Wireless issue

Unanswered Question
Nov 1st, 2007

I am troubleshooting a wireless network. On some spots, the signal strengths keep changing between -58dbm and -72dbm. Nobody moving around when I collected the data. I am WirelessMon running on a HP laptop with internal antenna. Is it normal? please advise. Thanks!

I have this problem too.
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George Stefanick Thu, 11/01/2007 - 20:36

Hi Sky,

Yes and no .... RF is dynamic... What is the nature of your issue, that may be a better way to start off...

sky_zhang Fri, 11/02/2007 - 06:27

Thanks for replying.

I am working on deploying Vocera wireless VOIP phone, which requires the signal strength to be more than 65dbm. On some spots the signal strength changes in the range from 58dbm to 72dbm so users get cut-off when they are talking through VOIP phone. Basically, I can't increase the AP transmit power since this will increase interferences. Any idea?


dennischolmes Fri, 11/02/2007 - 11:46

What you are probably running into is called multipath interference. It occurs when the RF wave pattern is disrupted and received at slightly different times at the receiver. The receiver radio attempts to use the best signal for proper decoding. What APs are you using? What external antennae if any? What is your RF environment? Hospital? Mfg.? Distribution? All those are just minimums on what is needed to solve your problem. I tend to like to use directional patch antennae 80% of the time using power to control the size and shape of my cells. I use omnis to fill in any coverage holes I have left. Verify coverage with a good site survey tool like Airmagnet or Ekahau.

sky_zhang Fri, 11/02/2007 - 12:56

Thanks Dennis.

We are using Cisco Aironet 1240 AP and Ant5959 antenna. I am a newbie and don't know how to exactly describe the RF environment I attached a diagram of the building. Basically, we are a hospital. I am having troubles on the red spots.

Scott Fella Fri, 11/02/2007 - 18:46

Are you using LWAP or Autonomous? Make sure you hard code the power (25mW) and the channel.

john.preves Fri, 11/02/2007 - 21:45

Hmmm, just eyeballing it and not knowing what your power settings are it looks (to me) like you may have to push AP004 further into that room. That is a lot of walls and stuff that the signal has to attempt to get through based on the location of that AP. The number 006 AP (Ch1) may have to be moved to pick up any slack that moving 004 caused and then the slack from that AP should be fixed by the one you will have to put in the remaining open space.

Anytime you see the signal strength swing wildly like that you can narrow it down to interference from another device or multipath.

I'll agree with Dennis on this one; low power smaller cells, more AP's.

sky_zhang Sat, 11/03/2007 - 06:59

Thanks for your suggestion!

I will try moving Ap04 to see the signal strength can be better.

sky_zhang Sat, 11/03/2007 - 06:57

We are using autonomous now. A vendor company suggests us to move to LWAP but I don't have that budget(100k) this year.

Scott Fella Sat, 11/03/2007 - 09:25

Going LWAP will not solve your issue. What it looks like from the drawing you sent is that maybe the user is bouncing from one ap to another AP4 to AP5. Since you are using Vocera, why don't you disable all the data rates below 11mbps on ap 4 and ap 5 to see if that helps. Also TX power setting should be set to 14dbm or 25mW. Client local power should be disabled. Try it on the two ap's first and see if that helps in the coner area that has issues. Also make sure that there are no mircowaves between the ap's and that particular area. I have seen where the RSSI can change due to RF path and interfrerence.

sky_zhang Sat, 11/03/2007 - 19:48

Thank you so much!

I'll give it a try next week and post the result here.


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