Digital subscriber lines (xDSL) are a set of technologies that use sophisticated modulation schemes to send data onto copper wires. They are called the last-mile technologies because they are used only for connections from a telephone switching station to a home or office, not between switching stations. xDSL modems are similar to ISDN. Both operate over existing telephone wires, and both need a short connection (less than 20 feet) to a central telephone office. There are several types of xDSL modems in use today such as ADSL, SDSL, HDSL:
• ADSL - Asynchronous DSL: Supports data transfer rates of up to 9 Mbps when receiving data (upstream), and up to 640 kbps when sending data (downstream).
• SDSL - Symmetric DSL: Supports data transfer rate of up to 3 Mbps in both direction, upstream and downstream.
• HDSL - High data-rate DSL.
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