ame is present in the current VC-4. User communication channel (F2 byte): The F2 byte supports a user channel that enables proprietary network operator communications between path terminating equipment. Appendix A: SDH/SONET Signaling A.5 Alarm Signals Alarm and performance information is included as part of the POH. Loss of signal (LOS): LOS state is entered when received signal level drops below the value at which an error ratio of 10-3 is predicted. LOS state ends when two consecutive valid framing patterns are received, pro- vided that during this time no new LOS condition has been detected. Out of frame (OOF): OOF state is entered when four or five consecutive SDH frames are received with invalid (errored) framing patterns. Maximum OOF detection time is therefore 625 us. Loss of frame (LOF): LOF state is entered when OOF state exists for up to 3ms. If OOFs are intermittent, the timer is not reset to zero until an in-frame state persists continuously for 0.25 ms. LOF state is exited when an in-frame state exists continuously for 1-3 ms. Loss of pointer (LOP): LOP state is entered when N consecutive invalid pointers are received where N = 8, 9, or 10. LOP state ends when three equal valid pointers or three consecutive AIS indica- tions are received. A.6 Multiplexer Section AIS Multiplexer section AIS is sent by RSTE to alert downstream MSTE (multiple section ter- minal equipment) of detected LOS or LOF state. It is indicated by STM signal containing valid RSOH and a scrambled "all 1s" pattern in the rest of the frame. Multiplexer section AIS is detected by MSTE when bits 6-8 of the received K2 byte are set to "111" for three consecutive frames. Its removal is detected by MSTE when three con- secutive frames are received with a pattern other than "111" in bits 6-8 of K2. Far end receive failure (FERF or MS-FERF): FERF is sent upstream by MSTE within 250 us of detecting LOS, LOF, or MS-AIS on incoming signal. Optionally, it is transmitted upon detection of excessive bit error rate (BER) defect (equivalent BER, b