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通信网正在逐步向综合业务数字网(ISDN)的方向发展,这一方面是由于有数字交换技术和数字传输技术的发展作为基础,另一方面也是由于对各种新业务的需求大量增加。在向ISDN演变的过程中,应逐步建立数字传输系统。以往的数字传输均采用四线制方式,然而在局部网中,用户线路一般都是二线线路,为了节省投资,避免敷设新电缆,必须考虑在原来传送模拟电话信号的二线用户环路上传送数字信号的问题。在二线用户环路上实现数字信
The communications network is evolving towards Integrated Services Digital Network (ISDN), on the one hand due to the development of digital switching and digital transmission technologies and on the other hand to the substantial increase in demand for new services. During the evolution to ISDN, digital transmission systems should be gradually established. However, in the local network, subscriber lines are generally two-wire lines. In order to save investment and avoid laying new cables, it is necessary to consider transmitting digital signals on the second-user loop that originally transmitted analog telephone signals The problem. Implement digital signaling on second-line subscriber loops