集中管理中心与集中管理式电话计费器之间可通过公众电话网以拨号方式建立双向通信链路，从而实现对集中管理式电话计费器的远程管理。
Between the center and the devices, the duplex communication links are created through the public switched telephone network, thus the remote management for the devices is realized.
在这里概述的题目包括点对点链路，电路交换，分组包交换，虚电路，拨号服务，和WAN设备。
Topics summarized here include point-to-point links, circuit switching, packet switching, virtual circuits, dialup services, and WAN devices.
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