DIGITAL LINE TEST SYSTEM

PURPOSE:To attain execution of a test of an information line with a part of device only by allowing the transmission side to define the test mode to a control field at test, sending a frame given with a test pattern and allowing the reception side to reflect the test pattern of an information field to the transmission side. CONSTITUTION:A remote test device 5 sends data to a DTE 1-1 by the loop 2 mode designation, then a DSU 3 gives information directly to the DTE as it is in the same way as other normal frame. The DTE 1-1 uses a field A to recognize the information to the DTE 1-1 and uses a field C to recognize the loop 2 mode. The DTE 1-1 returns the test pattern represented by one field as it is. In this case, as the return system by the DTE, it is considered that two ways such as the realizing system returning four fields A, C, I, FCS like the DSU 3 and the system processing fields A, C and any of the systems is used by deciding the system in advance between the DTE and the remote test device 5. Thus, the test is conducted without giving effects on a logical channel having no fault at the time of fault occurrence.