Get Your WLAN Ready for Carrier Wi-Fi Calling

To follow-up my last post where I expressed concern about marking cellular carrier Wi-Fi calls with the proper QoS class, I’m please to see that Cisco will include application signatures for Wi-Fi Calling in it’s upcoming AVC Protocol Pack 15 update. Other vendors should follow suit.

Keep in mind that changing the classification of VoWiFi packets on the WLAN only affects downstream packets from an AP. Upstream is up to the client.

Do Wi-Fi Calling smartphones mark upstream VoWiFi packets for the WMM AC_VO queue? If so, that could pose a problem in high density networks, as a large group of these clients will demand immediate airtime and limit other clients’ access to the medium. Imagine a future where Apple, AT&T, and Verizon all support WiFi Calling and enable it by default to off-load data from their LTE networks. This could happen as early as 2016. High density networks that were designed for best effort data suddenly have to deal with these demanding clients who can dominate the 802.11 contention window. Wireless engineers that haven’t handled voice on the WLAN in the past will now be forced to deal with it.

The first thing to consider is making WMM Admission Control (WMM-AC) mandatory for voice to prevent voice clients from dominating a channel’s contention window. I suggest doing this before all the major cellular carriers enable Wi-Fi calling and these clients show up en masse on your WLAN. To date, the Wi-Fi Alliance has certified 77 smartphones for WMM-AC. The elephant in the enterprise room is the iPhone, which lacks WFA certification for WMM-AC, although it may still support it. I suspect that most newer clients that support WMM probably are WMM-AC capable as well, but that is just a hunch. A client that doesn’t support WMM-AC just won’t gain access to the AC_VO queue, but it can still pass voice traffic without higher priority.

Wireless engineers may also choose to tweak the default WMM AC_VO AIFSN and contention window min/max settings to give these packets less airtime priority. Given today’s PHY rates, that may not cause a significant impact on the performance of these applications when channel utilization is low to moderate.

The goal will be to strike a balance between voice performance without significantly degrading the performance of best-effort data clients. WLAN’s that were designed with voice in mind will have an advantage as they provide higher minimum SNR and therefore higher minimum PHY rates, as well as better roaming characteristics. If your WLAN doesn’t provide fast roaming now, expect it to be a requirement in the future. (Queue the lack of client support for 802.11r rant, with a hat tip to Apple)

What other approaches are out there for dealing with a sudden increase in voice clients?

Update 10/9/2015

Yesterday, AT&T enabled Wi-Fi calling for iPhones on its network. AT&T is by far the largest carrier in the US to enable this feature, so expect to see an increase in Wi-Fi calling on your WLAN soon. Twitter user @wirelessguru posted this packet capture, which shows an iPhone with service from AT&T sending Wi-Fi voice packets with WMM AC_VO QoS markings (and some odd layer 3 markings as well).

The time is upon us to flip the WMM-AC mandatory bit for the voice queue, and consider enabling AVC QoS markings for downstream Wi-Fi calling traffic if available.