t systems had to be replaced. In addition, BPON and GPON operated on the same upstream and downstream wavelengths, SO they could not coexist on the same optical fiber. Each system required separate fibers on the ODN. The passive OSP of a GPON network consists of the fiber optic cables, optical splitters, and other passive devices that make up the physical layer of the access network. To enable a graceful upgrade process, GPON and next-generation PON systems such as XGS-PON and NGPON2 have dedicated noninterfering wavelength assignments. In combination with the new software archi- tecture, this will allow an elegant transition from GPON to XGS-PON and NGPON2. Network element hardware can be designed to support both XGS-PON and NGPON2. With open hard- ware specifications and hardware abstraction, the modular NFV and SDN software developed for GPON and XGS-PON can be modified and extended to support NGPON2 and future advanced PON access technologies. The key to this elegant transition is the use of coexistence element (CE) devices or passive wave- length multiplexers in the central office where the OLTs are located, along with wavelength blocking filters or tunable optics at the customer location. This will allow GPON, XGS-PON and NGPON2 to operate simultaneously on the same fiber in the ODN, optimizing both the flexibility and extensibil- ity of the new access network, while providing additional bandwidth management capabilities and common services across all of the technologies (see Figure 11.5). The new access architecture coupled with XGS-PON provides a flexible access network suited to support: Best-effort consumer broadband services at 1 Gbps (and beyond) symmetric for both sin- gle family and MDUs Best-effort business broadband services at 1 Gbps (and beyond) symmetric The Coexistence Element combines all wavelengths at the CO ONTs/ONUs with "fixed" such that multiple optics require blocking filters PONs can be "stacked" to protect the receiver from on the ODN. the wrong wavelengths. XGS-PON OLT GPON ONT NGPO