Satellite latency

Wed Feb 27 16:54:43 UTC 2002

"Tim Devries" <zsolutions at 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