# Wi-Fi Calling

## Summary


Wi-Fi Calling is a technology that allows users to make or receive voice calls via a local Wi-Fi hotspot rather than via their mobile network operator's cellular radio connection. Voice calls are thus carried over the Internet, implying that Wi-Fi Calling relies on VoIP. However, unlike other VoIP services such as Skype or Viber, Wi-Fi Calling gives operators more control.

Wi-Fi Calling is possible only if the operator supports it, user's phone has the feature and user has enabled it. Once enabled, whether a voice call uses the cellular radio link or Wi-Fi link is almost transparent to the user. With cellular networks going all IP and offering VoLTE, Wi-Fi Calling has become practical and necessary in a competitive market.

Wi-Fi Calling is also called *Voice over Wi-Fi (VoWi-Fi)*.

## Discussion

### In what scenarios can Wi-Fi Calling be useful to have?

In places where cellular coverage is poor, such as in rural residences, concrete indoors, basements, or underground train stations, users will not be able to make or receive voice calls. In these scenarios, the presence of a local Wi-Fi network can serve as the "last-mile" connectivity to the user. Wi-Fi can therefore complement the cellular network in places where the latter's coverage is poor.

For example, a user could be having an active voice call via the cellular network and suddenly enters a building with poor coverage. Without Wi-Fi Calling, the call might get dropped. With Wi-Fi Calling, the call can be seamlessly handed over to the Wi-Fi network without even the user noticing it. Astute users may notice that their call is on Wi-Fi since smartphones may indicate this via an icon. More importantly, user intervention is not required to switch between cellular and Wi-Fi. Such seamless handover has become possible because cellular network's IP and packet switching: VoWi-Fi can be handed off to VoLTE, and vice versa. 


### Isn't Wi-Fi Calling the same as Skype, Viber or WhatsApp voice calls?

Many smartphone apps allow voice (and even video) calls over the Internet. They are based on VoIP technology. We normally call them over-the-top (OTT) services since they merely use the phone's data connection and operators bill for data usage and not for the service itself. However, many of these systems require both parties to have the same app installed. Even when this constraint is removed, the service is controlled by the app provider.

Wi-Fi Calling gives cellular operators greater control. Driven by competition from OTT services, Wi-Fi Calling gives operators an opportunity to regain market share for voice calls. Voice packets are carried securely over IP to the operator's core network, thus allowing the operator to reuse many resources and procedures already in place for VoIP calls. Likewise, messages and video–*Video over LTE (ViLTE)*–can also be carried over Wi-Fi. 

From an architectural perspective, Wi-Fi Calling is served by operator's IP Multimedia Subsystem (IMS), whereas Skype calls are routed out of the operator's network into the Internet.


### Isn't Wi-Fi Calling the same as Wi-Fi Offload?

Not exactly. Wi-Fi Calling can be seen as a form of offload but they have different motivations. Wi-Fi Offload came about to ease network congestion and improve QoS for users in high-density areas. The offload is transparent for users whose devices are authenticated via EAP-SIM/AKA. 

Wi-Fi Calling is in response to OTT services stealing revenue from mobile operators. Even VoLTE was deployed by operators, voice calls couldn't be made over Wi-Fi and OTT services was what users used when they had access to Wi-Fi. Wi-Fi Calling aims to overcome this problem. 


### What are the possible benefits of Wi-Fi Calling?

For subscribers, benefits include seamless connectivity and mobility between cellular and Wi-Fi. The selection is automatic and transparent to users. Data is protected using IPSec from mobile to core network, along with traditional SIM-based authentication. Users can potentially lower their monthly bills through service bundles and reduced roaming charges. Sometimes calling home from another country could be free depending on the subscribed plan and operator. 

Moreover, the user's phone will have a single call log (likewise, for message log). The default dialler can be used along with all saved contacts. Those receiving the call will see the caller's usual phone number. These are not possible with a third-party installed app.

For operators, Wi-Fi complements cellular coverage and capacity. T-Mobile was one of the early adopters because it had poor indoor coverage. Wi-Fi Network performance is optimized by allowing bandwidth-intensive traffic to be offloaded to Wi-Fi when so required. All their IMS-based services can now be extended to Wi-Fi access rather than losing out to OTT app/service providers. 


### How does the network architecture change for Wi-Fi Calling?

Two network functions are involved: 

    + **Evolved Packet Data Gateway (ePDG)**: Serves an untrusted Wi-Fi network. An IPSec tunnel protects data between mobile and ePDG, from where it goes to Packet Gateway (PGW). The mobile needs an update with an IPsec client. No changes are needed for the access point.
    + **Trusted Wireless Access Gateway (TWAG)**: Serves a trusted Wi-Fi network, which is typically under operator's control. In this case, data between mobile and TWAG is encrypted at radio access and IPSec is not used. From TWAG, data goes to PGW. No changes are needed for the mobile but Wi-Fi access point needs to be updated.If the network is not an Evolved Packet Core (EPC), then Tunnel Termination Gateway (TTG) is used instead of ePDG; Wireless Access Gateway (WAG) is used instead of TWAG; GGSN is used instead of PGW.

The untrusted mode is often used for Wi-Fi Calling, since public hotspots can be used without updating the access point. It's the operator who decides if a non-3GPP access can be considered trusted. 


### How is an end-user device authenticated for Voice over Wi-Fi service?

Within the network, *3GPP AAA Server* is used to authenticate end devices. Authentication is based on SIM and the usual network functions located in the Home Subscriber Server (HSS). 3GPP AAA Server does not maintain a separate database and relies on the HSS. 

