rm Telecom Router A Router D Telecom Telecom Switch B Switch C Figure 21-6. Red alarm Figure 21-7 shows another red-alarm scenario. In this example, Telecom Switch C is sending garbled frames to Router D, which sees consecutive OOF problems and de- clares a red alarm. When Router D declares the red alarm, a yellow-alarm signal is sent back out the link. The way these alarms behave can be a bit confusing. A red alarm generally indicates that something serious is happening on the local equipment, but in the example Figure 21-7, Router D is receiving SO many OOF errors that the signal is useless. Because Router D cannot figure out how to read the frames from the far side, it triggers a local red alarm and sends out a yellow alarm. Yellow Alarm A yellow alarm is also called a remote alarm indication (RAI) and indicates a remote problem. A yellow alarm is defined in RFC 1232 as follows: Chapter 21: T1 Locally undetected 00F signal Red alarm problem Yellow alarm Telecom Router A Router D Telecom Telecom Switch B Switch C Figure 21-7. Yellow alarm A Yellow Alarm is declared because of an incoming Yellow Signal from the far-end. In effect, the circuit is declared to be a one way link. One vendor specifies that a yellow alarm be declared for superframe links when bit six of all the channels has been zero for at least 335 ms, and that it be cleared when bit six of at least one channel is not zero for less than one to five seconds. For an ESF link, a yellow alarm is declared if the signal pattern occurs in at least 7 of 10 contiguous 16- bit pattern intervals, and is cleared if the pattern does not occur in 10 contiguous 16- bit pattern intervals. Wow, what a mouthful! The simple truth is that unless you're designing T1 CSU/DSUs, you don't need to know all of that. Here's what you need to know: a yellow alarm does not necessarily indicate a problem with your device; rather, it's a problem being re- ported by the device to which you are connected. Figure 21-8 shows a simple network. Router A has a T1 link to Router D. The T1