Knowledge Base

ISDN alarms and what they mean.

AIS (Alarm Indication Signal) CFA
The AIS is also known as a “Keep Alive” or “Blue Alarm” signal. This consists of an UNFRAMED all-ones signal sent to maintain transmission continuity. The AIS CFA signal is declared when both the AIS state and RED CFA persist simultaneously.

OOF (Out-Of-Frame) Condition

Occurs whenever Network or DTE equipment senses errors in the incoming framing pattern. Depending upon the equipment, this can occur when 2 of 4, 2 of 5, or 3 of 5 framing bits are in error. A reframe clears the OOF condition.
Red CFA (Carrier Failure Alarm)
Occurs after detection of CONTINUOUS OOF condition for 2.5 seconds. This alarm state is cleared when no OOF conditions occur for AT LEAST 1 second. Some applications (AT&T DACS services) may not clear the CFA state for UP TO 15 seconds of NO Out-Of-Frame occurrences.

Yellow CFA (Carrier Failure Alarm)

When a Terminal/Network equipment enters a RED CFA state, it transmits a “Yellow Alarm” in the opposite direction.
A Yellow Alarm is transmitted by setting Bit #2 of each timeslot to a 0 (zero), SPACE state for D4 Framed facilities. For ESF facilities, a Yellow Alarm is transmitted by sending a repetitive 16-bit pattern consisting of 8 MARKS (1) followed by 8 SPACES (0) in the Datalink bits. This is transmitted for a MINIMUM of 1 second.
LOS (Loss Of Signal)
A LOS condition is declared when no pulses have been detected in a 175 +/- 75 pulse window (100 to 250 bit times).

Notes taken from Digiums Realease notes

Alarm Types

An alarm indicates that a port is not available for some reason. Thus it is probably not a good idea to try to call out through it.

Red Alarm

Your T1/E1 port will go into red alarm when it cannot maintain synchronization with the remote switch. A red alarm typically indicates either a physical wiring problem, loss of connectivity, or a framing and/or line-coding mismatch with the remote switch.

When your T1/E1 port loses sync, it will transmit a yellow alarm to the remote switch to indicate that it’s having a problem receiving signal from the remote switch.

The easy way to remember this is that the R in red stands for “right here” and “receive”… indicating that we’re having a problem right here receiving the signal from the remote switch.

Yellow Alarm

(RAI — Remote Alarm Indication)

Your T1/E1 port will go into yellow alarm when it receives a signal from the remote switch that the port on that remote switch is in red alarm. This essentially means that the remote switch is not able to
maintain sync with you, or is not receiving your transmission.

The easy way to remember this is that the Y in yellow stands for “yonder”… indicating that the remote switch (over yonder) isn’t able to see what you’re sending.

Blue Alarm

(AIS — Alarm Indication Signal)

Your T1/E1 port will go into blue alarm when it receives all unframed 1s on all timeslots from the remote switch. This is a special signal to indicate that the remote switch is having problems with its
upstream connection. dahdi_tool and Asterisk don’t correctly indicate a blue alarm at this time. The easy way to remember this is that streams are blue, so a blue alarm indicates a problem upstream from
the switch you’re connected to.