XRT86L30
SINGLE T1/E1/J1 FRAMER/LIU COMBO
REV. 1.0.1
12.6
T1 Brief discussion of alarms and error conditions
As defined in ANSI T1.231 specification, alarm conditions are created from defects. Defects are momentary
impairments present on the DS1 trunk. If a defect is present for a sufficient amount of time (called the
integration time), then the defect becomes an alarm. Once an alarm is declared, the alarm is present until after
the defect clears for a specified period of time. The time it takes to clear an alarm is called the de-integration
time.
Alarms are used to detect and warn maintenance personnel of problems on the DS1 trunk. There are three
types of alarms:
•
•
•
Red alarm or Service Alarm Indication (SAI) Signal
Blue alarm or Alarm Indication Signal (AIS)
Yellow alarm or Remote Alarm Indication (RAI) Signal
A simple DS1 system model is shown in Figure 103 to explain the error conditions and generation of different
alarms, let us create. In this model, a DS1 signal is sourced from the Central Office (CO) through a Repeater to
the Customer Premises Equipment (CPE). At the same time, a DS1 signal is routed from the CPE to the
Repeater and back to the Central Office.
FIGURE 103. SIMPLE DIAGRAM OF DS1 SYSTEM MODEL
CO
Repeater
CPE
DS1
Transmit
Section
DS1
Receive
Section
DS1 Receive
Framer Block
DS1 Receive
Framer Block
DS1
Receive
Section
DS1
Transmit
Section
DS1 Transmit
Framer Block
DS1 Transmit
Framer Block
Simple DS1 System Model
When the DS1 system runs normally, i.e., when there is no Loss of Signal (LOS) or Loss of Frame (LOF)
detected in the line, no alarm will be generated. Sometimes, intermittent outburst of electrical noises on the line
might result in Bipolar Violation or bit errors in the incoming signals, but these errors in general will not trigger
the equipment to generate alarms. They will at most trigger the framer to generate interrupts which would
cause the local microprocessor to interrupt as well as add statistics in the performance monitoring accumulator
registers.
Now, consider a case in which the DS1 line from the Repeater to CPE is broken or interrupted, resulting in a
complete loss of incoming data or a severely impaired signal quality. Upon detection of Loss of Signal (LOS) or
Loss of Frame (LOF) condition, the CPE will generate an internal Red Alarm, also known as the Service Alarm
Indication. This alarm will normally trigger a microprocessor interrupt informing the user that an incoming signal
failure is happening.
When the CPE is in the Red Alarm state, it will transmit the Yellow Alarm to the Repeater indicating the loss of
an incoming signal or loss of frame synchronization. This Yellow Alarm informs the Repeater that there is a
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