Thanks for asking this question. Wifi calling is an instance 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 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. Their is messenger, email, what app, twitter, skype in fact we are at a stage where its 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

Thanks for asking this question. Wireless networking is one of those things that is easier to do in practice than it is to learn in theory. You will learn more by jumping in right ahead rather than poring through the books. But if you’re an absolute beginner you need to start somewhere. So I’ll give an overview of the practical aspects of wireless networks first and towards the end of my answer I’ll give suggestions from where you can learn the theory.

All wireless networks have three basic components

1. Antennas that send out wireless signals

2. A hardware that provides input signal to the antennas which it sends out wirelessly

3. An operating system that controls the hardware, which sends the input signals to antennas, which then sends it out wirelessly.

Now if you look at something as basic as a wifi router you’ll note that it has

1. Antennas (either external or on chip)

2. A chip to which those antennas are connected

3. An OS (that is accessible through the web admin panel) that controls the chip

So as a start you should try to read the specifications of wifi routers. What kind of chip does it use? What kind of antenna it has? What kind of OS runs on it? Buy a cheap wifi router and tear it apart. Watch tear down videos on youtube. Try to absorb as much information from a complete working system as you can.

Later on you’ll come to an understanding that a wifi router is simply a specialized computer. At that point you should try to build a wifi computer of your own. In this article I give an overview of how you can build your own wireless base stations

https://www.quora.com/q/jcmpkzyhqciulode/Integrated-custom-base-stations

The concepts used in wifi base station is exactly the same as the ones used in base stations for 5G and 4G networks. So all your skills are transferable.

By the way, next time you go out and keep an eye on the telephone towers you come across. Try to understand how the different components on the towers namely

1. The wire

2. The antennas

3. The base stations

are connected to each other. At this point you don’t have sufficient theory to fit all the pieces together. But when you start to learn theory seeing things first hand is going to help you get a better grasp of underlying concepts.

How to study theory?

Don’t try to learn everything all at once. Think of the subjects as separate courses that you take in college. Divide your time into “semesters” and try to get a hold fundamental concepts first.

Learn the OSI model before you do anything else, so that you have a framework of how to arrange the different components of wireless network in your mind. For me nothing has helped better than this video by drunk engineer. It’s 18 minutes long Check it out

https://www.youtube.com/watch?v=3b_TAYtzuho

At this point with the OSI model in place can you fit the telecom tower and all of its components in it?

What to study in theory?

In theory you want to learn all about antennas. How many different kinds there are? How do they work and what are their radiation patterns? What are their radiation patterns?

Try to co-relate your learning with the on field deployments in mobile towers. At this point you should be asking questions like

Given the length of the antenna can I guess what frequencies it operates on?

Given the shape of the antenna can I guess its radiation pattern?

Given the placement of the antenna can I guess its coverage?

Some youtube channels that will help you learn more about antennas

Antenna and wave propagation playlist

https://www.youtube.com/playlist?list=PLgwJf8NK-2e7tzLIDL4aXUbtRFY3ykmkT

Antenna theory by NPTEL

https://www.youtube.com/playlist?list=PLzJaFd3A7DZsL9dZDCeA3ijHZwwBb6R8y

stan gibilisco’s videos (must watch)

https://www.youtube.com/user/stangibilisco/playlists

And tonnes and tonnes of videos by ham radio operators (search for them) along with DIY antenna builders, you can learn a lot by watching people build antenna. Build one yourself and see if you can improve reception in your home

And this great Practical Antenna Handbook for when you need it

https://archive.org/details/PracticalAntennaHandbook

Alt link

http://s1.nonlinear.ir/epublish/book/Practical_Antenna_Handbook_0071639586.pdf

Then you want to know as much as possible about the chips that provide signal to it. Learn about motherboards. Building computer systems from components. If you’re a gamer it should be easy for you. If not watch videos of people building gaming pc. To build a base station for wireless network you’ll just have to switch a few components. You’ll know them if you’ve taken the time to learn about chips. This guy is good

https://www.youtube.com/user/AwesomeSauceNews/playlists

WARNING: Remember you are here to learn about how to build wireless networks not to play games. The urge will be strong. Resist it … if you can….

And finally know how to operate a network. That you can understand by playing around with a wifi router admin panel. Whatever is in there should be more than enough.

Some More Material

1. I have found sunny’s classroom videos invaluable in helping me learn as well as refresh software part of the wireless networks. You can see his playlists here https://www.youtube.com/user/sunnylearning/playlists . Be sure to check out the videos on modulation schemes.

2. Wireless Networking for the developing world has great book that helps with both concepts as well as the business aspects of running a network http://wndw.net/

3. Start your own wisp is a great community resource for both learning about practical operations as well as sourcing of materials https://startyourownisp.com/

Remember that the industry is moving towards IP based networks. From 4G onwards all cellular networks run on IP. So spending some time studying that will help a lot.

