XRT86L30
SINGLE T1/E1/J1 FRAMER/LIU COMBO
12.0 ALARMS AND ERROR CONDITIONS
REV. 1.0.1
The XRT86L30 T1/J1/E1 quad Framer can be configured to monitor quality of received DS1 frames. It can
generate error indicators if the local receive framer has received error frames from the remote terminal. If
corresponding interrupt is enabled, the local microprocessor operation is interrupted by these error conditions.
Upon microprocessor interruption, the user can intervene by looking into the error conditions.
At the same time, the user can configure the XRT86L30 framer to transmit alarms and error indications to
remote terminal. Different alarms and error indications will be transmitted depending on the error condition.
The section below gives a brief discussion of the error conditions that can be detected by the XRT86L30
framer and error indications that will be generated.
12.1
AIS Alarm
As we discussed before, transmission of Alarm Indication Signal (AIS) or Blue Alarm by the intermediate node
indicates that the equipment is still functioning but unable to offer services. It is an all ones (except for framing
bits) pattern which can be used by the equipment further down the line to maintain clock recovery and timing
synchronization.
The XRT86L30 framer can detect two types of AIS in DS1 mode:
•
•
Framed AIS
Unframed AIS
Unframed AIS is an all ones pattern. If unframed AIS is sent, the equipment further down the line will be able to
maintain timing synchronization and be able to recover clock from the received AIS signal. However, due to the
lack of framing bits, the equipment farther down the line will not be able to maintain frame synchronization and
will declare Loss of Frame (LOF).
On the other hand, the payload portion of a framed AIS pattern is all ones. However, a framed AIS pattern still
has correct framing bits. Therefore, the equipment further down the line can still maintain frame
synchronization as well as timing synchronization. In this case, no LOF or Red alarm will be declared.
The Alarm indication logic within the Receive Framer block of the XRT86L30 framer monitors the incoming
DS1 frames for AIS. AIS alarm condition are detected and declared according to the following procedure:
1. The incoming DS1 frames are monitored for AIS detection. AIS detection is defined as an unframed or
framed pattern with less than three zeros in two consecutive frames.
2. An AIS detection counter within the Receive Framer block of the XRT86L30 counts the occurrences of AIS
detection over a 6 ms interval. It will indicate a valid AIS flag when twenty-two or more of a possible twenty-
four AIS are detected.
3. Each 6 ms interval with a valid AIS flag increments a flag counter which declares AIS alarm when 255 valid
flags have been collected.
Therefore, AIS condition has to be persisted for 1.53 seconds before AIS alarm condition is declared by the
XRT86L30 framer.
If there is no valid AIS flag over a 6ms interval, the Alarm indication logic will decrement the flag counter. The
AIS alarm is removed when the counter reaches 0. That is, AIS alarm will be removed if over 1.53 seconds,
there is no valid AIS flag.
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EXAR [ EXAR CORPORATION ]
