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New Member

Help with larger sized voice wlan.... design considerations.... tips...etc...

Hi Everyone,

     I'm hoping that you guys and gals can help me with an ongoing problem that we have at one of our sites. We're working on areas of the location due to it's size and phone load. We have an area, that I will refer to as building A, that is roughly a square that is 240' x 240'. The inside of the building has some pallets inside for storage(they allow RF to penetrate through them) and also some metal production lines. There is also a mezzanine  / elevated area in the middle that the users can walk under. It's not very large, but it would affect a phone if you walked underneath it while making a call. So, now that I have a brief description of the environment, I will tell you the equipment I'm running...

1 x 4402 50AP Wireless Controller with (MR3) installed.

We currently have nine AP's installed in this area, with 4 up front so that way they cover the office areas better. Most are 1231G's, but some are 1242's.

Since we do not have A radios everywhere (budgetary decision) we are running all of these phones on 2.4Ghz (Yikes! I know!)

The 7920's use LEAP and the 7921's use PEAP MS-CHAP v2 with CCKM enabled on the controller.

I also have 802.1p wired QoS enabled for the voice QoS profile and it is applied to our voice WLAN.

We have conference rooms in the front area that will need to support roughly 20-30 maximum mixed 7920 and 7921G phones in a roughly small area. (Yikes! I know!)

We also have a 2106 with mesh .54M installed, but it is for outdoor AP's and should not be affecting this area.

So, I guess my questions are....

Has anyone ever operated the 7920 and 7921G's in mixed mode?

I'm thinking about separating the 7920s on 2.4 and tell the 7921's to prefer the A band or just use A. This will require A radios / surgery, but we've dealt with different code trains, TAC configs, and even added a few more radios. I think it's time to say we need to redesign this area. It doesn't help that phones keep getting purchased either....

What rule of thumb would you guys / gals say would be appropriate for this phone count in terms of the number of AP's I should use?

Since we're dealing with two different phone models, it makes it hard to simply just read the deployment guide. I know these phones can coexist, I just think we're running into over capacity and problems with 802.11B in the mix. In the conf room right now, there are most likely signals from at least two AP's. This doesn't seem like enough bandwidth for just the 2.4 Ghz range when 20 - 30 phones are in there. Not all of them are calling, but the associations / mgmt traffic alone must be horrendous since the phones are 802.11B

We are going to do a manual survey with a 1242 since that AP will support both phone models. I think we may need to survey twice if we're going to go this route; One survey for the 7920's on 2.4Ghz with a 1242AG and one survey on 5Ghz with a 7921G. I think that will provide the info we need to get the AP's repositioned where they need to be.

What kind of power levels should I be using in an area like that? We've originally had a survey at 50mW, but since then we turned the power down to pwr lvl 3 in some spots due to the additional AP's. I've seen references of roughly 1 AP per 3000 sq. ft at pwr level 4, but that seems overkill. 

Can we run mixed power levels on AP's with the phones? Or will that cause one way audio due to the transmit power diffrences in the cells?

I'm thinking that we should pick a power level, survey the -67 cell size at the power level, add more AP's so that way they are overlapped 15-20%, and then actually implement the design. I'm pretty good at getting decent channel assignments in place. I know of non-overlapping channels, RRM, etc. I also have an AirMagnet laptop with an Aironet Adapter. It is good for finding noise, interference, etc...

I understand this is practically a book, but at this point, we've been trying a LOT of different things in order to get this to work properly. I think it's finally time for me to "strongly suggest" that we do the following...

1. Choose ONE phone model.
2. Choose ONE AP model.

3. Make sure to implement the 5Ghz band for all AP's so we have complete coverage.

4. Choose ONE power level for each band. This will affect the coverage and placement of AP's. I'd imagine that we'd need to survey with A first, and then survey with b/g. Typically we can use the 1242 and get about the same cell size on both bands, but 5Ghz is a higher frequency and may not penetrate as much as 2.4 Ghz..

5. Come up with a new coverage map based on a manual site survey with the phones mentioned above.

6. Implement the design.

7. Use it.

8. Try not to have a heart attack when the system actually supports that many phones in that area...

So, please, let me know your thoughts and if you have any suggestions. It would be greatly appreciated. We've been slowly working out the gremlins in the phones there over the past few years. I'm more of a data wireless person myself, but I do have good luck with a low to moderate phone count (usually no more than 7 calls per AP). Once we start doing craziness like trying to get 20-30 802.11B phones to work in the same area on only one or two AP's, then things start to become a challenge, especially when we don't have the 5Ghz cells to help with the bandwidth requirements for voice....

Thank you for your time,


Cisco Employee

Re: Help with larger sized voice wlan.... design considerations.

I think the silence means that everyone agrees with your plan. It sounds good to me at least !

If you have meeting rooms with 20/30 phones that include 7920, you might look into having 3 APs in that room (covering all b channels) with statically configured extremely low power, and the rest of the building normally covered and APs placed so that the meeting room is not covered by other APs than the meeting room ones.

After that, you can't pull much more bandwtidh out of the b/g :-)

New Member

Re: Help with larger sized voice wlan.... design considerations.

There's a pretty recent (last few months) Voice over Wlan design guide published (was published for the 9971 phones - but all great advice) and it recommends just about all the settings required for an off the shelf wireless voice network.