**1. Introduction**

The exponential growth of consumer demands and machine-to-machine network traffic coupled with the downward trend in revenue per bit transported is challenging network operators to adopt a strategy which tackles a twofold problem. The dual nature of the problem, on one hand lies in selecting a network architecture/technology which can efficiently transport

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

traffic originating from multiple sources, be it time division multiplexing (TDM) or Packet, and on the other hand to make use of an increasingly flexible and heterogeneous optical layer, where the characteristics of the optical light paths to be set up (e.g., modulation format, spectral width) are customized to the specific path properties.

Today, typical transmission networks are a layered combination of dense wavelength division multiplexing (DWDM) equipment (the lowest layer above the optical fiber layer), a subwavelength aggregation and grooming layer and an internet protocol (IP) layer. These layers form server/client relationships and are independent of each other. From a technological point of view, the functions of each layer are very different. Higher layer equipment is typically more expensive per bit transported because it needs to do more processing, so the use of the layers must be carefully balanced to deliver cost-optimized networks. The introduction of coherent 100G (100 Gigabits per second) optical transport was a key catalyst which offered massive performance gains over incumbent technologies to exploit more capacity from a single fiber. Telco operators achieved considerable gains as the cost per bit started going down with the introduction of 100G. But this was not the end, as they eyed newer improved scalable architectures to strengthen network operations and total cost of ownership (TCO). These nextgeneration network architectures aimed at efficiently grooming and aggregating sublambda data streams resulting in cost-optimized well-packed 100G wavelengths which would allow telco operators to survive within challenging capital expenditure (CAPEX) and operational expenditure (OPEX) cost targets in the near future. However, due to their relatively compact and short reach topologies, an abundance of optical fiber and the requirement to interconnect network elements at 10G rates or less, metro networks still mostly relied on a direct-detect 10G optimized optical transport infrastructure, though this situation is bound to change dramatically [1].

Importantly, the abundant deployment of 100G coherent systems in core networks has been attained at the expense of relatively costly line interfaces, performing electrical-optical (EO) and optical-electrical (OE) conversions. In the meanwhile, optical transport network (OTN) emerged as a key building block to complement the capacity gains unleashed by 100G and coherent optics. Efficiency, predictability, and reliability of the transport world and the agility, programmability of the packet world were blended into the ITU G.709 standard (OTN protocol) [2] and thus became an automatic choice. The demanding service level agreements (SLAs) of private E-LANs, E-Lines, and other packet traffic along with wavelength services from the near future could be met by the many features that OTN was offering. Moreover, traffic processing on IP packet level (layer 3) is much more expensive (per GByte) than switching the same amount of traffic in optical channel data unit (ODU) containers by OTN switches. Therefore, OTN provides a more cost-effective platform for subwavelength services to be multiplexed not only at their source node, but also at selected intermediate nodes and, as a result, reduce the amount of expensive (WDM) line interfaces used without having to resort to expensive router equipment to perform this task [3].

Nevertheless, a continuous steep inverse trend between the data volume and cost per bit being carried and the explosive growth of the traffic between data centers has supported the development of higher-order modulation formats, namely, 8-quadrature amplitude modulation (8-QAM) and 16-QAM, which provide 50% (150G) and 100% (200G) more capacity than standard quadrature phase shift keying (QPSK) albeit at the expense of reducing transparent reach to around half and one-third, respectively. This catered to the need of extremely high equipment density and maximizing optical transport capacity per fiber and per transceiver. However, there was another need growing up from the continuously shifting traffic pattern. And one answer to all these needs mentioned above was the flexible-rate interface modules, which grant software-switchable modulation (QPSK, 8-QAM, and 16-QAM supported in the same device), flexible channel spectral width, and flexible frequency tunability to provide the ideal balance between performance, capacity, and reliability across the most challenging networks [4].

traffic originating from multiple sources, be it time division multiplexing (TDM) or Packet, and on the other hand to make use of an increasingly flexible and heterogeneous optical layer, where the characteristics of the optical light paths to be set up (e.g., modulation format, spectral

