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Everything Snax said. This is then in addition:
I'm not sure what detail you'll get from ATT but there's a few things that you should be trying to find out.
If you have a login into your modem you can get most of it.
a) Distance from the node/dslam. To get a decent 1.5Mbps down I'd say less than 18,000 feet on 26AWG copper. 20,000 if you're on 24AWG (unlikely).
b) wiring routes within the house from the demarc. If your telco provider service ends at their demarcation point outside, run a dedicated pair to the modem. Avoid splitters like the plague. They corrode and get noisy. Check all connections for signs of corrosion. Wall jacks go bad for no reason, even when they look good. Older jacks lose the springiness in their pins and make for noisy connections.
c) 3-way splitters from Radio Shack and Walmart are evil and for some reason cause problems with DSL. Throw them away. They all seem to be made the same and have crosstalk problems.
d) What transmission mode is your ISP providing? G.Lite? G.DMT? ADSL2+? (Three of a possible many). At distances over 16,000' if your modem is trying to train G.DMT and be throttled you'll get abysmal throughput, even with Reach-Extended ADSL2. Some modems can work a lot better at distance by being switched onto "Interleaved" mode. This is at the ISP end, and I'm not sure how willing your ISP will be to discuss the way they make their system work for you.
e) What's your SNR and attenuation? Ideally you're looking to get the entire-line attenuation below about 31dB. in the mid-to-upper 20's is good, low 20's to about 15dB is excellent. 6dB on the SNR is about the minimum to give a solid connection, albeit slow, over 12dB will give a much better connection. Forget not that you eat into the SNR margin the greater the bandwidth is being able to be used. The better the SNR, the more bits per tone can be encoded accurately, and the faster your connection will be, and the better the error correction and recovery. See if you can find out if your HEC (Header Error Count, basically packets that arrive so mangled they can't be repaired and are discarded) climbs steadily. It should hardly move- on my line here my HEC is 1350 packets in four months of operation.
DSL can be wonderfully reliable, given good line characteristics. You just have to make allowances for the fact it's a newer technology piggybacking over a very old technology. Wireless is fickle compared, and subject to transmission problems totally out of your control. You can control the copper within your house and improve it. You can't stop your neighbor arc-welding...
Clean the line up inside the house and you should see an improvement. Also note that if your modem has been fighting poor line conditions for several years the amplifiers might not be in tip-top shape any more, having run at max amplitude for a long time.
Hope that helps.
--Phil
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