inets in an access network, as shown in Figure 3.3. The last patch panel is generally located inside buildings, at least in city arrangements, where every pair is distributed to its own user. A critical characteristic of a copper-based access network is the average length of the user line. As a matter of fact, the maximum capacity that can be conveyed on a copper pair via digital subscriber lines (DSL) techniques critically depends on the copper pair length (see Figure 3.4, where as an example the bit-rate distance relation is shown for the very- high-speed digital subscriber line2 [VDSL2] format in typical conditions [7]). Average length of copper lines in different European countries and in the United States is reported in Figure 3.5 [8]. Networks Fundamentals and Present Architectures 200 pairs cable 2400 pairs cable 20 pairs cable Central First partition office cabinet Second partition cabinet Building partition cabinet equipment FIGURE 3.3 Block scheme of the copper pair-based access network. 30 MHz-copper cable bandwidth FIGURE 3.4 Example of the dependence of the VDSL2 capacity Meters from the copper pair length in typical conditions. From the figure, it is possible to see that relevant differences exist among European coun- tries, while the copper length is longer in the United States with respect to all European networks. This is another reason, besides regulatory differences and harder competition, that explains why U.S. carriers are much more advanced in deploying fiber optics in the access loop. The exact replica of the copper-based structure using fiber optics would require direct connection of each end user with the local exchange using a dedicated fiber optics cable. A similar architecture would have several advantages since it would assure maximum transmission capacity, physical separation of signals belonging to different users, and, last but not least, it would allow unbundling to be replicated in the new infrastructure with the same strategy used in the copper network. Telecommunication Network