The Blueprint of Synchronous Transport

At the heart of the SDH/SONET revolution is a highly organized and predictable data structure known as the frame. The fundamental frame in the SDH hierarchy is the Synchronous Transport Module, level 1 (STM-1). Think of it as a standardized, digital cargo container that is dispatched with clockwork precision, carrying various types of data across the optical network. Its structure is the key to solving the main problems of the older PDH systems, such as difficult channel access and limited management capabilities.

Anatomy of the STM-1 Frame

The STM-1 frame is best visualized as a two-dimensional block of bytes, which is then transmitted serially, byte by byte. This structure is precisely defined to ensure that every device in the network knows exactly where to find information.

Core Specifications

• 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 exactly every $125$ microseconds ($125 \mu \text{s}$). This precise timing is fundamental to the "synchronous" nature of SDH. It's derived directly from the sampling rate 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:
  $\text{Bit Rate} = 2430 \frac{\text{bytes}}{\text{frame}} \times 8 \frac{\text{bits}}{\text{byte}} \times 8000 \frac{\text{frames}}{\text{second}} = 155,520,000 \frac{\text{bits}}{\text{second}} = 155.52 \text{ Mbps}$

Main Sections of the Frame

The 270 columns of the STM-1 frame are divided into two main, functionally distinct areas: the transport overhead, which acts as the frame's "control panel," and the payload, which is the "cargo hold" for the actual data.

Detailed Look: The Section Overhead (SOH)

The 81-byte Section Overhead is so important that it is further divided into two sub-layers, each processed by different types of network equipment. This layered approach to overhead is a hallmark of SDH's robust design. The first 3 rows of the SOH are the Regenerator SOH, row 4 contains the vital AU Pointer, and rows 5-9 make up the Multiplex Section Overhead.

Regenerator Section Overhead (RSOH) - Rows 1-3

RSOH is used for communication between adjacent regenerators. It ensures the integrity of the signal on a hop-by-hop physical link.

• 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 4-9

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

• AU-4 Pointer (H1, H2, H3 - in Row 4): This is arguably the most important part of the overhead. These 9 bytes point to the exact starting location of the payload (the Virtual Container) within the frame. This allows the payload to "float", which is the key mechanism for handling timing variations.
• B2 (BIP-N*24): Similar to B1, but this is a more extensive parity check for error monitoring across the entire link between multiplexers.
• K1, K2 (Automatic Protection Switching - APS): Bytes used to coordinate the rapid switching to a backup fiber line in case the primary one fails, ensuring high network reliability.
• D4-D12 (Data Communication Channel): A 576 kbps data channel for more advanced network management between multiplexers.
• S1 (Synchronization Status Message - SSM): A critical byte that carries information about the quality level of the timing source used by the equipment. This allows the entire network to synchronize to the best available clock.
• M1 (Remote Error Indication - REI): Allows a multiplexer to report back to the far-end multiplexer that it has detected errors (using the B2 byte), providing end-to-end link performance information.