The optical transport network interface standard is

The optical transport network (OTN) interface standard is defined by ITU-T Recommendation G.709, which specifies the interfaces, frame structures, and client signal mapping for OTN networks.

The ITU-T G.709 standard establishes a framework for optical transport networks, enabling the transport, multiplexing, switching, and management of client signals over optical fiber networks . It defines the interfaces used within and between subnetworks, including:

• UNI (User-Network Interface): Marks the boundary between a user's network and the OTN operator's domain, allowing service provisioning, monitoring, and fault isolation .
• NNI (Network Node Interface): Connects OTN nodes within the operator's network, supporting interoperability and efficient signal transport across multiple nodes .
• MNI (Multiple Network Interface): Facilitates interconnection between multiple operator networks or vendor domains, ensuring seamless multi-operator operation . The standard also specifies OTN hierarchy, frame structures, bit rates, and formats for mapping client signals such as Ethernet, SDH, Fibre Channel, and IP/MPLS into OTN frames . This ensures interoperability across multi-vendor and multi-operator environments and supports advanced optical network architectures, including point-to-point, ring, and mesh topologies . Additionally, G.709 provides mechanisms for management, supervision, and survivability of optical channels, and it has evolved to support high-speed line rates beyond 100 Gbit/s, including OTUCn and FlexO interfaces for modern DWDM networks . In summary, ITU-T G.709 is the authoritative standard for OTN interfaces, defining how client signals are encapsulated, transmitted, and managed across optical transport networks while ensuring interoperability and scalability.