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Modem vs. Modem event 2: Dropped Call Chess

Ever tried to make a phone call, only to get the dreaded “call failed” message? What about seeing you have a new voicemail, but knowing your phone never rang?

You probably blamed your mobile network for those calling issues. Thing is, your operator could be innocent. The culprit may actually be the modem inside your phone.

Modems are the wireless chips that connect our phones to cell networks. Any time you communicate with the outside world over cellular — whether that’s sharing a photo, checking your Twitter feed, or making a phone call — it’s the modem that handles getting your data and calls over the air.

Of course, not all modems are created equal. Even when implementing the same features, some modems do a better job than others. And Qualcomm Snapdragon LTE modems are designed to provide your phone with the highest calling performance and reliability possible.

In our continuing “Modem vs. Modem” video series, we highlight these differences and the impact of modems on our everyday mobile experiences. The premise is simple: Each team is made up of a human athlete and a “modem athlete”. The first modem athlete is a Snapdragon LTE modem, and the second is from a competitor—let’s call them Team Blue.

This week’s challenge: how reliable is each modem at handling phone calls.

In this next event, called “Dropped Call Chess,” human athletes competed in a heated game of speed chess—while sitting above a dunk tank. Each time the human athlete passes the turn to his opponent, the opponent’s turn begins with a call placed to his phone. The modem athletes’ one job: successfully connect all attempted phone calls to keep their human teammates from being dunked.

While the consequences are comical, the underlying challenge is very important. That’s because it demonstrates modem performance in implementing the technology used for handling voice calls on the vast majority of LTE networks around the world. In most 4G LTE networks, mobile Internet connectivity is handled through LTE, but phone calls are still handled via 3G or even 2G (we tested 3G in this case). The technology that enables this is called Circuit-Switched Fallback, or CSFB for short.

Let’s say you’re streaming a video over LTE and a friend calls. Once she places the call, a very rapid series of events takes place. The LTE network “pages” your phone, and once located, it tells your phone to “fall back” to 3G mode in order to receive the call. The modem inside your phone physically disconnects from the LTE network and reconnects via the 3G network. It then receives the incoming call and your phone rings. This little dance happens in just a few seconds, but the timing is absolutely critical (read our CSFB whitepaper for more information).

If the modem inside the phone bungles this fallback process, your phone won’t ring. And the only way you’ll even know that the call failed is if she leaves you a voice message, or if she asks you later about why you’re not picking up your phone.

The Snapdragon modem successfully connected all 100 attempted calls. Team Blue’s modem, on the other hand, failed to connect 13 out of the 100 tries

So how do we test modem performance in this scenario? Qualcomm Technologies ran industry-standardized tests in a controlled lab environment. To really put the modems through their paces, the test system emulated attempting to call the phones while they are in a moving car, with interference from multiple cell towers. We then attempted making 100 calls using CSFB technology.

The Snapdragon modem successfully connected all 100 attempted calls, hence keeping the Team Snapdragon human athlete out the dunk tank. Team Blue’s modem, on the other hand, failed to connect 13 out of the 100 tries—with rather messy consequences for its human teammate.

So what is Team Blue’s modem doing wrong? Why is it dropping more than one out of every eight incoming calls? We can’t be sure exactly, but we have some ideas. It may be unable to process the incoming pages fast enough. Or possibly, in a fast-moving car scenario, it’s trying to connect to the wrong 3G tower instead of the one the LTE network instructed it to. Either way, an incoming call that never even rings the phone is a big deal.

Think real world. What if one of those calls was from a friend stranded by the side of the road because his car broke down? What if a prospective employer was trying to call you for a phone interview? Or what if it was your mother? “Sorry mom, not my fault—I have Team Blue’s modem in my phone.”

And what does it say about the OEM that chooses to use Team Blue’s modem in their phone? Are they providing their customers with the superior mobile experience they deserve? Tolerating a high call failure rate would indicate that they are compromising. On the other hand, when an OEM chooses to put a Snapdragon LTE modem inside their phone, they send a message that they want a superior connected experience for their customers.

Stay tuned for more “Modem vs. Modem” madness. Next up: two teams, two white rooms, two giant balls of paint, and a metaphor for the twists and turns of life.

Qualcomm Snapdragon is a product of Qualcomm Technologies, Inc.

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