Re: Slow broadband..

I'm paying for the AT&T 600 kilobit DSL service. Actual speed-meter
testing shows my downloads averaging about 1.1 megabits. I think the
quality of the line and the distance to a substation dominates DSL
speed.

Unlikely.

Look at the connection speed at the status of your modem. You will see
something like 3...10 Mbits downstream. The transfer rate is trottled at
provider, not by the physical link.

It's both.

The physical quality (most significantly, the length) of the link
between the DSL modem at the home, and the DSLAM termination, sets the
upper limit of the connection speed. If the length is too great,
the attenuation of the signal (in particular, its higher-frequency
components) reduces the SNR at high bit rates too much, and the rate
of cell/packet loss becomes unacceptable. The link has to be run at a
lower symbol rate in order to get the data through at all.

For shorter links, the transfer rate is set/throttled by the provider
at the DSLAM.

I've been in both situations with my DSL connection. When I
originally set it up, the DSL termination was provided at the telco
central office in my city, which is several miles away. I was getting
around 512 kbits/second on an account which was sold as "up to 1.5
megabits"... I was just too far away from the DSLAM to get a higher
rate, no matter what I paid for.

A couple of years ago, the telco installed a "remote terminal"
(mini-DSLAM connected to the central office via fiber) about a quarter
mile from here, and I asked my provider to switch me over to it. I'm
paying for a 3-megabit/second downstream connection, and have a solid
connection at that rate (actual throughput is around 2700 kbits/second
due to ATM overhead). When the telco guy set up and tested the new
line connection, he said "I'm getting a solid 6-megabit signal
quality"... so I could go up to that speed if I wanted to pay a bit
more (i.e. the line is currently throttled at 3 megabits at the
mini-DSLAM).

I've checked my DSL-modem status, and it says that it has negotiated a
3-megabit modulation rate with the DSLAM. Apparently, the provider
doesn't simply throttle the connection at the ATM cell level - they
also limit the modulation negotiation speed. I suspect that this
can make the connection a bit more reliable/consistent, as there's
less risk that an occasional noise burst would cause the modem/DSLAM
to go through a modulation-and-rate renegotiation.

--
Dave Platt <dplatt@xxxxxxxxxxxx> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
.

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