Data Center Cabling Solutions

Specifying Fiber Infrastructure as a Critical Network Component

Published October 2014, updated March 2019


Network hardware designers must prioritize the acquisition and placement of the most critical network hardware such as active switches, servers, and storage devices as they implement their company’s data center design plan. Historically, the design and quality physical layer of the network has often been an afterthought.

This last-minute thinking has led to the inadvertent purchase of fiber optic cable assemblies of poor quality, sometimes with non-licensed connectors, and glass from unknown sources and of unknown performance levels. As network speeds have increased, however, the assumption that any fiber optic cabling will do can be fatal to the ROI of the data center.

Through education and need this inattention to the infrastructure of the cable quality and design is fading fast, ushering in a new era of global network design excellence for which world-class cabling is required.

The notion of treating fiber optic cabling as a last-minute purchase is changing due to multiple factors. These include next-generation transmission speeds with far lower allowable losses, network virtualization initiatives, and the most consequential, the technical advances of the light transmitters inside the active devices.

In the 1980’s and early 1990’s, the only network light transmitter available was the light emitting diode (LED). With the transition to Vertical Cavity Surface Emitting LASERs (VCSELs), the light within the fiber optic core area became more focused, lighting up a much smaller area of the core, enabling higher speeds, greater bandwidth, and overall faster Ethernet and fiber channel protocols.

These newer light transmitters (VCSELs) demand far superior fiber optic cabling. The light launch nature of the VCSEL makes the end face geometry and the polish far more significant than in the past. As a result, most network designers today need to view the physical layer fiber optic cabling to be of equal, importance as the active components of their network.

Current trends show data center active network gear is usually refreshed or replaced in three to five years from date of purchase. Replacements bring the need for greater speeds, more data movement, more feature use.

This drives the need for high quality fiber optic infrastructure that will allow for the growth of your data center needs. The proper layer one design today, can easily last far into the future, if good design practices are followed and emphasis is placed on the quality of the cabling.

Although very important and a good starting place, Industry standards are the minimum criteria to which a cable assembly should be manufactured. Most cable assemblies are built to these standards, but this may not be enough to future-proof your data center.

Make sure the product you buy can support any planned upgrades you have, and note that, some unreliable sources may cut short of meeting even the lowest industry standards.

Today’s data center environment is seeing a shift in design philosophy. Green initiatives are allowing for data centers to run at hotter temperatures than previously thought necessary. Cooling has long been an area of focus in data center architecture.

In a 2007 Environmental Protection Agency (EPA) study [1], results indicated that target temperatures of most data center computer rooms are lower than they actually need to be. By raising their target temperature a few degrees, power usage can be reduced by 20% over the course of one year. As this recommendation gains ground, the hot aisles will be even hotter than they are today. Clearly, purchasing cabling that exceeds industry minimums for thermal cycling tests will be in the best interest of data center architectures going forward.

Fiber optic cable manufacturing includes industry standards for connector loss, thermal aging, humidity, thermal shock, thermal cycling, vibration and other test criteria. The issue is that most fiber optic and copper cable manufacturers, especially those imported from overseas, are not capable or simply do not have proper testing equipment in place.

Given the long-life potential of a cable assembly and given the potentially harsh environment in which they will be utilized, it is good design practice to purchase assemblies that exceed the industry standards and include the below performance testing after the below characteristics are applied to the assemblies, most of which are not included the standards.

These testing units should be:

  • Thermal Aging
  • Humidity
  • Thermal Shock
  • Thermal Cycling
  • Vibration
  • Off axial twist and pull

Logically, it would be best practice to select a cabling manufacturer that has invested in equipment to test for these environmental industry standards. Having this equipment onsite would enable the manufacturer to periodically re-verify that the materials supplied to them by their OEM vendors continue to meet the standards they expect and promise to data center managers.

