WHEN our ancestors built fires on distant hilltops to signal to one another, they were using an early form of optical communication. This idea of using light to send information began to be developed scientifically in the 1800s, when British physicist John Tyndall demonstrated that he could (although just barely) direct light down a stream of water. He found that light could be guided by transparent materials if those materials were denser than air, and his insight, when followed by other scientific inquiry, culminated in fiber-optic technology.|
Fiber-optic technology takes electric signals from our phones, computers, and televisions and transmits them more efficiently than other methods, making it possible to deal with the volume and variety of communications that constitute modern life. The information-carrying capacity of fiber optics is so great that it is far from fully exploited. It is being counted on to help solve problems such as the traffic bottleneck on the Internet.
The members of the National Transparent Optical Network (NTON) consortium are among those counting on fiber optics. The consortium (Figure 1) has received matching funds from the Department of Defense's Advanced Defense Research Projects Agency to test and demonstrate advanced optical components in a high-speed, all-optical communication network. The network is based on existing Sprint and Pacific Bell fiber-optic lines and has been operational since February 1996. Currently, it is being tested by users of large, emerging applications, and the consortium is actively soliciting the interest of other such test users.
The project's two components--next-generation optical technologies and the emerging applications used to test these technologies--are bound into one ambitious objective: to provide a transmission capability for a multitude of complex, advanced uses, at speeds of billions of bits per second, with complete security and reliability. NTON members are thinking beyond the needs of the Internet cruiser's ability to download large graphics files. They are envisioning users such as physicians of the future, who will be able to retrieve a host of complex medical records from various remote locations, perform remote telesurgery, or practice space telemedicine.
The Context for Livermore's Work|
Lawrence Livermore leads the work on the prototype network, which is to integrate new, developing technologies into a logical and efficient working system. It is a fitting role not only because of the Laboratory's broad expertise in optics and large-scale computing but also because of its neutral perspective on work that ultimately must be commercialized.
Integrating the new technologies into a high-service-quality, high-speed network on which new high-capacity applications can be tested will promote the advanced applications and demonstrate the commercial feasibility of the new technologies. One important goal of NTON is to convince private-sector investors that the new optical components are worthy of commercialization and that the fiber infrastructure should be upgraded. But as Bill Lennon, Lawrence Livermore's project leader from the Advanced Telecommunications Program, points out, "While these innovations are necessary for technological advancement and global competition, change is costly and investors are fiscally conservative. Investors must be totally assured of good returns on their money."
Making More of Optical-Fiber Bandwidth
The high speed and great capacity of the network are based on the inherently large bandwidth of optical fibers. Bandwidth is the expression of a medium's communications capacity. Optical bandwidth offers, of course, the speed of light. But it also offers the whole rainbow of light frequencies. Having this capacity range can be likened to having a musical keyboard of many octaves, which can be used to play far more complex melodies than a keyboard of one octave.|
NTON enlarges optical bandwidth capacity even more through a technique called wavelength division multiplexing (WDM), wherein each optical fiber is used to carry more than one wavelength. The various wavelengths do not interfere with each other, so each can be used as a different communication channel. (In the keyboard analogy, this characteristic would be tantamount to simultaneously playing a different song with each available octave.) The use of wavelength division multiplexing increases fiber capacity without the need to install more fiber cable.
The NTON fiber carries four wavelengths at present, but plans are to expand to eight ultimately. The capacity expansion that occurs with WDM requires new devices for regulating the resulting voluminous traffic. One of the new devices used in the network is being developed into a new product by Uniphase Telecommunications Products, a consortium member. It is an acousto-optic tunable filter (AOTF), whose function is to route the multiple wavelengths through the different regions of the network. Made of lithium niobate glass, the four-port filter selectively and simultaneously switches many wavelengths on their way to different destinations. Some other wavelengths are isolated by routing them to network-access equipment that "maps" their signals to a different wavelength. Because those signals are isolated by this blocking, their former wavelengths can be used elsewhere in the network. This wavelength "reuse" makes the system scalable, that is, able to indefinitely increase the volume of information being switched through (Figure 3).
A Flexible, Transparent Network|
NTON is intended to be an open network; it must therefore be easily accessible to heterogeneous systems and formats (including future ones such as high-definition television), and users should work at their desktops without any awareness of its operations. In short, the network must be flexible and transparent.
These characteristics are achieved through the use of standards, the rules that enable systems to "talk" to each other. When different systems use different local formats, standards provide them with a common interchange language. Various standards are used in different layers of the network architecture to provide a hierarchy for signal transmission. The hierarchical process may be compared to having sheets of paper packaged into envelopes and delivered to an envelope handler who repacks them into boxes of envelopes, which are delivered to a box handler to turn into boxes of envelopes inside trucks, and so on through the delivery sequence until the packages arrive at their destination, where the reverse process yields the sheets of paper to the addressee.
NTON uses two standards developed specifically for advanced networks. First, signals from various user formats such as video, data, and voice are fed into the network and converted into a standardized common format by means of the Asynchronous Transfer Mode (ATM) standard. ATM not only makes the signals insensitive to transmission format, it also assigns transmission space and priority according to the needs of the terminals, thereby making best use of network capacity and efficiency. After ATM, the signals must undergo another conversion to package them for optical-fiber transmission. This packaging is the function of the Synchronous Optical Network (SONET) standard.
SONET is particularly efficient. It keeps a signal and its management information together, and it synchronizes signals to a common clock to simplify handoff between the networks worldwide. These features make the signals easily and quickly extractable for distribution or routing. The SONET signals are the ones that are transmitted over one of the switchable wavelengths of the optical layer.
Key Words: acousto-optic tunable filter (AOTF), Asynchronous Transfer Mode (ATM), fiber optics, National Transparent Optical Network (NTON), remote visualization, standards, Synchronous Optical Network (SONET), wavelength division multiplexing (WDM).
For further information contact William Lennon (510) 422-1091 (firstname.lastname@example.org).