DWDM

Wavelength Division Multiplexing is a technique that allows multiple frequencies (or wavelengths) to be transmitted over the same fiber simultaneously. This is accomplished by the use of optical transmitters or transceivers with outputs tuned to individual and specific wavelengths so that there are distinct and non-overlapping transmission channels.

Coarse Wavelength Division Multiplexing (CWDM) uses wavelengths between 1260nm and 1670nm (the O, E, S, C, L and U transmission bands) and allows up to 18 individual channels to be created within this region, carrying any combination of voice, data or video with channels spaced 20nm apart. CWDM is a cost-effective solution for relatively low-bandwidth deployments however, because CWDM signals cannot be amplified, there are no broadband optical amplifiers capable of supporting that range, distances are limited to 80km.

Dense Wavelength Division Multiplexing (DWDM) takes WDM to the next level by decreasing channel spacing to 0.8nm or less and shrinking the operational wavelength range. This can produce 80 or more channels or lanes of traffic, opening the door to more high speed, high bandwidth applications.

Amazingly, all DWDM wavelengths reside within the narrow 1525nm to 1565nm region known as the C-Band. This area is utilized due to the relatively low (0.25dB/km) signal loss (fiber attenuation) compared to lower wavelengths found in the O or E-bands for example. As a result of the narrow channel spacing higher-precision lasers and filtering processes are required to maintain channel integrity and minimize interference.

Passive DWDM network architecture begins with a transponder or transceiver accepting data inputs of various traffic types and protocols. This transponder performs the essential function of mapping input data onto an individual DWDM wavelength. Each individual wavelength is fed to an optical multiplexer (MUX) which filters and combines multiple wavelengths onto a single output port for transmission over the main/core/common DWDM fiber. At the receiving end wavelengths can then be separated to isolate the individual channels by using an optical demultiplexer (De-MUX), each channel is then routed to the appropriate client-side output through an additional wavelength matched transponder.

Because DWDM technology overlaps the CWDM frequency band, a “hybrid” solution can also be selected. This type of hybrid system leaves the CWDM MUX and deMUX hardware in place, inserting DWDM wavelengths on top of existing channels in the 1530 to 1550nm range, thereby creating up to 28 additional channels. A hybrid CWDM/DWDM system can provide a significant capacity boost without requiring new fiber installation or wholesale infrastructure changes.

An Optical Add Drop Multiplexer (OADM) is an optional component of DWDM architecture that can be added to either passive or active networks to facilitate the addition or subtraction of a specified wavelength from a mid-stream location on the main/core/common DWDM fiber. Bidirectional architecture includes transmitters and receivers at both ends of the circuit as well as combination MUX/De-MUX devices.

For long-haul networks, DWDM architecture gains complexity with the addition of active system components needed to compensate for optical losses that will make signal reception and data recovery impossible. An Erbium Doped Fiber Amplifier (EDFA) can be used as a booster or launch amplifier to boost the optical power levels just as they leave the MUX while a pre-amplifier performs the same function prior to entering the DeMUX. Additional inline amplifiers might also be included. Passive networks, without EDFA, minimize this complexity. 

As the appetite for bandwidth continues to expand, it is no longer a question of if, but how providers will meet these requirements. Multiplying fiber capacity translates to a higher volume and diversity of services, more endpoints/users and countless monetization opportunities. Laying additional fiber is one obvious strategy but is often the most disruptive and costly option for addressing bandwidth constraints. So why not sweat the existing assets (fibers) already laid?

CWDM and DWDM were both standardized in 2002 by ITU-T G.694.2 and  G.694.1, respectively. Until recently, the installation and ongoing operating expenses associated with DWDM laser, transponder, MUX, De-MUX and OADM components have negated the comparative financial benefits. As economies of scale and efficiency improvements continue to drive down the cost of DWDM networks, the case for dense wave division multiplexing has become more compelling.

If CWDM has managed to keep up with bandwidth demand in some instances, the benefits/reasoning of DWDM deployment or conversion might not seem immediately obvious. With 300% per year growth in Internet traffic, providers are seeing bandwidth demands double every six to nine months. As this ramp continues to steepen, more of this traffic will land in low latency categories like VOIP, live UHD video streaming, cloud-hosted gaming, and emerging 5G fronthaul/backhaul applications, for things like autonomous vehicles, that create similar capacity demands. Optimizing and maximizing fiber bandwidth through this technology is a proactive, cost-effective solution to the capacity dilemma.

The close proximity of neighboring channels inherent to dense wave division multiplexing introduces challenges that necessitate intelligent maintenance and test practices. To maintain channel integrity, precision temperature control of lasers and reliable DWDM MUX/De-MUX devices are required. Even the slightest drift in wavelength can create offsets that interfere with adjacent channels and reduce signal quality. SPF/SFP+ transceivers provide the benefit of a lower price point but may be less effective in managing the integrity of wavelengths.

Noise is an additional challenge for active DWDM networks used for metro deployments. EDFA and Reconfigurable Optical Add Drop Multiplexers (ROADM), which also contain amplifiers, can add noise to a network and there is a fine balance to maintaining good Optical Signal to Noise Ratio (OSNR) in order to maximize bandwidth utilization of a DWDM channel and minimize bit errors which can result in data losses and retransmissions.

Passive DWDM applications found more commonly in access networks do not suffer from noise issues, there are no amplifiers to contribute noise and the shorter distance mean it’s more about minimizing optical power loss (attenuation) and getting good optical power level at the receiving transponder/SPF/SPF+ so fiber and connector losses and reflections are important concerns.

