On Thu, 9 Sep 2004, at 10:37am, [EMAIL PROTECTED] wrote: > What's the latency like on that? I'm assuming with a 48K mile round-trip > minimum for every packet, latency must be rather high?
It's closer to 100K mile. Geostationary orbit is about 25K miles up. For a round-trip, that means 25K from you to orbit, 25K to the ground station, then 25K from ground to orbit and then 25K back to you. That's roughly 550 ms of latency *at the speed of light*. And that's not even including latency introduced by the regular terrestrial networks. When doing ping tests with a Starband feed a year or two ago, I never saw a RTT less then 700 ms. It frequently went to well over 1000 ms. That is *over one full second* round-trip time for a 64 byte ICMP datagram. For comparison, I typically see between 200 and 300 ms RTT latency on a POTS dial-up link. Latency on satellite really, really sucks. Anything user interactive (SSH, games, VoIP, etc.) is going to suck, period. Satellite works great for streaming (large file downloads, multimedia, etc.), if the protocol allows for high-bandwidth, high-latency links. In TCP terms, that means you need a very large window size. Recall that TCP has to complete a three-way handshake before any data flows. That means applications that make use of many short-lived TCP connections also count as interactive. Think web browsing. You have to wait a minimum of about 2.5 seconds for each page element to load. This makes interactive web browsing nearly unusable. To solve the TCP problem, satellite carries play games with the protocols. They configure a transparent proxy to sit between you and the satellite uplink. The proxy intercepts all the TCP requests and fakes the TCP handshake locally, before the bits even hit the air. This lets the HTTP request get into the air without waiting for the "real" TCP handshake to finish. Of course, this is an incredible kludge. I expect it could conflict with other types of TCP usage, and I know it makes trouble-shooting a bear. Back when I had to deal with it, Starband had also just switched from doing that protocol magic in the "satellite modem" to doing it in software. That meant you had to run their funky and annoying Windows-only software. Satellite may be the best option if high-speed terrestrial links are not available at the location, *and* latency is not a factor. Otherwise, I strongly recommend avoiding it. -- Ben Scott <[EMAIL PROTECTED]> | The opinions expressed in this message are those of the author and do | | not represent the views or policy of any other person or organization. | | All information is provided without warranty of any kind. | _______________________________________________ gnhlug-discuss mailing list [EMAIL PROTECTED] http://mail.gnhlug.org/mailman/listinfo/gnhlug-discuss
