nels means lower optical connec- tors failure rate and lower routing error probability. Moreover, it is much easier to realize automatic patch panels if they are small, further reducing the local exchange mainte- nance cost. Last, but not least, reducing the number of fiber arriving at the central office for a given capacity allows the switching machine located in the central office to interface with the network with a small number of high-bit-rate interfaces. Using a small number of high- bit-rate interfaces optimizes the working of the local exchange switch machine, while min- imizing its cost [9]. The earlier discussion shows that there are several reasons for pushing toward the reduction of the amount of fiber in the access network. The solution to this problem is constituted by the so-called passive optical network (PON). In a PON [6], the fiber deployed in the field constitutes a logical tree, with the root in the local exchange and leafs at the end users' locations (see Figure 3.7). At the branching points, an optical passive element is present, either a splitter in time-division multiplexing (TDM) PON or a wavelength-division multiplexing (WDM) mux/demux in WDM-PON. In the PON tree, every branch can be constituted either by a single fiber or by a fiber couple, depending on the adopted transmission technique. In particular, a single fiber is Telecommunication Networks Passive branching point Optical distribution network (ODN) Local exchange FIGURE 3.7 PON topology. enough where bidirectional transmission is carried out, otherwise a fiber couple is used, one fiber for the upstream and the second fiber for the downstream. In order to effectively use a PON infrastructure, the optical signal traveling through the tree root has to be shared among the different end users. This sharing can be realized either via WDM or via TDM multiplexing. 3.1.3 Metro Network and Core Network Architectures The metro network collects signals from the access central offices and delivers it generally to the core nodes. This is the 