There are two main mechanisms that can be put in place to increase network survivability: protection and restoration. Protection involves pre-allocating and reserving backup resources so that they are ready in case of a failure. Restoration involves allocating backup resources upon failure and using a 1+1 protection mechanism to recover each trail. This ensures that the network is able to re-route traffic in the event of a failure, increasing the overall survivability of the network.

Comprehensive and Detailed Explanation From Nokia Optical Networking Fundamentals:
The Optical Transponder (OT) is the essential interface component in a WDM system that bridges the gap between the client-side equipment and the WDM line-side. Client signals, often referred to as "colorless" or "black and white" because they typically use standard 1310nm or 1550nm short-reach optics, cannot be directly multiplexed into a DWDM fiber because they would interfere with one another.
The Transponder performs an O-E-O (Optical-Electrical-Optical) conversion process: it receives the client's optical signal, converts it to an electrical format to perform 3R functions (Re-amplification, Re-shaping, and Re-timing) and often wraps it into an OTN (Optical Transport Network) frame, and then re-transmits it using a high-precision, ITU-T grid-compliant colored wavelength. In the Nokia 1830 PSS portfolio, these can be dedicated transponders for a single high-speed service or Muxponders, which aggregate multiple lower-speed client signals into a single high-speed "colored" line interface. Other components like the SFD are used for multiplexing those colors, and the DCM is used for managing fiber impairments, but only the Transponder performs the initial frequency conversion.

According to the Nokia Optical Networking documentation, a Shared Risk Group (SRG) is defined as "a set of network resources that share a common failure risk. When a resource in an SRG fails, the other resources in the group are also affected." This can include fibers, boards, nodes, and other network resources. The SRG concept is used in network design and protection mechanisms to ensure survivability and minimal impact on service in case of a failure.

A trail is a transparent transport of a client signal. A trail is a physical link between two points in an optical network, allowing for the transport of a client signal from one point to the other. It is a low-order signal, such as a 10G Ethernet or a Fibre Channel signal, encapsulated into a high-order container, such as a 40G or 100G signal. This allows for the transport of the client signal over longer distances, increasing the power budget of the optical link.