The underlying network architecture can be divided into three layers (Device layer, Transport layer, and Control layer) plus the service layer and will be introduced from bottom to top respectively.

基础网络体系结构可分为三个层次(设备层、传输层和控制层)，其上还有服务层，我们将按照从下向上的顺序分别进行介绍。

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While the network layer manages communication between hosts, the transport layer manages communication between endpoints within those hosts.

尽管网络层负责管理主机之间的通信，但是传输层需要负责管理主机内部各端之间的通信。

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In addition, the transport layer allows IMS devices to make and receive calls to and from the PSTN network or other circuit-switched networks via the PSTN gateway.

此外，传输层允许IMS设备通过PSTN网关呼叫PSTN网络或其他电路交换网络和接收来自这些网络的呼叫。

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