnets on each vlan

in this case what about network performance will that be smooth or slow ? because broadcast will be increased with a large number of subnet on one vlan???

please advise me about this

many thanks in advance

Hall of Fame Super Gold

Interference

and if i will shut down the 802.11 b radios and put warning sign on 802.11 b/g is not supported in that case inteference will not happen??

For the 802.11b radios?  Yes, zero interferance.  For 802.11a, little or no interference because 802.11a has more channels to play with.

but i have voice over wireless too what about that than?

should that work fine ?

If your wireless voice can operate with 802.11a, then yes.

New Member

Interference

thanks for your reply

leolaohoo could you please tell me that from i can shutdown the 802.11a and b channels in access points after joining the controller i tried to do that but i did not find that options there?

i have cisco wireless phone 7925 series shall this works on 802.11 a channels? or b channels?

any idea about this please?

also i have apple iphones and ipads do you have any idea they use which 802.11 channel a or b or g ?

many thanks in advance

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