Voice-over-Wifi QoS Issues Explained

So, you’ve set QoS up on your WiFi network & it’s still failing? What’s wrong? Well, to answer that you first need to understand how WiFi networks work. Modern day Wired networks are Full Duplex, meaning they can transmit & receive at the same time & do not share the medium (the wire) with anyone else.

WiFi is not anything like that. It is a half-duplex shared medium. Only one device or Access Point(AP) can transmit at a time. This isn’t per AP either, it’s per channel. So, all APs & devices within earshot on the same channel must take their turn.

The protocol that manages access to the WiFi Channel is called Carrier-Sense, Multiple-Access with Collision Avoidance (CSMA/CA). Prior to transmitting a frame, CSMA/CA requires each device to monitor the wireless channel for other Wi-Fi transmissions. If a transmission is in progress, the device sets a back-off timer to a random interval and tries again when the timer expires. Once the channel is clear, the device waits a short interval (the arbitration inter-frame space) before starting its transmission.

Since all devices follow the same set of rules, CSMA/CA ensures “fair” access to the wireless channel for all Wi-Fi devices. The Wi-Fi standard defines a distributed system in which there is no central coordination or scheduling of clients or APs.

When you setup QoS for WiFi (802.11e standard, or WMM as it’s also known) what happens?

Well, two of the CSMA/CA parameters are adjusted – the random back-off timer & the arbitration inter-frame space. These are changed according to the type of traffic “seen”. Higher priority frames are given shorter values & lower priority frames must wait longer. Four priority levels are defined in WMM in descending priority: Voice, video, best effort & background.

VoWiFi Troubleshooting

So, to have Voice over WiFi(VoWiFi) operate better than other types of traffic on your network it first needs to be marked/tagged correctly on the upstream direction(your phone to the AP)by your device/Application so it can be classified correctly & go into the correct priority queue. If this doesn’t happen then it will just fall into one of the other queues. The same needs to happen in the opposite direction – travelling through the network back to the AP, otherwise the AP won’t know to prioritize it over the air.

You used to have a VLAN/SSID for Voice traffic & specific hardware that would tag packets. But nowadays most companies value mobility & use devices(think tablets/smartphones/laptops) that transmit voice/video/data at the same time & you can’t VLAN them. So, you need devices that can tag packets preferably. If not, the APs need to be able to provide this based on Application signatures.* But then you’ve already lost one big QoS opportunity(from the client to AP)This really only works in the downstream direction – from AP to client.

Also, the whole LAN needs to support QoS – from the Firewall, switches down to the APs. this. This needs to be the case for both directions.

Common issues with Voice over a WLAN include Choppy Audio, Clipping, Echo, Gaps in Audio & no Audio when roaming between APs.

*Note – some APs/controllers can do perform this classification, but many can’t.

As you can see, QoS over WiFi is not straightforward & many things could go wrong. Also, having QoS when it isn’t required can adversely affect the performance of your other network traffic.

On top of that we must go back to the fact that wireless is a shared medium. So higher priority traffic cannot be guaranteed to be able to transmit before lower priority traffic from other devices, or even internally within a device’s internal queues. WMM/802.11e only provide for a statistical advantage for higher priority traffic. In the main, higher priority frames will wait less time until transmission, but in a shared medium, there is no such thing as a guarantee!

So, a good design for coverage & capacity is required:

• Does each AP have a channel to itself (no Co-Channel interference)?
• Have power & data-rates been optimized?
• Have you calculated the device to per-AP ratios?

By David Ballard, Director at Performance Networks