ADSL  vs.  SDSL

What is ADSL?
Asymmetric Digital Subscriber Line (ADSL) is currently the most popular flavor of DSL. It is widely deployed and used by many home consumers and Small Businesses. ADSL is called "asymmetric" because most of its two-way bandwidth is devoted to the downstream direction that sends data to the user. Only a small portion of bandwidth is available for upstream. Most Internet users, and especially those who use graphics- or multi-media intensive Web data, need lots of downstream bandwidth. For these applications, user requests and responses are small and require little upstream bandwidth. Using ADSL, up to 15 Mbps of data can be sent downstream and up to 1 Mbps are available for upstream. The high downstream bandwidth means that your telephone line will be able to bring large data files, video, audio, and 3-D images to your computer or hooked-in TV set. And, since it supports Plain Old Telephone Service (POTS), a small portion of the downstream bandwidth can be devoted to voice, rather than data, so you can make phone calls without having to use another separate line.

What is SDSL?
Single-Line, or Synchronous, Digital Subscriber Line (SDSL) supports symmetric service. A single line can carry anywhere from 192 Kbps to as much as 1.5 Mbps in each direction equally. SDSL is often used by business customers that require a lower cost solution then T1 or EoC but still require that Mission Critical Reliability.

SDSL is symmetrical (upstream and downstream bandwidth). For this reason, the maximum data rate is lower than that for ADSL. It can carry large amounts of data in both directions.

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