This variable indicates what type of code is being sent across the X.21/HSSI interface by the device. The values mean: x21NoCode: No loopback. x21SendLoopACode: Start sending Loop A code. Valid only if the backcard is HSSI and the line-type is DTE. This command loops the line module of the remote device. x21SendLoopBCode: Start sending Loop B code. Valid only if the backcard is HSSI and the line-type is DTE. This command loops the digital section of the remote device. x21SendLocalLoopCode: Start sending CCITT X.21 Loopback Type 3 code. This is valid only if the backcard is X.21 type. x21SendRemoteLoopCode: Start sending CCITT X.21 Loopback Type 2 code. This is valid only if the backcard is X.21 type. x21SendUnLoopCode: Sending a loopback termination request. Valid for both HSSI and X.21 lines. To execute any of the loop-codes the x21LineLoopbackCommand should be set to x21RemoteLoop. Once the x21SendUnLoopCode succeeds the x21LineLoopbackCommand would transition back to x21NoLoop state. Default is x21NoCode.