Skip to main content

WHAT IS STM-N?

The basic transmission format of DH is the STM-1 frame. The frame duration is 125 μs and there are 8000 frames per second. The STM-1 frame consists of an overhead and a payload.

Each frame is 270 bytes long. Each frame consists of 9 lines. The first nine columns in each frame are the section header SOH (SectionOverhead). The next 261 columns are the payload. The STM frame is transmitted line by line, with bytes in serial order.

In the SOH part, the first three lines are Regenerator Section Overhead (RSOH) and the last 5 lines are Multiplex Section Overhead (MSOH) bytes. The 10th column in the frame is the Path Overhead (POH).

VCs (Virtual Containers) are packets that are independent of the SDH payload and carry signals at a lower rate level. The location of VCs is indicated by pointers.

STM-1 HIERARCHY

At the lowest level the containers are input to the VCs. The purpose of this is to create a uniform VC payload by bit stuffing to bring all inputs to an overall bit rate ready for synchronous multiplexing. The various containers, ranging from VC-12 at 2048 Mbps to VC-4 at 150.336 Mbps, are organized in the SDH hierarchy. The next VCs are placed in sub-units where the indication operations are located. These initial functions allow the multiplexing of the payload into TUgroups (TUGs). The next step is the multiplexing of TUGs into higher level VCs. TUG-2 and TUG-3 are multiplexed into VC-3 and VC-4. VCs are multiplexed to administrative units (AUs) with fixed byte padding. AUs are multiplexed to AUGs. This load is then multiplexed to the STM.

TUs are grouped so that different types of TUs can coexist mixed. A VC-4 carrying lower level units is divided into 3 TUG-3s, each containing 7 TUG-2s or a single TU-3. There can be a mix of different TU groups. For example, the first TUG-3 contains 12 TU-12s and 3 TU-2s, making a total of 7 TUG-2 groups. TU groups do not have headings or indicators, they are just a multiplexing and organization of different TUs in VC-1 of an STM-4. The columns in a TU group are not consecutive in the VC. Compared to other TU-groups, they are sorted column by column by byte-interleaved column by column. If the TUG-2 structure is carried, the NPI (Null pointer indicator) is used rather than a TU-3 combined with the TU-3 indicator.

The first TUG-2 group in a TUG-3, called group-1, is located every 7th column, starting from column 3, skipping columns 1 and 2 of TUG-3. The columns of the lower level units48 within a group are not placed in consecutive columns. The columns of the TUs in the TU group are also byte-interleaved. TUs are optimized in different sizes to accommodate different signals. Each size of a TU is known as the type of TU. A 36-byte structure, or 4 columns with 9 rows, can accommodate a 2.048 Mbps signal. This particular TU is simply designed as a TU-12. In this case the 4 columns, together with the bass, provide a speed of 2,048 mbps. Other signals require different sized TUs. The numbers of each column of the different low order TU types are factors of 12.

AS A RESULT, A TU GROUP MAY CONTAIN ONE OF THE FOLLOWING COMBINATIONS:

  • 3 TU-12 (each with 4 columns).
  • 1 TU-2 (each with 12 columns).