Friday, March 9, 2007

Building a Voice Capable WiFi Network

Building a wireless network that supports data traffic is hard enough but trying to support VOIP over your WLAN (also known as VoFi) can be a nightmare. To make matters worse, when you ask your vendor how to make Voice work on your WLAN they explain you will need 2X-3X as many APs as you needed for data. "Sure I do", you respond. Confident that the sales person from your vendor just wants to sell you more APs. Or, better yet, you turn to your trusted VAR and he suggests another site survey. "Right, another one", you say, with that knowing look in your eye and a sinking feeling that you are being strung along. You feel like the guy who brings his car in for a tune-up and gets told he needs a complete overhaul.

Well, I have nothing to sell you and no agenda that I will benefit from by saying this but your infrastructure vendor and your VAR are absolutely correct. You probably will need more APs and you sure as heck will need another survey. Lets find out why, shall we?

Unlike Email and web access, slight lags or delays in traffic or small losses in connectivity will completely destroy calls. A person who has access to the Internet durring a meeting in a conference room is far less likely to lose his cool for small delays than when he is on the phone with an important client.

You see, wireless handsets are much lower powered compared to the access points they talk through. A typical AP is usually set to communicate at 100milliwatts (mw) whereas the typical handset is roughly 5mw. This makes it very easy for the handset to hear the AP but very hard for the AP to hear the handset when it is far away. Also they are far less resilient to fragmented packets, retries, packet loss etc.

So what can I do? Well the simplest thing to do would be to ensure that the handset is always at the same power as the AP. That means either increasing the power on the handset or, more likely, lowering the power on the AP. This will mean, of course, that you will need more APs to cover the same area.

For example here are 4 APs at 100milliwatts:

Here are the same APs but now set to 5mw instead, notice the gaps in coverage:

In order to compensate, we must add many more APs to fill in the holes, all configured to run at 5 mw:

As you can see, much better. Now, though, our main issue is channels. APs that overlap thier signal on the same channel take away from the usable bandwidth. We want to ensure we do not trample the signal from another AP so we must adjust the channel plan.
Also, remember we only have 3 channels to work from.

Cisco, at this point recommends the following:

That explains why I limited the seen signal to -67dbM making all the other signal fall off and appear grey.

In a week or two, we will discuss debugging Voice issues and setting MOS scores.

1 comment:

  1. Hello,

    I very much enjoy and benefit from your blog on wireless issues. I would suggest that the graph showing the Cisco suggested separation of same channel cells should read 19 dB not 19 dBm. The relative rather than the absolute.

    Cheers from downunder in New Zealand.