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当前,发达国家已经实现电话“普遍服务”(住宅用户普及率近100％),电信运营公司已将其注意力转向网路宽带化,纷纷提出改造或重建用户网路的计划。 80年代末,为了能实现B-ISDN,人们提出了全数字、宽带的光纤用户环路(FITL);1992年美国FCC着手审批本地电话公司提供视像业务后,开始开发在现有电话用户线上传输视像信号的非对称数字用户线(ADSL);为了适应管制的
At present, the developed countries have already implemented telephone “universal service” (nearly 100% penetration of residential users) and telecom operators have turned their attention to broadband internet. They have put forward plans to transform or rebuild user networks one after another. In the late 1980s, in order to realize B-ISDN, an all-digital broadband optical fiber subscriber loop (FITL) was proposed. After the United States FCC approached the local telephone company to provide the video service in 1992, it started to develop the FITL Asymmetrical Digital Subscriber Line (ADSL) for transmitting video signals; for regulatory purposes