North Carolina’s Research Triangle is known for solving mysteries, in particular technical ones. That’s what happens when three tier-one research universities — North Carolina State University (NCSU), Duke, and the University of North Carolina (UNC) at Chapel Hill — and cutting-edge tech companies such as Sumitomo Electric Lightwave (SEL) Corp. (SEL) are within the triangle.

One of SEL’s areas of expertise is deciphering fiber-optic connectivity challenges. In keeping with Research Triangle tradition, SEL lays claim to these innovations:

  • a patented Vapor Phase Axial Deposition manufacturing process;
  • lowest attenuation Z-fiber at 0.154dB/km;
  • bend insensitive fiber PureAccess
  • fiber-optic ribbon cable “Easy Split & Peel” technology, allowing access by hand; and
  • the industry’s first MPO Splice-On Connector.

I ran across SEL while researching IEEE’s 802.3ba 40Gb/s and 100Gb/s Ethernet fiber-optic standard. Why was I putting myself through that? Well, before too long every commercial data center on the planet will have portions of its fiber-optic network migrated to 40Gb/s or 100Gb/s to stay competitive business-wise. Finding the paper The Optical Fiber Ribbon Solution for the 10G to 40/100G Migration (PDF) written by SEL’s Bill Charuk, product manager, data center solutions, was especially fortuitous, as it answered several perplexing questions.

IEEE 802.3ba

The 802.3ba 40G/100G Ethernet standard was ratified in June 2010. The standard provides guidance for 40G/100G data transmission using single-mode and multi-mode fiber-optic cables.

  • Single-mode: This classification allows one light-transmission to propagate. This lowers light attenuation, allowing the signal to travel faster and farther.
  • Multi-mode: This classification is capable of four optical modes (OM1-OM4). It has a larger diameter core than single-mode cable, which allows multiple light-transmissions to propagate together — this, in turn, means more data throughput, albeit for a shorter distance.

Data centers operators, always concerned about bandwidth, opt for multi-mode fiber-optic cables because of its greater throughput.

Why ribbon-style cables?

Ribbon-style cabling comes into play because OM3 and OM4 — the only multi-mode fibers included in the 802.3ba standard — use parallel-optic transmission. According to an article on the Cabling Installation & Maintenance site that means by design optical/electronic interfaces allow data to be transmitted and received over multiple fibers. It also means 40G Ethernet interfaces consist of four 10G channels on four fibers per direction, and 100G Ethernet interfaces use four 25G channels on four fibers per direction as shown in the diagram below.

Bottom line: parallel runs are used to increase throughput bandwidth using either multiple fiber-optic cables or multiple fibers in a ribbon cable. To the point of employing a ribbon cable over individual cables Charuk writes, “The use of ribbons allow for easier connectorization (less opportunity to cross fibers in an MPO connector), and perhaps more importantly, achieve easier polarization continuity regardless of the polarity method selected for the system.”

Cable considerations

Before deciding on a specific cable design, Charuk advises considering these characteristics:

  • Adequate fiber count for future need
  • Available duct/conduit space
  • Installation path
  • Ruggedness of design
  • Termination method
  • Fire codes
  • Cost

“Ribbon cables have been used in the telecom industry for over twenty years,” writes Charuk. “They were introduced to increase the fiber density in a given cable and to reduce cable costs. Of particular importance is fiber density, as fiber counts increase in the data center, it is an attractive feature.”

Consider future upgrades

Fiber-optic ribbon cables seem like a logical choice. “The overall combination of ruggedness of the ribbon design, fiber density, size, and relative cost points to ribbon as being most suited to both new and retrofit installations in the data center,” concludes Charuk. “Additionally, the ribbons in fiber optic cables are best suited for future expansion, since the transmission protocols progress to higher and higher data rates.”