Another factor to consider is the outer diameter of the cable assemblies. Lower diameter cables improve air flow, making cooling more efficient. Smaller diameters also enable more cabling to fit in the cable tray.

Why is low optical fiber loss now strongly recommended when considering manufacturers?

Consider that most manufacturers do not recommend or support more than six connections in an optical fiber channel.

The Industry Standard for LC connector loss is specified at 0.5dB maximum per mated pair. And with a 10 Gb/s LC duplex maximum allowable channel loss of 2.6dB (loss budget), and 25/40/100G at 1.5dB, one is challenged to stay under 2.6dB or 1.5dB with the maximum six (recommended) connections per channel.

With MPO-style optical fiber connectors, choosing a very low-loss MPO option is essential, especially since the MPO-style connector is one of the industry’s chosen connector for 40G, 100G, 200G, and 400G.

  • Industry standard MPO loss is 0.75dB max.
  • An Industry leading low-loss MPO connector is 0.20dB MAX or lower.
  • A low-loss MPO is also strongly recommended if the MPO connector is used as part of an LC connector-based backbone solution.

What is thermal cycling and humidity cycling?

If capable, cable manufacturers will test and certify their cabling products by qualifying them in a temperature and humidity chamber. The temperature and humidity are cycled up and down to stress the cable and simulate aging.

Exceeding the industry standard in this area is highly recommended as it dictates the overall quality and lifespan of the cable assembly.

Why is it important to demand licensed LC connectors?

It is highly recommended to use only licensed connectors. There are many LC connectors, often manufactured off-shore, that are made with unlicensed LC connectors. These have been known to be DOA, fail quite early in their life, or fail with little or no stress applied to them in a cross-connect environment.

They have also been known to have outer ferrule diameters that are out of tolerance on the high side. This defect results in a connector that, when plugged into a coupler, does not disconnect without breaking first. It is very difficult to know the quality and performance of an unlicensed product.

What is important to look for in an MPO connector?

With respect to the MPO connector, there are several different options. The average loss MPO cable assembly uses average performance parts. Consistently, the low-loss MPO connectors use higher-quality parts, equating to better performance. Investing in high-performance MPO connectors will ensure a robust layer one infrastructure.

The MPO connector end face is polished during manufacturing. This is always a proprietary “secret sauce” recipe of a combination of polishing films, times, and pressures. Not all manufacturers excel in polishing.

The metric for this is in the fiber protrusion and uniformity of the end face. Some fiber protrusion is necessary to guarantee physical contact at connector matings. Too much protrusion will allow for physical contact in the short-term life of the cable but will result in fiber breakage soon after it is deployed. Too little protrusion will lead to poor mating and significant signal loss.

Fiber protrusion and uniformity can only be measured with a tool called a profilometer. One cannot assume that all manufacturers have a profilometer, and one cannot assume that those who do have one use it to keep their processes in check on a daily basis. It is important to note that there is no industry standard for the fiber protrusion of an MPO connector. It is left up to the manufacturer to decide and test for what works best.

Is all fiber optic glass the same?

Lower costs can point to manufacturers cutting corners, resulting in poor bandwidth rating, non-bend-insensitive glass, and other transmission problems. In a multi-mode solution, currently OM4 glass or above is recommended, especially when a data center is considering moving to a 25/40G/100/200/400G environment in the future.

After all, the glass is where the information of the network travels from device to device, essentially the core of any data center architecture. Be wary of situations in which the glass manufacturing is several vendors removed from the company providing the final product. It’s imperative to use caution when unable to verify glass performance from the original glass manufacturer.

Conclusion

Given the demanding nature of the switches and active devices in our networks today, the physical layer is of greater importance than ever in the history of data center architectures. High-performance sports cars demand high-performance tires. Similarly, a high-performance network demands world-class layer one cabling.

Plan ahead, ask questions, do your research. Ensure your company’s network can be optimized to its full potential.

References

[1] Cited from the ANSI/BICSI 002-2011 Data Center Standard


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