Hi Colin & Treggers, There are basically 3 alarms on a T1 and ISDN-PRI trunk. A red alarm is declared whenever an out-of-frame condition is present for 2.5 seconds, typically indicating a loss of signal. When the receiver declares a red alarm, the transmitter sends a yellow alarm, also called RAI (Remote Alarm Inidication), back down the trunk to indicate to the originating equipment that it's information is not being received. If the device declaring the red alarm is series type equipment, such as a repeater or CSU, then it will also transmit an AIS alarm (Alarm Indication Signal), also called blue alarm, to the downstream eqiupment to indicate that there is a problem earlier in the trunk.
The AIS alarm consists of unframed all 1's so that downstream equipment can maintain synchronization/timing with the trunk. Note that this will technically result in a red alarm in addition to the AIS at equipment receiving the AIS due to the out-of-frame condition presented by the unframed all 1's. That's why earlier I said a red alarm "typically" indicates a loss of signal. However, a red alarm condition should not be declared in the presence of an AIS. Japanese J1 trunks operate very similarly, but transmit the yellow alarm pattern a little differently. I believe E1 trunks are also similar. ANSI T1.403 and T1.231 define and describe the operation of these alarms. I'm not aware of any requirement for a visual indication of the alarm state (i.e. a red LED for red alarm, yellow LED for yellow alarm, etc), but most devices provide them. Some devices will use a red LED to indicate a loss of frame condition, which can be due to either a red or AIS alarm. Hope this helps! -- Mike > -----Original Message----- > From: [email protected] [mailto:[email protected]] > Sent: Wednesday, December 08, 1999 4:13 AM > To: [email protected] > Subject: Reserve of Red LEDs > > > I've been told that in telecom related equipment the red LED > indicates a > particular alarm. Can someone help me here with more specific > information, such > as industry or international standards which identify the use > of red for some > error condition? (Any such "official" information would be helpful.) > > Many thanks, > > Colin McGeechan, > Product Regulations Specialist. > > TNTD, > Agilent Technologies UK Ltd. >
