GPON and EPON, described at length above in Chapter 3 and Chapter 4, respectively, represent the two most important Gigabit-class passive optical networks (PONs) that are widely deployed in the United States, Europe, and Asia Pacific region. Given the ever increasing bandwidth demand from consumer and business applications, current PONs are expected to evolve into next-generation PONs (NG-PONs) over the next couple of years. GPON and EPON are expected to coexist for the foreseeable future as they evolve into NG-PONs. Clearly, one way to realize NG-PONs is to increase the line rate of current Gigabit-class PONs to 10 Gb/s. A good example of this approach is the IEEE 802.3av 10G-EPON standard, which was approved in September 2009 (see Section 4.4). NG-PONs are mainly envisioned to (i) achieve higher performance parameters, e.g., higher bandwidth per subscriber, increased splitting ratio, and extended maximum reach, than current GPON/EPON architectures, and (ii) broaden GPON/EPON functionalities to include, among others, the consolidation of optical access, metro, and backhaul networks, and the support of topologies other than conventional tree structures. Network operators are seeking NG-PON solutions that can transparently coexist with legacy PONs on the existing fiber infrastructure and enable gradual upgrades in order to avoid costly and time-consuming network modifications and stay flexible for further evolution paths.