"Tim Devries" <zsolutions@cogeco.ca> wrote:
I think this question may have been asked before, but what is the minimum latency and delay I can expect from a satellite connection? What kind of delay have others seen in a working situation?
As others have pointed out, GEO is about 22,000 miles, plus offsets for both sending and receiving station, both in lat. and long. There are a lot of satellites up there, on the equator, spaced every 0.5 degrees or so. Another latency factor is something not normally factored in land-based systems. Doppler shift! Satellites are seldom in perfect orbits - they drift up and down, returning to zero (thus in stable orbit), but the relative motion results in clocking issues. Satellite modems usually have 8ms receive buffers to accommodate drift, and when local clocking is not available, clocking transmit from receive requires at least double. On the down-swing, the buffer drains; on the up-swing it fills (both rates are slow, but last a long time). Regardless, the buffer itself adds 8ms to each way delay. A good working value is 280ms each way, *plus* any router serialization delay and terrestrial backhaul at both ends (unless you live at an earth station).
What factors should be considered in end to end connectivity architecture when utilizing a satellite link?
If you have end users, put in a huge web cache, as big as you can afford. Not only will it reduce bandwidth usage, it lowers average latency considerably. Doesn't help non-cacheable content, or other applications, directly, but lowering usage minimizes latency at least. Another approach is asymmetric - buy a low-speed terrestrial link for return path. Round-trip time is the sum of transmit and receive paths, so reducing even one of them helps most applications. Brian Dickson