It's been 18 months since Apple announced its Wi-Fi calling feature in iOS 8, a good time to take stock. While the rollout has been a bit slower than anticipated, all the major operators now support Wi-Fi calling. The technology promises to transform the way enterprises support cellular devices.
Wi-Fi calling is a standard protocol that takes a cellular voice stream (and SMS texts) and encapsulates it for transmission over a Wi-Fi-to-Internet connection. It's tied to the cellular operator: the Internet connection terminates on a gateway to the operator's network, allowing the call to switch between cellular and Wi-Fi at will, under control of the network.
When the cellular connection is LTE, the architecture is very clean, as the voice-over-LTE and Wi-Fi links both use IP packets. All that is required is for the phone to switch the IP packets between the two interfaces, and the gateway in the operator's network to mirror this function. Some operators use Wi-Fi calling over non-LTE connections, but it's more difficult to switch the connection seamlessly between Wi-Fi and 3G because the cellular side is not IP-based.
When it works well – and like any new technology, there are users who love it and others whose experience is not so good – the phone and network decide which medium to utilize without disrupting the user. All calls, incoming and outgoing, are to and from the user's cellular phone number, and call-switching should be seamless.
How did Wi-Fi calling get to (mostly) universal support?
Similar technologies have been around for years – UMA (Unlicensed Mobile Access) deployments started around 2007 – but three recent developments have changed the Wi-Fi calling game. On the cellular side, LTE (with voice-over-LTE) is a pure-IP architecture, making the translation between Wi-Fi and cellular encapsulation much easier. And while it's not yet affecting their revenue, mobile operators see the rise of over-the-top voice services, whether Republic Wireless and Google Fi with Wi-Fi-first strategies or Skype and WhatsApp alternatives, as a significant threat to their traditional voice business. But perhaps most importantly, Apple brought the technology to iOS 8 in mid-2014, creating a new wave of user demand that the cellular operators could not resist.
In early 2014, T-Mobile was the only operator with Wi-Fi capable voice, and a limited range of Android-only smartphones. The technology had some enthusiastic users, but with limited device and carrier support it was destined to remain a niche application. By the end of 2014 we saw Wi-Fi calling support extend to the iPhone, voice-over-LTE rolling out over cellular networks, and significant market demand from users… but while many operators expressed intentions to roll out the feature, their support remained limited.
It took a while, but Sprint, AT&T, and now Verizon have joined T-Mobile, and as of early 2016 all major operators in the U.S. support Wi-Fi calling.
Technical challenges are not completely vanquished: seamless call handover is still not universal, particularly where LTE coverage is poor. And the algorithms for moving off a Wi-Fi connection when it gets intermittent, switching between Wi-Fi access points when moving around indoors, and deciding when to switch the call back to LTE can all be improved. But most of the time, users in most places report the experience is very good.
How the enterprise can take advantage
What does this mean for enterprise network design? While Wi-Fi calling was originally consumer-driven, allowing users to save money by staying off the cellular network and using their home access points to compensate for poor cellular coverage, it can solve similar problems for enterprises.
The primary benefit is build-your-own-coverage. Many organizations still have campuses with poor cellular coverage from one or more cellular operators, and in-building coverage is becoming ever-more challenging as new cellular spectrum has worse propagation characteristics and energy-efficient buildings incorporate more wireless-shielding coated glass. Wi-Fi calling allows smartphone users entering the building, whether employees or visitors, to switch seamlessly to the in-building WLAN for all cellular services: voice, SMS, and data. It is inherently multi-operator, given that all operators offer Wi-Fi calling. Since Wi-Fi access points are already indoors, they don't suffer building-penetration problems, and indoor WLAN design is a mature discipline. A modern WLAN should already have the bandwidth and other features necessary to support Wi-Fi calling. Indeed, it takes active measures to prevent authenticated smartphones from using an existing WLAN for the service.
As Wi-Fi calling rolls out, the first victim will be DAS (Distributed Antenna Systems). This is the traditional way to extend cellular coverage indoors, but it is expensive compared to a WLAN, and additional operator support carries an incremental price tag. Anecdotally, operators are already de-emphasizing DAS for in-building coverage in favor of Wi-Fi calling because they can't recover the full costs of a DAS installation from the customer.
Meanwhile, equipment vendors are busy developing small cells combining LTE and Wi-Fi radios. Most of these have some form of Wi-Fi-to-cellular integration on the roadmap, such as LAA (Licensed Assisted Access) from the 3GPP. But even these may struggle to establish a business case compared to a WLAN with Wi-Fi calling.
The final piece of the Wi-Fi calling puzzle, universal operator support is now in place. The remaining issues are how quickly the smartphone installed base can be refreshed, the configuration required to enable Wi-Fi calling and connect to the WLAN (although most users are comfortable with this already), and the inter-access point and inter-network handover decisions.
Any organization faced with cellular coverage problems should take a look at this alternative technology. Pick a few users with a diversity of smartphones and carriers, consult your vendor on whether you need to strengthen your WLAN, then run a pilot to see how it goes. Or ask your users – many are already using Wi-Fi calling over your network.
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