Fedora Eight is out on the streets!

VirginSnow at vfemail.net VirginSnow at vfemail.net
Sat Nov 10 18:46:11 EST 2007


> Date: Sat, 10 Nov 2007 13:27:52 -0500
> From: "Ben Scott" <dragonhawk at gmail.com>

>  The classic example is BitTorrent on an asymetric feed.  The feed
> can suck down a lot, but can only send out a little; meanwhile, the
> rest of the swarm is trying to suck a lot from you.  If you don't
> properly cap the outgoing rate, you'll cause congestion, which impacts
> everything.  For example, if I don't cap BitTorrent's outgoing rate, I

The situation is even more bleak with technologies like RADSL.  The
"RA" in "RADSL" stands for "Rate-Adaptive".  The difference between
RADSL and ADSL (Asymmetric DSL) is that, with RADSL, the bandwidth
allocated to the upstream and downstream channels is changed according
to which channel is used more.  As downstream traffic increases (say,
because you're downloading something like, oh, Fedora 8), bandwidth is
reallocated to the downstream channel and taken away from the upstream
channel.  This is in contrast to ADSL, in which there are fixed-size
channels for each of up/downstream communication.

Now, the RADSL scheme would be fine, if it weren't for one very
important (and very very BAD!) fact: With RADSL, the amount of
downstream bandwidth lost is about 10 times the amount of upstream
bandwidth gained.  So, if you start uploading another 1KB/s, you lose
about 10KB/s from your download bandwidth!

So, if you have a 1Mbps/128kbps RADSL, you'll either get the full
1Mbps down and 0bps up, the full 128kbps up and 0bps down, or a 10:1
weighted combination of the two.  So, if you're downloading at 512kbps
and uploading at 64kbps, you've actually SATURATED you're supposedly
1Mbps/128kbps link.  In this case, you're really only getting HALF the
stated speeds.

The situation is even worse if you want to try and have equal up/down
stream rates.  Doing the math: 10*D+U=1Mbps, with U=D, you get: U = D
= 91kbps.  That is to say: if you have a 1Mbps/128kbps RADSL link
*and* are uploading at the same rate that you are downloading, your
bandwidth will top-out at a mere 91kbps on each channel.  In this
case, you're getting a total bandwidth of 182kbps, or just 18% of the
link's stated speed.

Unfortunately, many companies that sell so-called "ADSL" are actually
selling you "RADSL".  Verizon is one of them.  They'll tell you that
you're getting (to continue the above example) a 1Mbps/128kbps "ADSL"
service.  But, when you plug in your equipment and test it, you'll
more likely than not discover that they've actually given you RADSL.
The salespeople don't know the difference.  They don't know what the
"R" means (let alone how important the distinction is), so they just
leave it out and call their product "ADSL".

When I shop for DSL, I make sure to *explicitly ask* if what they're
offering me is ADSL or RADSL.  And if they've never heard of RADSL, I
ask to talk to someone who has.  Because if you don't ask, you may
only be getting 20%-50% of what you think you're paying for.


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