STM frame structure
SDH hierarchy has STM-0, 1, 4 , 16, 64, 128. STM-128 and the higher levels of hierarchy are not used and preferred because of the toughness of implementation and higher cost.
SDH- Synchronous Digital Hierarchy
STM - Synchronous Transport Module
In this post I will explain you the STM-0 frame structure with which every other frame structure can be derived easily.
As we have already discussed STM-0 frame will have 90 x 9 bytes of information which includes data and control information.
Th above figure gives the rough view of STM-0 frame structure with 90 columns and 9 rows. Here, I have written the names of bytes for the columns 1, 2 and 3. These three columns contain the overhead bytes in it. In short, the first three columns (excluding the third row) of STM-0 frame is called overhead section.
This overhead section is divided into two namely - Regenerated section overhead and Multiplexed section overhead.
RSOH ( Regenerator Section Overhead )
This section includes 9 bytes namely A1, A2, J0, B1, E1, F1, D1, D2 and D3. (The bytes which intersect the first three rows and first three columns).
This section has got its name from its function. The main aim of this section is to help an intermediate node (the pass through node through which the frame goes to reach the destination) receiving this frame pass through the frame to the other node without reading it completely.
This section helps in identification of the frame, checking for errors and boosting or regenerating the entire frame structure for travelling long distances without noise issues.
MSOH( Multiplexed Section Overhead)
This section includes 21 bytes namely B2, K1, K2, D4 - D12, S1, M1 and E1. (The bytes which intersect the last 4 rows and first 3 columns).
The bytes present in the section are used in higher level of STM-0 from 1, 2,..64, 128. And more than that, they have some more functions like synchronisation.
Multiplexed section is read only by the node to which the frame is destined to. This section is for extracting the lower order VCs (Virtual Connection) and TUs (Tributary Unit) from STM frame using the MSOH bytes.
To conclude,the destination node of a frame processes both RSOH and MSOH whereas the intermediate node processes only the RSOH.
Pointer
The bytes H1, H2 and H3 are used for pointing the address where the data starts in the frame. The pointer range is from 0 to 782.
The pointer address 0 starts at 4th column of 1st row (after J0). The first row will have address from 0 to 87 (4th to 90th column of 1st row). The address 88 will start from 4th column of 2nd row and so on. In short, there are 87 columns and 9 rows of data which resulted in 783 addresses (0 to 782 ) in STM frame.
Payload
Payload section refers to the bytes to which contain the data (i.e.) all 9 rows of columns 4 to 90. In other terms, the addresses of pointer 0 to 782 is the payload area of an STM frame.
In the next post, we will see the function of every single byte in the overhead region and pointer bytes.
SDH- Synchronous Digital Hierarchy
STM - Synchronous Transport Module
In this post I will explain you the STM-0 frame structure with which every other frame structure can be derived easily.
As we have already discussed STM-0 frame will have 90 x 9 bytes of information which includes data and control information.
STM-0 frame structure
Th above figure gives the rough view of STM-0 frame structure with 90 columns and 9 rows. Here, I have written the names of bytes for the columns 1, 2 and 3. These three columns contain the overhead bytes in it. In short, the first three columns (excluding the third row) of STM-0 frame is called overhead section.
This overhead section is divided into two namely - Regenerated section overhead and Multiplexed section overhead.
RSOH ( Regenerator Section Overhead )
This section includes 9 bytes namely A1, A2, J0, B1, E1, F1, D1, D2 and D3. (The bytes which intersect the first three rows and first three columns).
This section has got its name from its function. The main aim of this section is to help an intermediate node (the pass through node through which the frame goes to reach the destination) receiving this frame pass through the frame to the other node without reading it completely.
This section helps in identification of the frame, checking for errors and boosting or regenerating the entire frame structure for travelling long distances without noise issues.
MSOH( Multiplexed Section Overhead)
This section includes 21 bytes namely B2, K1, K2, D4 - D12, S1, M1 and E1. (The bytes which intersect the last 4 rows and first 3 columns).
The bytes present in the section are used in higher level of STM-0 from 1, 2,..64, 128. And more than that, they have some more functions like synchronisation.
Multiplexed section is read only by the node to which the frame is destined to. This section is for extracting the lower order VCs (Virtual Connection) and TUs (Tributary Unit) from STM frame using the MSOH bytes.
To conclude,the destination node of a frame processes both RSOH and MSOH whereas the intermediate node processes only the RSOH.
Pointer
The bytes H1, H2 and H3 are used for pointing the address where the data starts in the frame. The pointer range is from 0 to 782.
The pointer address 0 starts at 4th column of 1st row (after J0). The first row will have address from 0 to 87 (4th to 90th column of 1st row). The address 88 will start from 4th column of 2nd row and so on. In short, there are 87 columns and 9 rows of data which resulted in 783 addresses (0 to 782 ) in STM frame.
Payload
Payload section refers to the bytes to which contain the data (i.e.) all 9 rows of columns 4 to 90. In other terms, the addresses of pointer 0 to 782 is the payload area of an STM frame.
In the next post, we will see the function of every single byte in the overhead region and pointer bytes.
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