In order to carry out maintenance in the public switched telephone network of switched 64-kbit/s digital circuits@ digital circuits at sub-rates of 64 kbits/s@ and N ??DS0 digital circuits (up to the maximum payload of a primary rate facility)@ a digital circuit test system is defined for digital exchanges and digital PBXs. Currently@ only digital loopback testing is defined in this standard. Further study is required to develop other types of digital test lines (e.g.@ responder type testing). Purpose This standard specifies a loopback test line capable of being used in the measurement of error performance of switched digital circuits for purposes such as: a) The analysis of reports by customers of degraded error performance. b) A repair verification check of error performance prior to a return to service of circuits previously reported as faulty. c) A pre-service check using enhanced test patterns to ensure proper implementation of DS0/DS1 equipment options that affect circuit operation. d) A pre-service check of the proper provisioning of switches supporting N ??DS0 services. N ??DS0 channels require that bit sequence integrity be maintained. This may be incompatible with some types of switching equipment and so special routing must be provisioned for these calls. e) A pre-service check to establish a performance benchmark. f) An off peak hours Auto Routine Test for the purpose of maintaining network performance standards.