Hi Eric,
My main concern with the satellite feed is from a recent comment of a possible ~0.5 seconds latentcy for getting the data from satellite (the satellite is located over the equator, south of Houston). Can anyone confirm this for certain?
Your data is traveling at the speed of light or very close thereto.
Since that is 186,000 mi./sec., the uplink and downlink theoretically require [(2 x 22,700 + 130) /186,000)= .245 seconds or 245 milliseconds.
Where 22,700 is the height of the bird above the equator and 130 is the calculated hypotenuse of a triangulation from 29 Deg. N, 95 Deg. W
to 0 Deg., 95 Deg. W. In other words, from Houston to the spot on the Equator directly under the bird.
For video signals this is perfectly acceptable. For the transmission of your data it is not quite as bad as the 15 minute delay some of us find acceptable. (Hey, no twitch instinct here!)
For full duplex (two way) voice communication this 240 ms. delay is anathema in that the human ear can detect latency greater than ~150milliseconds. The PSTN (Publicly Switched Telephone Network) is set up so the average latency when you are in converastion on a long distance call is ~75 milliseconds. So geostationary satellites will nver be useful for replacing the existing PSTN. One of the bugaboos of trying to establish VoIP is the latency issue. This should be satisfactorily resolved by mid 2000.
Back to your PC quote issue. The only way to reduce the latency inherent in the use of GEO birds is to stick to a land line solution today. And possibly make use of a LEO solution when it become available (Both Skybridge and Teledesic are in the planning stages right now.)
HTH, Ry
|
