Subnetwork connection protection

In telecommunications, subnetwork connection protection (SNCP), is a type of protection mechanism associated with synchronous optical networks such as synchronous digital hierarchy (SDH).

SNCP is a dedicated (1+1) protection mechanism for SDH network spans which may be deployed in ring, point to point or mesh topologies.^[1]

It is complementary to Multiplex Section Protection (MSP), applied to physical handover interfaces; which offers 1+1 protection of the handover.

An alternative to SNCP is Multiplex Section Shared Protection Ring (MS-SP Ring), which offers a shared protection mode, and Multiplex Section Dedicated Protection Ring (MS-DP Ring), which offers a dedicated protection mode.

SNCP's functional equivalent in SONET is called UPSR ^[2]

SNCP is a per path protection. It follows the principle of Congruent Sending Selective Receive, i.e., Signal is sent on both paths but received only where the Signal Strength is best. When the working path for Signal receiving is cut, the receiver detects SD (Signal Degradation) and the receiver of the other path becomes active.

SNCP is a network protection mechanism for SDH networks providing path protection (end-to-end protection). The data signal is transmitted in a ring structure via two different paths and can be implemented in line or ring structures. The changeover criteria are specified individually when configuring a network element. A protection protocol is not required. The switchover to protection path occurs in the non-revertive mode, i.e. if traffic was switched to the protection path due to a transmission fault, there is no automatic switch-back to the original path once the fault is rectified, but only if there is a fault on the new path (the one labeled as "protecting" and currently services traffic).^[3]

SNCP is a 1+1 protection scheme (one working and one protection transport entity). Input traffic is broadcast in two routes (one being the normal working route and the second one being the protection route).

Supongamos un estado sin fallos para una ruta desde el nodo B al nodo A. El nodo B reenvía la señal destinada a A desde otros nodos del anillo, tanto en rutas activas como de protección. En el nodo A, las señales de estas dos rutas se monitorizan continuamente para detectar defectos en la capa de ruta y se selecciona la señal de mejor calidad. Ahora consideremos un estado de fallo en el que se corta la fibra entre el nodo A y el nodo B. El selector conmuta el tráfico a la ruta de reserva cuando falla la ruta activa entre el nodo A y el nodo B.

Para evitar conmutaciones de protección innecesarias o espurias en presencia de errores de bits en ambas rutas, la conmutación se produce normalmente cuando la calidad de la ruta alternativa supera la de la ruta activa por un umbral determinado (por ejemplo, una tasa de error de bits un orden de magnitud mejor). En consecuencia, cualquier fallo se descarta en el mecanismo de decisión de SNCP.

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