HISTORY OF FIBER OPTICS
This week’s History of Fiber Optics is continued from George Gilder’s book, Telecosm.” We ended last week’s segment with…Today’s engineers and scientists starkly confront the absolute speed of light and everything collides with that—computer architectures, network topologies, (branch of mathematics concerned with those properties of geometric configurations [as point sets] which are unaltered by elastic deformations {as twisting or stretching}; configuration of a molecule or of a magnetic field), satellite systems, software conventions and the entire time and space grid of telecommunications.
The most immediate collision is in satellite technology. Light speed (186,000 miles per second) requires at least 250 milliseconds for a signal to make a round trip from earth to satellites in geosynchronous (stationary) orbit, 23,600 miles above earth in the Arthur C. Clarke belt. Two hundred and fifty milliseconds is a tolerable delay for voice communications, but it’s untenable for interacting with data. In 250 milliseconds, a modern computer can transmit the equivalent of a thousand large books before it would receive back any acknowledgement, error correction, or any other feedback. A gigabyte of data would be afloat, stored en rout, buffered in air, wires or other media, and impervious to the wishes and keystrokes of the user.
The response to this problem has been a new industry of low-earth-orbit satellites. Loral—Qualcomm’s Globalstar, Alcatel’s Sky Bridge and Gates—McCaw’s Teledisic are collectively launching devices into tracks twenty-five to fifty times closer to Earth than the Clarke belt. One fiftieth of 250 milliseconds—five milliseconds—is a level of delay as good as the latencies of local fiber-optic systems already operating between New York and Washington.
On the ground, network delays stem chiefly from the distances between internet routers that take a particular message packet, read its address and forward it to a particular destination by the best available path.
In the year 2000, the fastest routers took about ten microseconds (millionths), but soon it will be one-tenth that time. The problem is that across the internet an average message flows through seventeen routers and some messages hit as many as forty. Most web pages contain diverse elements that collectively take as many as twenty back-and-forth trips to download. Many of the routers are thousands of miles, or about ten milliseconds apart. The delay budget for voice communications is 150 milliseconds and any more than that causes us to step on each other’s conversations.
The dream of ubiquitous (being everywhere at once), voice and video delay over the internet, depends on reducing delays across the network. Worldcom’s UUnet guarantees 80 milliseconds latency to its business customers, but the average internet packet takes over 160 milliseconds in the U.S. and twice as much overseas. The net needs a new topology. Similarly on microchips, computer operations entail accounting carefully for the speed of light. In a medium, the light-speed limit varies in proportion to the medium’s index of refraction or dielectric constant. In metal wires, this translates to nine inches—twenty-three centimeters—per nanosecond, against about one foot per nanosecond in a vacuum. Such calculations deeply affect the newest microprocessors. A 600-megahertz Pentium 2, for example, may execute one computer instruction in a nanosecond, the time needed for a signal to move nine inches in a metal wire. Nine inches per billionth of a second might seem ample on a microchip with features measured in the microns (millionth of a meter), but a leading edge chip today bears as much as seven miles of wires. Even the Pentium 2 in your desktop PC has some 400 meters of wire on chip, about a quarter of a mile.
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