 is actually terminated locally at the T1 provider's central office, where it is usually ag- gregated into a larger circuit, hauled to the remote CO, and then converted back to T1 for delivery to the remote location. Telecom Switch B is fine, but Telecom Switch C has experienced a failure. Yellow alarm Red alarm Yellow alarm Telecom Router A Router D Telecom Telecom Switch B Switch C Figure 21-8. Another yellow alarm Telecom Switch B receives a yellow alarm from Telecom Switch C. This alarm may be forwarded to Router A. When diagnosing the outage, remember that the presence of a yellow alarm usually indicates that some other device is at fault. Here, Telecom Switch C is the cause of the outage. Alarms Watch out for assumptions. Router D will receive a yellow alarm, as might Router A. In this case, the admin for Router A may blame Router D (and vice versa), as he probably has no idea that telecom switches are involved. Blue Alarm A blue alarm is also called an alarm indication signal (AIS). There is no definition in RFC 1232 for this condition. A blue alarm is representative of a complete lack of an incoming signal and is indicated by a constant stream of unframed ones. You may hear someone say that the interface is receiving "all ones" when a blue alarm is active. If you're receiving a blue alarm, there is a good chance that a cable is disconnected or a device has failed. Troubleshooting T1s 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 you de- termine what steps to take. All of these tests are invasive, which means you must take the T1 out of service to perform them. Loopback Tests 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 21-9 shows how a loopback test might fail. Loopback 1111111111111111