Obviously it is very easy for me to list out all of the things here. No doubt you’ll find yourself stuck in places. Feel free to ask questions on quora and the community will do its best to help you. Take things as slow as you can. It sounds overwhelming but its not. Once you tune yourself in.

Building your own network

Now if you do things right, not only will you have the knowledge but also very valuable insights that will tell you what most internet companies are doing wrong. And that insight will help you start a successful wireless network of your own. As a final note don’t get washed away in industry trends.

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A base station is the brain behind a wireless network. It is responsible for data transmission lines,authentication, routing,computing.

Although it looks much different, a base station is simply a desktop computer packaged in a way that it can be placed outdoors.

So that if you can build a gaming pc that can play Metal Gear Solid V on max settings you can build a very powerful base station.

Now of course there are some differences. For one instead of graphics card you would go with a network card. Instead of an ssd you might want to get the latest wifi card.

You may ask, is this how the base station for 5G and 4G is created?

Yes but they use different wireless chips and the software to control these chips are proprietary.. for now. But inside the components are the same. A motherboard, RAM, processor, peripheral components.

Luckily there are alternatives. WiFi that can work on both 2.4 and 5 GHZ bands is quite open.

Coupled with the advantage that you don’t have to pay anything to use wifi spectrum a wifi base station becomes very promising investment.

In fact there is a full open source operating system that allows you to customize your wifi network to your heart’s content — openwrt

Link aggregation,quality of service, dual band networks, VPN,DHCP, ssh, custom web services. Anything that you can do with linux you can do with openwrt.

Infact openwrt is linux, complete with its own package management. So that you won’t have to use an outdated firmware on your router ever again.

With openwrt you can turn a base staion into a full blown application server.

Besides it has an appealing web interface that you can use to configure your networks without getting into the terminal at all.

With advancement in single board computers you can even create an embedded router that can distribute your network to 10s of Kilometers.

Gigabit speeds without having to deal with installation of fiber optic networks is a big win. It slashes your infrastructural investment by 1/10,maybe more. Giving you faster returns.

If you are a small ISP or WISP looking for cheap ways to reach more customers build your own wireless base station. Install openwrt and you’re ready to go.

Thanks for asking this question. Before we get to the designing part lets quickly review a few basics.

1. Antennas can broadly be classified into directional and Omni directional. An Omni directional antenna radiates in all directions which makes it wider in coverage but at a smaller distance. A directional antenna on the other hand is focussed. Meaning the beam goes a longer distance but has lesser area of coverage. As you can see there are trade offs to be made.

2. An antenna is the last part of the system working to transmit or to receive. To make something useful you need other components as well. The chip that will generate the input signal to create EM waves on the transmit end or the one that will demodulate at the receiver end.

3. The size of the antenna depends upon the frequency of your signal. You don’t want to choose one that will make you antenna too big. Luckily this problem has been solved for you. The ISM band has GHZ frequencies especially ones in the wifi range that will be a few centimeters.

Now coming to the actual building part. For testing You can use any wifi chip to create an input signal. These chips are quite cheaply available online. For example

https://www.mouser.in/m_new/embedded-solutions/wireless-rf-modules/wifi-80211-modules/n-6l7qa

Once that is done you’ll need to add a receiver to your drone. The same wifi chip can be used to receive the signals.

Your drone remote control system would consist of a wifi chip at transmitter and receiver.

This setup would work fine at small distances like a few feet. This is good enough for you to test your setup in a lab and fine tune things if needed.

But for a production remote control drone you want an antenna that is able to transmit at receive signals at longer distances. So the first thing you need to while designing your antenna is to account for that.

The built in antennas for the chip are patch and not able to transmit to long distances.

What you’ll need to do is drive the input signal into an antenna with better gain.

This step is experimental and you’ll need to try with many different antennas. Unfortunately I can’t give a definitive answer…. but I can give you some points to get started

If you want to build your own antenna you’ll need

1. A wire

2. Cut to an appropriate length (depending upon the frequency of your signal,don’t ignore this step because it’s important)

3. A means to drive current to it . (Which you can do by connecting it to chip)

That’s it.

Before you make your own antenna it would be good to check out what other people have built.

Some References

Drone FPV antenna gain and range explaination– https://www.dronezon.com/learn-about-drones-quadcopters/learn-about-uav-antenna-fpv-live-video-transmitters-receivers

Some commercial drone antennas to study —

https://www.getfpv.com/fpv/antennas.html

A nice video explaining the effect of antenna gain in a drone

https://youtu.be/MigTBIsk6IA

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