esent Architectures On the other side of the layered network architecture, optical transmission in almost every part of the network is out of discussion. No other technology allows such a large capacity to be transmitted at such a long distance with an affordable CAPEX. Only in the last mile, copper pairs are generally still used, but a slow trend exists also in this part of the network to substitute copper cables with fiber. If selecting the technology for the upper and the lower layers of the network is quite natural and there is a large agreement on the choice, the situation is not the same for the intermediate layers. As a matter of fact, even if conceptually it is not impossible to directly carry IP data- grams on the OTN frame, this solution has several issues if realized in the simpler way. A very-poor layer-two functionality is not easy to accept in the design of a carrier net- work. In this case, for example, since all the QoS would have to be managed by Internet layer protocols, IP could not be used to convey the greater part of carrier services and some other Internet layer protocol has to be devised, partially loosing the advantage of the con- vergence on a widely used network layer. Moreover, the typical content source, for example, a VoD server, does not format its out- put as a steam of pure IP datagrams (or the equivalent in some other Internet layer pro- tocol), but frequently it is designed to format the output as a stream of Ethernet frames. With this choice, it adopts the most diffused format for local networks (even the big local networks adopted, e.g., by TV producers) and leverages on the low cost of Ethernet subsys- tems due to the high volume production. Also data-oriented services, like network storage or network servers mirroring are based on transmission of Ethernet frames. Last, but not least, the network is generally evolved starting from a legacy situation in which, in the vast majority of cases, a telephone circuit switched network was served by a SDH/SONET transport. As a conseque