数位用户回路(DSL)
全名Digital Subscriber Line
宽频数位通讯线路，在标准的电话铜线上运作。用户需要数位用户回路数据机，将传输分割为两个频带：较低频给语音用（一般电话），较高频给数位资料，特别是网际网路的连线。资料经由数位用户回路传送比起普通拨接数据机服务要快得多，不过数位用户回路信号的范围极小。线路范围只能在发送站附近几公里之内。数位用户回路和「某某数位用户回路」是由许多不同协定和技术的统称。「非对称数位用户回路」（ADSL）是数位用户回路常见的类型，把大部分的线路频宽用来从网路下载资料，只留下一点连线来上传资料。「高速数位用户回路」（HDSL）和「对称数位用户回路」（SDSL）的资料流是对称的，也就是说上行和下行的速率相同。「单向数位用户回路」（UDSL）和「超高速数位用户回路」（VDSL）等还在研发阶段，试图提高资料传输速率。
English version：
DSL
Broadband digital communications connection that operates over standard copper telephone wires. It requires a DSL modem, which splits transmissions into two frequency bands: the lower frequencies for voice (ordinary telephone calls) and the upper band for digital data, especially for connection to the Internet. Data can be transferred via DSL at much higher rates than with ordinary dial-up modem service; the range of DSL signals, however, is very small. Connections can be made only within a few miles of the nearest transmitting station. DSL and "xDSL" are umbrella terms under which a variety of protocols and technologies fall. ADSL (Asymmetric DSL) is a popular type of DSL in which most of the bandwidth of the connection is devoted to downloading data from the network to the user, leaving only a small-scale connection for uploading data. In HDSL (High bit-rate DSL) and SDSL (Symmetric DSL) the data stream is symmetric; that is, the upstream and downstream rates are the same. UDSL (Unidirectional DSL), VDSL (Very high data rate DSL), and others still under development are intended to offer even greater rates of data transmission.
