Wifi calling is an example of what are known as generic access networks (GAN) [1]. GANs allow you to access the networks of cellular providers through an external IP network.
It is important to keep in mind however that Wifi Calling in the traditional sense is still a cellular call. By that I mean you are calling a number. Authentication, call connection , call routing all works as it would if you called directly.
“How does it work in sync with the cellular provider?”
For that we need to look into something called the IP Multimedia Subsystem (IMS).
[I’ll limit this discussion to 4G networks but note that GAN also works for 2G networks just that the handoff method is a bit different requiring a GANC (C here stands for controller referring to a specific hardware device that transforms and forwards the data traffic from WiFi to cellular network processor)]
IMS is a system for delivering communication services over an IP network. Before 3G ,services were delivered using a circuit switched network. IMS performs routing,authentication ,authorization and accounting over internet protocol. Several protocols are available for performing individual tasks but they all work over IP. All these tasks are performed on the server level (or the base station level in telephony terms) provided by the network operator.
Now it's common knowledge that cell phones work wirelessly. These wireless signals form the “Physical Layer” of the communication network stack. 4G onwards all cellular communication is IP based [2].
What is other wireless communication technology that works on IP?
Wifi!!
So since wifi understands IP pretty well. Since wifi is widely deployed. And since nearly all smart devices support wifi it is an easy choice for infrastructure providers to use wifi calling to not only offload the strain on their core network but also to enable their services where towers don’t yet exist.
Now 4G is an all IP network and wifi understands IP pretty well, all that is needed to be done is to forward the wifi packets to the 4G network provider over the wifi connection. This is handled by the operating system of the handset when you enable the wifi calling feature.
For example check out this line on android open source repository
https://android.googlesource.com/platform/frameworks/base/+/master/telephony/java/android/telephony/ims/ImsMmTelManager.java#69
How does it work on the hardware level?
The easy answer is it will work in the same way as you receiving a call on your phone while watching youtube connected to your wifi router. At that point in time you are using both wifi and cellular network services.
For wireless communication to work you need radios and antennas. Now if you have been following the developments in mobile chip technology you must have noted that a single modem encapsulates 3g,4g,5g,wifi and bluetooth in a package.
Such a package contains both radios as well as antennas for various technologies to work simultaneously. The antennas and radios for wifi are separate from the antennas and radios for 4G even though all the components are on a single chip. They are all distinct subsystems.
You can have your wifi,bluetooth and 4G on all at the same time and use all these services at once.
As described above the handoff or the sync will have to be handled by the OS (android or IOS depending on your smartphone). It will determine when to handoff and to what network to handoff to.
The handoff can only work when both cellular and wifi radios are “on”. Which can lead to more load on the battery.
Wifi calling is not without its faults however. The way current wifi systems are deployed ,range can often be an issue. On the other hand , at least as a personal observation, most people seem to prefer an asynchronous mode of communication.
Email me I’ll reply when I have the time.
What’s app me I’ll reply when I check your message
Voice message me and I’ll get back to you.
Direct telephone calls, unless they are from someone close to you or pre scheduled are seen as intrusive.
Now this makes wifi based offloading a very attractive option especially considering the pricing of wireless equipment (remote radio heads for example). Even if a user does not use wifi calling feature natively, they can converse on several other apps. There is messenger, email, what app, twitter, skype in fact we are at a stage where it's hard to keep a track of all the ways in which we can communicate!
When GAN was proposed in 2005 there were not as many alternatives. Android was not popular. Smartphones were in their infancy. At that time it was a brilliant idea. But today not so much.
That is not to say that the idea has no merits. It does. Especially when you consider that nearly all telecom companies are pushing for wifi deployments. Wifi calling may or may not be successful. But it definitely will reduce the operational cost of service providers by reducing the load on their towers.
I’m of the opinion that in the near future we are going to see more and more open systems like wifi taking over the functions that were previously provided by core cellular deployments. The cellular networks are IP based but they don’t offer significant advantages over the default networking open source stack [3] on linux which has already proven its networking capabilities in data centers. Communication technology is going to be more open. That’s what I feel at least. And wifi calling is a step towards that direction.
References
[1] https://en.wikipedia.org/wiki/Generic_Access_Network
[2] 4G is all IP https://electronics.howstuffworks.com/4g2.htm
[3] It is relevant here to point out that VOLTE is basically SIP. Here are a few references that explain how Voice over LTE works
https://www.quora.com/Is-SIP-required-in-Voice-over-LTE
https://www.netmanias.com/en/post/blog/10907/lte-volte/part-3-what-happens-when-a-user-performs-a-voice-call-from-an-lte-4g-network-volte (Jump to Section 3.2)
https://www.quora.com/How-do-4G-and-2G-users-call-each-other
http://www.eventhelix.com/lte/volte/volte-originating-call.pdf
It is quite interesting to note that android natively supports SIP
https://developer.android.com/guide/topics/connectivity/sip
Also freeswitch an opensource linux application can basically be used to replicate the same features over unlicensed bands.
https://en.wikipedia.org/wiki/FreeSWITCH
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