SDH (Synchronous Message Blocking) is structured into block frames according to a specific pattern, called Synchronous Transmission Modules (STM). These modules transmit serially at a rate synchronized with the network. The most basic and important module signal in SDH is STM-1, with a rate of 155.520 Mbit/s. The higher-level module STM-N is formed by multiplexing N basic module signals (STM-1) synchronously after byte interleaving. Its rate is N times that of STM-1, where N is a positive integer (1, 4, 16, 64, 256). Detailed rate levels are shown in the table. The STM-N optical interface line signal is simply the result of electro-optical conversion of the STM-N signal after scrambling; therefore, its rate remains unchanged.
| SDH Rate Levels | Bitrate (kbit/s) | Rate (bit/s) |
|---|---|---|
| STM-1 | 155,520 | 155 M |
| STM-4 | 622,080 | 622 M |
| STM-16 | 2,488,320 | 2.5 G |
| STM-64 | 9,953,280 | 10 G |
| STM-256 | 39,813,120 | 40 G |
