ing WDM). Each wavelength in WDM is called an optical channel (or simply, 4 QoS for Fixed Ultra-Broadband a channel). With this notation, if one channel carries, for example, 10 Gbit/s and WDM multiplexes 160 channels on a single fiber cable, then the available bitrate is 1.6 Tbit/s (= 160 channels X 10 Gbit/s/channel). In this phase bitrates of 40 Gbit/s and 100 Gbit/s per channel (i.e. per wavelength) are also being developed, which further boosts the bitrates over a single cable to tens and even hundreds of Tbit/s (with dense WDM schemes). The higher bitrates per cable offer the possibility to use the already installed fiber cables infrastructures and to provide higher bitrates by changing the operating equipment (at the cables' ends), which is a prerequisite to increase bitrates also in the access parts and with that to improve QoS (because the bitrates are one of the most important QoS parameters in all types of networks). Terabit per second speeds are targeted for transport networks, not for access networks, at least in the 2010s and 2020s. However, with the evolution of fiber-based technologies, they are expanding from the transport networks toward the access networks, including the last mile. There are different architectures for fiber implemen- tations and design in the access part. The general name for fiber access networks is FTTx (fiber-to-the x), where X stands for home (FTTH), curb or cabinet (FTTC), building (FTTB), premises (FTTP), desk (FTTD), etc. With FTTH, optical connection reaches the end-user's home premises (Figure 4.3), while FTTB or FTTC can be used for differ- ent solutions, including HFC access networks. However, deployment of fiber requires larger capital investments in the network, therefore the penetration of fiber is gradual, starting from hot traffic zones such as urban areas (e.g. major cities and their down- towns) and then going toward suburban areas. One of the challenges for cost-effective FTTx deployments is minimization of the number of fibers (e.g. by using WDM over existi