 Router B Payload Remote Remote payload Figure 20-11. Loopback points in CSU/DSUs Data port/DTE This point loops the signal from the directly connected data device (in our case, Router A) back to that device without using the T1 framer logic. This tests the V.35 cable and the router. Local This point loops the signal from the directly connected data device back to that device after it has been processed by the T1 framer logic. This tests the CSU/ DSU, in addition to the V.35 cable and the router. Payload This point loops the signal coming from the T1 back onto the T1 after it has been processed by the T1 framer logic. This test would be administered on the (B) side in our example, but the loopback would be set locally on CSU/DSU (A). This point loops the signal coming from the T1 back onto the T1 before it has been processed by the T1 framer logic, effectively testing the T1 line without testing the CSU/DSU. In our example, a line loopback on CSU/DSU (A) would be tested from the (B) side, though the line loopback would be set locally on CSU/DSU (A). Remote line Remote line loopback is a feature available on some CSU/DSUs that allows a local CSU/DSU to set a line loopback on the far-end device. In our example, though the loopback would exist on CSU/DSU (B), the command to initiate the loopback would be entered on CSU/DSU (A). Remote payload Remote payload loopback is a feature available on some CSU/DSUs that allows a local CSU/DSU to set a payload loopback on the far-end device. In our exam- ple, though the loopback would exist on CSU/DSU (B), the command to initiate the loopback would be entered on CSU/DSU (A). Troubleshooting T1s Say we're having problems with the T1 in Figure 20-11 and we need to trouble- shoot. Looking at the error stats in CSU/DSU (A), we see many OOF, BPV, and CRC6 errors. Actual testing of the T1 line would typically proceed as shown in Figure 20-12. Remote Remote Data port Local payload CSU/DSU CSU/DSU Router A Router B Data port Local Remote line [ BPV, 00F, CRC6 ] Remote payload Figure 2