## 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
GMPLS can be configured to handle networks with different switching options/solutions (as far as the data plane is concerned) and different means of managing the network (the control and management planes). The GMPLS protocol allocates the resources of the optical transport network as per connectivity requests coming from a client packet-switched network [73]. Another robust technique that characterizes F5G is impairment awareness embedded within the routing and wavelength assignment (IA-RWA) process in optical network design/operation. Physical-layer impairments (PLIs) can be dealt with mainly by two different methods: (a) allowing system margin to withstand the worst-case scenarios caused by various physical layer impairments and/or (b) calculating the impact of these impairments as precisely as possible, with respect to the physical quality of the signals (as measured by e.g., the optical signal to noise plus interference ratio (OSNIR) and eventually the bit-error-rate of a connection). In case (a), sufficient system margin could be set to withstand the interactions of signals at one connection (lightpath) with others supporting other connections, or the signal degradation due to noise accumulation or inter-symbol interference effects resulting from the characteristics of optical components and fibers used across the optical connections (lightpaths) in an optical network. In this case, two different types of margin can be set [64]. The first is the _power margin_, which is used to compensate the power degradation attributed to a number of physical layer effects, such as a drop in the output power of the transmitters due to aging; the excessive losses of optical components; imperfect channel equalization, etc. The second method is the _BER margin_, which ignores the actual physical layer effects and sets a higher OSNIR threshold value for a channel to be admissible. In case (b), the optical performance and, as a consequence, the reach of a certain usable wavelength along a given path is calculated depending on the state of the neighboring wavelengths at the time a communication request is sent. The choice between the two methods (a) and (b) can be influenced by how regenerations are performed and by the existence of accurate and time reasonable analytical expressions that calculate the impact of the most catastrophic physical layer effects. If regeneration is allowed in the network, the impact of using a worst-case impairment scenario would most likely require additional, costly regenerations because the device's optical distance is significantly limited. However, as long as the optical reach (distance at which the signal is transmitted with acceptable quality) remains reasonable, minor changes in distance have little effect on the amount of regeneration needed and, as a consequence, at the total number of regenerators on the network. The post-2015 backbone network investigations were mostly based on the concept of (flexible) elastic optical networking (EON), as it is illustrated in Figure 27. In EONs, the _line-rate of a \"sub-channel\"_ allows us to flexibly allocate a portion of the available bandwidth according to the connection's needs. With single-carrier channels, this is performed by tweaking the symbol rate of the source or by invoking a higher-order modulation format. With multi-carrier channels (such as Nyquist-WDM or optical-OFDM), this is realized either by increasing the bitrate of each individual frequency component/sub-channel or by altering _the number of sub-channels_ while the bitrate per \"sub-channel\" remains constant. Network protocols, such as GMPLS in ASON and the path computation feature communication protocol (PCEP) in PCE, and also the OpenFlow protocol in SDN, are examples of network control protocols (NCP) that are used to orchestrate the network operation, while they can also incorporate capabilities for physical layer awareness.