nd dedicated. The network edge is described in more detail in Chapter 12. 2.5.9 NETWORK CORE Chapter 13 describes in more detail, core networking for optical (layer 1) and packet (layers 2 and 3) in the OSI layer model. The optical layer provides fiber optic interconnect between remote cen- tral offices; this entails electrical to optical conversion, optical amplification, transport controls, and multiplexing/demultiplexing of multiple signals. The packet core acts as a "backbone" that ties together the periphery allowing packet traffic to enter the network virtually anywhere and be eco- nomically forwarded to its intended destination. The electro-optical equipment used for modern fiber communications falls into two basic types: the reconfigurable optical add/drop multiplexor (ROADM) and the optical amplifier (or OA). The former is placed typically in operator central offices to allow traffic to be added and removed (simi- lar to on and off ramps of the highways). The latter is responsible for increasing the strength of the light SO that it can continue toward its destination. Modern high-capacity optical systems work by combining a number of separate high-rate (cur- rently 100 or 200Gbps, but soon to be 400Gbps) optical signals onto the same fiber, using a tech- nique known as dense wavelength division multiplexing (DWDM) where each signal is assigned a unique wavelength. The ROADM adds onto this by providing the ability to add or drop specific wavelengths at each node. This forms an optical network allowing a large number of locations to send optical wavelengths between each other without requiring separate fiber optic cables. With the advent of SDN, the optical layer is being further advanced in two key ways- disaggregation of functionality and global optimization of transport resources in conjunction with the packet layer. Global optimization is done by using SDN. The centralized controller keeps track of all of the wavelength entry and exit points and uses routing optimization algorithms and any per waveleng