TELECOM Digest OnLine - Sorted: Uplink Requirements, was: ADSL and SDSL?

Uplink Requirements, was: ADSL and SDSL?

Danny Burstein (dannyb@panix.com)
Wed, 16 Feb 2005 05:10:01 UTC

In <telecom24.70.7@telecom-digest.org> bonomi@host122.r-bonomi.com
(Robert Bonomi) writes:

[ lots snipped ]

> Separately, there is the matter of the speed of the 'upstream' link
> from the DSLAM, and the number of users (and at what speeds) sharing
> that uplink. The 'uplink' is *almost*always* "over-subscribed" --
> ie. the aggregate bandwidth for the 'sold' DSL lines is _much_ higher
> than the bandwidth of the upstream link. This "works" because it is
> *very*rare* for all the end- users to be *trying* to use all their
> bandwidth at the same time.

Keep in mind that when downloading at that higher speed, you're also
using the upstream channel for various handshakes. The exact amount of
bandwidth required for those acknowledgements will vary _very_
dramatically depending on what you're pulling over and the protocols
in use, but a realistic number would be five percent.

What this means it that if you and a bunch of your neighbors are
pulling stuff down at 1 meg, you're each using up 50k on the
upside. (Again, with _mucho_ variation). With, say, that typical
advertised 1.5meg/128k (which, on a good day, may be 80% of that), you
and the folk down your block don't have much spare room for that VOIP
phone which requires bi-directional throughput.

Knowledge may be power, but communications is the key
dannyb@panix.com
[to foil spammers, my address has been double rot-13 encoded]

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