SDSL (Symmetric Digital Subscriber Line) is high-speed Internet access service with matching upstream and downstream data rates. That is, data can be sent to the Internet from the client machine or received from the Internet with equal bandwidth availability in both directions. Normally, DSL service is asymmetrical (ADSL), with the bulk of the bandwidth reserved for receiving data, not sending it.
Normally, SDSL is used by businesses with a Web presence, VPN, extranet or intranet needs. In these cases, the client server may be required to upload large streams of data to the Internet on a regular basis. ADSL would be slow and inadequate for this purpose, as the bandwidth available for uploading is normally less than 1 megabit per second (mbps). SDSL bandwidth can be as high as 7 mbps in both directions.
An Internet service provider offering SDSL may offer different grades for varying prices. The faster the data rate, the more expensive the service. Usually, long-term contracts are required for service, regardless of the grade chosen.
SDSL utilizes a digital frequency traveling across existing copper telephones lines to send and receive data. When using the telephone line for this service, phone and fax services on that line must be suspended. Therefore a dedicated second, or additional, line is typically needed. This differs from ADSL, which "leaves room" for both standard analog telephone equipment and the digital signal, so that one can talk on the phone or use a fax machine while online.
This is an "always on" service, meaning that the computer is actively connected to the Internet whenever powered up. If the computer always remains on, the Internet connection will be continuously active.
SDSL service requires a special modem, normally supplied by the Internet service provider, and the equipment is often proprietary. The SDSL modem will likely require same-vendor equipment in the LAN, or common DSL chipsets.
Apart from businesses, SDSL can also serve individuals that require high upload speeds. Network sharing, for example, has become very popular, and with it the need for uploading programs and files — often extremely large files. SDSL is a good choice for heavy network sharing, as long as the user has a second telephone line to dedicate to the service or chooses to suspend telephone services while online.
SDSL is not available in all areas and speeds might vary depending upon the user's physical distance from local hubs. The service is also more expensive than ADSL, but well worth the difference for those with demanding upstream needs.