Selected Network Issues

Wavelength division multiplexing (WDM) represents a dedicated (physical) layer in the network. Optical transport network (OTN) adds cross connectivity, fault location, and protection/restoration at the optical level. WDM technology has enabled unregenerated transmission over large distances. Several signal parameters can be derived from the optical spectrum. Optical spectrum analyzers (OSAs) can be used to measure the optical signal‐to‐noise ratio (OSNR) and WDM channel power imbalance. Monitoring can be classified into two categories: optical performance monitoring (OPM) and optical impairment monitoring (OIM) techniques. OPM techniques are signal quality related. Bit error rate (BER) and Q‐factor are comprehensive measures of signal quality. Centralization of the client layer by means of flexible WDM helps reducing cost. One prerequisite is that the client layer lifetime is significantly shorter than that of the transport layer. Efficient restoration in a WDM network requires colorless, directionless, contentionless reconfigurable optical add—drop multiplexers (ROADMs).