NATO announced recently that it's adopting a standard protocol for underseas communication. What's JANUS and how does undersea communication differ from in-air data transmission?
Why JANUS Was Needed
Thanks to standardized communication protocols, devices can communicate effectively. Wi-Fi, 4G, and Li-Fi are all examples of protocols that allow for reliable data transmission, enabling the growth of the mobile industry and the ubiquitous Internet of Things.
The intersection of the electronics industry and maritime industry—an intersection that includes buoys, submarines, and other autonomous underwater devices—heavily relies on wireless communication. This is partly because of the vastness of our oceans and the untethered nature of most seacraft. For example, a submersible craft that needs to map the bottom of a trench in the ocean at a depth of 8km and then explore crevices and caves cannot reliably use a cable. In this scenario, wireless communication would be ideal as it would allow for complete freedom of movement and would not require the host (a ship for example) to carry 10km of cable.
Alvin, one of the most famous Deep Submergence Vehicles in the world. Image courtesy of the Woods Hole Oceanographic Institution.
However, there is a serious problem with traditional wireless communication technologies: they rely on electromagnetic waves to transmit data. We tend to take for-granted that electromagnetic radiation does very well in the atmosphere (primarily due to the fact that air is mostly empty space) and, as a result, we can get radio signals to travel great distances with decent reliability. For a matter of perspective, the ESP8266 module can transmit a Wi-Fi signal up to 5km (with the aid of a telescopic antenna) while traditional radio stations can broadcast their signals up to 50 miles. The radio equipment current aboard the Voyager 1 and 2 probes are transmitting their data over a distance larger than the size of the solar system!
So if EM waves can travel such distances, why can they not be used for underwater communication?
Light and Sound and Water
It has been established that electromagnetic waves can travel for some distance in the atmosphere. But how far can they travel underwater?
The answer is not far at all. Typical Wi-Fi devices having a range of anywhere between one to three feet. Water molecules are actually quite close together with molecule sizes being approximately 0.29nm with average distances between two water molecules being 0.31nm. This small separation is one of the reasons why water is very good at absorbing radiation and also one of the reasons why a bright light will struggle to go further than 100 meters underwater. In fact, the light from the sun does not typically penetrate further than 200 meters into the ocean, making photosynthesis impossible at greater depths.
Light cannot penetrate very far into water. Image courtesy of NOAA
But water's density makes it ideal for a different type of wireless transmission: acoustic. The denser a material is, the better it is at transmitting sound. (Picture the classic tin can phone using a piece of taut string to transmit sound.) Not only does sound travel further in a denser medium but it also travels at a greater speed (for example, the speed of sound in the air is 343 m/s while the speed of sound in water is 1,500 m/).
Communication via sound is so practical underwater that the blue whale, the largest and loudest mammal, can communicate over thousands of miles with other whales just by making low-frequency rumbles. Sound is currently used in many applications underwater with one of the biggest uses being SONAR (Sound Navigation And Ranging), where a sound pulse is emitted from a source and the reflected pulse from objects underwater are received. Using multiple receivers, the size and distance of objects can be determined, which can greatly help with generating an image of the environment when there is little to no light.
But ranging and navigation is not the only use for sound. Data transmission is also possible in a near identical matter to normal wireless communication involving error correction and parity bits.
Acoustic Data Transmission
Underwater acoustic digital data transmission is currently manufacture-specific—there is no one specific protocol. While each specific protocol has its own unique frequency and/or message structure, they usually use one of the following encoding methods for actually defining a 1 or 0:
Frequency shift keying
Phase shift keying
Frequency hopped spread spectrum
Orthogonal frequency-division multiplexing
The use of multiple different protocols has resulted in devices not being able to communicate and share information. When it comes to the underwater world, we now have a situation similar to the pre-IBM standard days when there were many different computer manufacturers specifying their own data protocols and file structures.
JANUS could be the solution, a unified underwater protocol that will allow for all devices to communicate.
The Unified Protocol
JANUS is an underwater protocol developed by The NATO Science and Technology Organization’s Centre for Maritime Research and Experimentation. It was adopted recently by NATO to help unify all underwater communications and allow for devices made by different manufacturers to cross-communicate.
What makes JANUS a real contender as a unified protocol is its ability to allow for devices to announce themselves on a commonly-used frequency, 11.5kHz. Then, when two devices have agreed to connect, they can switch to a different frequency or transmission method for faster data rates. This is incredibly important as data transmitted using sound waves has a seriously limited baud rate where the baud rate is affected by the frequency of the carrier wave, reflections of near by objects, sound-to-noise ratio, and much more.
Joao Alves, one of the key researchers, made an interesting analogy as to the purpose of JANUS: Imagine two people meet in a foreign country and want to communicate with each other. Chances are that they would both speak English to establish a common universal language. However, the two discover that they both speak Spanish fluently and then switch languages for ease and speed. In this case, JANUS provides the common platform for individuals to announce who they are and how fast they can communicate.
JANUS could allow for all devices to communicate in a similar fashion to the Internet. Image courtesy of CSIRO [CC BY 3.0]
The JANUS protocol also defines how sound should be encoded in a carrier wave (specifying the use of FH-BSK), as well as error correction and redundancies. Some of the specifics of the protocol can be found in this 2010 conference paper but here are a few key points:
Three pulses are used as the “wake-up” announcement for other devices to begin listening. The time between the pulses should be no less than 0.4s as reverberant energy needs to dissipate (i.e., reflections of objects and medium boundaries).
Four inputs are required for the first 64 bits of a JANUS message, which includes sample frequency, output file name, output format, and the name of an external file.
Up to 4096 bits can be transmitted as a payload for data transmission.
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JANUS could really revolutionize the underwater industry with devices that can communicate and share information which is never a bad thing (remember how the internet has impacted lives with the availability of free information?). The JANUS protocol could enable for an underwater IoT as well as mesh networking which could potentially increase data rate speeds if a data block is split up and sent on different frequencies from different sources.