t failed Figure 20-9. Conceptual loopback test failure Troubleshooting T1s When you perform a loopback test, failed results won't typically be as clean as ones being changed into zeros. Usually, the problem is more electrical in nature, like the scenario shown in Figure 20-10, or framing errors cause the data to become entirely unreadable. Loopback All ones (proper polarity) CSU/DSU CSU/DSU Router A Router B Loopback test failed (BPVs) Figure 20-10. BPVs seen during loopback test When performing loopback tests, the logical way to proceed is to start at one end of the link and move across it until the symptom appears. CSU/DSUs generally offer the option of setting multiple types of loopback, which greatly assists in this process: you can usually set a loopback at the interface connect- ing the T1 to telco (called a line loopback), or after the signal has passed through the CSU/DSU's logic (called a payload loopback). Many CSU/DSUs allow these loopbacks to be set in either direction, which can fur- ther aid in trouble isolation. Telecom engineers look for trouble in a line when they troubleshoot. This may sound pedantic, but when you listen to telecom engineers troubleshooting, they will use very specific terms like no trouble found (which is often abbreviated as NTF). Remember, there are more than 100 years of standards at work here. Let's look at an example. In Figure 20-11, Router A is connected to Router B by a T1 using two CSU/DSUs. From the point of view of CSU/DSU (A), we can see the possi- ble loopback points available on the CSU/DSUs themselves. Bear in mind that if we were to look from the point of view of CSU/DSU (B), all of the points would be reversed. Not all models of CSU/DSU have all of these options, and not all CSU/DSUs call these loopback points by the same names. The following list of terms describes the loopback points that are usually available. Remember that the descriptions are based on the network as seen from Router A in Figure 20-11: Chapter 20: T1 Data port Local CSU/DSU CSU/DSU Router A