STM-1

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. The other levels are STM-4, STM-16 and STM-64. Beyond this we have wavelength-division multiplexing (WDM) commonly used in submarine cabling.

Frame structure

The STM-1 frame is the basic transmission format for SDH. A STM-1 signal has a byte-oriented structure with 9 rows and 270 columns of bytes with a total of 2430 bytes (9 rows * 270 columns = 2430 bytes). Each byte corresponds to a 64kbit/s channel.

File:STM 1 frame structure.GIF

The frame consists of two parts, the transport overhead and the path virtual envelope (virtual container with a(VC-4) capacity).

Transport overhead

• RSOH: Regenerator Section OverHead.
• MSOH: Multiplex Section OverHead.

Frame characteristics

The STM-1 base frame is structured with the following characteristics:
- Length : 270 column x 9 row = 2430 bytes
- Duration (Frame repetition time): 125 μs i.e. 8000 frames/s
- Rate (Frame capacity): : 2430 x 8 x 8000 = 155.520 Mbit/s
- Payload = 2340 bytes i.e. 149.760 Mbit/s

1 byte i.e. 64 kbit/s (e.g. speech channel)

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, before 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 beyond a Regenerator Section 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 STM-N frames

MSOH (multiplex section overhead)

X = Bytes reserved for national use.

The Multiplex Section OverHead uses rows from 6-9 & columns from 1-9 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) Synchronization 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 synchronization

- E2 Engineering Orderwire (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 bytescvbnm,./dfhjkl;lkjhgfdfghk;lkjhgfklkjhgfddhjkjhgfd