Can you hear me now?

Voice over IP (VoIP) phones demand a ton from a wireless network.

They need Quality of Service (QoS) to make sure voice packets receive priority over other traffic. Roaming has to be lightning fast (sub 50ms) so that nothing gets dropped. They need low latency and jitter to maintain consistent quality so you don’t lose a single note of your favorite muzak songs when you're on hold.

One of the most common fallacies with voice over Wi-Fi (and really anything related to Wi-Fi) is that any coverage or connection problem can be fixed with more power. It can't. Just look at a Wi-Fi phone vs. a laptop.

Each has a radio transceiver, each has antennas and both speak 802.11. So why do my phones get crappy signal on the Wi-Fi network? Is it because they can’t hear the APs.  So I just need to give the APs more power? Right?

Wrong. It’s all about the battery.

APs typically don't come with batteries.  They have AC power or PoE. APs can transmit all day long and never have to worry about running out of power. An AP can easily afford to transmit at the maximum allowed power. In the states this would be an EIRP (Equivalent Isotropically  Radiated Power) of 4W or 36dBm. An AP transmitting at 1W (30dBm) with a 6dBi antenna, is at the maximum, 36dBm (30dBm + 6dBi).

But a laptop is built to run without AC power. With a battery, there is a definite limit on available power. That’s why most laptop wireless adapters max at about 20dBm (100mW).

Then there are very small, ultraportable devices such as a Wi-Fi phones, PDAs or RF scanners. They have very small batteries and are required to go an entire day without a recharge. They are extremely aggressive when it comes to saving power. The Wi-Fi radio is a big part of that. These babies might max out at just 16dBm or 40mW. That’s one hundredth of an AP’s available power. Again, most Wi-Fi phones transmit at just 1/100th the power of an AP.

It’s rare for a phone not to hear an AP. More likely, it’s that the AP can’t hear the phone. It’s like going to a baseball game and listening to the announcer in a press box on the other side of the stadium. He’s using a loudspeaker and I can hear him just fine. But the reverse is not true. If I start yelling at the top of my voice it’s very unlikely he would hear me. That’s exactly what happens with Wi-Fi phones – or any wireless device.

How to solve this?

First, some reality: poor Wi-Fi voice performance can and will happen if you don’t take preventative measures up front. Second, if you know you’ll have Wi-Fi voice, do a site survey. This is not optional.

Pay attention to the handset manufacturer’s minimum required signal strength from the AP and ensure you survey to that minimum everywhere a phone might go (by the way, phones imply 100% coverage, so you can’t expect them to roam from hot spot to hot spot).

Third, when doing a site survey, use one of the handsets to check coverage – many handsets include a “site survey” mode. Don’t rely on a higher powered device (like a laptop).

Finally, understand the signs of a Wi-Fi voice deployment in trouble: voice calls are dropped, start clicking or the phone simply can’t connect. If this happens and other clients (like laptops) are fine, it’s not the AP transmit power that’s a problem, it’s the phone received power.

Oh...one last thing: notice at no point did we even say "dynamic beamforming"  Of course, this solves pretty much all your voice problems.

Comments

[excITingIP.com]

I guess the discussion is about VoWLAN phones or Dual-mode handsets which do operate at lower power levels... The loud speaker analogy was clear. I wonder how much of those disconnections happen because the user is too close or too far away from the access point (with the wi-fi phone). I thought most of the problems with the wi-fi phones are because of improper planning of adequate wireless coverage due to which signals are lost when the user is roaming around.

excITingIP.com