Wi-Fi Calling

Goes Mainstream


Everything that Apple does is big news in the mobile world, as they are at the top of
the food chain. The announcement in September of 2014 that they are supporting
Wi-Fi Calling on the iPhone sent shockwaves throughout the industry, and it certainly
reinforces Wi-Fi’s position as a cornerstone technology in the mobile world. Apple
didn’t invent Wi-Fi calling as it’s been around in various forms for many years (Skype,
Lync, UMA, etc.), but they have made it a mainstream technology by integrating it into
their smartphones. Wi-Fi already carries close to 80 percent of smartphone data trafic
according to the latest reports from Mobidia. Now with carrier-class Wi-Fi Calling, we
can expect a great deal of smartphone voice trafic to also move over to Wi-Fi. This
announcement does not bode well for voice-centric technologies like femtocells, which
have always been a struggling market.
December 2014

Figure 1:Mobidia Survey on U.S. Data Usage of Wi-Fi on Smartphones


Figure 2: Tunneling Wi-Fi Calling Tr afic into the Evolved Packet Core



So what exactly is Wi-Fi Calling as deined by Apple? It is the ability to place a cellular voice call using a cellular voice stack in a smartphone (in this case LTE) over a Wi-Fi network and then terminating it on the IP Multimedia Subsystem (IMS) complex in the mobile network operator’s data center. This is very different from over-the-top services like Skype, because it is native to the smartphone (not a 3rd party app), and it works in exactly the same way that a voice call over LTE would work. It also supports seamless handoff as the user moves between Wi-Fi and cellular coverage areas.So what is required for a MNO to support Wi-Fi Calling? The Apple solution utilizes the GSMA’s IR-92 Proile for VoLTE
(Voice-over-LTE), which has an option that allows the entire LTE stack to be tunneled over Wi-Fi using IPsec. This is very similar to unlicensed mobile access (UMA), which tunnels the entire GSM voice stack over Wi-Fi using IPsec. In both cases, the voice stack is encrypted in the handset before being sent across the Wi-Fi network. The payload then needs to be decrypted in the MNO data center using an ePDG Gateway (and vice versa for trafic going the other way). The ePDG is a massively scalable IPsec
concentrator that then tunnels this trafic into the evolved packet core using the GPRS tunneling protocol (GTP).Since all VoIP trafic is encrypted, the Wi-Fi access network can’t see the payload and has no idea what is going on here… almost. Ruckus has the ability to prioritize Wi-Fi Calling trafic by looking at the TOS (type of service) bits set by the smartphone in the IP header, or by using heuristics. In the latter case, Wi-Fi Calling trafic is detected by looking at the size and frequency of packets in a “ow (even an encrypted “ow). This combination of capabilities in a Ruckus Zone”ex access point is known as SmartCast(™). When this is combined with Ruckus’ Adaptive Antenna Technology, the result is a very compelling Wi-Fi Calling experience in almost any situation. Adaptive antennas can increase the signal gain as seen by the user’s device, which of course leads to a much better voice experience. This combination of technologies is essential to the delivery of a true low-latency, low-jitter carrier-class Wi-Fi Calling experience. Since this is a carrier-class offering, MNOs can charge for this service in much the same way they charge for voice-over-LTE calls. It also enables the MNO to provide a much better overall
voice experience by leveraging both LTE and Wi-Fi radio access networks, and there will always be situations where Wi-Fi simply provides a better signal.So what else must an MNO do to implement Wi-Fi Calling besides deploy an ePDG? They need an evolved packet core to terminate trafic coming from the ePDG and they need an IMS complex, as VoLTE is only deined for IMS. Can a non-MNO also provide a Wi-Fi Calling service using IR-92? The answer here is yes, if they have an EPC, an IMS complex, smartphones, SIM cards, and an HLR to authenticate the user. A non-MNO can always provide
the Wi-Fi access network that is used to tunnel Wi-Fi Calling trafic from the user back to the MNO’s home network, and it can enhance that experience by prioritizing voice packets. As with other Wi-Fi Calling solutions, IR-92 based implementations can be used to eliminate voice roaming charges when placing calls from anywhere in the world.

Preparing the Network to Offer a Carrier-Class Wi-Fi Calling Experience
So what must service providers do to insure that their Wi-Fi network is ready to support a carrier-class Wi-Fi Calling experience? There are several elements to a successful deployment.

On the Importance of a Strong Radio Signal
Everything starts with a really strong radio signal. To this end, Adaptive Antenna Technology was developed for the transporting of delay-sensitive video and voice trafic over Wi-Fi.Adaptive Antenna Technology provides a much stronger signal as seen by the user’s smartphone, which equates to a better
modulation and coding scheme (MCS). A better MCS means higher data rates, and a higher data rate means it takes less time to send a speciic amount of data (or voice) allowing client stations to spend less time accessing or ighting for access to the Wi-Fi medium. It also reduces contention for the RF channel, as well as reducing the likelihood of collisions (increased jitter), frame loss, or packet retransmissions (increased latency). In other words, providing a stronger signal at the smartphone increases the overall airtime eficiency of the device for sending voice as well as other types of trafic.

Figure 3: Enhancing Downlink Performance with BeamFlex Adaptive
Antenna Technology



Another important innovation beneiting Wi-Fi Calling is the ability to enhance the uplink signal from the client to the AP by receiving the client’s signal on both horizontally and vertically polarized antenna elements. When polarization diversity is combined with maximal ratio combining (PD-MRC), the end result can be up to 5 dB of additional gain. This is especially important when considering single stream/single antenna mobile devices (the vast majority of smartphones and tablets, including all models of the iPhone), which transmit with a single polarization.

Figure 4: Enhancing Uplink Performance with Polarization Diversity


Adaptive smart antenna technology is able to effectively extract or construct the strongest possible Wi-Fi signal regardless of the client’s orientation relative to the AP. Because real-time voice is inherently bidirectional, it is important that both the downlink and uplink support the best possible MCS and highest possible data rates.

Prioritizing Voice Trafic with SmartCast
Getting a strong signal is only the irst step — next comes prioritizing “ows through the Wi-Fi access network. There are several methods by which an encrypted “ow can be prioritized so as to guarantee a high-quality voice call and they are all part of the Ruckus SmartCast feature.At the heart of SmartCast is a sophisticated trafic inspection, classiication, and optimization engine that works in software to provide per-client, per-trafic-class queuing.The SmartCast quality-of-service (QoS) engine inspects each packet and automatically classiies it into one of four queues — voice, video, best effort, and background. SmartCast can inspect a variety of headers including those of Ethernet frames (both TCP
and UDP), VLAN tags, and IPv4 and IPv6 packets.If the type-of-service (TOS) or 802.1p priority !eld is used, SmartCast maps packets to an equivalent internal !eld. If no tag is provided, SmartCast employs heuristics to classify traf!c. With heuristics based quality-of-service techniques, Wi-Fi Calling “ows
can be detected by looking at the size and frequency of packets in a “ow, even an encrypted “ow.Once classi!ed and queued, traf!c is scheduled using a weighted round robin method based on airtime and throughput potential as well as prioritization de!ned for the WLANs. Rate limits can also be applied on a per-WLAN basis for every client.The net result is that SmartCast can maximize the reliability and
performance of delay-sensitive Wi-Fi Calling traf!c by minimizing jitter and latency.

Figure 5: SmartCast Smart Scheduling of Low-latency Voice Trafic