## 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 advancements in TDM systems, along with the development of sophisticated hardware that was able to perform pulse stuffing and sample incoming channels at a higher rate, made feasible the introduction of presiochronous digital hierarchy (PDH) systems. The introduction of the PDH system made possible to avoid having TDM mux/demux down to the fundamental rate when a tributary was to be extracted from the aggregate rate channel between two nodes in tandem. This innovation dropped the cost of switching and, hence, of channel transportation. The basis for PDH was a transmission rate of 64 kbps/channel and with TDM, the rates could scale up from 1.544 Mb/s (or 2.048 for E1) to some dozen Mb/s, depending on the available hierarchy of channel multiplexing in the time-domain. It would take, of course, another 25 years before the standardization of the ISDN, which brought digital TDM multiplexing to the consumer side, besides backbone networks. The main standards of the five generations on the backbone part of the network are summarized in Table 8 and will be discussed in the following paragraphs as part of the evolution timeline. The historical evolution of modern backbone networks is depicted in Figure 21, and it is elaborated in the rest of this section. The emergence of digital technologies marked the start of the _first generation (\"F1G\")_ of backbone networks and had a major impact on the subsequent evolution, as it made possible the use of advanced TDM techniques for circuit-switched networks on the one hand and efficient packet switching for IP networking on the other, while still maintaining copper cables or co-axial cables as the main transmission medium. Figure 22 shows the timeline for the main standards of the past five generations of fixed networks. The beginning of each arrow marks the year the corresponding transport standards of Table 8 were made publicly available. The proliferation of digital multiplexing and switching techniques increased the attainable capacity and brought co-axial cables to  
Figure 20: Parallel architecture proposed in [57]. their physical limits in terms of the distance they could transport the high-capacity signals without being irrevocably distorted. This was the point where cables embraced the use of optical fibers as the dominant transmission medium, thus giving way to the rise of the next generation of broadband networks. The _second generation (\"F2G\")_ of backbone networks relied on fiber-optic transmission systems to transport digital signals. The corresponding systems exploited the advances \begin{table}
\begin{tabular}{c c c} \hline \hline
**Year** & **Name** & **Standard** \\ \hline
1976 & X.25 & Standardized by CCITT in the Orange Book \\
1981 & IPv4 & RFC 791 \\
1983 & Ethernet & IEEE 802.3 family \\
1984 & OSI reference model & ISO 7498 \\
1988 & Frame Relay & I.122, I.441 \\
1988 & PDH & G.702, G.703, G.705 \\
1991 & ATM & I.362, I.363 \\  & & Multiple ITU, ETSI and ANSI standards \\  & & ([[https://www.itu.int/ITU-T/studygroups/](https://www.itu.int/ITU-T/studygroups/)]([https://www.itu.int/ITU-T/studygroups/](https://www.itu.int/ITU-T/studygroups/)) \_COM15/otn/SDH-rec.html) such as G.707, \\  & & G.708 \\
1998 & WDM over SDH & G.692 \\
1998 & IPv6 & RFC 2460 \\
2001 & OTN & Multiple, such as G.709, G.798, G.872, \\
2001 & MPLS & RFC 3031 \\ Early 2000s & ASON & Multiple ITU standards ([[https://www.itu.int/](https://www.itu.int/)]([https://www.itu.int/](https://www.itu.int/)) itudoc/gs/promo/tsb/80686.pdf) and RFC 6827 \\
2003, 2004 & GMPLS & RFC 3471, 3945 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Timeline of backbone protocols and standards. Figure 21: Timeline of five generations of backbone network standards.