## 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
With the use of network-controlled OADMs/OXCs, only add/drop traffic was processed by the IP routers. Optical cross-connects proved efficient in large transit-traffic cases where IP routers connected on OXCs provided cost savings and increased cost/power-consumption efficiencies. This is because OXCs provide router bypassing for transit traffic, limiting the number of digital ports and the amount of cross connection hardware needed to implement efficient routing. Now, the transit traffic is transparently routed through a node, and only the fraction of the traffic that is locally dropped has to go through the digital switch, as shown in Figure 25.  
Figure 25: Improvement of performance versus transit traffic. Moreover, in F4G systems, all client signal-related processes took place in the IP/MPLS layer, whereas the required traffic aggregation, switching and traffic restoration in case of failure could be implemented by the optical layer at the line-side. However, with the introduction of transparent optical networking, besides the huge benefits, some notable drawbacks related to the analogue nature of optical networks arose, due to the limited ability to perform error monitoring, signal quality tracing and fault isolation. Despite these downsides, the optical transport and switching layers emerged as key parts of the backbone networks (as shown in Figure 26), allowing for significant improvements of optical mesh networking [68, 69]. Subsequently, IP/MPLS over OTN established the network resilience capabilities that modern data services required [70]. OADMs and OXCs in post-2005 OTN deployments could be reconfigurable (i.e., ROADMs), either by the use of wavelength-selective architecture or by considering a broadcast-and-select architecture. Topologies with higher degrees of connectivity and versatility can be realized using WSSs with port counts N > 2 and allow optical mesh networking. OXCs exploiting 1 \(\times\) 8 or 1 \(\times\) 9 WSSs are commercially available, offering up to 8 degrees of connectivity at a given node. The layout for the N-degree ROADM is similar to that of the 2D ROADM, except that the \"broadcast\" power splitter on the drop path becomes a 1 \(\times\) N and the \"select\" WSS on the add path is a N \(\times\) 1. The WSS based ROADMs provide optimal bandwidth management and support better network resilience in mesh network architectures. The \"current\" _fifth generation (\"F5G\")_ of backbone networks is based on the innovative generalized multi-protocol label switching (GMPLS) and ASON concepts, while introducing software-defined networking (SDN) and network function virtualization (NFV) capabilities gradually over time at the optical transport networks. In the late 2000s, ITU-T introduced the ASON standard in order to harmonize the control plane in optical networks. Figure 26: Data networking over all-optical infrastructure (IP over optical networks). In ASON, the network performs management, control and transport from top to bottom. ASON features the connection control interface (CCI) and the network management interface for the transport network (NMI-T) [71]. The novelty of ASON lies in the control plane, which incorporates significant innovative features, such as topology discovery, setup and tear-down of connections, as well as protection and restoration and traffic engineering capabilities. Moreover, the control-plane, the management of network elements is handled by the management-plane via NMI-T and the network management interface handles all network resources for the ASON control plane (NMI-A). The simple network management protocol (SNMP) can be employed; however, additional options are available [72]. The GMPLS architecture provides convergence between the transport and client networks. The aim of this protocol was to reduce design complexity and provide simplicity in implementation.