of FTTx field tests based on GPON are ongoing. In the field of Ethernet-based PON, IEEE has emitted an important standard (called GEPON) that has been largely adopted mainly in the Far East. NTT, the major carrier in Japan, and one of the largest telecommunications carriers in the world, began deploy- ing GE-PON-based FTTH access network equipment in 2003. In 2004, the IEEE ratified the IEEE 802.3ah Ethernet in the First Mile specification that fully defined the GE-PON technology deployed by NTT and later adopted by other carriers in Japan and Korea. A synthesis of the main TDM-PON standards is reported in Table 11.1, with some key optical parameters specified for each standard. In the following discussion, we will mainly focus on GPON, due to its adoption in North America and probably in Europe also, where it is quite improbable that an ITU-T standard would be neglected in preference for an Ethernet-based GEPON. Several considerations on transmission impairments and other elements are, however, common between GPON and GEPON. 11.2.2 GPON Architecture and Performances The GPON standard [5], as all other standards for TDM-PONs, is based on unidirectional transmission, conveying downstream and upstream signals on different wavelengths on the same fiber. In this case, the effect of reflection is limited by the great wavelength sepa- ration between the two signals. Telecommunication Networks Isolation Wavelength Upstream Upgrade band Downstream video bands FIGURE 11.1 GPON standard wavelength plan including the case in which the video overlay is used. In addition to the GPON version with two wavelengths, there is a GPON standard con- veying two downstream wavelengths. One of them (1490nm) is used to convey the GPON downstream signal, the second wavelength (1550 mm) is conceived for the distribution of analog CATV. This solution has been adopted in North America where CATV is practically the TV standard, and this solution allows the customers to maintain their home TV sets. The frequency plan of the GPON standard is sh