In a T1 telephone circuit, repeaters are located at intervals along the line to boost the signal.  Collectively, these components comprise what is referred to as a span.  Occasionally, individual repeaters may fail and some means of testing and locating a defective repeater is required. 
Until now, elaborate analog circuitry and extra wiring was required to perform such testing.  Such arrangements are costly.  It would be useful to have a simplified and cost-effective method of accessing individual repeaters in a span so that tests could be run to check performance.