d frames) These frames' framing bits are used to store a six-bit CRC value for each superframe. Any time a T1 circuit is ordered for use as a WAN link for data networks, the T1 should be provisioned with B8ZS encoding and ESF framing. Performance Monitoring CSU/DSUs report on 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 confus- ing 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. Chapter 20: T1 Different vendors often define performance events differently, and finding detailed descriptions of these events can be challenging. One place where the event types are outlined is in RFC 1232, titled "Definitions of Managed Objects for the DS1 Inter- face 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 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) Loss of signal 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 that the line is dead. There are no alarms, no signals, etc. LOS is equivalent to having no cable in the T1 jack. Out of Frame (00F) An out-of-frame condition (also called loss of frame, or LOF) indicates that a certain number of frames have been received with framing bits in error. In this case, the data cannot be trusted because the synchronization between the two sides of the T1 is invalid. Excessive OOF errors will trigger a red alarm. An OOF condition is described in RFC 1232 as foll