Four Practical Considerations for Distributed Access Architectures

Driving digital optics deeper into the network presents key challenges for operators
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With the dramatic expansion of applications and OTT content, the demand for bandwidth is constantly rising. As a result, MSOs are currently undertaking a major network transformation towards Distributed Access Architectures (DAA), such as Remote PHY, Remote MAC, and Fiber Deep to drive digital traffic over fiber closer to the customers.

Deploying WDM technologies allow existing fiber infrastructure to be leveraged for DAA.

Driving digital optics out of the headend and into the outside plant presents practical challenges for network operators. DAA nodes exist in the cable plant, outside the relative comforts of the headend, where robust environmental control systems with backup power do not exist. WDM technologies are especially at risk in these environments.

Operation at the extreme temperatures in these environments is critical to network performance, and so there are a few practical considerations that need to be taken into account by MSOs.

Practical Consideration No. 1: Know The Lingo

It’s essential to understand the commonly used terminology that governs how network devices are recommended for deployment, especially in regards to temperature.  

The key terms to keep in mind are Commercial Temperature (C-Temp), Industrial Temperature (I-Temp) and extended temperature. C-Temp is the most common envelope for standard use in controlled environments and stretches from 0°C to +70°C, while I-Temp, which stretches from -40C to +85C, is recommended for devices deployed outside of climate controlled environments, such as customer premises sites, outdoor electronics cabinets, mobile antennae sites, and cable television return path nodes. Extended temperature is not a recognized envelope, but is used when devices can operate properly outside the C-Temp envelope, but fail to meet full I-Temp requirements.

Practical Consideration No. 2: Optical Transceivers are Impacted by the Physical Environment

Wavelength drift impacts all types of optics, WDM and standard gray optics alike. Fiber optic wavelengths fluctuate around their center wavelength over time. Common factors that impact the drift are time and environmental temperature conditions.

WDM technology uses passive WDM devices to multiplex multiple circuits on to a single fiber or fiber pair.  WDM passive devices have a ‘passband’ that essentially acts like lane markers or dividers in a swimming pool. The passband ensures that each wavelength stays in their own lane as it is transmitted across the fiber optic circuit. Like a swimming pool lane divider, there is a certain amount of room for WDM wavelengths to drift within the lane.  As in a swimming competition, should a wavelength drift outside of its assigned lane or passband, the passive device will penalize by dropping the transmission.

Practical Consideration No. 3: don’t Take Wavelength Performance for Granted

Performance over temperature over time cannot be taken for granted. Research has shown that temperature cycles over time can indeed impact the long-term performance of WDM optical transceivers. The behavior of the WDM laser may become less predictable with multiple temperature cycles.

Practical Consideration No. 4: Not All Transceivers are Created Equal

The transceiver marketplace offers two approaches to ensuring performance over temperature. First is the “should be good enough” approach, where transceivers are screened in the factory. The screening may include reading optical test results or perhaps placing the transceiver in an environment chamber. The key point for network operators to consider is that the transceiver may perform at temperature at “birth,” but it does not consider the effect of temperature cycles over time on the transceiver.

The second option available is transceivers that incorporate wavelength stabilization technology to ensure WDM wavelengths remain in their “swim lanes” over time and temperature.

Two of the common wavelength stabilization technologies are heat-assisted and thermo-electric cooling. Both technologies are integrated on to the micro-controller of the transceiver and are transparent to the network switch or element.

In conclusion, practical considerations for distributed access archictectures must account for the impact of the environment on digital optics deployed in the field. Ensuring that your WDM optics deployed in the field incorporate stabilization technology is one practical step to network performance over temperature over time.

Ray Hagen is Americas Product Manager at ProLabs, a maker of OEM-compatible optical transceivers and connectivity products