nce, a lot of SDH/SONET machines are deployed in the field, some of them out of the last generation of TDM machines, thus with a long operating lifetime in front of them. If the telephone switches cannot reused, but have to be substituted, it is desirable to reuse, at least for an important part of their lifetime, the most modern SDH/SONET machines. All these considerations bring to the conclusion that a more complex layering is needed in between the IP and the WDM layer. 3.3.1 Packet over SDH/SONET Model Packet over SDH/SONET (POS) [41,59] is a layer 2 technology that uses two protocols: Point-to-Point Protocol (PPP) [60,61] and High-Level Data Link Control (HDLC) [20,61] to encapsulate IP packets into a frame. It is generally used to adapt IP packets to be trans- ported by an SDH/SONET network, but this is not the only possible use. POS enabled machines can be also directly connected by a bidirectional fiber link, thus generating the simpler example of direct IP over fiber. Since this solution has its wider application in the United States, from now on we will adopt the SONET language, even if, all we will describe is also valid for SDH with the needed names and acronyms adaptation. When this will not be true for some specific aspect it will be remarked clearly. Since a POS-based IP network uses SONET transport, if underlying WDM transmission systems exist, then the interface is well defined, since it is the interface between SONET and WDM transmission. Telecommunication Networks Adopting POS, the reuse of last generation SONET machines and the reuse at physi- cal layer of SONET management and survivability mechanisms that are well known and appreciated by carriers, is immediate. Moreover, if a legacy data layer has to be integrated into the network besides the Internet layer, it can be done simply using SONET multiplexing capability. As an example, if an overlay Asynchronous Transfer Mode (ATM) network has to survive besides the IP one, it can be inserted via the ATM/SONET adaptation function in a suitable VT 