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Synchronous Transport Module level-1 (STM-1) is a fiber optic network standard for synchronous digital hierarchy. The transmission format is considered the fundamental building block for synchronous digital hierarchies. Apart from the fact that higher rate signals are created by multiplexing numerous STM-1 signals, lower rate payloads are also mapped into STM-1.
The Synchronous Transport Module level-1 frame structure has nine rows and 270 columns of bytes, totaling 2,430 bytes. It is a byte-oriented arrangement with a bit rate of 155.52 Mb/s. The frame is transmitted at 125 µs, resulting in 8,000 frames per second on the circuit. The frame of STM-1 consists of payload blocks, overhead blocks and pointers. The ratio of these components largely depends on the initial payload that needs to be transmitted. While the last 261 columns of the frame provide the information payload, the first nine available columns contain the overhead and administrative pointers. The administrative pointers can contain one or more virtual containers which have the path overhead or the virtual container payload information.
The STM-1 signal is separated into two categories: the regenerator section overhead and the multiplex section overhead. The regenerator overhead takes up the initial three rows and nine columns in the STM-1 frame, whereas the multiplex section overhead takes up five to nine rows along with the initial nine columns in the STM-1 frame. The regenerator section overhead monitors the network sections that present on the fiber optic cable network. The multiplex overhead has the information that allows data packets to be transmitted on the same network compared to other data packets.The regenerator section overhead supervises the sections of a network that exist on a fiber optic cable network. Both overheads provide information on the transmission system and its management functions like failure detection, service channels and monitoring transmission quality. One of the salient features of Synchronous Transport Module level-1 is that can be multiplexed to generate higher order Synchronous Transport Modules.