Digital Subscriber Lines - internet over a phone line. TurtleSpot provides G.SHDSL and ADSL2+ over new telephone lines. These are not the same lines used by the existing telephone service provider, but seperate newly installed copper lines.
The advantages of DSL over WiFi are speed, stability and reliability. With the ever growing number of access-points, the 2.4GHz frequency for WiFi has become extremely busy, and the newer concrete buildings often have no coverage inside. DSL eliminates all these factors.

ADSL is available in Sairee and the direct vicinity. ADSL, ADSL2 and ADSL2+ are supported. This service is provided for a minimum contract period of 1 year. Installation costs depend on the distance to one of the nearest exchanges.

G.SHDSL is available in Sairee and Mae Haad. This service is provided for a minimum contract period of 1 year. Installation costs depend on the distance to one of the nearest exchanges.

The main question: how fast is it?
The actual line speed is 2.3Mbps for G.SHDSL and up to 24Mbps/2Mbps for ADSL2+. Internet speeds depend on the traffic (see shaping in the FAQ) and the destination. Domestic traffic in Thailand is usually fast, international traffic is limited to the available bandwidth at the international internet exchanges. Most important factor: internet is a shared medium. That means you will share the bandwidth with others.
The total available bandwidth to Koh Tao is 4Mbps.