In the Synchronous Digital Hierarchy( SDH), the transmission rates are divided into hierarchy levels. These hierarchy levels are standardized and are designated STM-n(Synchronous Transport Module), where "n" results from multiplexing "n" STM-1 frames, the first hierarchy level. STM data frames can be used to transmit a group of different data streams synchronized over a clock. In the Synchronous Digital Hierarchy (SDH), this can be used to identify time slots and channels within a T1 or E1 line.
The SDH frame of the first hierarchy level (STM-1) transmits a data frame every 125 µs, consisting of nine lines of 270 bytes each (9 bytes plus 261 bytes), which means 2,430 bytes in length. Each byte of the payload represents a 64 kbit/s channel. The gross transmission rate is 155.520 Mbit/s, which is transmitted over optical fiber using OC-3(Optival Carrier). The overhead is 90 bytes, which corresponds to a total overhead of 5.760 Mbit/s or 3.7 percent per frame. During transmission, the bytes are sent line by line one after the other. The repetition frequency of the data frame results in a repetition frequency of 8 kHz. Each byte of a frame has a transmission rate of 64 kbit/s.
The higher transmission rates are always formed from the fourfold multiplier of the basic bit rate (STM-1) of 155 Mbit/s and are 622 Mbit/s for STM-4, 2.5 Gbit/s for STM-16, 9.9 Gbit/s for STM-64 and 40 Gbit/s for STM-256.
Below the first hierarchy level, there are STM-1/3 with a speed of 51.840 Mbit/s, STM-1/6 with 26.920 Mbit/s and STM-1/12 with 12.00 Mbit/s. In addition, there are still some STM-n classes, which, however, do not correspond to the STM multiplex hierarchy and are used for the adaptation of the Sonet transmission rates. For example, STM-3 is the transmission rate of 466.560 Mbit/s, which corresponds to OC-9 for Synchronous Optical Network( SONET), STM-6 is the transmission rate of 933.120 Mbit/s (OC-18), and STM-8 is the transmission rate of 1.244160 Gbit/s (OC-36). The specified data rates refer to the complete bandwidth and include the user data rate and the header overhead.