Peer-to-peer systems are often implemented as an abstract overlay network built at the application layer on top of the native or physical network.
对等系统常常实现为一个抽象的覆盖网络，构建于原生或物理网络之上的应用程序层上。
The two bottom layers, called the physical layer protocols, take care of the actual signal transfer over the network cable.
称为 物理层协议的两个底层处理网络电缆上的实际信号传输。
Each layer employs the services of the layer underneath, which enables a top layer protocol to transmit messages over the physical network.
每个层都使用下面一层的服务，这使得顶层协议能够在物理网络上传输消息。
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