Parallel-optics technology evolving with higher-speed transmission

The physical interfaces that support parallel optic transmission adapt from 12-fiber bases to 8- and 16-fiber bases to accommodate current and future transmission schemes.

From the March, 2015 Issue of Cabling Installation & Maintenance Magazine

The physical interfaces that support parallel optic transmission adapt from 12-fiber bases to 8- and 16-fiber bases to accommodate current and future transmission schemes.

The capabilities and uses of the MPO connector continue to evolve and, literally, take shape as higher-speed fiber-optic transmission specifications come into being. MPO-style connectors, widely available in 12- and 24-fiber variants, provide the physical-layer support for parallel optic transmission, including notably at 40 and 100 Gbits/sec. In parallel-optic 40-Gbit/sec Ethernet transmission, four separate lanes of 10 Gbits/sec travel in one direction (transmit) and four other separate lanes of 10 Gbits/sec travel in the other direction (receive).

A parallel-optic 100-Gbit/sec communication comprises 10 lanes of 10-Gbit/sec transmission and 10 lanes of 10-Gbit/sec reception. A pair of 12-fiber MPOs or a single 24-fiber MPO can accommodate. Many cabling and networking experts have pointed out that the deployment of MPO-based connectivity in a data center is a way to equip an environment currently running 1- or 10-Gbit/sec Ethernet so it is prepared for a later upgrade to 40- and/or 100-Gbit/sec Ethernet parallel-optic transmission.

Unused lanes

Dr. Casimer DeCusatis, an IBM distinguished engineer emeritus and an assistant professor at Marist College, took to the OFC Blog last summer to discuss 40- and 100-Gbit Ethernet issues. In his blog post, Dr. DeCusatis explained, "Although most of the Ethernet market is still running around 1-10 Gbit/sec, there is a small but growing interest-about 10 percent of the market-in significantly higher data rates of 40 to 100 Gbits/sec ... Historically a higher incremental data rate for Ethernet will begin to see high-volume deployments within about five to six years. In other words, 40G should be going mainstream anytime now."

After explaining that the MPO is the connector interface of choice for most parallel-optic deployments, Dr. DeCusatis stated, "For various historical reasons, these connectors were standardized in rows of 12 fibers each; this isn't a good match for data communication systems ... [In a 40G system] if we held up a standard 1x12 fiber MPO connector and looked back into the cable, the four leftmost fibers are used to transmit data, the middle four fibers are left unused, and the four rightmost fibers are used to receive data ... We can do something similar to concatenate 10-Gbit/sec links into a duplex 100-Gbit/sec channel, but we need an MPO connector with two rows of 12 fibers each. We leave the outermost fibers on either end of the rows vacant, and use the remaining 10 fibers in the upper row to transmit data, and the remaining 10 fibers in the lower row to receive data."

Over the past few years, providers of fiber-optic connectivity have introduced systems to combat the apparent inefficiency of MPO connectors' multiple-of-12 orientation. For example, Sumitomo Electric Lightwave (www.sumitomoelectric.com) offers 12-fiber-to-8-fiber conversion cassettes. The cassettes feature 12- or 24-fiber MPO connectivity in the rear and 8-fiber MPO in front. According to Sumitomo, the cassettes "are designed to enable a quick and easy transition from existing 1G/10G 12-fiber network infrastructure to 40G/100G capability."

And Corning Optical Communications (www.corning.com/opcomm) offers the Pretium Edge Advanced Optics solution set, which includes conversion models and harnesses that the company says "allow networks to fully utilize base-12 fiber count trunks when migrating to 40G, which uses base-8 fiber counts. Without this conversion, data centers running 40G parallel optics on their existing fiber backbone only use 66 percent of the installed fiber."

Other organizations have pointed to the practical advantages of using 24-fiber-based systems for efficiency. (See "A 24-fiber interconnect solution: The right migration path to 40/100G," September 2012 issue; and "Are you ready for 40 and 100G?" October 2011 issue.)

These physical-layer principles now also can apply to 100-Gbit/sec Ethernet, with the development of the "4x25" parallel-optic iteration of 100G, incorporating four transmit and four receive lanes, each of 25 Gbits/sec.

Looking ahead to 400G

As Dr. DeCusatis pointed out, the market for 40G systems in data centers, and the optical connectivity to accommodate it, is just beginning to ramp up. But while that's happening, the world's leading-edge communication-system users are looking beyond 40G and even beyond 100G. Efforts to standardize 400G Ethernet are underway, and the MPO connector is in the midst of the discussion. In September 2014 a presentation was made to the IEEE P802.3bs 400-Gbit/sec Ethernet Task Force on the topic of 400GBase-SR16 cabling. Of note, the "16" in the preceding nomenclature denotes 16 lanes of transmission (as well as 16 lanes of reception). So the 400GBase-SR16 setup will call for 16 lanes of 25-Gbit/sec transmission and 16 lanes of 25-Gbit/sec reception.

The presentation to P802.3bs focused heavily on efforts to define 16-fiber (single-row) and 32-fiber (dual-row) MPO-style connectors to accommodate 400G. Within North America, those efforts are being taken up in the Telecommunications Industry Association (TIA) TR-42.13 Subcommittee. The presentation to IEEE noted that US Conec (www.usconec.com) has 1x16 and 2x16 MT ferrule production tooling.

US Conec has started sampling ferrules and connector components in the new 16- and 32-fiber formats currently under the standardization process. US Conec explains, "Next-generation parallel transmission protocols are arranged in increments of 8 or 16. Whether the application requires aggregation of multiple 8-fiber QSFP links (4 Tx/4 Rx) or direct coupling to emerging 32-fiber parallel-optic links (16 Tx/16 Rx) as proposed in the IEEE P802.3bs 400 Gbit/sec initiative, US Conec's X-16 fiber MT format is an ideal solution for high-density interconnect applications.

"The new, X-16 MT format utilizes the same external ferrule footprint as the existing traditional 12-fiber products, leveraging all of the proven features and technology of US Conec's PPS MT ferrules in a higher density format. The ferrule comprises a smaller diameter guide pin and hole with a longer pitch and is compatible with existing MT-based connector platforms."

Within TIA TR-42.13 work continues on what will become FOCIS-18, Fiber Optic Connector Intermateability Standard 18, which will specify these 16- and 32-fiber interfaces. One consideration in this connector's development is the fact that, as proposed, its outer dimensions will be the same as those of the MPO defined in FOCIS-5. As such, the possibility exists that technicians will try to mate a 16- or 32-fiber/FOCIS-18 male with a 12- or 24-fiber/FOCIS-5 female. The developers of FOCIS-18 are working to ensure against events like this. One possibility is for FOCIS-18 to specify different keying than what is found in FOCIS-5, but details such as these remain tentative and very much under development. We will continue to follow the technical and standards developments related to 16- and 32-fiber connectivity, and keep you informed of progress.

Patrick McLaughlin is our chief editor.

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