CSU/DSUs convey the status of T1 links by reporting the incidence of a set of standard events. Some of the events and the alarms they trigger can be a bit confusing to data professionals who have not been exposed to the technology before. To make matters worse, most CSU/DSUs report errors using not words, but rather the well-known (to telecom engineers) abbreviations of the errors.
Different vendors often define performance events differently, and finding detailed descriptions of these events can be challenging. The event types are outlined in RFC 1232, titled “Definitions of Managed Objects for the DS1 Interface Type.” This RFC defines the standards for use in SNMP traps and does not describe the electrical properties of these alarms. These descriptions are not binding for manufacturers, and thus may not be accurate for any given device. Still, the RFC does contain some of the clearest descriptions of these events.
Loss of signal (LOS) is the state where no electrical pulses have been detected in a preset amount of time. RFC 1232 describes LOS as:
This event is declared upon observing 175 +/− 75 contiguous pulse positions with no pulses of either positive or negative polarity (also called keep alive).
In English, that means the line is dead. There are no alarms, no signals, etc. LOS is equivalent to having no cable in the T1 jack.
An out-of-frame (OOF) condition (also called loss of frame, or LOF) indicates that a certain number of frames ...