Dense wave division multiplexing has aligned cutting edge laser optics, electronics and modulation technologies to maximize the efficiency of optical fiber data transmission. This successful conglomeration has been the byproduct of a coordinated, end-to-end approach to development, installation, testing and maintenance practices.

During all these phases of a network lifecycle inspection of the fiber end faces and connectors is critical to ensure reliable operation. In order to be efficient inspection tools must not only allow you to see the fiber end face they must also automate the whole test process, the FiberChek Probe microscope is a handheld solution capable of auto-focus, auto pass/fail analysis, auto data storage and automated fiber inspection workflows.

Fiber testing is essential during network construction, both before and after MUX/De-MUX equipment is installed in order to ensure first time service activation and reliable networks. Conventional fiber testing tools such as VFL and fiber end inspection tools can be used with conventional OTDR testing equipment, that uses standard 1310/1550nm test wavelengths, to characterize distances to and losses from connection/splice points and find issues such as excessive optical losses and bends in the main/core/common fibers in DWDM networks.

Once MUX/De-MUX connections have been completed, standard/conventional OTDR tools become less useful as by the very nature of the MUX/DeMUX devices those 1310 & 1550nm wavelengths are blocked (filtered out). What is required to characterize the DWDM links end to end are more specialized OTDR that operate at the exact DWDM service wavelengths so they can validate specific routes. For example the VIAVI DWDM OTDR module is a tunable C-band OTDR which will do just that and allow characterization of DWDM links end-to-end through MUX and De-MUX. Having an integrated tunable laser source (via the OTDR test port) also enables a basic continuity test before service turn-up. The Smart Link Mapper (SLM) feature provides an icon-based view of the OTDR trace to simplify the interpretation of test results and to clearly identify common DWDM link components/elements and any faults.

DWDM Turn-up Testing

To verify channel performance and wavelength provisioning over live metro/access links a DWDM Optical Channel Checker Module can be used to accurately assess wavelengths and power over the complete spectrum. 

An Optical Spectrum Analyzer (OSA) is an additional tool for active systems that can verify transmitted wavelengths and power levels and most importantly OSNR. The OSA-110 series OSA module is a compact CWDM and DWDM test solution that is compatible with the T-BERD/MTS-6000A and -8000 platforms. The OSA-110 features full-band measurement capability, high optical resolution and built-in calibration to ±.05nm accuracy.

Network Monitoring with a Remote Fiber Test System and DWDM

Hardware: A remote fiber test system can provide around-the-clock OTDR monitoring in a rack-mounted solution.  Automated, rack-mounted optical test units can be applied to test fiber carrying DWDM transmissions via scan routine or on demand test for specific troubleshooting and restoration use cases for 5G, FTTH, and high-speed business services.

VIAVI test equipment such as the OTU-5000 with 1625-1650 nm is designed to test out of band wavelengths that do not interfere with the active DWDM transmissions. These wavelengths are reserved for test. The OTU-8000 with a tune-able DWDM module enables testing multiple branches in a DAA network such that testing can be conducted in band on a specific transmission wavelength assigned to a node beyond a DEMUX or out of band with a wavelength reserved for tests. With scalability beyond 1000 ports, both Optical Test Heads free up valuable technical resources while providing detailed, instant fault alerts with location details with integrated mapping functionality.

Software: The ONMSi remote fiber test system for core, access, metro and FTTH applications is  software that controls and tracks all data obtained by the OTUs.  It is designed to give your team a network wide view to fiber health and team resolution progress as an essential test solution that establishes a centralized, high-visibility portal for overall network integrity data. This includes construction tests, long-term performance monitoring and intrusion detection (security).

For smaller, private or single links in the  network such as data centers and industrial sites, the SmartOTU software is a stand-alone solution for continuous in-service fiber or dark fiber monitoring and fault detection. The SmartOTU can be deployed out of the box without a server or training requirements.

DWDM Troubleshooting

Locating and repairing faultyDense Wavelength Division Multiplexing (DWDM) network links quickly, and without disrupting existing traffic, is the key to avoiding excessive downtime or SLA penalties. OTDR tests at specific wavelengths can be performed on the live network to avoid service interruption. Optical Channel Checkers (OCC) and OSA can also become valuable troubleshooting tools by using power and wavelength analysis to pinpoint anomalies. Test solutions which include additional features to validate SFP/SFP+ transceivers and even program tunable transceiver devices will significantly reduce Mean Time To Repair (MTTR).

End-to-end test solutions from VIAVI are found in the earliest stages of manufacturing and lab environments and continue to add value throughout the network lifecycle. In the field, remote fiber testing and monitoring solutions like ONMSi  and XPERTrak help to minimize ongoing service issues, OPEX and MTTR by helping locate issues via alarming, to demarcate between fiber vs. network elements and to troubleshoot on demand for specific wavelengths.

With the ability to assess any channel quickly and accurately, operators can gain confidence in correctly installed network links and contractor sign-off that coincides with ongoing performance assurance. DWDM OTDR modules, Channel Checkers and OSA test solutions collectively enhance this certainty and improve first-time activation success rates.

By certifying fiber and channel integrity through installed DWDM MUX/De-MUX and validating new wavelength provisioning, the operational requisites of any DWDM network topology can be satisfied. Spectral and drift testing on different wavelengths are additional capabilities that extend from the lab to service launch and eventually to the monitoring, maintenance and live network troubleshooting practices of a successful DWDM network deployment.

Performance requirements for today’s networks are more exacting than ever, and the need for testing of equipment and networks is critical — from the lab and production environment throughout the entire network lifecycle.