Networki ng in the 21st Century: The Sky's the Limit

	Do LEOs Have Too Much Latency? Even for LEOs, flying 400 to 1,000 miles from the earth's surface, some round-trip delay is inherent-although experts disagree on just how much. Fred Baker, chairman of the Internet Engineering Task Force, puts round-trip terrestrial delay at about 100-150 milliseconds; LEOs at 200 milliseconds; and GEOs at about 600 milliseconds. That's in the same ballpark as a recent Booz-Allen report prepared for NASA, with corresponding minimums and m aximums of 42/446 milliseconds for LEOs and 274/1068 for GEOs. That report concludes that only terrestrial services are suitable for interactive media, although LEOs come very close to making the grade. The NASA report reenforces the opinion of experts who contend that latency-sensitive applications--such as voice, LAN-to-LAN interconnection, interactive video and gaming-will perform best when kept out of the sky altogether, especially for short hops. For longer transcontinental journeys, almost everyone agrees that terrestrial fiber and LEOs will be quite competitive. A second camp, however, argues forcefully that most LEO delays will be attributable to processing, not propagation, and that the use of high-speed laser-based switching -like that planned by Teledesic--will make LEO delay at least comparable to terrestrial delay. Hans-Werner Braun, architectural consultant to Teledesic, says "a transmission halfway around the world through Teledesic could be faster than one through a fiber optic cable, because although the arc is a bit longer for the former, light traveling through fiber propagates at around two-thirds the speed of light in a vacuum." Teledesic expects typical round-trip latency to fall between 40 and 100 milliseconds, with a round trip of 5,000 kilometers at about 150 milliseconds and a round trip of 500 kilometers at about 20 milliseconds. Motorola, too, contends that LEO delay will be comparable to terrestrial-and generally below the 100 millisecond threshold for voice. Scott Clavenna, with Pioneer Consulting, notes that while a satellite system typically doesn't outperform a fiber optic network, there are instances where the number of hops between routers and switches can be reduced by a satellite system. "If you design a system with fewer routing points on the network, it can have less latency," he says.

For the Side Bar on LEOs Dance The Jitterbug GEOs Turn Up The Speed Of Light You Say GEO, I Say LEO, Oh My, Oh MEO The Big Five Planned Constellations High Hopes, High Hoops A Q&A With Teledesic Mobile Voice Blazes an Astral Trail for Broadband Other Features Slicing Through the Hype of IP Switching By Joel Conover