## 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
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Section** & **Line** & **Path** \\ \hline \multirow{6}{*}{Description} & A section is a single fiber run that can be terminated by an optical generator or network feature (line or path). & One or more portions of a line signal are originated or terminated by line-terminating equipment (LTE). & Non-SONET equipment is connected to the SONET network via path-terminating equipment (PTE). The payload is mapped and demapped into the SONET frame at this layer. \\ \cline{1-1}  & To correctly format & & \\ \cline{1-1}  & SONET frames and to convert electrical signals to optical. The section header overhead can be originated, accessed, modified or terminated using section terminating equipment (STE). & On SONET frames, the LTE performs information synchronization and multiplexing. & This layer is responsible for end-to-end data transport. The optical interface layers are arranged in a hierarchical order, with each layer building on the services offered by the layer below. Each layer communicates with peers in the same layer, processes data and then transfers it up or down to the next layer. \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1} \cline{2-2} \multirow{4}{*}{Aim} & \multirow{2}{*}{(ATS-1 frame is in size. The section and line header overhead is made up of the first three bytes of each row).} & & \\ \cline{1-1}  & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Details of SONET hierarchy levels. its extension into metropolitan-area networks (MANs) was quite reasonable. RPR was proposed as a MAC protocol designed to optimize bandwidth utilization and facilitate services over a ring network while providing carrier-class required attributes like 50-ms ring-protection switching. It is worth pointing out that the aforementioned transportation methods (i.e., SONET and RPR) are fully compatible with WDM transportation technology, as the latter is operating as a multiplexing overlay method. To encapsulate the zoo of client signals (i.e., SONET, Ethernet, ESCON, fiber channel, ATM, IP, etc. ), advanced mapping and multiplexing schemes and new framing procedures are utilized to allow carriers to handle the heterogeneity. The bit rates of the data tributaries accessing the metro-access network varied greatly (e.g., from OC-3 to OC-196 for SONET and from 100 Mb/s to 10 Gb/s for Ethernet traffic). Therefore, network nodes at the edges of the metro network should also be capable of performing traffic aggregation in order to improve the efficiency of the transport network by combining the multiple low-bit-rate signals into a single high-bit-rate wavelength channel. Moreover, the network should be able to support bandwidth provisioning with various levels of granularity. All these requirements were met with the employment of WDM systems alongside next-generation SONET add/drop multiplexers. Standardized in 1998 with the G.692 standard, WDM brought an undeniable novelty as it skyrocketed the transportation capacity. WDM facilitated the design of systems where multiple signal transmissions occurred over multiple wavelengths over a single fiber, increasing the overall system capacity to a significant degree. Thus, transmission of data streams of 10 Gb/s and later 40 Gb/s was accomplished by multiplexing different wavelength channels over a single fiber. Combined with the emergence of optical amplifiers in the mid-1990s, WDM provided a revolutionary breakthrough in transmission reach x capacity products as well. The first generation of WDM systems was meant to simply scale the number of SONET/SDH rings using multiple wavelength channels. The network savings that aroused from WDM system offerings could be exploited, in a first approximation, in a reduction of the number of required optical fibers and SONET network overlays that were needed to accommodate the traffic demands.