The STM-1 is the SDH ITU-T

ITU-T

The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....

 fiber optic network

Telecommunications network

A telecommunications network is a collection of terminals, links and nodes which connect together to enable telecommunication between users of the terminals. Networks may use circuit switching or message switching. Each terminal in the network must have a unique address so messages or connections...

 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-4

The STM-4 is a SDH ITU-T fiber optic network transmission standard. It has a bit rate of 622.080 Mbit/s....

, STM-16, STM-64 and STM-256. Beyond this we have wavelength-division multiplexing

Wavelength-division multiplexing

In fiber-optic communications, wavelength-division multiplexing is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths of laser light...

 (WDM) commonly used in submarine cabling.

Frame structure

The STM-1 frame is the basic transmission format for SDH (Synchronous Digital Hierarchy). A 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 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 frame/s
• Rate (Frame capacity): 2430 x 8 x 8000 = 155.520 Mbit/s
• Payload = 2349bytes x 8bits x 8000frames/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, before scrambling of the actual STM frame. This check is carried out with a Bit Interleaved Parity check (BIP-8
  BIP-8
  BIP-8 is an abbreviation for bit-interleaved parity 8. BIP-8 consists of a parity byte calculated bit-wise across a large number of bytes in a transmission transport frame. BIP-8 bits are set such that the overall data stream, including the BIP-8 byte, has even parity...
  
  ).
• 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 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) Synchronization quality level:
  • 0000 Quality unknown
  • 0010 G.811
    G.811
    G.811 is a recommendation from the ITU Telecommunication Standardization Sector . It specifies requirements for primary reference clock devices in synchronization networks....
    
    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 bytes