Vendors who sell AAA servers usually give the ability to do authentication of devices that don't have SIM. For legacy networks, they can interface with HLR rather than HSS. They support AAA protocols such as RADIUS and Diameter. They support various EAP methods including TLS, PEAP and CHAP. 


### What are the 3GPP standards covering Wi-Fi Calling?

Documents that specify "non-3GPP access" are applicable to Wi-Fi Calling. The following are some relevant documents (non-exhaustive list):   

    + TR 22.814: Location services
    + TR 22.912: Study into network selection requirements
    + TS 23.402: Architectural enhancements
    + TS 24.234: 3GPP-WLAN interworking: WLAN UE to network protocols, Stage 3
    + TS 24.302: Access to EPC, Stage 3
    + TS 29.273: 3GPP EPS AAA interfaces
    + TS 33.402: System Architecture Evolution: security aspects
    + TR 33.822: Security aspects for inter-access mobilityIn addition, GSMA has released a list of Permanent Reference Documents on VoWi-Fi. 

Wi-Fi Calling is a technology that comes from the cellular world. From Wi-Fi perspective, there's no special IEEE standard that talks about Wi-Fi Calling.


### Are there commercial services offering Wi-Fi Calling?

In June 2016, it was reported that all four major operators in the US support Wi-Fi Calling, with T-Mobile supporting as many as 38 different handsets. In November 2016, there were 40+ operators offering Wi-Fi Calling in 25+ countries. Moreover, even affordable phones or devices without SIMs are supporting Wi-Fi Calling. An operator will normally publish a list of handsets that are supported, which usually includes both Android and iPhone models. In September 2017, it was reported that AT&T has 23 phones and Verizon has 17 phones that support Wi-Fi Calling. 

Wi-Fi Calling may involve regulatory approval based on the country's licensing framework. For example, India's TRAI commented in October 2017 that Wi-Fi Calling can be introduced since licensing allows telephony service to be provided independent of the radio access. 


### Within enterprises, how can IT teams plan for Wi-Fi Calling?

Some access points have the ability to prioritize voice traffic and this can be repurposed for Wi-Fi Calling. Examples include Aerohive, Aruba, Cisco Aironet and Ruckus. Enterprises can also work with operators to deploy femto/pico cells or distributed antenna systems. 

A minimum of 1 Mbps may be needed to support Wi-Fi Calling although Republic Wireless in the US claims 80 kbps is enough to hold a call, although voice quality may suffer. In reality, voice needs just 12 kbps but can scale down to 4.75 kbps. 


### How will users be billed for Wi-Fi Calling?

This is completely operator dependent and based on subscriber's current plan. For example, Canada's Rogers says that calls and messages are deducted from airtime and messaging limits. Roaming charges may apply only for international roaming. Verizon Wireless states that a voice call will use about 1 MB/minute of data; a video call will use 6-8 MB/minute. Billing is linked to user's current plan. 


### What are some practical issues with Wi-Fi Calling?

Back in 2014, T-Mobile had handoff problems but it was improved later. The service was also not offered by other operators and not supported by most handsets. Even when a handset supports it, operators may not offer the service if the handset has not been purchased from the operator. 

Since any Wi-Fi hotspot can be used, including public ones, security is a concern. For this reason, all data over Wi-Fi must be protected and subscriber must be authenticated by the cellular operator. Seamless call continuity across cellular and Wi-Fi could be a problem, particularly when firewalls and VPNs are involved. Some users have reported problems when using Wi-Fi behind corporate firewalls. Likewise, IT teams in enterprises may have the additional task of ensuring Wi-Fi coverage and managing traffic. 

Since Wi-Fi Calling often uses public hotspots, there's no QoS control. However, it's argued that in places where cellular has poor coverage, QoS cannot be guaranteed anyway. In addition, QoS on Wi-Fi can often be achieved implicitly because of excess capacity. With the coming of 802.11ac and the ability to prioritize traffic via Wi-Fi Multimedia (WMM), QoS is unlikely to be a problem. 

## Milestones

2007

T-Mobile in the US launches something called "HotSpot @ Home". This is based on a technology named *Unlicensed Mobile Access*, which is a commercial name of a 3GPP feature named *Generic Access Network*. GAN operates in the IP layer, which means that access can be via any protocol, not just Wi-Fi. UMA does not take off because of lack of handsets that support it. It also had other operational issues related to interference, handover and configuration setup. 

Nov  
2011

Republic Wireless, a mobile virtual network operator (MVNO) in the US, rolls out "Hybrid Calling". Calls are primarily on Wi-Fi and cellular will be used as a fallback option. Their General Manager, Brian Dally, states,

> Every other mobile carrier talks about offloading to Wi-Fi, we talk about failing over to cellular.

Sep  
2014

T-Mobile introduces Wi-Fi Calling in the US. This comes at the heels of the operator's rollout of VoLTE. Meanwhile, Apple iPhone starts supporting Wi-Fi Calling. 

Apr  
2015

Sprint introduces Wi-Fi Calling in the US. EE does the same in the UK. Meanwhile, Google gets into telecom by launching *Project Fi*, which allows seamless switching between Wi-Fi and cellular. Google doesn't have its own cellular network but uses those of Sprint, T-Mobile, and US Cellular. 

Oct  
2015

In the US, AT&T obtains regulatory approval to launch Wi-Fi Calling. By 2016, all four major US operators rollout Wi-Fi Calling nationwide. 

Jun  
2017

UMA, which may be called first generation Wi-Fi Calling, is decommissioned by T-Mobile in the US. 

Nov  
2018

Researchers discover security vulnerabilities with Wi-Fi Calling due to various reasons. They propose possible solutions to overcome these.