User`s manual

Use the AT commands to send test patterns through loopback paths in order to test the operation of network components.
The TM LED is on during a locally or remotely initiated test.
Local Loopback (LL) With Test Pattern (TP)
An LL/TP performs a self test of the CSU/DSU. A 511 bit test pattern from the CSU/DSU pattern generator passes through
the the transmit logic (Tx) and the receive logic (Rx) to the test pattern detector.
Figure 9, LL/TP (Local Loopback with Test Pattern)