The fit loopback tests the cable by looping the signal back after it reaches the DTE port.
A data terminal equipment( DTE) recognizes a virtual circuit by its associated LCN.
数据终端设备(DTE)或数据电路端接设备(DCE)使用的信息包，用来表示暂时无法接收虚拟呼叫或永久虚拟电路的附加信息包。
A signal from a modem ( data set ) to a DTE indicating that it is ready to operate.
A unique reference number that identifies a logical channel. A data terminal equipment ( DTE ) recognizes a virtual circuit by its associated LCN.
