[NCLUG] DSL Throttling or General Congestion?

[Stephen Warren]

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Sean Reifschneider wrote:
> On Tue, Nov 21, 2006 at 11:56:25AM -0700, Stephen Warren wrote:
>> The thing about ATM is that it's time division multiplexing, so there's
>> an assigned fixed-bandwidth link all the way from your DSL modem at home
>> all the way to the ISP.
> 
> Not at all true.  We had more than 25 users, which at 1.5mbps is more than
> the 35mbps we had on the DS-3, after you took off the 10mbps that was used
> for transit.

OK. Looks like I was slightly confused...

ATM is a time-division protocol at the lowest level; the cells - they
get clocked out with a precise timing.

I had assumed that the mapping from virtual circuit content onto the
cells was also strict time division, giving fixed-bandwidth virtual
circuits. Further reading shows that the mapping is dynamic, so you can
have variable bandwidths, congestion, etc. in general.

That said, I'm sure it's possible to configure the mapping to be strict
TDM, but then you wouldn't be able to over-subscribe bandwidth, which
would raise costs too much.

> Cross-talk issues in a single cable bundle with
> multiple DSL lines on it are the reason they switched encoding years ago,
> and are the reason that you had to switch from Cisco 675s to Cisco 678s.
> IIRC, one of the encodings was called CAP.

Out of curiosity, do you happen to know the difference between "G.DMT
Annex A" and "T1.413"?

>> I suppose it's possible for the ATM session to be terminated directly in
>> the CO, and then to have, say, a GigE backhaul to the ISP, but I don't
> 
> ATM isn't a line, it's an encoding.  They wouldn't normally transit it over
> GigE, because the phone company can't transit ANYTHING over Ethernet.  They
> instead pass it over a T1, DS-3, OC-3, or other sort of transit to the ISP.
> The one we had at NCIC was over an OC-3 that was then broken out into 3
> DS-3s for the last dozen feet in our facility.

I was meaning something a little more generic; eventually some piece of
equipment terminates the ATM session, extracts the packets (IP over PPP
it seems in my case) and then routes the content as IP packets. I was
simply referring to the ATM session termination, and GigE was just a
random (possibly incorrect!) example.

I guess what you're saying is that the telco itself only transports ATM
to the ISP, over some irrelevant physical cable.

Isn't it possible for the ISP to co-locate their ATM->xxx equipment in
the CO (or wherever) and choose to run say GigE over fiber back to their
location? They'd just be leasing the raw fiber from Qwest, or whoever,
for this, so Qwest wouldn't care about the protocol on the fiber.
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