A communication system extending Ethernet Operation and Management (OAM) Loopback Message (LBM) and Loopback Replay (LBR) functionality into a time division multiplexing (TDM) domain has a first interworking function (IWF) of an Ethernet network that transmits an Ethernet Operation and Management (OAM) Loopback Message (LBM) having a first type length value (TLV) field. The first IWF controls the first TLV field to indicate that a connectivity test is to extend beyond the Ethernet network. The communication system further has a second IWF of the Ethernet network that receives the Ethernet OAM LBM and interprets the first TLV field. In addition, the second IWF performs a TDM connectivity test across a TDM connection in response to the Ethernet OAM LBM and based on the first TLV field and transmits a reply for the Ethernet OAM LBM to the first IWF, the reply prompted by the LBM and comprising data based on the TDM connectivity test.