DSL modem

Jeffrey Wong jmwong at math.ed.hawaii.edu
Thu May 10 00:22:52 PDT 2001


On Wed, 9 May 2001, Jimen Ching wrote:

>
> Also, last I checked, the bandwidth in the DSL package is guaranteed
> bandwidth between you and your CO.  If your switch is dropping packets,
> then that is false advertisement.  Are these ISP's allowed to do that?
>

I'm not really an expert on this, but this is my understanding of how it
works.  You have a dedicated line from you to the DSLAM at the CO, and
thus have the full available data rate here (Actually higher, than what
you pay for since this hop is the same for all service levels.  Unless
you're paying for the top level already).  From the DSLAM to your ISP you
run over a shared dedicated circut, this is where the CIR comes into play.
The DSLAM throttles back your connection to whatever the max data rate for
your service level before even trying to send it to the ISP.  If there is
enough available bandwidth on the circut at that time, you get your full
data rate, if not then it throttles back even more, but not less than your
CIR.  Generally, the dedicated circut will have at least enough bandwidth
for every CIR on every DSL modem on that DSLAM.  From what I hear Verizon
is pretty good at keeping it that way, but some of the mainland providers
have been accused oversubscribing to the point that they couldn't maintain
CIR.  From the ISP to the internet, you share with everyone else and if
you only get a 2400bps connection there, well it's past the part of the
connection covered by the CIR.

Jeff Wong



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