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Multiple wireless access points on the same channel

I have multiple access points that are all configured with the same SSID and on the same channel.  Users can roam from AP to AP and still stay connected.  Everything seems to be working, but I'm having people tell me that the WAP's should be on different channels.  I did some research and I'm reading that people have done it both ways.   Is there any downfall to the way I have set this up?
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denver218
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denver218
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2 Solutions
 
John HurstBusiness Consultant (Owner)Commented:
Not really. I do that in a big and somewhat complex space with 3 routers and one channel. It works. The occasional downside is that if people actually roam around, they may stay on the original router. The majority of times there is no issue with approach.

... Thinkpads_User
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denver218Author Commented:
I do have some users who roam around with their laptop.  I haven't tested this yet, but should the laptop hop onto another AP when its out of reach from the original AP it was connected to?
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John HurstBusiness Consultant (Owner)Commented:
If you are using the same setup and channel, it depends. Sometimes they will hop and sometimes they try to stay connected. If they roam entirely out of range of one point, then they will hop to the next. There is usually a grey area, however.

Broadly speaking, you will not experience any significant difficulty with what you are proposing. I do it myself.

.... Thinkpads_User
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Darr247Commented:
The only difference would be slightly less bandwidth available.
Is your internet connection faster than 50Mb?

Do you transfer large files across the LAN often?
What is the maximum wired speed on the LAN?

Get some baselines on transfer speeds from different locations with them all on the same channel, then test again after spreading the APs among channels 1, 6 and 11.

Seamless roaming should work the same both ways, pre-authenticating via RADIUS before the switch so they keep the same IP when they change APs... doesn't matter what channels are used for that, as long as they don't overlap and cause interference. When using the same channel and SSID, they will gracefully take turns using Carrier Sense Multiple Access, and NOT interfere with each other.
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denver218Author Commented:
"Is your internet connection faster than 50Mb?"
-No, my internet connection is 10Mbps.  

"Do you transfer large files across the LAN often?"
-We do not transfer large files across the LAN.  User who use the wireless are normally just entering information into an application.  We have 10/100 Cisco 2950 switches.
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John HurstBusiness Consultant (Owner)Commented:
So then you should be just fine proceeding as you suggest in your original post.
.... Thinkpads_User
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Craig BeckCommented:
If your APs can cover any part of each other APs coverage area you MUST set them to different channels - no questions.  Co-channel interference will cause drop-outs and can severely decrease the amount of available bandwidth and the effective size of the coverage area.

If the APs don't cover any part of each other's area you can set them to whatever you like and they will be fine.

The only time it is acceptable for overlapping APs to be on the same channel is if you're using a SCA-based system such as Meru's controller-based system (for example).
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Darr247Commented:
You only get same-channel interference if they are different SSIDs.

Same Channel+Same SSID is exactly what the Carrier Sense Multiple Access scheme covers.

Without CSMA you wouldn't be able to have more than one laptop at a hotspot because they would all be using the same channel and interfere with each other.
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Craig BeckCommented:
Darr - you're wrong!  You don't need to tell me how CSMA/CA works - I'm a Cisco certified Advanced Wireless LAN System Engineer.

CSMA/CA is a mechanism which attempts to ensure that no two clients try to transmit on the same channel at the same time - it has nothing to do with the SSID.  Clients listen to see if any other client is talking, then send their data if the air is clear.  Because of this mechanism, single-channel WiFi is half-duplex.  Also, Wireless LANs HAVE to use CSMA/CA as they can't do collision detection like wired Ethernet can.

RF doesn't care about the SSID, and it doesn't even know the concept of an SSID - it is just the medium over which the data is carried.

Consider Cisco's Wireless LAN system (for example).  Each AP reports what RF it can see to the controller.  If an AP is on the same channel as an adjacent AP it will be forced to change by the controller.  If the AP can't be changed to a suitable non-overlapping channel because there is too much RF interference (from its own APs broadcasting the same SSID, or APs, on the same system or a different one, broadcasting different SSIDs) the controller will reduce the power level of that AP in an attempt to decrease the APs coverage area and therefore minimize the effects of interference from other APs.
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denver218Author Commented:
Let me explain my wireless network.  I have a building that has three floors.  Each Floor has a long hallway, with office's and room's on each side of the hall.  There are three AP's in each Hall that provide internet access throughout the hall, offices, and rooms.  Every AP on every floor is on the same SSID and channel.  I do believe these AP's may overlap.  With that being said should I put them on different channels?  Thanks.
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Craig BeckCommented:
Yes, you should put them on different channels.  You have to plan carefully as you can still interfere with APs on different floors.

Consider this (each [ ] is an AP and each number inside denotes the AP's operating channel at 2.4GHz)....

Floor 3            [1]          [1]          [1]

Floor 2            [1]          [1]          [1]

Floor 1            [1]          [1]          [1]


In this scenario, APs on Floor 3 can see APs on Floor 2 (and maybe on Floor 1 too depending on the structure), as well as APs on the same floor.  These APs will all interfere with each other on the same floor and will degrade performance, and they may well interfere with APs on other floors too.


This deployment would work much better...

Floor 3            [1]          [11]         [6]

Floor 2            [11]         [6]          [1]

Floor 1            [6]           [1]          [11]


Here you are spreading the available channels throughout the building whilst still providing coverage.  This maximises the potential to cover your entire area at a good power level whilst causing minimal co-channel interference between your APs.


Have a look at this...

http://www.cisco.com/en/US/prod/collateral/wireless/ps5678/ps10981/design_guide_c07-693245_ps2797_Products_White_Paper.html#wp9001231
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denver218Author Commented:
Thanks.
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John HurstBusiness Consultant (Owner)Commented:
Thank you and good luck going forward with your wireless project.  ... Thinkpads_User
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Darr247Commented:
I would give you links to a couple pages on cisco.com that support precisely what I said in http:#a38402186 but you already closed this, so continue following deliberately false advice.
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Craig BeckCommented:
I'd be interested to see them Darr :-)
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Darr247Commented:
You're the "Cisco certified Advanced Wireless LAN System Engineer" - look them up yourself.
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Craig BeckCommented:
I have but couldn't find them.  My friend who works at Cisco couldn't find them either!
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