PAUL MOMTAHAN DWDM LINE SYSTEMS
SFP-based OTDRs have provided a more compact but single-fibre alternative to higher-performance OTDR form factors that support multiple fibres via an optical switch. SFPs are now available that integrate the OSC and OTDR, with the SFP acting as an OSC until there is a fibre cut then switching to an “out-of-service” OTDR. Coherent OTDRs are another recent innovation. While traditional OTDRs can measure loss, coherent OTDRs can also measure parameters such as chromatic dispersion, polarisation mode dispersion, and state-of-polarisation changes. And with the ability to pass through amplifiers, they can be used to monitor the entire length of a repeatered transoceanic fibre. Other potential applications for coherent OTDRs include pre-warning of terrestrial fibre cuts based on vibrations from construction activity and submarine monitoring of seismic activity. As another example, OCMs provide the ability to monitor the power level of each wavelength. This information can then be used by the link control to attenuate each wavelength with WSSs at ROADM sites or DGEs at ILA sites to optimise the power level of each wavelength. OCMs can also be used to troubleshoot the network. Recent innovations include flexible-grid OCMs and higher-resolution coherent OCMs. Coherent OCMs offer sub-GHz accuracy and highly accurate power monitoring of fine spectral slices independent of adjacent channel power. They reduce C-band scanning time from seconds to hundreds of milliseconds, and they provide advanced processing of spectral characteristics, such as valid channel detection, center wavelength, and optical signal-to-noise ratio. As a final example, the OSC provides a communication channel between adjacent nodes that can be used for functions including link control, in-band management, control plane (i.e., ASON/ GMPLS), and span loss measurement. OSC data rates have evolved from ~2 Mbps to ~100-155 Mbps, and more recently to 1 Gbps. The location of the OSC has moved from the shelf controller to the ROADM card, and more recently to SFP pluggables that also enable different flavors of OSC that meet specific application and interoperability requirements.
Figure 2: Open line system
are now able to provide full line system functionality including ROADM, in addition to a wide range of other functions (i.e., transponder/muxponder) in self-contained sleds. In terms of line system footprint, we have gone from around 6RU per ROADM degree for long-haul and 3RU per ROADM degree for metro in the 2005-2010 timeframe to two ROADM degrees in 1RU with modern 600-mm compact modular platforms. 300-mm compact modular is the next evolution. An additional aspect of this platform evolution is from application- specific platforms to a single common platform that can support applications from metro edge through metro core to long-haul and SLTE. 4: OPEN LINE SYSTEMS With benefits that include accelerated innovation, optimised networks, and transformed economics, open optical networking is being embraced by many network operators. Line systems are therefore becoming more open, with integrated OCMs, WSS-based attenuation, and alien wavelength-friendly link control simplifying support for third- party wavelengths, while flexible-grid capabilities provide a path to spectrum services. Management interfaces such as TL1 and SNMP are evolving to open APIs (NETCONF, RESTCONF, gRPC, gNMI, etc.) with standardised data models (Open ROADM MSA, OpenConfig). The Open ROADM MSA has also provided standards for line interoperability between metro ROADMs from different vendors, with specifications covering the SDN domain controller, link control, power levels, laser safety, performance monitoring, fault detection, and OSC. 5: ENHANCED OPERATIONS AND MANAGEABILITY A number of innovations address the need for reduced operational costs and enhanced manageability. For example, an OTDR transmits pulses of light into the fibre under testing and then analyses the light that is returned through scattering and reflections. Use cases include identifying the location of fibre cuts, detecting increased fibre loss, and intrusion detection. Integrated OTDR started to appear as a ROADM option around 2015. More recently,
being able to mix and match from a wide range of amplifiers, but it typically results in a high footprint as well as more complex installation, with the need to correctly cable all these modules together. An alternative approach called “ROADM-on-a- blade” emerged around 2007, with all the components required for a ROADM degree (WSS, input amplifier, output amplifier, OCM, OSC, etc.) in a single module. A third alternative that emerged in 2015, originally purely for fixed WDM with WSS coming later, was putting the individual line system components into compact optical layer pluggables. 2: AMPLIFIER INNOVATION Amplifiers have evolved in terms of the amount of gain they can deliver. One key contributing factor to this higher gain is the adoption of integrated ROADM- on-a-blade architectures with internal connections to the amplifiers, allowing higher power levels. They have also improved in terms of the amplified spontaneous emission noise added for a given gain. Another evolution has been from fixed-gain amplifiers to variable-gain amplifiers. Variable-gain amplifiers typically cover a specific span loss range, with at least three types required (e.g., 0-18 dB, 14-25 dB, 22-35 dB). This later evolved to switchable-gain amplifiers with a single part number able to cover a very wide span loss range (i.e., 0-32 dB). In addition, there has been a trend toward hybrid amplification combining erbium-doped fibre amplification (EDFA) with Raman to reduce noise. A final amplifier innovation is the recent availability of hardened (i.e., -40°C to + 65°C) EDFAs, simplifying the deployment of amplified DWDM in network facilities that lack temperature control, for example, in metro edge networks and some specific long-haul environments (e.g., ILAs in the desert). 3: COMPACT MODULAR Line system platforms have evolved from traditional chassis-based transport systems with sub-300-mm depth and side-to- side airflow. The first evolution was from large (12+ RU) to more compact (~5RU) chassis circa 2007, around the same time as ROADM-on-a-blade. 600-mm-deep compact modular platforms with front-to- back airflow were launched in 2015 and
LOWERING OPTICAL NETWORK TCO
These innovations are providing network operators with a number of key benefits including enhanced coherent optical engine capacity-reach, increased fibre capacity, greater degree and add/drop flexibility, reduced footprint, higher network availability, lower operational costs, and the accelerated innovation of open optical networking. Together, these benefits are enabling network operators to significantly reduce their optical network TCO.
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ISSUE 30 | Q3 2022
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