Today, typical transmission networks are a layered combination of dense wavelength division multiplexing (DWDM) equipment (the lowest layer above the optical fiber layer), a subwavelength aggregation and grooming layer and an internet protocol (IP) layer. These layers form server/client relationships and are independent of each other. From a technological point of view, the functions of each layer are very different. Higher layer equipment is typically more expensive per bit transported because it needs to do more processing, so the use of the layers must be carefully balanced to deliver cost-optimized networks. The introduction of coherent 100G (100 Gigabits per second) optical transport was a key catalyst which offered massive performance gains over incumbent technologies to exploit more capacity from a single fiber. Telco operators achieved considerable gains as the cost per bit started going down with the introduction of 100G. But this was not the end, as they eyed newer improved scalable architectures to strengthen network operations and total cost of ownership (TCO). These nextgeneration network architectures aimed at efficiently grooming and aggregating sublambda data streams resulting in cost-optimized well-packed 100G wavelengths which would allow telco operators to survive within challenging capital expenditure (CAPEX) and operational expenditure (OPEX) cost targets in the near future. However, due to their relatively compact and short reach topologies, an abundance of optical fiber and the requirement to interconnect network elements at 10G rates or less, metro networks still mostly relied on a direct-detect 10G optimized optical transport infrastructure, though this situation is bound to change dra-

Importantly, the abundant deployment of 100G coherent systems in core networks has been attained at the expense of relatively costly line interfaces, performing electrical-optical (EO) and optical-electrical (OE) conversions. In the meanwhile, optical transport network (OTN) emerged as a key building block to complement the capacity gains unleashed by 100G and coherent optics. Efficiency, predictability, and reliability of the transport world and the agility, programmability of the packet world were blended into the ITU G.709 standard (OTN protocol) [2] and thus became an automatic choice. The demanding service level agreements (SLAs) of private E-LANs, E-Lines, and other packet traffic along with wavelength services from the near future could be met by the many features that OTN was offering. Moreover, traffic processing on IP packet level (layer 3) is much more expensive (per GByte) than switching the same amount of traffic in optical channel data unit (ODU) containers by OTN switches. Therefore, OTN provides a more cost-effective platform for subwavelength services to be multiplexed not only at their source node, but also at selected intermediate nodes and, as a result, reduce the amount of expensive (WDM) line interfaces used without having to resort to expensive router equipment

Nevertheless, a continuous steep inverse trend between the data volume and cost per bit being carried and the explosive growth of the traffic between data centers has supported the development of higher-order modulation formats, namely, 8-quadrature amplitude modulation

width) are customized to the specific path properties.

212 Optical Fiber and Wireless Communications

matically [1].

to perform this task [3].

A key aspect of transport networks is their capability to withstand failure scenarios, given the very large amount of traffic they carry. This requirement is usually met via protection and restoration techniques [5]. Protection is a static mechanism to protect against failures, where the resources for both the primary and the backup paths are reserved prior to the data communication. Restoration is a dynamic mechanism where the backup path is not set up until the failure occurs. Survivability using these techniques is usually provided to handle single/ multiple link or node failures in the network with each scheme, claiming a different stake in terms of network resources and recovery time. Moreover, protection/restoration can be supported at different layers of the OTN and the selection of which layer to utilize, either the ODU or the optical channel (OCh) layer, also involves similar trade-offs [6].

This chapter is organized as follows. Section 2 overviews the current role of OTN switching in transport networks and introduces the concept of the universal OTN switch, highlighting the motivation behind it and the key benefits of adopting it. In order to support the case studies presented in the remaining of the chapter, a routing and grooming framework is detailed in Section 3. Section 4 addresses the relevance of mechanisms for failure survivability in transport networks, presenting a case study comparing the cost-effectiveness of supporting restoration at different layers of the transport network. Moreover, the benefits of combining universal traffic switching and flexible-rate line interfaces are investigated in Section 5 and are quantitatively assessed via a case study using reference transport networks. Section 6 elaborates on the prospects of further cost savings in metropolitan and core networks as a result of adopting coherent-detection technology in both network segments. Finally, Section 7 presents the concluding remarks.
