Editing Digital Signal 1 (section)

===Alarms===
Alarms are normally produced by the receiving terminal equipment when the framing is compromised. There are three defined [[alarm indication signal]] states, identified by a legacy color scheme: red, yellow and blue.

'''Red alarm''' indicates the alarming equipment is unable to recover the framing reliably. Corruption or loss of the signal will produce "red alarm". Connectivity has been lost toward the alarming equipment. There is no knowledge of connectivity toward the far end.

'''Yellow alarm''', also known as remote alarm indication (RAI), indicates reception of a data or framing pattern that reports the far end is in "red alarm". The alarm is carried differently in SF (D4) and ESF (D5) framing. For SF framed signals, the user bandwidth is manipulated and "bit two in every DS0 channel shall be a zero."<ref name="yel">American National Standards Institute, ''T1.403-1999'', ''Network and Customer Installation Interfaces – DS1 Electrical Interface'', p. 12</ref> The resulting loss of payload data while transmitting a yellow alarm is undesirable, and was resolved in ESF framed signals by using the [[data link layer]]. "A repeating 16-bit pattern consisting of eight 'ones' followed by eight 'zeros' shall be transmitted continuously on the ESF data link, but may be interrupted for a period not to exceed 100-ms per interruption."<ref name="yel"/> Both types of alarms are transmitted for the duration of the alarm condition, but for at least one second.

'''Blue alarm''', also known as alarm indication signal (AIS) indicates a disruption in the communication path between the terminal equipment and line repeaters or [[Digital cross-connect system|DCS]]. If no signal is received by the intermediary equipment, it produces an unframed, all-ones signal. The receiving equipment displays a "red alarm" and sends the signal for "yellow alarm" to the far end because it has no framing, but at intermediary interfaces the equipment will report "AIS" or [[Alarm indication signal|Alarm Indication Signal]]. AIS is also called "all ones" because of the data and framing pattern.

These alarm states are also lumped under the term Carrier Group Alarm (CGA). The meaning of CGA is that connectivity on the digital carrier has failed. The result of the CGA condition varies depending on the equipment function. Voice equipment typically coerces the [[robbed-bit signaling|robbed bits for signaling]] to a state that will result in the far end properly handling the condition, while applying an often different state to the customer equipment connected to the alarmed equipment. Simultaneously, the customer data is often coerced to a 0x7F pattern, signifying a zero-voltage condition on voice equipment. Data equipment usually passes whatever data may be present, if any, leaving it to the customer equipment to deal with the condition.