Transport Networks
Transport networks in the SRA Composite exam cover optical and OTN transport: WDM, ROADM, coherent optics, protection, and management. Expect questions on how Nokia 1830 PSS and photonic service switches are modeled and how client signals are carried.
Optical and OTN layers
- WDM carries multiple wavelengths over a fiber; ROADM nodes add, drop, or express wavelengths without O-E-O conversion.
- Flexgrid uses narrower frequency slots than fixed 50 GHz, improving spectral efficiency.
- OTN adds ODUk containers, FEC, and OAM; it can switch at the optical or electrical layer.
- Protection options include optical line protection (OLP), OTN SNCP, and mesh restoration.
Worked example
A 100G client must survive a single fiber cut with minimal latency. At the optical layer, OLP can switch the working fiber to a protect fiber, but it protects the line, not the client path. If the requirement is path-level protection across a ROADM mesh, OTN SNCP or GMPLS restoration is appropriate. Choose based on whether the failure domain is fiber, wavelength, or ODUk path.
Traps
Do not assume OLP protects against transponder failure. Do not confuse wavelength spacing with baud rate. Do not ignore FEC and optical reach when selecting a coherent mode.