In a previous article, Mxsmanic said:
Paul Tomblin writes:
I fly with a Garmin 296 handheld GPS. In my experience, nearly always
just around the same time it says I need to start my descent if I want to
descend at 500fpm to my destination, ATC clears me down to a lower
altitude without being asked.

I have noticed this as well. I suppose if they know the route well,
they know when the descent usually starts.

Except they know where to start my descent whether I'm flying a 100 knot
Archer or a 140 knot Lance, or on one occasion, a Piper Dakota with a 70
knot tail wind.

I suspect there is software they use to handle this.

--
Paul Tomblin http://blog.xcski.com/
#define sizeof(x) ((int)rand()*1024)