The SDH technology makes use of the STM (155Mbit/s) transport module. Different multiples of the STM rate (eg. STM-1, STM-4, STM-16, etc) give the different data rates available via SDH.
SONET is an alternative to SDH and makes use of the STS (51.84Mbit/s) transport module. SONET rates are generated as multiples of the STS rate and are often known by the corresponding Optical Carrier (OC) designation.
There is correlation between different SDH and SONET rates which allow interfacing between the two similar technologies. (eg. STM-1/OC-3 = 155Mbit/s, STM-16/OC-48 = 2.5Gbit/s, etc)
For both SDH and SONET the higher data rates are achieved using fibre optic cable as the physical medium.

Global Approvals
Devices making use of the SDH and SONET technologies are predominantly telecommunications devices and as such are subject to the relevant Safety and EMC requirements around the world. Currently Telecoms performance requirements are usually in the form of conformance tests.

Testing Services
Regulatory Testing
As an example, the table below lists the necessary regulatory requirements to gain market access to Europe and the US for SDH products.

	European Requirements	US Requirements
EMC	EN55022, EN55024	CISPR22, CISPR24
Telecoms	-	TIA-968-A
Safety	IEC 60950-1	UL950-1

Conformance Testing
In addition to regulatory requirements, the performance testing provides confidence that the product is operating as intended and adds value to prospective customers.

	Standard	Description
Telecoms	ITU-T G.707	Network node interface for the synchronous digital hierarchy (SDH)
	ITU-T G.825	The control of jitter and wander within digital networks which are based on the synchronous digital hierarchy (SDH)
	ITU-T G.957	Optical interfaces for equipments and systems relating to the synchronous digital hierarchy