Standard SDH optical interfaces defined by ITU-T
STM-1 155M optical port
STM-4 622M optical port
STM-16 2.5G optical port
STM-64 10G optical port
STM-256 40G optical port
And of course the STM-1 155M electrical port.

These interface rates, signal frame structure, optical power and other optoelectronic indicators are all well defined by ITU-T (International Telecommunication Union - Telecommunication Standards Department). SDH equipment manufacturers around the world must strictly comply with. So that the global SDH equipment can be seamless.
The original PDH equipment, there is no globally unified interface standard, the interface of each production is different, different manufacturers can not be directly docked. It is very inconvenient for users to organize networks, which seriously limits the development of communication.

SDH is a synchronous transmission system, the smallest module unit is STM-1, 155.520Mbit/s. All transmission modules defined by SDH are strictly 4-fold relationship, as shown above. There is never any STM-2, STM-3, STM-7 or anything like that, there are only five modules, 1, 4, 16, 64 and 256.

Sometimes you will see a module is STM-0, this module is not the standard module of SDH, it is the OC-1 under the SONET system in North America.

The corresponding relationship between SONET and SDH is:
OC-1/STM-0 52M optical port (in fact, the smallest module of SDH is STM-1, and there is no such thing, it is just for the purpose of correspondence)
OC-3/STM-1 155M optical port
OC-12/STM-4 622M optical port
OC48/STM-16 2.5G optical port
OC192/STM-64 10G optical port
OC768/STM-256 40G optical port
As you can see from the above, the correspondence between SDH and SONET is a strict 3-fold relationship.

The MSTP equipment (multi-service transport platform) in SDH equipment also has FE and GE electrical ports and FX, GE and 10GE optical ports.