First, let’s clarify what we mean by Wi-Fi VoIP (VoIP for short) in the context of this blog.
| Wi-Fi VoIP is... | Wi-Fi VoIP isn’t... |
|---|---|
| SIP / H.323 voice and video traffic. |
This is the protocol used by internal Wi-Fi extensions from the likes of https://www.ascom.com/products-and-services/mobile-devices/dect-and-vowifi-phones/i63/, https://www.mitel.com/products/devices-accessories/wireless-devices/mitel-5634-wifi-phone, https://www.spectralink.com/products/wi-fi/wi-fi-feature-phones/, https://www.cisco.com/c/en/us/products/collaboration-endpoints/wireless-ip-phone-8821-ex/index.html, http://www.grandstream.com/products/ip-voice-telephony/wifi-cordlessor even the Star-Trekesque https://www.vocera.com/product/vocera-badgebadges. Normally there is an on-premise or (rarely) cloud-based PBX system managing all the handsets. | Microsoft Teams, Skype, Zoom, WebEx or any kind of internet-based conferencing system.
Mobile-operator driven Wi-Fi Calling.
Anything which uses other wireless technologies like DECT or Bluetooth and then converts to wired VoIP via a dedicated bridge. |
Contrary to popular belief, VoIP is the #1 most difficult protocol to work with in the Wi-Fi world. Much harder than streaming video, VR/AR, cryptomining, you-name-it. This is because VoIP requires exactly what Wi-Fi doesn’t deliver, and doesn’t care about what Wi-Fi does best! Wi-Fi is great at delivering high speeds and it can handle thousands of devices. It’s also decentralized, in that all devices on the network - APs and clients - have an equal right of access to the medium.
VoIP doesn’t need a lot of bandwidth - each audio call only uses about 100kbps (that’s kilobits with a k), and you rarely have more than 2-3 phones within coverage of a single AP. This means that capacity and bandwidth are useless to VoIP. What VoIP does require is predictable reliability, and Wi-Fi is 100% best effort and unpredictable.
VoIP also needs very fast roaming, faster than any other application. Think about it - it’s common to see people running in a hospital hallway while talking on the phone, but not while watching a YouTube video or writing an e-mail on a laptop. While Wi-Fi has improved over the years, it still cannot offer 100% predictably reliable roaming.
<aside> ⚠️ Always keep in mind: there is no “FixIt” button for VoIP! Optimizing VoIP relies on fine-tuning several aspects and missing out on just one aspect could cause detectable issues.
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VoIP support should be factored in when designing for VoIP. Make sure there is continuous coverage at -65dBm throughout all walkways, doorways, elevators, and stairways. Often VoIP is relied upon as part of emergency procedures so make sure any escape routes or sheltering areas have good Wi-Fi coverage.
Ruckus also recommends using tunnelling for the VoIP WLAN. This is not strictly necessary, but depending on the switch architecture behind the APs, tunnelling traffic will help with Roaming.
Let’s move onto the controller setup. I’ll be focusing on SmartZone, but almost all of these recommendations can be transposed to other Wi-Fi controllers
Noise and interference causes packet loss, retries and high jitter - all three being killers to your VoIP MOS Score. Use the AP list to look at the Airtime Utilization on your VoIP band and fine tune the RF to keep this value to a minimum.
<aside> ⚠️ Rule of thumb, when an AP has zero clients connected, it should report: ≤ 20% on 2.4Ghz ≤ 1% on 5Ghz
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Use the Gear icon to show the Airtime Utilization columns, then click on the title bar to sort the table.