Optical Network Elements

The dramatic shift in optical-network architecture that began in the 2000 time frame is chiefly due to the development of advanced optical network elements. These elements are based on the premise that the majority of the traffic that enters a network site is being routed through the site en-route to its final destination as opposed to being destined for the site. The deployment of optical bypass technology, which allows transiting traffic to remain in the optical domain, greatly reduces the amount of required network equipment. The optical-bypass-enabling network element known as the reconfigurable optical add/drop multiplexer (ROADM) is the focus of this chapter. Various ROADM architectures and their trade-offs are presented, along with numerous designs that illustrate operational alternatives. The chief attributes that affect the flexibility, cost, and efficiency of ROADMs are covered, including the colorless, directionless, contentionless, and gridless properties. Integration of elements within the nodal equipment is a more recent development, motivated by the desire to eliminate individual components, reduce cost, and improve reliability. The range of possible integration levels is illustrated by the discussions regarding integrated transceivers, integrated packet-optical platforms, and photonic integrated circuits.