DSL

DSL¹ - an acronym meaning "Digital Subscriber Lines" - provides high-speed Internet access over your existing telephone wiring². Data transmission takes place using hardware attached to both ends of the telephone line, utilizing frequencies above the normal telephone bandwidth (from 300Hz to 3,200Hz). The speeds achieved are much faster than those you can get by modem over so-called "plain old telephone service" (a.k.a. POTS).

The connection is constantly on (a.k.a. 24/7) so you don't have to "dial in" to connect. Connection speeds vary with distance from the telco's Central Office (CO). The closer you are, the faster the service, and vice versa. Bandwidths for ADSL, the most common DSL, are typically in the 150Kbps (K/second) range for download, and 10Kbps for upload (again, this works best if you're close to the CO). Farther from the CO, download speeds may be as low as 32Kbps. DSL is still under development, and speed improvements are inevitable.

A word about ADSL: There are two main reasons for the Asymmetric DSL implementation. First, the technical reason: Due to the high density of wire terminations at a CO, there can be a lot of interference between lines, called crosstalk in telephony circles. The second reason is that an asymmetric design mirrors actual usage - there is considerably more download than upload for most users.

With some DSL service, a "POTS splitter" is required. The splitter is an unpowered device that insures that the traditional POTS voice service is available on the phone line. Note that with DSL your normal phone line is available for voice use - callers do not get a busy signal.

Note that certain packet-based applications, such as two-way video conferencing, do not work very well over DSL. DSL is ideal for most gaming applications, however.

To hook up to a DSL service, you will usually need a 10Base-T adapter which connects to the external DSL device. The telco will usually provide, and even install, this adapter.

The cost of DSL is roughly equivalent to that for a second phone line and dial-up Internet access. (Note that DSL does not require a second line.)

Risks: If you have file sharing enabled, data on your hard disk is potentially at risk. You can protect yourself by using a firewall (on another PC or with a hardware device), or at minimum you can disable file sharing if you have a standalone computer. For step-by-step instructions on this, see: http://www.ehow.com/eHow/eHow/0,1053,13617,FF.html?src=bre4. Note that even then you are not safe, see cable modem by Blue_Bellied_Lizard for more on security with a persistent connection.

For more details about DSL, see http://homepage.interaccess.com/~jkristof/xdsl-faq.txt, which was the primary research source for much of the forgoing content.

Noded by request

One key advantage that telcos and ISPs who provide DSL (Digital Subscriber Lines) push, other than the higher speeds of course, is that you only need one phone line for both data and voice transmissions. This is fundamental to the way DSL works. I will explain it briefly here.

The basic idea is dividing up the frequency range on the copper wire, and using different frequencies for regular telephone usage and DSL. This works because voice transmissions are already limited to the frequency range of 0-3400 hertz. This was done originally to lower analog interference from phone lines to each other. (Note that fax machines and dialup modems also operate at this range, thus you can receive fax transmissions while online with DSL, and technically, if you wanted to, even use a dialup modem at the same time although you'd have to reset some configurations in your computer usually.)

Phone lines can actually handle much higher frequencies. DSL takes advantage of that, and allocates those higher frequencies for digital data transmissions. The two popular implementations are CAP and DMT. In CAP, the frequencies are statically assigned to each task. Upstream data is carried on the 25-160kilohertz range. Downstream is carried from 240khz up to whatever the line can handle. In DMT, the entire band is divided up into 247 channels of 4kilohertz each. Each of these can be used for downstream and upstream, some even bi-directional. The signals switch dynamically between the channels depending on what the line quality on each channel is like at the time.

Usually customers will need to install low-pass filters to telephony devices other than the DSL modem. What these are, are simple filters that block out any signal over, say, 3400khz or 4000khz or so. Basically it lets vocal transmissions through while keeping DSL signals from causing static interference on the line.

usage:

bonhomme_warez: Quelle connection as-tu? Câble ou DSL? Envoie-moi Photoshop 6!

me: dsl, je n'ai que 33.6.

Before the "Digital Subscriber Line," DSL used to mean something different. DSL was used to refer to oral sex and stood for "Dick Sucking Lips," and it was used in this context in several rap songs.

Once DSL and fast access internet became widely used and popular, many people began coming up to me and asking if I had "DSL." I couldn't help but laugh.