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Synchronous Digital Hierarchy 87

Figure 4-1 270xN bytes
The STM-1 frame
9xN bytes 261xN bytes
formats

Section STM-N Payload
Overhead
9 rows

Data Transmission Rates

In the standards, a number of transmission rates are defined or
possibly based on multiplexing rates. Not all rates are available
commercially; however, the rates are there in case a new rate is
needed. Similar to the SONET standards, SDH defines the syn-
chronous transport signal level n (STS-n) and the SDH level n
(SDH-n). These define the electrical rate of multiplexing and the
optical rate of multiplexing, both working at the appropriate rate of
speed necessary to map and multiplex the higher rates of speed.
The typical rates of speed and the appropriate STS and STM rates
are shown in Table 4-2.
Only three of the hierarchical levels are actually defined in the
standard and are available commercially: STM-1, STM-4, and STM-
16. Other rates will become available as needed.As the multiplexing
occurs and the overhead is subtracted from the payload, the actual
throughput is shown in Table 4-3. This table reflects the aggregates
of throughput in the three commercially available STM payloads.