[LINK] The PLAN, and broadband speeds?

[Saliya Wimalaratne]

On Tue, Jun 26, 2007 at 04:46:57PM +1000, Stewart Fist wrote:
> 
> Geoff's point about satellite delays, only apply to roughly symmetrical
> interactive services, where the back-channel is also over the satellite.
> One way broadcast doesn't suffer delay problems.

Well, it does - but the delay's always the same (e.g. watching sat TV). 
But let's assume we're talking about two-way traffic when we're talking
about 'interactive' (that is, interactive != changing the channel)...

On an unloaded connection: 

A 14.4-33.6k modem PPP connection connected via external serial port used
to average about 150ms round-trip between modem endpoints. 
A similar 'internal modem' would shave about 20-30ms off that.
56k PPP modem, 90-120ms.
64k ISDN, 30ms. 
<n> bandwidth ADSL, 20-50ms depending on architecture. 

For geostationary satellite, the delay is about 450ms (one-way sat, one-way
terrestrial) or about 1000ms (two-way sat). 
Or, expressed another way, the fastest satellite is about triple
an average modem latency or 10x ADSL latency. That's very noticeable to
a user. It's the difference between click -> content and click ... ...
... -> content for interactive sessions. 

> Asymmetrical services can use terrestrial low-bandwidth back-channels

They can, to a point. In general the up:down ratio is about 1:5 or so 
depending on what people do; with queue tweaking and other TCP segment 
size and window trickery (which I would guess all satellite providers use) 
I've seen it at 1:20 or so for typical usage. 

Still means that usable satellite bandwidth is about 600kbps or so if 
you're using a low-bandwidth 33.6k modem backchannel. And of course P2P is 
a whole 'nother ballgame again...

Regards,

Saliya