ly aggregated into a larger circuit, hauled to the remote CO, and then converted back to a 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 20-8. Another yellow alarm Chapter 20: T1 Telecom Switch B receives a yellow alarm from Telecom Switch C. This alarm may be forwarded to Router A. When diagnosing the outage, 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. 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 (AIS) 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 active. If you're receiving a blue alarm, there is a good chance that a cable is discon- nected, or a device has failed. Troubleshooting T1s The first step in troubleshooting a T1 is to determine where the problem lies. Usu- ally, 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 Loopback tests involve setting one piece of equipment to a loopback state and send- ing 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. Loopback 1111111111111111 CSU/DSU CSU/DSU Router A Router B 1110110101110101 Loopback tes