Wi-Fi Calling with iOS8 and T-Mobile

I was sitting at home watching an Apple oriented video podcast that was discussing how Wi-Fi calling was now available in iOS 8 and that T-Mobile was one of the first carriers rolling this out. I realized that this may help me specifically with some of the dropped call issues I had been experiencing. I then grew curious, T-Mobile was offering a ‘customized’ router to subscribers that professed to offer home users better connectivity than their current gear. How did that work I wondered. With that I set about finding out by switching on Wi-Fi calling in my phone and doing some packet captures with the handy Remote Sniffer provided in my home Aerohive AP-370.

First of all I investigated how Wi-Fi calling actually worked. Digging around on the Internet turned up that it was a form of GAN/UMA that T-Mobile was using. Essentially this sends packets that would normally be sent over the GSM network via the Internet. With a bit more digging, however, I discovered on Reddit that T-Mobile used to use UMA but are now deploying IMS. The essential point for WLAN engineers such as myself is that we now have encrypted voice traffic going over our networks to the Internet which route to whichever provider is allowing their subscribers to use this service. As an aside, this isn’t a new thing as T-Mobile has been doing it since 2007 for Android phones.

Wi-Fi Calling Capture

So lets break down what’s going on here. T-Mobile being focused on the home use case is providing a high end router to customers which essentially provides QoS enabled connectivity so voice packets from their phones using Wi-Fi calling are prioritized properly for home users. That’s a win for them in that they will get great voice calls from their phones on their new router. What happens when those same users bring their phone into work and connect to the enterprise WLAN cause their office ‘just never had good cell reception’? Now we have a lot more devices doing what looks like ESP encrypted traffic that has voice priority (6) set on the packets.

Wi-Fi Calling QoS Capture

I would say as a WLAN designer, you should be taking into account that a lot of BYOD devices, specifically phones, will mean an increase in voice traffic on both your WLAN and the rest of your LAN. Read up on how it works at my pal Andrew’s blog, Revolution WiFi. I especially recommend his series of posts on Voice-Enterprise and Roaming.

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