client roaming issue

Unanswered Question

I am trying to track down some performance issues on my wireless network, and hope someone out there will help educate me!  My environment:

  • Access Points are all 1231, 1242, and 1252
  • The 802.11g radios are active
  • All APs are configured to use Channel 4
  • WEP security (I know, I know ...)
  • Clients are either laptop users (mostly Dells) or Intermec vehicle-mounted terminals with Prism NICs
  • Clients are VERY active as far as moving around the building

Here is what I *think* is happening:

  • The clients don't "roam" as quickly as I think they should - in other words, they remain associated to the active AP too long.  Their signal gets too low before they pick up another AP.

I wonder if I need to adjust the data rates on the APs (like maybe disable the lowest ones).  For example, if I disable the 1.0, 2, and 5.5 Mb/sec rates, does that mean that a client will look for another AP when the "speed" drops below 6 Mb/sec?  If so, then by disabling the lowest data rates, the client would be forced to find another AP before the data rates drops to nothing, right?

I also don't understand the power ratings - I've just left them at the default settings - does this affect performance as well?

Thanks in advance ...

I have this problem too.
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George Stefanick Sun, 02/14/2010 - 07:50

Hi Susan,

How are you doing today ... Sounds like you have a little mess on your hands. Lets sort through some of these issues. It also sounds like you are open to feedback, that is always great to hear!

First. I need to point out your channel selection and this could be a the likly cause for your roaming issue. Follow along and let me explain why.. There is a BIG miss understanding when doing wifi that folks think that all the AP needs to be on the same channel. In fact, you should only use channels 1,6 and 11. When you use the same channel off channels like 4 you cause interference.

Let me explain why with a real world example. Think back to the late 80s when cordless phones were hitting the market. Do you remember when you would pick up the phone and you would hear your neighbor's conversation? Well that is becuase you were on the same channel you shared the same freq. Now what happens when 4 people (on 2 calls) try to all talk at the same time!? It becomes a mess, right. "Can you say that again", "I didnt hear you", "What was that !?".

Well the same holds true with WiFi. If all the aps are on the same channel you have one big cordless phone call going, for exmaple.So the first thing we need to do is correct the channel issue before you do anything else. It WiFi channel uses 22 MHz of RF. Thus what you can only use 1,6,11. You also want to make sure you dont put like channels close together. For example, don't line up a hallway with all channel 1... here is some reading

Thanks for your quick response, and your good explanation, and also for pointing me to the Cisco docs on channels!

Sounds like my 1st step is the reconfigure the APs with a "5-away" channel configuration.  Do you think I might notice an immediate improvement?  Or do I need to tackle some other issues, like power settings, before it will all come together?

And a question about the channels:  What happens on the client side when the client switches over from 1 AP to another, assuming the APs are using different channels?  Is there any loss of connectivity?  Or to ask the question differently, if the client is running an application that isn't so smart about negotiating, like a mainframe session or a telnet session, will the session get disrupted when the client roams from 1 AP to another?

Thanks again ...

George Stefanick Sun, 02/14/2010 - 11:07

Not sure what you mean with a 5 way channel. You should only use channels 1,6 and 11 on your access points.

Specific to power. Have you had a professional site survey done? What type of enviroment are you working in? I never like to design my surveys with more then 50mW (17 dBm), but most times at 25mW (14 dBm). But a site survey dictates ap placment and power.

Regarless if you are on the same channel or different channel there is a momentray loss of network as your client roams. In some cases you may lose a ping. However, most if not almost all apps today can handle this roam. If you are using 802.1x security (EAP-PEAP,LEAP,TLS) etc you need to make sure you are using WPA2/AES with 802.11i or WPA/TKIP w/ CCKM.

Can you share what security you are using, type of wireless network, how many ap, how many clients, what type of clients, etc ?


All I meant by "5-away", was to reiterate your recommendation to use channels 1, 6, and 11, trying to keep them as far apart as possible - that an AP adjacent to one using channel 1, should be at least 5 channels away - meaning 6 or 11. Sorry for that confusion!  But I think I understand that concept.

I am currently using WEP security.  In about 8 weeks, I will migrate to EAP-PEAP / WPA-TKIP (per Corporate directive, and also, just a good idea!).

Our facility is about 80% manufacturing and 20% office.  The manufacturing "shop floor" presents a number of challenges:  floor-to-ceiling racking in some places, and wide open spaces in other places - plus, the ceilings are 30 - 40 feet high.

I did have a site survey done a number of years ago, but so much has changed since then.  In particular, a lot of the floor-to-ceiling racking has been moved.  I know that I need to have another survey done, and need to have a professional come in who knows more than I, but it will be weeks before I can make that happen.  In the meantime, I am trying to make the environment functional - I'll leave it to the pros to make it optimal!

Um, clients:  mobile computers, mostly using Prism 802.11b/g adapters.  The adapters are configurable as far as "roaming aggressiveness".  The applications used most frequently are a telnet session and a DB app that connects to a MS SQL database.  There are no voice clients.

The APs are a mixture of 1231, 1242, and 1252 models, and they are configured for "max" power, which is the default, I believe.

George Stefanick Sun, 02/14/2010 - 19:23

If you plan to move to PEAP you will want to do your home work with EAP. You may want to test your devices prior to implementation. You may want to insure your devices support CCKM. If not, your devices will do a FULL 802.1z AUTH each time they roam! This will likely cause you issues, if you have sensitive apps..

You can read more about it here..


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