The STM-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.
Beyond this we have wavelength-division multiplexing commonly used in submarine cabling.
Frame structure The STM-1 frame is the basic transmission
format for SDH.
A STM-1 frame has a byte-oriented structure with 9 rows and 270 columns of bytes, for
a total of 2,430 bytes.
Each byte corresponds to a 64kbit/s channel.
TOH: Transport Overhead MSOH: Multiplex Section Overhead
RSOH: Regeneration Section Overhead AU4P: AU-4 Pointers
VC4: Virtual Container-4 payload POH: Path Overhead
Frame characteristics The STM-1 base frame is structured with the
following characteristics: Length: 270 column × 9 row = 2430 bytes
Byte: 1-byte = 64kbit/s speech channel Duration: 125 μs i.e. 8000 frame/s
Rate: 2430 × 8 × 8000 = 155.520 Mbit/s Payload = 2349bytes × 8bits × 8000frames/sec
= 150.336 Mbit/s RSOH
1st row = Unscrambled bytes.
Their contents should therefore be monitored X = Bytes reserved for national use
D = Bytes depending on the medium 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 A2: 0010 1000 = 28
J0: Path Trace.
It is used to give a path through an SDH Network a "Name".
This message 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.
E1 Engineering Orderwire.
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.
This channel is used to transmit management information via the STM-N frames
MSOH
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
K1, K2 Automatic Protection Switching.
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 protocol
K2 MS_RDI: Multiplex Section Remote Defect Indication
D4 to D12 Data Communication Channel at 576 kbit/s.
S1 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.
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.
Z1, Z2 Spare bytes References