There are several variations on DSL technology. In fact, there are so many that you will often see the term xDSL, where x is a variable, when the discussion is about DSL in general.
- Asymmetric DSL (ADSL) - It is called "asymmetric" because the download speed is greater than the upload speed. ADSL works this way because most Internet users look at, or download, much more information than they send, or upload.
- High bit-rate DSL (HDSL) - Providing transfer rates comparable to a T1 line (about 1.5 Mbps), HDSL receives and sends data at the same speed, but it requires two lines that are separate from your normal phone line.
- ISDN DSL (ISDL) - Geared primarily toward existing users of Integrated Services Digital Network (ISDN), ISDL is slower than most other forms of DSL, operating at fixed rate of 144 Kbps in both directions. The advantage for ISDN customers is that they can use their existing equipment, but the actual speed gain is typically only 16 Kbps (ISDN runs at 128 Kbps).
- Multirate Symmetric DSL (MSDSL) - This is Symmetric DSL that is capable of more than one transfer rate. The transfer rate is set by the service provider, typically based on the service (price) level.
- Rate Adaptive DSL (RADSL) - This is a popular variation of ADSL that allows the modem to adjust the speed of the connection depending on the length and quality of the line.
- Symmetric DSL (SDSL) - Like HDSL, this version receives and sends data at the same speed. While SDSL also requires a separate line from your phone, it uses only a single line instead of the two used by HDSL.
- Very high bit-rate DSL (VDSL) - An extremely fast connection, VDSL is asymmetric, but only works over a short distance using standard copper phone wiring.
- Voice-over DSL (VoDSL) - A type of IP telephony, VoDSL allows multiple phone lines to be combined into a single phone line that also includes data-transmission capabilities.
The chart below provides a comparison of the various DSL technologies:
As you can see, VDSL provides a significant performance boost over any other version. But for VDSL to become widely available, it must be standardized.