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1 旧DN出现的鲁是 社会的发展对通信服务不断提出新的需求,除传统的电话电报通信外,各种非话新业务,例如数据、传真、可视图文、电子信箱、可视电话、会议电视等方面的需求急剧增长,这些业务要求电信网直接提供端到端的数字连接;另一方面,随着经济的快速发展,各专用网的互连需要有效的通信手段来建立多点之间的远程连接;其次,由于数字网的用户网仍然是模拟同,传输数字信号要借助于调制
1. The emergence of the old DN is a social development. With the continuous development of new demands for communication services, various non-verbal new services such as data, fax, video, e-mail, videophone The demand for conference television and the like has dramatically increased. These businesses require the telecom network to directly provide end-to-end digital connections. On the other hand, with the rapid economic development, the interconnection of private networks requires effective means of communication to establish a multi-point Between the remote connection; Second, because the digital network user network is still analog with the transmission of digital signals by means of modulation