STM-N frames MSOH (multiplex section overhead) X = Bytes reserved for national use. The Multiplex Section OverHead uses the 5th through 9th rows, and first 9 columns in the STM-1 frame. B2 : Bit Error Monitoring. The B2 Bytes contains the result of the parity check of the previous STM frame, except the RSOH, before scrambling of the actual STM frame. This check is carried out with a Bit Interleaved Parity check (BIP24) K1, K2 Automatic Protection Switching (APS). In case of a failure, the STM frames can be routed new with the help of the K1, K2 Bytes through the SDH Network. Assigned to the multiplexing section protection (MSP) protocol K2 (Bit6,7,8) MS_RDI: Multiplex Section Remote Defect Indication (former MS_FERF: Multiplex Section Far End Receive Failure) D4 to D12 Data Communication Channel at 576 kbit/s (DCCM). (See also D1-D3 in RSOH above) S1 (Bit 5 - 8) Synchronisation quality level: 0000 Quality unknown 0010 G.811 10-11/day frequency drift 0100 G.812T transit 10-9 /day frequency drift 1000 G.812L local 2*10-8/day frequency drift 1011 G.813 5*10-7/day frequency drift 1111 Not to be used for synchronisation E2 Engineering Order wire (EOW). Same function as E1 in RSOH M1 MS_REI: Multiplex Section Remote Error Indicator, number of interleaved bits which have been detected to be erroneous in the received B2 bytes. (former MS_FEBE: Multiplexing Section Far End Block Errored) Z1, Z2 Spare bytes References Synchronous optical networking de:Synchrone Digitale Hierarchie#SDH Hierarchiestufen