The STM-1 frame

The STM-1 (synchronous transport module level - 1) frame is the basic transmission format for SDH or the fundamental frame or the first level of the synchronous digital hierarchy. The STM-1 frame is transmitted in exactly 125 microseconds, therefore there are 8000 frames per second on a fiber-optic circuit designated OC-3 (Optical Carrier-3). The STM-1 frame consists of overhead and pointers plus information payload.

The first 9 columns of each frame make up the Section Overhead and Administrative Unit Pointers, and the last 261 columns make up the Information Payload. The pointers (H1, H2, H3 bytes) identify administrative units (AU) within the information payload.

Carried within the information payload, which has its own frame structure of 9 rows and 261 columns, are administrative units identified within the information payload by pointers. Within the administrative unit is one or more virtual containers (VC). VCs contain path overhead and VC payload. The first column is for path overhead; it’s followed by the payload container, which can itself carry other containers. Administrative units can have any phase alignment within the STM frame, and this alignment is indicated by the pointer in row four,

The section overhead of a STM-1 signal (SOH) is divided into two parts: the regenerator section overhead (RSOH) and the multiplex section overhead (MSOH). The overheads contain information from the system itself, which is used for a wide range of management functions, such as monitoring transmission quality, detecting failures, managing alarms, data communication channels, service channels, etc.

September 20, 2011

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