T1 and E1 Loopback Signals

The control signal on a T1 or E1 link is the loopback signal. Using the loopback signal, the operators at the network control center can force the device at the remote end of a link to retransmit its received signals back onto the transmit path. The transmitting device can then verify that the received signals match the transmitted signals, to perform end-to-end checking on the link.

Two loopback signals are used to perform the end-to-end testing:

• The loop-up command signal sets the link into loopback mode, with the following command pattern:

  ....100001000010000100...

• The loop-down signal returns the link to its normal mode, with the following command pattern:

  ....100100100100100100....

While the link is in loopback mode, the operator can insert test equipment onto the line to test its operation.