DWDM is moving into access networks at a higher rate than ever before. This includes cable service providers upgrading to the newer distributed access architecture (DAA), both cable and traditional telecom service providers to deliver business services, and wireless service providers as part of their centralized RAN (C-RAN) approach as well as a key part of their 5G deployments. Even owners/operators of passive optical networks (PON) are looking at how to operate DWDM over the same PON where there may already be regular E-PON or G-PON services delivering fiber-to-the-home (FTTH).

So, why is everyone looking at DWDM? The answer is simple, really: capacity and cost.

With DWDM, you can transmit multiple wavelengths of light on the same fiber, effectively multiplying the number of services (1G, 10G or 100G) you can deliver and support more end points/devices or customers. Because this is over a single fiber you can really leverage your assets if you already own the fiber or reduce CaPEX if you are deploying new fiber… or minimize your operational costs if you are leasing dark fiber.

Of course, the deployment and use of DWDM does present new challenges. For example, there are additional components, MUX and DeMUX devices, which are required at each end of a link or where a service is to be dropped off or terminated. These devices have wavelength-filtered ports for the individual service wavelengths and a common port which connects to the main, or ‘core,’ fiber in a link.

For access network applications, these DWDM links are usually passive, as much for complexity reasons (or rather simplicity reasons) as for cost, but this will limit the distance over which a DWDM service can be delivered. The way to extend the reach of a DWDM network would be to use optical amplifiers to boost the optical power levels and overcome the optical losses in the network. However, once you go that route there is a lot more to consider: the cost of the amplifiers, the practical issue of housing and powering the amps (more cost), and the need to set up and maintain the amplifiers — which means longer commissioning times and the ongoing operational maintenance (yet more cost). So, for complexity and cost reasons, most applications of DWDM in access type networks are passive.

How do you ensure that you have built a network that will be reliable, require minimal maintenance and not cause delays with service activations? The answer is to test and certify the build (of course!).

During construction it’s fairly easy to test the core fiber. The basic recommendation would be bi-directional IL, ORL and OTDR certification using standard 1310/1550nm wavelengths. However, once the MUX/DeMUX devices are connected and you want to check or verify the end-to-end links, the standard wavelengths won’t help you. Access network DWDM operates in the C-band which is between 1520 – 1565nm, so 1310nm won’t help much as the filtering of the MUX/DeMUX ports will simply block that wavelength (plus, 1310 is not a DWDM service wavelength). A standard 1550nm OTDR won’t help much either, as the test pulse is too spectrally broad to be passed by a port, the majority of the OTDR pulse energy will be blocked or removed (filtered out) so you’ll never get a good OTDR test result, and there is definitely not a decent end-to-end view of a specific wavelength route through both MUX and DeMUX.

What’s needed is a dedicated DWDM OTDR that operates at the specific DWDM wavelengths, which will allow testing of the MUX/DeMUX before installation and each service port — to certify each wavelength route end-to-end once the MUX and DeMUX are installed. There is also an argument for using a DWDM OTDR for certifying the core fiber in the first place, instead of a standard OTDR, in order to guarantee that there are no issues which will affect specific or groups of wavelengths. After all, once the network is built and services are active, any remedial work required on the core fiber means scheduled downtime or an outage for all service on that link.

One challenge that techs do encounter with a little too much regularity is missing or incorrect labeling of MUX/DeMUX ports or labels that are worn and illegible. A lot of time can be wasted manually setting a DWDM light source or DWDM OTDR to each wavelength to try and identify the port channel, not to mention it is a tedious task. In this situation, what is needed is a fast way to identify the port channel and automatically launch into the OTDR testing, a capability that VIAVI has introduced to its DWDM OTDR with a feature called Wavescan®. As the name suggests, it enables the OTDR to automatically scan through wavelengths and identify the port channel, usually in under 10 seconds, and then carry out the OTDR test, all with a single button push.

The same argument for having a tool that is specific to DWDM applies to optical power meters, too. Once you are operating in a multiple wavelength environment, then standard wavelength power meters and especially broadband power meters are no good (unless you can guarantee that there is only one wavelength present at your measurement point). This is because they cannot select or isolate an individual wavelength to report accurate power level and wavelength. What’s needed is a dedicated DWDM optical channel checker (OCC) or optical spectrum analyzer (OSA), but we’ll leave discussing those for another time.

Being able to check (or shoot) individual DWDM wavelengths also means that a DWDM OTDR can be used once a DWDM network is live. as it won’t interfere with any of the other active services. This means that you can perform continuity and wavelength routing checks before additional services are turned up, ensuring that activation happens right the first time around or as part of fault-finding/maintenance activities.

So when deploying DWDM, save yourself future operational headaches (and cost) by fully certifying core fibers bi-directionally during the build and wavelength routes end-to-end, through MUX and DeMUX, once the build is completed. This ensures the best, first-time DWDM service turn-up success rate.

For more information on DWDM, take a look at our DWDM solutions guide for Cable MSO and for Wireless Service Providers or take a look at our DWDM technology page.

This is my final post in the fiber construction series. If you missed them, my previous posts include:

Part 1: Comparing True Bi-directional Analysis and Loopback Testing

Part 2: How to Improve Efficiency and Accuracy When Certifying PON

Part 3: Certifying PON with Unbalanced Splitter Architecture


Douglas Clague is currently solutions marketing manager for fiber optic field solutions at VIAVI. Doug has over 20 years of experience in test and measurement with a primary focus on fiber optics and cable technologies, supporting the telecommunications industry. Prior to VIAVI, Doug held positions as manufacturing engineer, solutions engineer and business development manager. Doug has participated on numerous industry panels around fiber and cable technology trends. He attended Brunel University in London and graduated with an honours degree in electrical and electronic engineering.

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