Data Centers

Wanted: A new fiber-optic standard that meets data centers' needs

The length of cable runs in massive data centers is driving the creation of a new fiber-optic standard. Several consortiums are taking on this challenge.

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Image: Slide from Intel/Arista Networks presentation about 100G CLR4.

Moore's Law is a good example of how scientists and engineers have marched right past what were once considered manufacturing and material limits. Networking technologists are seemingly like-minded. They intend to produce a 100 GbE optical interface that is low cost, uses less power, and provides greater range.

If you're wondering what's driving this effort, look no further than data centers. Current fiber-optic technology does not meet their needs or budgets. Dr. Mario Paniccia, Intel fellow and general manager of Intel's Silicon Photonics Group, in an Intel/Arista Networks presentation (PDF) noted the increased footprint of data centers is taxing connectivity and networking capabilities.

For our discussion, let's focus on footprint. The largest data center in the world is an ever-moving target. That said, the top five on the list are all over 1 million square feet (approximately 15 US football fields) in size. Those who keep track of this sort of thing place the Switch SUPERNAP data campus located in southern Nevada with an aggregated total of close to 2 million square feet at the top of the list. Being a campus with interconnected buildings, it is conceivable there are long fiber-optic runs.

The challenge

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Image: Multicom
To get an idea of the challenge facing engineers, let's take a quick look at the construction of a fiber-optic cable. The digitized light signals travel along a core made from thin glass or plastic fibers using an optical engineering trick called total internal reflection. The slide to the right (courtesy of Multicom) depicts the core surrounded by several protective layers.

Two other terms will come up in the discussion: Single-Mode fiber-optic cable and Multi-Mode fiber-optic cable. Multicom provided the following definitions:

Single-Mode fiber-optic cable has a small diametral core that allows only one mode of light to propagate. Because of this, the number of light reflections created as the light passes through the core decreases, lowering attenuation and creating the ability for the signal to travel faster, further.

Multi-Mode fiber-optic cable has a large diametral core that allows multiple modes of light to propagate. Because of this, the number of light reflections created as the light passes through the core increases, creating the ability for more data to pass through at a given time. Because of the high dispersion and attenuation rate with this fiber, the quality of the signal is reduced over long distances.

That's the basics. Now to what data centers are looking for: a fiber-optic interface that will transmit 100 gigabits of digital traffic per second, minimal fiber-optic signal loss allowing a readable signal 2 kilometers away, and of course cheap, relatively speaking.

The Intel slide above shows the sweet spot, right between two existing fiber-optic interfaces, SR4 and LR4. Here are their specifications:

  • SR4 is the Short-Range multi-mode 50-micron fiber-optic interface. The larger diameter makes working with the fiber optics easier, but the larger diameter also increases light attenuation limiting SR4's range to 100 meters.
  • LR4 is the Long-Range single-mode 9-micron fiber-optic interface. The smaller diameter reduces light attenuation allowing a much longer range (10 kilometers) and higher bandwidths. But the small size makes it difficult to manufacture and work with, thus expensive.

All sorts of interest

There is an intense interest in developing a 100 GbE optical interface that will ultimately become the standard. Several parties have formed Multi-Source Agreements (MSAs) towards that goal.

  • The 100G CLR4 alliance spearheaded by Intel and Arista Networks with support from several major companies in the IT space is designing an affordable, low-power optical interface for a QFSP transceiver. The intent is to create a multi-mode fiber optic with four lanes of 25 Gb/sec.
  • OpenOptics MSA is a consortium headed by Mellanox Technologies. "New MSA will define 1550nm silicon photonics based QSFP optical transceiver for 100G data-center networks on single-mode fiber infrastructure."
  • A third consortium CWDM4-MSA consists of Avago Technologies, Finisar Corp., Oclaro Inc., and JDSU, a cable manufacturer. This group is also working with single-mode optical interfaces.

Will it be enough?

In 2011, the Chinese company Range Technology Development announced that it was building a data center in Langfang, China. The building will measure more than 6.2 million square feet (roughly the same size as the Pentagon). The data center is not expected to be completed until 2016. With that size building, one has to wonder if the two kilometers specified in this round of 100 GbE standards is enough.

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