In the time since Apple’s revelation that iOS 8 would support a form of Wi-Fi calling, the industry has seen a barrage of announcements, even TV commercials, around Wi-Fi calling. Come to find out that many of them are actually existing products and technologies simply re-spun. A deeper dive into Wi-Fi calling’s history and characteristics reveals what is truly needed to properly support this exciting new Apple capability, as well as other Vo-Fi services.
So What is Wi-Fi Calling?
Sort of like the term “cloud,” Wi-Fi calling often means different things to different people.
The iOS form of Wi-Fi calling is different from so-called over-the-top (OTT) services like Skype or Lync because it is integrated within the OS’s dialer (not a third- party app) and is architected to work in the same way a Voice over LTE (VoLTE) call works. It’s also being developed to support the transparent handoff of a call as the user moves between Wi-Fi and LTE coverage areas, something OTT approaches simply can’t do. It’s more of an evolution of about older UMA (unlicensed mobile access) based services, which were some of the first to support cellular voice services over Wi-Fi. Other implementations would include services from MVNOs such as republic wireless and Scratch Wireless.
While these are all examples of “Wi-Fi Calling,” they have very different characteristics, raising a number of important questions:
- Where does the voice session terminate?
In an IMS core, at a standalone SIP server/gateway, or on a MSC, does the voice session have to enter an operator’s core network? If so, how is the ‘untrusted/trusted’ border transited?
- What codec is used?
How is it encapsulated for transmission?
- Is the voice session encrypted?
If so, what are the encryption endpoints?
- Is the calling service integrated into the native dialer?
Or does it require a separate app?
- Does the service support call handoff to and/or from a cellular service?
If so, does handoff work with Circuit-Switched networks, VoLTE/IMS networks, or both?
While there are obviously a lot of possibilities to enable Wi-Fi calling, it’s also encouraging that there are so many ways to support voice over Wi-Fi – underscoring Wi-Fi’s flexibility to support a myriad of IP-based services.
What’s REALLY changing with Apple’s iOS integration and mobile operators lining up to support it is that Wi-Fi Calling will no longer be just a so-called OTT service, or only be offered by upstart MVNOs like republic wireless or Scratch Wireless. Wi-Fi calling is going mainstream.
The question now becomes: How do Enterprises, Operators, and Venues optimize their Wi-Fi networks to support this service?
Voice as an IP Service
Voice is a low bitrate, but very finicky, data service because real-time, bidirectional voice demands a narrow set of operating parameters from the network in to ensure a high quality calling experience.
As such, the requirements for latency, jitter, and packet loss are much tighter for voice than for standard business or Internet applications.
Wi-Fi Never Really Designed for Voice
Wi-Fi utilizes a shared medium (unlicensed spectrum in 2.4 or 5 GHz) for all the stations in a service set (including the Access Point). Access to the medium is not directly coordinated between the stations, but is performed using mechanisms that seek to minimize simultaneous access attempts and indicate to a transmitter if the intended receiver did not receive its frames. In addition to the contention for access to the medium, Wi-Fi can also be subject to interference by other uses of the same unlicensed spectrum.
While on the surface, Wi-Fi might not seem like an appropriate access network for quality voice services, advances in Wi-Fi technology make it possible.
Stronger Voice with Smarter Wi-Fi
Addressing many of these issues that can hinder good Wi-Fi calling, new adaptive antenna technology was conceived for transporting delay-sensitive video and voice traffic over Wi-Fi to enable a highly optimized signal for each client.
A stronger signal equates to a better Modulation and Coding Scheme (MCS). Better MCS means higher data rates and higher data rate means it takes less time to send a specific amount of data allowing client stations to spend less time accessing or fighting for access to the Wi-Fi medium.
This 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 better signal at the receiver increases the overall airtime efficiency of the service set for stations sending voice and those sending other types of traffic.
Adaptive antenna array technology utilizes smart directional Antennas within a single array, automatically controlled by fancy software that picks, for every packet, the best antenna combination to focus the RF energy towards the intended receiver. This results in a 5 to 6 dB of gain of signal on the downlink connection.
In addition, smart antennas help mitigate interference from other access points operating in the area by only directing RF energy towards the intended receiver, not simply blasting it everywhere. The impacts from the receiver gain and interference mitigation are cumulative and quite pronounced in dense deployments such as office buildings or high capacity public venues.
Looking ahead, 802.11ac Wave 2 introduces the concept of Multiuser MIMO (MU-MIMO). Multiuser MIMO effectively allows concurrent Wi-Fi conversations to occur for different clients. The ‘grouping’ of clients into MU-MIMO sets will be essential to maximizing the benefits of this innovation. Good grouping will enhance the ability of a given set of clients to simultaneously receive a transmission and effectively interpret their individual data streams.
Due to the uniform nature of Wi-Fi calling payload sizes, this will make Wi-Fi calling clients prime candidates for grouping with each other (assuming they meet other grouping criteria), benefitting the Wi-Fi calling experience by servicing multiple downstream clients simultaneously.
Another important innovation benefiting 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 the horizontally and vertically polarized antenna elements. Because they are able to implement polarization diversity with maximal ratio combining (PD-MRC), smart antennas can provide up to 5 dB of uplink gain. This is especially important when considering single stream/antenna mobile devices (the vast majority of smartphones and tablets, including all models of the iPhone), which transmit with a single polarization. Adaptive smart antenna technology is able to effectively extract or construct the best 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 data rates.
Beyond The Antenna
Beyond antennas, recent technical advances have also been made in how traffic is handled within Wi-Fi access points to ensure the best possible quality of service for Wi-Fi calling.
Since traffic is often encrypted with Wi-Fi calling, the Wi-Fi access network has no real visibility into the payload to determine what type of traffic is being served.
With more innovative heuristics-based quality of service, different traffic types can be automatically identified, prioritized, scheduled and queued even without the ability to inspect the inner contents of the packets and detect that they are part of a voice session. This is achieved through sophisticated algorithms that constantly examine the characteristics and behavior of the traffic such as the size and frequency of packets in a flow (even an encrypted flow).
Such sophisticated traffic inspection, classification, and optimization technology works in software to provide per-client, per-traffic-class queuing. So traffic is mapped into the various queues based on existing L2 or L3 tags received from the upstream network or these heuristic-based identification algorithms.
What’s more, sophisticated schedulers implement advanced algorithms to transmit the frames based on airtime and throughput potential or even WLAN prioritization settings that have been configured. If a client doesn’t receive a frame, the scheduler ensures that the frame gets priority for retransmission, eliminating head of line blocking issues.
The Holy Grail for Wi-Fi Calling?
Ultimately for Wi-Fi calling to work as everyone wants it to, the combination of these technology innovations is essential to delivering a true low-latency carrier-class Wi-Fi calling experience so good that you’ll be able to hear a pin drop (over Wi-Fi).