## 4 Network Layer: The Main Building Block of Fixed Networks
### Five Generations of Backbone Networks: From PDH to OTN/SDN
#### 4.2.1 Intro on the Evolution of Backbone Networks over the Past 50 Years
The additional cost of WDM system offerings was related to the use of multiplexers/demultiplexers, amplifiers and devices for signal conditioning management (e.g., variable optical attenuators or dispersion compensating modules). In addition to the higher capacity of a single fiber, WDM systems offered the capability of wavelength routing. Furthermore, SONET ADMs were replaced with Optical Add Drop Multiplexers (OADMs) and Optical Cross Connects (OXCs) that enabled wavelength switching and routing without costly O-E-O conversions, thus bypassing costly and energy-consuming electronic functionalities (a quest that is still on-going). In Figure 23, we summarize the aforementioned discussion, illustrating the evolution of backbone network architectures until the early days of optical networks relying on interconnected WDM rings. The more advanced WDM transportation systems allowed the introduction of optical switching by means of optical add-drop multiplexers and cross-connects in ring and partially meshed connectivity network topologies. These systems emulate, at the optical layer, some of the functionalities the SDH/SONET systems provide at the digital layer without the scalability and capacity limitations the SDH/SONET digital technology supports. At the same time, ATM features related to efficient traffic handling were made possible with next-generation SONET (NG-SONET) systems, through new functionalities such as \"virtual concatenation\", which led to efficient grouping of lower rate bit-streams in a non-standard manner, making higher efficiency available. In addition, time slot inter-changes were made possible, so that switching between different tributaries in data streams became feasible. Of course, all the aforementioned data processing functionalities of SONET and ATM systems, which allowed transmission of information over optical fibers, required the frequent use of optical-to-electrical (and vice versa) conversions at various points of the network. Furthermore, both the advancements at the optical transport layer and the enhancements of capabilities at the higher network layers made some processing functionalities of the past SDH/SONET and ATM networks redundant. These facts resultedin some major limitations/inefficiencies which were reflected in the layering of digital/optic transport implementation [67], as shown in Figure 24. These limitations include a highly layered, complex network architecture with repetitions of functionalities and numerous O-E-O conversions, resulting in poor network scalability, slow service turn-up, and last but not least, high operational and capital costs for network operators. Figure 23: Evolution of transport network architectures from single channel SONET systems to WDM based networks supporting various transport protocols. Figure 24: F3G network layered architecture. The _fourth generation (\"F4G\")_ of backbone transport networks was marked by the emergence of advanced optical transport networks (OTNs) which incorporated advanced optical switching features, alongside the multi-protocol label switching (MPLS) approach, which introduced the concept of label switching as a successor of ATM's \"virtual circuit/path identifier\" (VCI/VPI) based switching for connection-oriented forwarding of packet-switched traffic. The compatibility of the emerging F4G OTN automatically switched optical network (ASON) standards with MPLS, allowed for much better handling of network efficiencies of the F3G era and supported superior network capabilities. Overall, the fourth generation achieved:  
* higher signal-rate encapsulate multiple packet-based signal formats;
* service transparency (SDH/SONET, Ethernet, ATM, IP, MPLS etc. );
* superior restoration;
* improved FEC;
* use of reconfigurable optical add-drop multiplexers (ROADMs). On the network architecture side, the approach adopted was to use reconfigurable OXCs in a mesh topology, operating on individual wavelengths, where each could transport high-speed signals at OC-48 rates and above.