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31 May 2001

Digital communications are dead in the water. Acoustic networks are a prospect.

We've become accustomed to gigabit-per-second data transmission over fiber and megabit per second on copper. Cell phones that deliver e-mail and surf the Internet are commonplace. One doesn't see modems that operate at less than kilobits per second anymore.

All that history and technology is out the window when one considers communicating in the undersea world of Jacques Cousteau.

Wrote Mark Schrope in Business 2.0 magazine, "The problem is that nearly all radio and light waves used for land-based digital communication are largely blocked by the relative density of the sea.

"Sound waves, on the other hand, zip through water about four times faster than they can move through the air, making them a suitable alternative for underwater modems. Still, most digital signals travel at the speed of light: about 186,000 miles per second.

"Underwater transmissions are stuck at the comparatively clunky speed of sound in water — about 1 mile per second — which leads to delays. On top of that, the oceans also do a bang-up job of bouncing sound waves around, causing additional transmission problems."

Marine product manufacturer Benthos appears to be the leader in tackling this challenge and is working with the U.S. Navy. Benthos's Web site relates that, "Underwater modem technology has been slow to advance.

"It has been very limited in its usability and extremely expensive. But with new modems at 2,400 (bits per second), we're starting to get up to the areas where one can actually send usable data, pictures, and sound. Soon they should be working reliably at 9,600 (bits per second).

Convert bits to tones

"Progress thus far has centered on the ability to convert methods used in the cell phone industry to acoustic equivalents.

"The basic idea is to convert bits of information into tones, which then convert to digital data at the receiving end. Each bit of information transmits through the water as multiple tones to ensure that at least some will make it to their destination.

"In June, using the latest Benthos modem, the USS Dolphin sent out three e-mails from about 400 feet below the surface as part of an experimental project called Sublink. One message went to the squadron commander, one to the funding sponsor, and one to a sailor's father.

"The messages were relayed to several modems positioned on buoys, converted from there to radio signals, and relayed to the shore, where they were finally transmitted to the Internet."

Continued Schrope, "Though certainly useful, submarine communications is only one of several areas for which the military is developing acoustic technology. The main thrust of the initiative, in fact, is to develop deployable, possibly even disposable, wireless underwater networks.

Seeding spy networks

"Here is a possible scenario: Planes fly to a port of strategic interest, dropping a number of floating or submersible modem and sensor packages. The network could listen for enemy subs or mining activity automatically and inexpensively, each unit feeding data to a buoy for satellite transmission.

"Although similar information can be gleaned today, the process is encumbered by the number of people involved, prohibitive costs, and vulnerable underwater cables, for example.

"Beyond the hardware the military may retool for underwater transmission, there are pieces of scientific and commercial oceanographic equipment-seismic registers, pollution monitors, and so on-scattered in waters about the globe that, if fitted with wireless abilities, would prove extremely useful to businesses and nonmilitary researchers.

"Indeed, the larger market is probably the commercial one. Oil and gas exploration companies can use the modems to transmit data from sensors on their rigs. Otherwise, they must extend delicate cables thousands of feet down. Data transmission technology could also expand the capabilities of autonomous underwater vehicles. Newly outfitted vehicles could roam without tether, free to survey the ocean floor for tracks in which to lay new fiber-optic cables."

No smooth sailing

Drawbacks and obstacles to the technology are legion, however, because physics breathes differently underwater. Steven Phillips related in the Financial Times, "It's not all smooth sailing.

"While sound travels through water far better than other forms of energy, such as electromagnetic waves emitted by radios, the U.S. Navy is contending with several phenomena which potentially compromise the integrity of digital data encoded in sound waves.

"Firstly, sound waves are prone to attenuation, or a loss of intensity, as they travel from source, rendering weaker signals. To deal with this, the initial frequency of messages can tone down, but this comes at the price of losing bandwidth or data-carrying capacity.

"Meanwhile, because water is such a good transmission medium for sound, waves can travel for hundreds of miles, making it difficult to sift out the sound of a given digital bit from the general background cacophony. Additionally, interfering echoes can be generated from an effect called multipathing, when sound waves scatter on contact with subsea cliffs and canyons or different water temperature and salinity strata.

"The interaction of sound waves with obstacles on the sea bed is a particularly keenly felt problem in the shallow depths at which the (Navy) expects acoustic network nodes to operate.

Trudge through water

"The quest to optimize the data throughput of the channel is a key goal of the (Navy's) research, with various techniques being tested. Borrowing from the redundancy technique commonly used in compact disc production that encodes duplicate data on the disc to allow musical signals to be reconstructed if the surface is blemished, redundant coding may be included in acoustic signals to fill in if data is lost in transmission.

"Multipathing may be countered by a technique which lengthens the duration of acoustic signals to literally drown out disturbing echoes.

"Another shortcoming to be addressed before the technology is battle ready is sluggish data transmission rates that fall short of real-time data requirements in the heat of conflict.

"In trials, data typically trudged through the water at a sedentary 300 bits per second vs. 56 kilobits per second delivered by a standard cable modem. Theoretically, Benthos's acoustic modem could deliver a rate of 2,400 bits per second, but only part of the bit stream may be utilized for the message because of the need to encode duplicate data to offset attenuation or extraneous noise."

The Navy estimates it will take five years of tweaking before acoustic modems can take their place in its arsenal. Meanwhile, wireless undersea networking will hit the radar screen of rank-and-file submariners for the first time this year when commanders devote live craft and crews from the U.S. fleet to put the technology through its paces in the 2001 Seaweb trials.