Now that we've explained some of the differences between cable modems and DSL "modems," let's dig a bit deeper into DSL. Oliver asked:
"How do upstream speeds compare? I know the cable modem speed is set by the cable company -- e.g., you might only get 512kbps upstream although the modem is capable of 2mbps. Obviously both DSL and cable have more capacity downstream than upstream, but which is better in that respect?"Good question. Lets take a look at DSL uploads first. DSL is a generic term that encompasses a variety of technologies. Each of these technologies is similar, but different in how it compresses data. Compressing the data you send over your phone line allows you to have faster speeds. The better the compression, the faster uploads you'll have. The difference between an analog modem (at 56K or 33.6K or 28.8K) and a DSL modem isn't significant, but the results can be.
An analog modem is so "slow" because it only uses a small portion of the available amount of information that can be transmitted over copper wires (your phone line). The maximum amount of data that you can receive using an ordinary modem is about 56K (the fastest analog modem available). The download speed is limited by the fact that the telephone company filters information that arrives from the Internet as digital data, puts it into analog form for your telephone line to transmit, and requires your modem to change it back into digital information that your computer will understand. Basically, its dumb and ineffcient.
The reason DSL is so much faster using the same wires as analog modems is that digital data from the Internet doesn't need to be changed into analog form and back when travelling between you and the Internet. Data is transmitted to you from the phone company in pure digital form, leaving more room in the "pipe" that your phone lines offer.
Here are some of the "flavors" (different compression techniques) of DSL:
ADSL: Asymmetric Digital Subscriber Line - the most common form of DSL
Allows for good download speeds, but upload speeds are limited because most of the bandwidth is used for information going "downstream." For the majority of Web users, this is great, because they don't spend a lot of time uploading data. Typically, you'll see download speeds of 1.5Mbps at 18,000 feet, 2Mbps at 16,000 feet, 6Mpbs at 12,000 feet, and 8.5Mbps at 9,000 feet from the phone company. You'll see anywhere from 16 and 640Kps upload speeds, depending on the distance from the phone company (and therefore signal degridation).
HDSL: High Bit-Rate Digital Subscriber Line - oldest form of DSL.
HDSL allows for equal upload and download speeds, although it sacrifices overall speed to achieve equal speeds fror uploading and downloading.
SDSL: Symmetric Digital Subscriber Line -similar to HDSL but much faster.
Can carry a full T-1 speed (1.54Mps) in full duplex (both upstream and downstream). You have to be pretty damn close to the telephone company (or one of their switches) to get this, though, because the signal degrades in a hurry due to the high speeds.
VDSL: Very High Date Rate Digital Subscriber Line - if you're within spitting distance of you're phone company, this is the fastest option you can have.
You can typically get 55Mps with VDSLs compression algorithms. The trade-off is that you have to be within 800 feet of the phone company. This is more for business uses from what I've seen.
There are a number of other compression algorithms out there that add to the alphabet soup, but the above are the most common. And as my previous article mentioned, this is between you and the phone company, not between you and the Internet.
So to answer Oliver's question: your upload speed depends on what compression technique you're using, which directly correlates to how close you are to the phone company. You can get as high as 55Mps with DSL. The average user, however, enjoys upload speeds of between 128Kps and 540Kps. A typical cable modem user can achieve much higher -- or lower -- upload speeds. I can upload to my servers at work at about 1.2Mps in an off-peak time. Naturally, congestion can lower that to what I used to use for a modem when I had an Apple ][gs :)
Hope that answered your question and cleared up some of the confusion about DSL. If anyone has further questions about DSL or cable modems or would like to see more articles about this stuff, post a comment below.