Under the Hood: How We Engineered Next Gen WiFi Calling


Republic Wireless was born on innovation and known for being the pioneers of WiFi calling. In 2016, we created Bonded Calling – something never before seen in the tech industry. But what exactly
is Bonded Calling?

Simply put, this technology allows our network to sense if there is a problem with your call and fill in the gaps with your phone’s other data networks to catch dropped calls before they ever happen.

For those who really want to understand the ins and outs of the technology that makes this possible, we wanted to take a look back and dive into this revolutionary tech that gave Republic Wireless its name. Hear directly from our innovative engineers as they give us an inside look at how Bonded Calling actually works.

We care about call quality

When you’re using your Republic phone to keep in touch with your friend, your calling experience is measured by the content of the conversation, not the quality of the call. However, when your network zigs instead of zags and you lose some audio, your focus is shattered. The conversation suffers. This is a worst case scenario for a Republic Engineer. This is why back in 2016 we rolled out something we were – and still are – extremely excited about — Bonded Calling.

Bonded Calling is one factor in the Republic Adaptive Coverage offering. Adaptive Coverage itself describes the entire history of Republic Wireless WiFi calling: seamless handover from WiFi to Cell and back again, and advanced algorithms for determining the optimal time to handover your call. Bonded Calling uses the cell data connection of your phone to supplement a WiFi call during a rough patch. As we dive into how this works, we’ll follow the journey of your voice, as it is digitized and slung across the world.

Your voice disassembled

Your phone initially receives your voice as an analog signal. Through various hardware and software elements, the signal is transformed into a digital stream of bytes. When we talk about a stream, or frankly anything digital, it’s important to keep in mind that things that appear continuous in observation are in fact made up of tiny discrete pieces. For example, your laptop display is shining millions of tiny dots into your eye, which your brain interprets as a single image. Likewise, it’s not possible for us to transmit a truly continuous signal over the Internet, so the stream is packetized; we split the stream into segments that are each 20 milliseconds in duration (that’s 1/50th of a second, roughly comparable to the duration of a single video frame).

Your phone breaks up your voice’s audio signal into short pieces.

These audio packets must arrive on the Internet backbone, the critical network of physically connected computers that make it possible for you to stay connected to the rest of the world. But how do we get them there? The obvious choice is to send them via your home WiFi network; on average it’s your most reliable and fastest connection to the Internet. This is how all WiFi calls start, but your phone is mobile, and so are you.

For example, maybe you go out to the mailbox while you’re on the call. Your WiFi network may struggle to keep up with the increased range. Republic can detect the change in WiFi and hand the call over. But with Bonded Calling, the story is a bit different.

Cell! WiFi! on your marks

Your phone detects that your audio packets are missing, are out of order, or are coming in later than expected, and acts quickly! It establishes a mobile data link over the underlying cellular IP data network. We duplicate each audio packet and send one over each available transport — one over WiFi and one over Cell.

Bonded Calling begins by duplicating each audio packet and setting different transports for each.

The Republic server listens for both, and takes whichever wins the race. Fifty times per second we are conducting a race between Cell and WiFi and awarding the winner a spot on the final audio stream playback.

(above the line) The packets in flight have varying delivery times. (below the line) Republic takes the first packet to arrive and constructs the audio stream.

The resulting audio output is the amalgamation of packets received over WiFi and Cell, a tapestry stitched together with threads 1/50th of a second in width. The result is a crisp and smooth audio stream for playback on the other end. Your conversation doesn’t miss a beat. 

In this example, Bonded Calling has protected your voice from being garbled, and it’s not only one way. This entire process works in both directions at this breakneck speed, with your phone and the Republic server working in tandem to make smart use of supplemental data over the cell network. Your voice is protected, and your friend’s voice is protected.

We are always trying to stay conscious about smart use of WiFi and Cell networks. So your phone and the Republic servers are constantly measuring many aspects of your calling experience and toggling Bonded Calling only at the appropriate times. And keep in mind that the cell data used here is never attributed to your monthly usage.

When the race is just too slow

Sounds great so far. We’ve all hit WiFi rough patches, but we’ve all hit those cell data data spots as well. In the packet race, If WiFi and cell are both getting frequent DNFs, then we have some additional work to do.

When Bonded Calling is engaged, we continuously measure the performance of the cell data network. In the rare case that both the WiFi and Cell data network are together unable to maintain a high quality audio stream, we still have our tried-and-true handover in our back pocket. Your call transitions to the circuit-switched cell network, and your conversation continues without a hitch.

Here’s what really matters

Even as Engineers, we still think the Internet is full of magic. No one person can understand the full stack of technology that keeps us all connected. No one person should have to. We’ve scratched the surface of the Bonded Calling implementation, but it’s all for one thing: if you’re on a call and you can’t tell any magic is happening, then we’ve done well! This is the standard that we measure ourselves by, and it won’t stop with Bonded Calling. The search for quality is continuous and never-ending here at Republic Wireless. With this search, we are striving to empower your real experience — keeping in touch with those most important to you.

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