In the context of transport networks like OTN and SDH/SONET, a trail is a logical entity that represents the complete, end-to-end transport of a client signal between two access points. Its primary function is to provide a transparent connection, meaning it delivers the client's information without modification to its content. The trail is monitored for integrity to ensure the quality of the transport service. For example, an Optical Channel (OCh) trail in an OTN network represents the full path of a specific wavelength carrying a client signal from ingress to egress.

A. This describes the process of mapping or multiplexing, which places a client signal into a transport container, rather than defining the end-to-end path (trail).

C. This describes the function of an optical amplifier or regenerator, which are network elements used to overcome signal attenuation, not the logical connection itself.

D. This describes a physical span or an Optical Multiplex Section (OMS), which is a segment of the network, not the complete end-to-end logical trail.

1. Source: ITU-T Recommendation G.805, "Generic functional architecture of transport networks"

Reference: Section 6.3, "Trail".

Quote/Paraphrase: The document states that a trail is responsible for "the transparent transfer of client characteristic information" between its access points. It is terminated by a trail termination function which monitors the integrity of the transported information.

2. Source: Nokia, "Nokia Optical Networking Fundamentals Course Student Guide"

Reference: Module 2, "OTN Fundamentals", Section on OTN Layering.

Quote/Paraphrase: The course material defines the Optical Channel (OCh) trail as the end-to-end logical connection that transports a client signal over a specific wavelength. It emphasizes the trail's role in providing a transparent path and performance monitoring from source to destination.

3. Source: ITU-T Recommendation G.872, "Architecture of optical transport networks"

Reference: Section 6.1, "Layer networks".

Quote/Paraphrase: This recommendation describes the Optical Channel (OCh) layer network, where the fundamental purpose of an OCh trail is to provide "transparent transport of a variety of client signals." This reinforces the concept of a trail as a transparent transport entity.