factors such as the similarity between physical elements and their corresponding VNFs. 15.4.3 LEGACY AND SDN/NFV NETWORKS COEXISTING IN HARMONY There is a tremendous investment in the legacy, hardware-based network, and large numbers of different network elements that are to be virtualized. Thus, even with the most aggressive of virtu- alization deployment plans, it is unavoidable that we must live in a hybrid physical and virtual world for the foreseeable future. In addition, there are also functions within some network elements that simply cannot be fully virtualized or run on off-the-shelf compute hardware-optical transponders and the mechanics of the optical switching in ROADMs, cell site (eNodeB) remote radio units, and the physical infrastructure interconnecting servers, for example. ONAP was originally conceived to manage VNFs, but it can also manage PNFs. Many of the ONAP benefits-including being policy driven, automated change management, and enabling auto- mated responses to network conditions through control loops-can also apply to PNFs. ONAP Network Operations can scalably enable new functionality that does not widely exist in legacy OSSs today, including automating operations responses to network impairments. And of course, via ONAP, operators can specify this through policies, putting this definition into the operators' hands enabling faster intro- duction of new functionality than could be achieved through software updates to legacy OSSs. If ONAP is leveraged to manage physical network elements, it can enable legacy OSSs and BSSs to be retired, thus eliminating the cost of supporting them. There are numerous examples of where ONAP can be used to automate simple operator responsi- bilities on PNFs-from automating actions as simple as rebooting an impaired mobility cell site, to sophisticated traffic management in response to localized high network load, outages and planned maintenance activities, and to automated change management. In combination with intelligent algo- rithms, ONAP can also provide 