The first step in troubleshooting a T1 is to determine where the problem lies. Usually, it's cabling-, hardware-, or telco-related. Running some simple tests can help determine what steps to take. Note that all of these tests are invasive, which means you must take the T1 out of service to perform them.
Loopback tests involve setting one piece of equipment to a loopback state and sending data over the link. The data should return to you exactly as you sent it. When the data does not come back as expected, something has happened to alter it. Figure 20-9 shows conceptually how a loopback test might fail.
Figure 20-9. Conceptual loopback test failure
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.
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 connecting ...