severely errored frame (SEF) persists for a period of 3 ms. The LoF defect is terminated when no SEF defects are detected for a period T, when 1 ms<T<3 ms. An SEF defect is the occurrence of four or five contiguous errored frame alignment words. A frame alignment word occupies the A1 and A2 bytes of an STS frame. An SEF defect is terminated when two contiguous error-free frame words are detected. Chapter 3 Line Two rates are considered here: STS-1 Rate and Above Line BIP-N*8 A line BIP-N*8 error event is an anomaly that occurs when the locally computed BIP-N*8 does not agree with the BIP-N*8 sent by the signal source. Line BIP-N*8 is computed over all bits of the previous STS-N(c) except for the first three rows of the section overhead and is placed in the B2 bytes of the current S-N(c) frame before scrambling. Remote error indication-line (REI-L) The occurrence of an REI- L>0 is an anomaly. REI-L is a count of the difference between the computed line BIP-N*8 and the received B2 bytes. The count is placed in the MO byte (STS-1 line signal) or M1 byte (STS-N line signals where N>1) in the opposite direction of transmission. Alarm indication signal-line (AIS-L) An AIS-L defect is the occurrence of line AIS in five contiguous frames. An AIS-L defect is terminated when no line AIS is detected in five contiguous frames. Line AIS is an STS-N(c) signal containing a valid section overhead with bits 6, 7, and 8 of the K2 byte set to 111, and a scrambled all-1s pattern for the remainder of the signal. Remote defect indication-line (RDI-L) An RDI-L defect is the occurrence of an RDI-L signal in x contiguous frames, where x = 5 or 10. An RDI-L defect is terminated when no RDI-L signal is detected in x contiguous frames. An RDI-L signal is 110 in bits 6, 7, and 8 of the K2 byte in STS-1 number 1. It is set by the far-end source when it detects an AIS-L defect. VT1.5 Rate Line BIP-2 A BIP-2 error event is an anomaly that occurs when the locally computed BIP-2 does not agree with the BIP-2 sent by the signal source. Line BIP-2 