## 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
made by synchronous digital systems based on the synchronous optical network (SONET) of the synchronous digital hierarchy transportation systems (SDH) in the early 1990s, which boosted the backbone network capacity to new levels. The implementation of SDH/SONET allowed us to overcome the limitations of PDH in terms of (de)multiplexing complexity. Synchronous networks facilitate the extraction of lower-rate digital tributaries from the multiplexed signal in an effective way, as it is a very powerful technique when multiple point-to-point links are multiplexed into an aggregated form. In addition, this technique made the concept of \"robust networking\" feasible via exploiting the capabilities of digital add/drop multiplexers and cross-connects, which subsequently enabled the implementation of new network architectures (beyond the point-to-point ones), such as interconnected rings and mesh. Based on the features of those architectures, the backbone networks could perform self-healing operations featuring rapid network restoration times. In the meantime, the breakthroughs in optical transmission, the deployment of the first cellular mobile networks and the widespread adoption of the packet-switched techniques of data routing, opened new possibilities for the backbone networks. SONET, relying on optical fiber transmission, set the next generation of digital rates by defining the synchronous transport signal STS-1 rate. The STS-1 data rate was approximately 51.84 Mb/s. This was formed by combining 90 columns in nine rows, 8000 times per second, and 8 bits per octet, with any SONET/SDH frame repeated every 125 microseconds. The asynchronous sub-rate signaling was made feasible via bit stuffing, allowing for the introduction of flexible add/drop multiplexing as well [64]. Each level of the SONET hier  
Figure 22: The evolution of backbone (metro/core) transport network standards. archy terminated its corresponding fields in the SONET payload, as shown in Table 9[65]. The key novelties of SONET/SDH, as a key technology of F2G era were:  
* [noitemsep,topsep=0 pt]
* Solved multiplex bundling problem;
* Overcame the need for multiple P2P links;
* Provided rapid restoration time (typ.50-100 ms);
* Enabled the use of electronic static ADMs and digital cross-connect (DXC) for traffic switching [66]. Moreover, during the F2G era, the landmark innovation of ATM came along at the higher network layer with the introduction of connection-oriented means to transport packet traffic, via the concept of establishing _virtual circuits_. In this way, ATM allowed for circuit-switching-based robustness alongside the flexibility of packet-switched segmentation of data traffic. Beyond ATM, other protocols such as frame relay and X.25 found their niche as layer 2/3 techniques to frame/encapsulate high-speed traffic routing for transportation between central offices and mobile switching centers. These technologies were characterized by a variable rate that began at the solid broadband threshold of 1.5/2 Mb/s and went up to theoretically hundreds of Mb/s. SONET systems offered excellent reliability and network protection. However, they did not quite match the requirements of the evolving backbone networks, since they were using too many costly optical-electrical-optical (O-E-O) conversions and initially single-channel transmission per fiber. Eventually, they evolved to support WDM and new features like virtual concatenation and traffic grooming based on the generic framing protocol. This was the point when the _third generation (\"F3G\")_ of backbone networks was introduced. During the early 2000s, several metro system vendors proposed additional data-transfer optimized protocols, like 1G/10G Ethernet and resilient packet rings (RPR), that started dominating the transport market in metropolitan networks in the forthcoming years. Ethernet was already a dominant transport solution in local area networks, and