TECHNICAL
technologies. Key L1 services had not been delivered by IP/MPLS, including transparent services demanding higher speeds (gigabits/sec and beyond), such as Ethernet private lines with deterministic paths and bandwidth guarantees, and higher levels of transparency for control protocols, leased lines for common storage protocols, and optical transport networking (OTN) leased lines. This gap in the technology has been used as the main reason why communication service providers (CSPs) still deploy electrical OTN switching and DWDM networks with wavelength and subwavelength services. To address that gap and achieve full services convergence, routed optical networking introduces private line emulation (PLE) services over the IP/ MPLS network. PLE is being defined as a standards-based solution within the IETF and provides bit-level transparency for transport-like L1 services with a variety of client protocols, including Ethernet, OTN, and fibre channel at gigabits/second speeds. While PLE provides the client encapsulation to transport those bit-stream services using standard pseudowire techniques, the control plane for the emulated services, which controls the behaviour of the service in the network including path definition and protection, is also based on modern
Figure 2: Remember that routed optical networking with 400Gbps DCO pluggables works over any 75GHz-capable DWDM architecture, supporting both fixed optical add/drop multiplexers (FOADMs), mux-demux or ROADMs, with the latter allowing use of tactical bypass of wavelengths as an additional tool for network optimization whenever it brings economical value, without sacrificing other benefits of the simplified architecture.
3) IP layer protection: Traditional network designs typically deploy protection at the transport layer, even though the IP/MPLS layer is generally designed with traffic protection in mind. When additional protection is added over the already-protected IP/ MPLS layer (i.e., at the DWDM layer), it requires the use of more dedicated wavelengths that will be standing by idle in the network until a failure happens. An alternative approach is to use wavelength restoration, which shares spare wavelengths among multiple active ones. However, given the slow convergence of DWDM networks (multiple seconds or even minutes), wavelength restoration is often used in combination with protected wavelengths, meaning that instead of saving wavelengths, restoration requires even more wavelengths that are standing by to cover for failure scenarios. This redundant protection adds up to other inefficiencies of traditional network designs like low average interface and wavelength utilisation and results in a “snowball” effect, where relatively low traffic demands, in the order of a few 10s of Gbps, end up consuming multiple 100 Gbps of capacity at the transport layer. Conversely, an optimised IP/MPLS network design that takes both link and node failures into consideration during the network design phase provides enough redundancy without any need for additional transport resources. Add to that the ability of the IP/MPLS network to switch in less than 50ms (the
transport golden standard) using modern technologies like topology-independent loop-free alternate (TI-LFA), and the IP/ MPLS layer can be self-sufficient in terms of traffic protection and network availability. No additional wavelengths may be required for traffic protection or restoration, reducing the number of wavelengths consumed for a given traffic demand. As a result of maximising available wavelengths by using these three techniques, routed optical networking may also delay DWDM network upgrades. In some cases, it might even delay the need to deploy additional fibre pairs or L-Band DWDM systems, as the number of wavelengths required for a given traffic demand and related SLAs may be significantly lower. Full services convergence—only possible today with IP/MPLS IP/MPLS networks have proven their multiservice capabilities, and for decades, network operators across the globe deployed networks to deliver L3, L2, and L1 services based on this technology. While L3 and L2 services are well covered by a variety of implementation options, with EVPN being the latest and most integrated Ethernet service control plane technology for IP/MPLS networks today, L1 has been limited to lower speed services with PDH and SONET/SDH services emulated over the packet network using CEoPS and SAToP
Figure 3: Routed Optical Networking innovations: Digital Coherent Optics (DCO) and Private Line Emulation
MARCH 2024 Volume 46 No.1
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