In a bold, technologically retrograde move that will strike fear into anyone who surfed the internet before the advent of DSL and cable, Microsoft Research has devised an acoustic file transfer protocol for mobile devices. That’s acoustic, as in, the same way that your 56k modem connected to the internet via a stream of modulated bleeps and screeches. This technique, called peer-to-peer acoustic NFC, allows any two mobile devices to transfer files as long as they have a speaker and a microphone — no special hardware, such as NFC, WiFi, or Bluetooth is required.
NFC stands for near-field communication. Strictly speaking, NFC is a standard that builds upon RFID for short-range radio-based peer-to-peer communications between devices. Your smartphone, unless it’s an iPhone, is probably equipped with NFC hardware. In this case, though, the Microsoft researchers simply mean that their technology works over the same kind of range as NFC (up to around 10cm) — it is not a drop-in replacement for NFC; it will not let you interact with radio-based NFC devices.
With that said, here’s how the acoustic NFC technology (dubbed Dhwani) works. You both load the Dhwani software onto your phone. Then, when you want to transfer a file, you hold your phones together, and data is transferred acoustically from your speaker to the recipient’s microphone. The range is short (a few centimeters), but presumably enough that two-way communication is possible even if your speakers/microphones don’t align. To mitigate against someone eavesdropping on the file transfer, self-interference cancellation is used — a technique where, in essence, the receiver transmits a stream of random noise. Because the receiver is transmitting the noise, it can work out what the original transmission signal was — but for anyone listening in, all they can hear is the big mess of sound produced by both devices.
After you factor in losses due to self-jamming, environmental noise, restrictions on output power (you don’t want to interfere with other devices or, you know, humans trying to have a conversation), and the variable performance of the electronic components involved (some microphones/speakers are better than others), Microsoft Research managed a data rate of 2.4 kilobits per second — or about 20 times slower than your 56Kbps modem. You probably won’t be using acoustic P2P to transfer files or photos between phones, then, but it’s more than enough to transfer a business card or website address.
Really, though, the key here is enabling peer-to-peer communication between devices that lack any other way of transferring data. It is no surprise that Dhwani was developed by Microsoft researchers in India, where everyone has a mobile phone with a microphone and a speaker — but relatively few have access to more advanced wireless technologies, such as Bluetooth. The researchers claim that Dhwani, with its built-in jamming and scrambling, creates a “information-theoretically secure communication channel,” which means it could be used for mobile payments, too — a big growth area in developing markets. (Read: The dumbphone strikes back.)
Sadly, as exciting as acoustic P2P is, one question remains: We don’t actually know what Dhwani sounds like. The research paper says that they use 1KHz of bandwidth between 6-7KHz, with an OFDM modulation scheme. Human speech generally maxes out around 1KHz. 7KHz… well… I’ll just let you watch this video, and you can make your own mind up. As long as it’s quiet, it shouldn’t be too ear-splitting…
Research paper: Dhwani: Secure Peer-to-Peer Acoustic NFC