For science-fiction aficionados, the holodeck rooms used by starship crew members in the various versions of Star Trek are the epitome of unbounded data bandwidth. The creation of computer-generated lifelike 3D images that can interact with their surroundings represents the flow of trillions of pentabits of data. That's somewhere in the neighborhood of 1027 bits/s to and from the sensors and "emitters" in the imaginary holodeck. That data contains a combination of images pulled from distributed storage, as well as on-the-fly generated images.
Similarly, the famous "beam me up" transporter defines a process of moving solid matter nearly instantaneously from location to location. Such a process, though only imaginary, would involve the movement of equally large or still larger amounts of data in a matter of a few seconds.
Even though we're not anywhere near the stage of implementing such capabilities, the amount of data that has to be captured, routed, and delivered without error or data losses is staggering. Even the best network switches available today are just starting to address terabit speeds. Ongoing research promises to bump up the aggregate data rate to pentabit-per-second levels over the next few years.
Achieving that higher bandwidth means overcoming challenges that can typically be divided into three categories: data capture/extraction, data transmission/aggregation, and data manipulation/routing. Each of these areas has no choice but to improve.
In some ways, the Internet and World Wide Web is analogous to the starship—a large, distributed collection of computer systems that intercommunicate. But the current bandwidth available on the web is severely limited. That's partly due to the unorchestrated collection of servers, software, and interfaces used to handle all of the transactions. In contrast, all systems on the starship are designed to operate in unison and thus deliver top-notch performance.
We also suffer from a technology limitation. We haven't really figured out how to go beyond the 10-Gbit/s SONET backbones that manufacturers are putting into place. Data rates of 40 Gbits/s and beyond will take many more years of research to put into commercial use. In the meantime, data-transfer speeds are outstripping silicon's ability to support the transfers using today's architectures (Fig. 1). New architectures and technologies must be brought to bear.
The physical transport of the data streams has typically been done over microwave links, coaxial cables, and optical fibers. But as data rates increase, copper- and microwave-based schemes rapidly run out of bandwidth and give way to optical fibers. Those fibers provide the streams with much higher bandwidth.
One of the oldest techniques to increase throughput is to divide the information into multiple channels and transmit all of the channels in parallel. In copper-based interconnects, that requires a separate cable for each channel, making the cables bulky, heavy, and expensive. Plus, signal crosstalk between adjacent channels can corrupt the data.
Solutions based on optical fiber don't suffer from that crosstalk but, until recently, could only handle a single data stream (one laser beam) per fiber. That again leads to bulky, multifiber cables and limited upgrade options to increase the capacity. Running a second multifiber cable would entail a considerable expense.
Holding a lot of promise for the future, wavelength division multiplexing (WDM) and its next-generation, dense WDM (DWDM) promise ten- to thousand-fold increases in data throughput without adding fiber cables. Those increases come from two aspects. The base data rate will be increasing, first of all. And more channels of laser beams can be transmitted on a single fiber.
To deal with the first issue, higher-speed laser diodes, detectors, and the basic driver and logic circuits to support them are under development. Advances in material-deposition technology will allow companies to manufacture higher-performance gallium-aluminum-arsenide (GaAlAs) lasers that produce light at precise frequencies.
More optimized fiber-optic fibers are being developed as well. Tests by researchers at Lucent Technologies and Bell Laboratories have demonstrated the ability of a single laser diode to transmit 160 Gbits/s over 300 km of optical fiber. The single-wavelength system employs a semiconductor-based laser transmitter and demultiplexer. The high data-transmission speed represents a four-fold improvement over today's best commercially deployed system.
The optical signals were sent over the company's high-performance TrueWave RS fiber, which offers the lowest dispersion slope in the industry. With that low dispersion, the signal can retain its pulse shape over a longer distance, reducing the number of repeaters that must be used in the fiber's path when signals are sent over long distances.
By tuning multiple laser diodes so that each generates light at a different specific wavelength, designers found that they could simultaneously send multiple beams down the same fiber using WDM. It requires forming multiple laser diodes on the same substrate. The diodes are designed to generate their beams at frequencies (wavelengths) a specific distance apart in order to minimize any interference—typically between 50 and 100 GHz. Then, the beams are multiplexed onto the single fiber. On the receiving end, they're demultiplexed and the signals are recovered.