一、非对称数字用户线的由来20世纪末期起,电话通信网实行数字化,用户自己装置计算机的数量普遍增多。相应地,用户使用数据通信的业务量急剧增长。原来的电话通信网（PSTN）上由对绞铜线组成的用户线除了传送传统的用户电话（双方实时会话）外,近来还大量地组成“数字用户 First, the origin of asymmetric digital subscriber line Beginning of the twentieth century, the telephone communication network digital implementation, the user generally increases the number of computers installed. Accordingly, the amount of traffic the user uses for data communication increases dramatically. In addition to transmitting traditional subscriber lines (both real time sessions), subscriber lines composed of twisted pair copper lines on the original telephone communication network (PSTN) have recently formed a large number of "digital subscribers