1. M1 Byte Multiplex Section Remote Error Indication (MS-REI) byte: M1 è A return message from Rx to Tx when Rx find MS-BBE è By evaluating the 3xB2, the M1 byte can report back the number of parity code violations. è MS-REI will be generated in Tx. è M1 byte is one per STM-N frame. Find B2 Error: MS-BBE Rx Tx Traffic Return M1 Generate MS-REI

2. User Channel Bytes: F1, F2 è Provide a 64 kb/s data or voice channel for local maintenance purpose to network operator. è Only transmitted in STM-1 #1 of STM-N signal.

3. D1~D12 Bytes Data Communication Channel Bytes: D1~D12 è These 12 bytes are provided for the transport of monitoring & control data in Network Management System. è D1-D3 belongs to RSOH, bandwidth is 3x64 kb/s è D4-D12 belongs to MSOH, bandwidth is 9x64 kb/s è D1-D12 are transmitted in STM-1#1 of STM-N only. OAM Massages: performance, alarm, operation commands etc. DCC Channel NMS

4. Order Wire Bytes: E1 & E2 è Provide 64 kb/s digital telephone channels è E1 transmit RS order wire message è E2 transmit MS order wire message (express channel) è Only present in STM-1#1 of STM-N

5. K1 & K2 Bytes Automatic Protection Switching (APS) bytes: K1, K2 (bits:b1-b5) è Used for network multiplex protection switch function è K1 & K2 only transmitted in STM-1 #1 of STM-N è Multiplex Section Remote Defect Indication (MS-RDI): K2 (b6-b8) –Return alarm message from Rx to Tx –Indicate Rx receiving alarm –K2 (b6-b8) value is 110

6. K2 Detection Sending back MS-RDI Giving MS-AIS Found K2(b6-b8) Producing MS-RDI 111 110

7. SDHEquipmentSenderSDHEquipmentReceiver STM-N Find K2(111) produce: MS-AIS alarm event Sending back K2 Sending back K2(110) Receive K2(110) produce: MS-RDI alarm event