The STM-1 (Synchronous Transport Module level-1) is the SDH ITU-T fiber optic network transmission standard. It has a bit rate of 155.52 Mbit/s. Higher levels go up by a factor of 4 at a time: the other currently supported levels are STM-4, STM-16, STM-64 and STM-256. Above STM-256 wavelength-division multiplexing (WDM) is commonly used in submarine cabling. Frame structure The STM-1 frame is on the basic transmission format for SDH (Synchronous Digital Hierarchy). An STM-1 frame has a byte-oriented structure with 9 rows and 270 columns of bytes, for a total of 2,430 bytes (9 rows * 270 columns = 2430 bytes). Each byte corresponds to a 64kbit/s channel. TOH: Transport Overhead ( + + ) : Multiplex Section Overhead : Regeneration Section Overhead : AU-4 Pointers VC4: Virtual Container-4 payload ( + ) : Path Overhead Frame characteristic The STM-1 base frame is structured with the following characteristics: Length: 270 column × 9 row = 2430 bytes Byte: 1-byte = 8 bit Duration (Frame repetition time): 125 μs i.e. 8000 frame/s Rate (Frame capacity): 2430 × 8 × 8000 = 155.52 Mbit/s Payload = 2349 bytes × 8 bits × 8000 frames/sec = 150.336 Mbit/s RSOH (regenerator section overhead) 1st row = Unscrambled bytes. Their contents should therefore be monitored X = Bytes reserved for national use D = Bytes depending on the medium (satellite, radio relay system, ...) The Regenerator Section OverHead uses the first three rows & nine columns in the STM-1 frame A1, A2 The Frame Alignment Word is used to recognize the beginning of an STM-N frame A1: 1111 0110 = F6 (HEX) A2: 0010 1000 = 28 (HEX) J0: Path Trace. It is used to give a path through an SDH Network a "Name". This message (Name) enables the receiver to check the continuity of its connection with the desired transmitter B1: Bit Error Monitoring. The B1 Byte contains the result of the parity check of the previous STM frame, after scrambling of the actual STM frame. This check is carried out with a Bit Interleaved Parity check (BIP-8). E1 Engineering Orderwire (EOW). It can be used to transmit speech signals between Regenerator Sections for operating and maintenance purposes F1 User Channel. It is used to transmit data and speech for service and maintenance D1 to D3 Data Communication Channel at 192 kbit/s (DCCR). This channel is used to transmit management information via the