 Ethernet First Mile project. The IEEE also completed the IEEE 802.3av or 10G-EPON (10 Gigabit Ethernet PON) standard in 2009 which is backwards compatible with 802.3ah EPON. The standard supports two configurations: symmetric, operating at 10 Gbps data rate in both upstream and downstream directions, and asymmetric, operating at 10 Gbps in the downstream direction and 1 Gbps in the upstream direction. Commercial 10G EPON equipment is expected in 2011. GPON, EPON and 10G-EPON are the three key technologies that can be used to carry fiber to the premises. GPON is designed to address the limitations of previous PON solutions, namely EPON and BPON. Table 7.4 compares the capabilities of different PON standards. 7.4.1.2 SONET/SDH Synchronous optical network (SONET) or SDH (Synchronous Digital Hierarchy) is the dominant tech- nology deployed in all metro and long distance networks. It refers to a group of fiber-optic transmission rates that can transport digital signals with different capacities. Since their emergence from standards bodies around 1990, SDH and its variant, SONET (used in North America), have tremendously enhanced the performance of telecommunications networks based on optical fibers. TDM-based networks of PDH (the Plesiochronous Digital Hierarchy) and SDH/SONET have long served as standard transport platforms for cellular traffic. PDH and SDH/SONET are optimized to handle bulk voice circuits with maximum uptime, minimal delay and guaranteed service continuity. SDH was developed to replace PDH system for transporting even larger amounts of telephone and data traffic over the same cable, and to allow for interoperability between equipment from different vendors. The signal hierarchy defined several line rates among which STM-1 (155 Mbps), STM-4 (622 Mbps), STM-16 (2488 Mbps) and STM-64 (9953) and STM-256 (1327 Mbps) have been widely adopted. The details on SDH can be found in [17]. Transmission Networks 7.4.1.3 DWDM DWDM (Dense Wave Division Multiplexing) is considered as one of the best technologies to