free line signal

例句：

1. 1. The free linesignal is used to transmit data over long distances without the need for a physical connection.
   自由线信号用于在没有物理连接的情况下，通过长距离传输数据。
2. 2.In this case, the free line signal was used to establish a secure communication channel between two remote locations.
   在这种情况下，自由线信号被用来在两个远程地点之间建立安全的通信通道。
3. 3. The free line signal has proven to be an effective way of transmitting large amounts of data over long distances.
   自由线信号已被证明是一种有效的方式来在长距离上传输大量的数据。