DIGITAL NETWORK SYSTEM HAVING ARRANGEMENT FOR TESTING DIGITAL SUBSCRIBER LINE

摘要

The network terminal device includes a turn-around portion for forming a turn-around loop through which the turning-around of a signal to the subscriber line connected with the network termination device is carried out, and a discrimination portion for detecting that data transmission can be carried out and cancelling the formation of the turn-around loop. The turn-around loop is formed when a frame synchronisation bit signal from the exchange terminal device is received by the network terminal device and cancelled when the discrimination portion detects that data transmission can be carried out. Communication for either a monitoring of transmission or a bit error testing can be carried out between the network terminal device and the switch while the turnaround loop is formed.