exchanged across the core only by multiprotocol BGP and this information is not distrib- uted again to the core network, but only to the associated carrier LER, which retains the information in VPN-specific VRFs. Thus, routing across a MPLS network remains separate for each VPN. 3.2.6 Synchronous Optical Network (SDH/SONET) The synchronous optical network is a very important example of transport network model [41]. The introduction of SDH/SONET in substitution of the PDH, which was the previ- ous transport network technology, was a revolution in terms of network manageability and transmission quality control. The modern idea of the management plane was born Networks Fundamentals and Present Architectures TABLE 3.2 SONET SDH Acronym Correspondence SONET STS-SPE Higher order VC (VC-3/4/4-Nc) STS-1 frame STM-0 frame (rarely used) STS-1-SPE STS-1 payload STS-3c frame STM-1 frame AU-4 STS-3c-SPE STS-3c payload STS-12c/48c/192c frame STM-4/16/64 frame AU-4-4c/16c/64c STS-12c/48c/192c-SPE VC-4-4c/16c/64c STS-12c/48c/192cpayload C-4-4c/16c/64c in practice with the SDH/SONET and till now the resilience and management of SDH/ SONET transport network are the standard that carriers call "carrier class." While IP was developed by IETF, a quite informal standardization institute born from the union of the main routers vendors, the SDH/SONET standard was generated by the official standardization institutes for telecommunications. That is European SDH stan- dard was generated by ITU-T and European Telecommunications Standards Institute (ETSI) and the U.S. centric SONET by ITU-T and American National Standards Institute (ANSI) [42,43]. As far as Far Eastern countries are considered, generally they adopt the SDH standard. We will not dwell on differences existing from SDH and SONET, since they are suffi- ciently similar to be dealt with the same network model. For a detailed analysis of differ- ences and cross-compatibility the reader can start from Refs. [41,44], besides of course the official standardization documents. From now 