This read-write BITS object selects which physical line or lines, such as individual dsx1 T1/E1 tributaries or the ETH 10/100 board, will have a special BER test signal inserted for testing. Because its own description restricts which lines can be tested simultaneously, one T1/E1 line, or the T3/E3 line, or both Ethernet ports, or everything at full capacity, and requires proper loopbacks to already be set, an admin must plan the test combination carefully before setting this object. For example, setting only the dsx1-1 bit selects the first T1/E1 tributary for a BER test while the other tributaries keep carrying live traffic undisturbed.