Stands for "Symmetric Digital Subscriber Line."
Since data flows at the same speed in both directions, SDSL is better suited to customers that need a reliable data connection to send and receive equal amounts of data. For example, a business that needs an Internet connection for VoIP service will use both upload and download bandwidth equally.
The other type of DSL connection, ADSL, is asymmetric; it allows users to download much faster than it allows them to upload. Since most home Internet users download more than they upload, most ISPs offered ADSL to their home customers instead of SDSL.