SDSL stands for ‘symmetric digital subscriber line’. An SDSL line provides for transport of digital data simultaneously in both directions across the line – the same bitrate being available in both directions (thus ‘symmetric’). SDSL connections typically allow transmission of up to 6 Mbit/s in both directions, but usually require a 4-wire connection (equivalent to two standard telephone lines). SDSL service is typically more expensive than ADSL and is usually provided by telephone companies as a ‘business service’ – either as a private ‘leaseline’ connection between two different company locations or as a high speed Internet access line at a site which sends as much data to the Internet as it receives (e.g. the company’s web server is located at the site: so a high volume of data is sent upstream to the network. Meanwhile, Internet ‘surfing’ activity of company employees at the site demand that a high downstream data rate is also available – so that Internet pages appear quickly).