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.

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Hotspot 2.0 Can Disrupt the Cellular Marketplace

When it comes to cellular in the U.S. there are two major carriers, AT&T and Verizon, and everybody else. While Sprint and T-Mobile both also compete in the national market, they have far fewer subscribers and a reputation for poor coverage. This has essentially been the state of affairs since Cingular bought AT&T Wireless in 2004 and continued business using the AT&T brand. There are some smaller regional competitors, but their market share is limited, and their customers roam onto one of these national networks when they leave their regional service area.

I think the combination of Hotspot 2.0 and Voice-over-Wi-Fi (VoWiFi), or “Wi-Fi Calling” as it’s known has the potential to disrupt the current cellular marketplace dynamics.

Sprint and T-Mobile have been dropping their prices to try to attract customers away from the Big Two (AT&T and Verizon) for years, even offering to pay early termination fees and give trade-in credit for phones, but it appears that this has largely been unsuccessful. When you can’t make a call from within your own home or office, who cares how cheap the service is?

Part of the problem for T-Mobile is that a lot of the spectrum they own is higher frequency than their competitors, so it doesn’t penetrate buildings as well due to the increase in attenuation that occurs as wavelength decreases. That’s a tough problem to solve.

carriers

VoWiFi and Hotspot 2.0 can change all of that.

VoWiFi extends the network’s voice coverage into the subscriber’s home and office, where subscribers can easily connect their phone to the W-Fi network, which takes care of that concern. Sprint and T-Mobile could also partner with SOHO Wi-Fi router manufacturers so that Hotspot 2.0 roaming integration was preconfigured for their networks on these products. Imagine if a subscriber could buy a NETGEAR “T-Mobile Edition” router and have VoWiFi calling work out of the box, without any configuration on their phones.

Imagine if Sprint and T-Mobile aggressively pursued Hotspot 2.0 integrations with major public Wi-Fi providers. Their subscribers would have seamless VoWiFi coverage in the areas where they currently have the biggest problem: indoors. As public Wi-Fi continues to expand, the voice coverage for these carriers could expand right along with it.

In fact, if we assume a properly designed WLAN, in very high density environments the indoor service for these carriers could be superior to the Big Two. Ever go to a ballgame and been unable to make a call or use data in a full stadium? That’s a common experience and Wi-Fi roaming integration solves that. Wi-Fi was designed to meet LAN access needs like this. Why not actually use it that way?

This could make Sprint and T-Mobile attractive again. Although I don’t imagine the costs would be very significant as it doesn’t involve building new towers and deploying more of their own hardware, they would probably need to compensate large public Wi-Fi operators for the use of their networks. That would allow them to keep their service priced below the Big Two.

Cellular data offload is commonly thought of as a driver for the adoption of Hotspot 2.0. Voice coverage expansion for smaller carriers may be more important.