SDH STM-1 Frame | Teleinf Edu

Synchronous transport structure

SDH places signal octets in a fixed structure called a frame. The basic frame in this hierarchy is the Synchronous Transport Module, level 1 (STM-1). It has 9 rows and 270 byte columns, repeats every 125 microseconds, and provides a hierarchical bit rate of 155.520 Mbps. The layout identifies the section overhead, the AU pointer, and the information area.

The information area corresponds to AUG-1. AUG-1 can contain one AU-4 or three AU-3s. The VC associated with an AU has no fixed phase relative to the STM frame, so the AU pointer identifies the first VC byte. In byte-oriented transmission, the most significant bit is sent first.

STM-1 frame structure

The STM-1 frame is a 9-by-270 byte matrix that is transmitted serially. Transmission proceeds from left to right and then top to bottom, following the byte order defined for STM-N.

Core parameters

Dimensions: The frame is a grid of 9 rows and 270 columns. Since each cell in the grid holds one byte, the total size of the frame is $9 \times 270 = 2430 \text{ bytes}$ .
Duration: A new STM-1 frame is transmitted every $125$ microseconds ( $125 \mu \text{s}$ ). This period corresponds to 8000 frames per second and also follows the sampling period of a standard voice channel ( $1 / 8000 \text{ Hz} = 125 \mu \text{s}$ ).
Bit Rate: Based on its size and duration, the bit rate of the STM-1 signal is calculated as:
$\text{Bit Rate} = 2430 \frac{\text{bytes}}{\text{frame}} \times 8 \frac{\text{bits}}{\text{byte}} \times 8000 \frac{\text{frames}}{\text{second}} = 155,520,000 \frac{\text{bits}}{\text{second}} = 155.52 \text{ Mbps}$

STM-1 frame areas

The STM-1 structure contains 72 Section Overhead bytes, a separate 9-byte AU pointer row, and the information area. The first 9 columns hold SOH bytes in rows 1-3 and 5-9; row 4 holds the AU-4 pointer. The remaining 261 columns form the area associated with the AUG and VC-4.

1. Section Overhead (SOH)

The first 9 columns of the frame contain 81 bytes of auxiliary fields ( $9 \times 9 = 81 \text{ bytes}$ ). SOH occupies 72 bytes in rows 1-3 and 5-9, while the 9 bytes in row 4 carry the AU pointer. The SOH carries framing, error-monitoring, management, and protection fields. The first three rows form the RSOH, while rows 5-9 form the MSOH.

2. Payload Area / Administrative Unit Group (AUG)

This is the remaining 261 columns of the frame ( $9 \times 261 = 2349 \text{ bytes}$ ). This is the useful capacity of the frame, designed to carry the actual data traffic, such as multiplexed voice calls or internet data packets. The content of this area is called the .

Section Overhead (SOH) breakdown

The first 9 columns contain 72 SOH bytes and a separate 9-byte AU pointer row. RSOH occupies rows 1-3 and MSOH occupies rows 5-9. RSOH is processed on the section between regenerators, while MSOH serves the multiplex section.

RSOH and MSOH have different processing boundaries. RSOH is regenerated on successive regenerator sections, while MSOH follows the multiplex section. The AU pointer row remains a separate frame area.

Regenerator Section Overhead (RSOH), rows 1-3

RSOH is processed between adjacent . The regenerator section can therefore monitor each physical hop before forwarding the signal.

A1, A2 (Framing Bytes): These are special, fixed patterns used by the receiver to find the beginning of each STM-1 frame in the continuous stream of bits.
J0 (Regenerator Section Trace): A byte used to send a repeating message to continuously verify the connection between regenerators.
B1 (BIP-8): A parity check byte used for error monitoring. It allows the network to detect transmission errors occurring between regenerators.
E1, F1 (Orderwire & User Channels): Voice channels reserved for maintenance personnel to communicate along the link.
D1-D3 (Data Communication Channel): A low-speed data channel (192 kbps) for remote management of regenerator equipment.

Multiplex Section Overhead (MSOH), rows 5-9

MSOH is used for communication between adjacent . It manages the entire multiplex section.

AU-4 Pointer (H1, H2, H3, in row 4): The pointer value identifies the starting position of VC-4 in the information area. It allows the VC phase to change relative to the STM-1 frame.
B2 (BIP-24): A parity field used to monitor errors in the multiplex section.
K1, K2 (Automatic Protection Switching, APS): These bytes carry the information used to coordinate a switch to the protection path.
D4-D12 (Data Communication Channel): A 576 kbps data channel for more advanced network management between multiplexers.
S1 (Synchronization Status Message, SSM): Carries information about the quality of the synchronization source used by the equipment.
M1 (Remote Error Indication, REI): Reports to the remote equipment that errors were detected in the multiplex section.