Archived posting to the Leica Users Group, 2002/06/07
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]In regards to internet service and it being fast and seeming fast, the most important statistic is actually not bandwidth, but latency. The internet is built out of 2-way protocols. It is not like television, where the sender puts the signal out into the air without ever knowing if anyone is receiving it. The only internet protocol that you are likely to encounter that is one-way-only is streaming audio and video, such as RealAudio. All of the other protocols for web, email, and so forth are two-way. When you click on a URL and wait for delivery of a web page, there are many things that have to happen, and all of them involve a back-and-forth protocol: Your computer asks for something The server replies to set up a connection Your computer acknowledges that connection The server sends some data Your computer acknowledges receipt of the data The server sends an "I am done" message Your computer acknowledges receipt of that message. Typically there are 4 round trips between client and server in even the simplest transaction. Images, popup banners, frames, and other complex stuff can increase that. Bandwdith is good, but rapid response (low latency) is vital. You can get tremendous bandwidth by putting a few dozen DVD-ROMS into a FedEX envelope and sending it by air express. The number of bits per second can be in the millions. But the latency--the time before the first bit is received--is many hours, rather than a few milliseconds. Satellite links do not have good latency. A geosynchronous satellite is 35,000 km above the surface of the earth, which will typically give you latencies of about a 1/3 of a second. This is horribly slow for any interactive use. I find myself twitching when the latencies get above 20 milliseconds. SO: satellite-borne internet such as T-DSL is interesting, and useful, but it is not a cure-all. Low-earth-orbit satellites, just 250 km in the air, are better, but since they are not geosynchronous, you need more of them, and the electronics to coordinate them is very expensive. - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html