These terms refer to Subnetwork Connection Protection (SNCP) monitoring methods defined in Optical Transport Network (OTN) and SDH standards:

• SNC/I (Inherent Monitoring): Protection switching is triggered by defects detected at the server layer (the physical or lower transport layer), such as Loss of Signal (LOS) or Alarm Indication Signal (AIS). It uses information "inherent" to the server signal rather than checking the specific connection's data integrity.
• SNC/N (Non-intrusive Monitoring): Uses a non-intrusive monitor to "listen" to the signal's Path Overhead (POH) without terminating the connection. This allows the detection of Signal Degrade (SD) or bit errors on the specific path.
• SNC/S (Sub-layer Monitoring): Monitors a specific sub-layer or segment of the path, typically using Tandem Connection Monitoring (TCM) overhead. This is used to protect a specific operator's domain or segment within a longer end-to-end path.

ITU-T Recommendation G.873.1 (10/2017), Optical Transport Network (OTN): Linear protection.

See Section 3 (Definitions), specifically definitions for SNC/I (Subnetwork Connection with Inherent monitoring), SNC/N (Subnetwork Connection with Non-intrusive monitoring), and SNC/S (Subnetwork Connection with Sub-layer monitoring).

ITU-T Recommendation G.841 (10/1998), Types and characteristics of SDH network protection architectures.

See Section 7.2 (SNC protection with inherent monitoring), Section 7.3 (SNC protection with non-intrusive monitoring), and Section 7.4 (SNC protection with sublayer monitoring).

ITU-T Recommendation G.805 (03/2000), Generic functional architecture of transport networks.

Provides the foundational architectural definitions for Subnetwork Connections (SNC) and monitoring types.