SDH frame structure

 The SDH (Synchronous Digital Hierarchy) frame structure is a standardized format used in telecommunications to transmit digital signals over optical fiber and other media. SDH is widely used in synchronous networks, particularly in Europe and other regions outside North America, where SONET (Synchronous Optical Networking) is more common. The SDH frame structure is designed to support high-speed data transmission and efficient multiplexing of multiple lower-rate signals.


Key Features of SDH Frame Structure:

1. **Synchronous Transmission**: SDH uses a synchronous timing system, meaning all network elements are synchronized to a common clock source.

2. **Hierarchical Multiplexing**: SDH supports multiplexing of lower-rate signals (e.g., E1, E3) into higher-rate signals (e.g., STM-1, STM-4, STM-16).

3. **Standardized Rates**: SDH defines a hierarchy of transmission rates, such as STM-1 (155.52 Mbps), STM-4 (622.08 Mbps), and STM-16 (2.488 Gbps).

4. **Overhead Bytes**: SDH frames include overhead bytes for network management, error detection, and synchronization.


SDH Frame Structure:

The basic SDH frame is called an **STM-1 (Synchronous Transport Module level 1)**. It has a specific structure:


1. **Frame Size**:

   - An STM-1 frame consists of **9 rows** and **270 columns** of bytes.

   - Each byte is 8 bits, so the total frame size is **9 x 270 = 2,430 bytes**.

   - The frame is transmitted at a rate of **8,000 frames per second**, resulting in a bit rate of **155.52 Mbps**.


2. **Frame Composition**:

   - **Section Overhead (SOH)**: The first 9 columns of the frame are dedicated to overhead for network management and synchronization.

     - **Regenerator Section Overhead (RSOH)**: First 3 rows of the SOH.

     - **Multiplex Section Overhead (MSOH)**: Rows 5 to 9 of the SOH.

   - **Payload**: The remaining 261 columns contain the payload, which carries the actual data.

     - The payload includes the **Path Overhead (POH)** and the user data.

     - The POH is used for end-to-end communication and error monitoring.


3. **Virtual Containers (VCs)**:

   - The payload area is organized into **Virtual Containers (VCs)**, which are used to carry lower-rate signals.

   - VCs can be of different sizes, such as VC-12 (for E1 signals) or VC-4 (for E3 signals).

   - VCs are mapped into the payload area and can be multiplexed into higher-rate signals.


4. **Pointer Mechanism**:

   - SDH uses a pointer mechanism to locate the start of the payload within the frame.

   - This allows for flexibility in aligning the payload with the frame structure.



Example of SDH Frame (STM-1):

```

| RSOH (3 rows) | MSOH (5 rows) | Payload (261 columns) |

|---------------|---------------|-----------------------|

| 9 rows x 270 columns = 2,430 bytes |

```


SDH Hierarchy:

SDH defines a hierarchy of transmission rates, starting from STM-1 and scaling up:

- **STM-1**: 155.52 Mbps

- **STM-4**: 622.08 Mbps (4 x STM-1)

- **STM-16**: 2.488 Gbps (16 x STM-1)

- **STM-64**: 9.953 Gbps (64 x STM-1)

- **STM-256**: 39.813 Gbps (256 x STM-1)


Applications of SDH:

- **Telecommunications Networks**: SDH is used in backbone networks for voice, data, and video transmission.

- **Internet Backbone**: SDH provides high-speed connectivity for internet service providers.

- **Mobile Networks**: SDH is used in the transport layer of 3G, 4G, and 5G networks.


In summary, the SDH frame structure is a critical component of modern telecommunications networks, providing a robust and efficient way to transmit high-speed digital signals.

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