[LINK] The PLAN, and broadband speeds?

[Richard Chirgwin]

Saliya Wimalaratne wrote:

[snip]
> For geostationary satellite, the delay is about 450ms (one-way sat, one-way
> terrestrial) or about 1000ms (two-way sat). 
>   
'Scuse me, but 270 milliseconds both ways.

Geostationary orbit = 35,800,000 metres
Speed of light = 299,792,458 metres per second (in a vacuum)

Two-way trip = 239 milliseconds plus a little bit for atmosphere, which 
isn't vacuum.

Two two-way trips (ie, user > Internet > user) = around half a second.

RC
> 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
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