The several flavors of 100-Gbit optical Ethernet

Dec. 1, 2016
Specifications from the IEEE define the transmission of 100-Gbit/sec optical signals.

By Patrick McLaughlin

More than 10 years ago the path toward 100-Gbit/sec Ethernet transmission began within the Institute of Electrical and Electronics Engineers (IEEE), and in that time the IEEE’s 802.3 Working Group has developed multiple specifications for 100G transmission. This article will cover existing 100G specifications as well as current IEEE 802.3 activities to establish additional standards. It focuses on optical technologies.

The 802.3ba standard, approved in 2010, establishes an architecture for both 40- and 100-Gbit Ethernet. Shortly after the standard’s approval in June 2010, the Ethernet Alliance published a technical paper summarizing the new set of specifications. Within that paper is a section titled “Physical Media Dependent (PMD),” which spells out physical layer specifications. The paper explains that the 100GBase-SR10 PMD is “based on 850-nm technology and supports transmission over at least 100-meter Om3 parallel fibers and at least 150-meter Om4 parallel fibers.” The effective rate per lane is 10 Gbits/sec. As such, “the 100GBase-SR10 PMD supports the transmission of 100 Gigabit Ethernet over a parallel fiber medium consisting of 10 parallel Om3 fibers in each direction.”

100GBase-LR4 is based on 1310-nm dense wave-division multiplexing (WDM) technology. It supports transmission over at least 10 kilometers on singlemode fiber. “The effective data rate per lambda is 25 gigabits per second … In this way, the 100GBase-LR4 PMD supports transmission of 100 Gigabit Ethernet over four wavelengths on each singlemode fiber in each direction,” the Ethernet Alliance paper explains.

100GBase-ER4 is also based on 1310-nm WDM technology and supports transmission over at least 40 kilometers on singlemode fiber.

Nearly five years after 802.3ba was published, the IEEE finalized the 802.3bm specifications. In its Standards Informant Blog, Siemon provides detail on the “bm” specifications, including this insight: “The rapid growth of server, network, and internet traffic drove the need for higher data rate, higher density, and lower cost optical fiber Ethernet solutions, especially in data centers. The 100-Gbit/sec optical fiber Ethernet applications specified in IEEE 802.3ba include a 10-lane electrical interface (20 fibers total) for operation over multimode optical fiber cabling (10GBase-SR10). Advances in technology supported the specification of a new multimode 100-Gbit/sec physical layer (PHY) specification with reduced lane count, which will simplify upgrades from 40GBase-SR4 using the existing cabling plant.”

The Standards Informant adds one of the new PHY specifications in 802.3bm is 100GBase-SR4, which specifies 100-Gbit/sec transmission using a four-lane electrical interface (8 fibers total) for operation over multimode optical fiber cabling with reach up to at least 100 meters.

When the IEEE announced the availability of 802.3bm, David Law, the 802.3 Ethernet Working Group’s chair, commented, “IEEE 802.3bm demonstrates how IEEE 802.3 is taking a forward-thinking approach to stay ahead of global demands on Ethernet networking, which paves the way for innovation in devices and services.”

Dan Dove, chair of the 802.3bm Task Force, added, “Significantly higher Ethernet performance, capacity and reach in optical networking are needed especially inside and among data centers across metropolitan areas, given the ongoing proliferation of smartphones, video-on-demand, cloud computing and other bandwidth-intensive applications such as the Internet of Things. The project to develop IEEE 802.3bm addressed these market needs through a globally open, collaborative effort that drew contributions from a broad spectrum of engineers and end users across the Ethernet ecosystem.”

Later in 2015, 802.3 formed three study groups, which are exploring the development of standards for 25-, 50-, and 100- and 200-Gbit/sec Ethernet. Siemon also addressed this development in its Standards Informant, describing the efforts to develop IEEE 802.3cd. The Standards Informant forecasts a publication timeframe of approximately September 2018. Within the blog, Siemon notes, “Server interconnects in the data center, which represent the highest number of equipment connections, require cost-effective solutions. Advances in cost-optimized single-lane solutions and higher-speed multi-lane transmission solutions warrant reevaluating the signaling technology for 50-Gbit/sec and 100-Gbit/sec Ethernet.

“In addition, servers virtualizing more applications are driving additional bandwidth into the network and network uplinks need to progress to higher speeds to match server speeds. 200-Gbit/sec can support network infrastructure and oversubscription rates similar to 40- and 100-Gbit/sec Ethernet as servers migrate from 25 Gbits/sec to 50 Gbits/sec, while also enabling data center fabric topology.”

Among the PHY specifications in this amendment to 802.3 are the following.

  • 100GBase-DR: 100-Gbit/sec serial transmission over one wavelength (2 fibers total) for operation over singlemode optical fiber cabling with reach up to at least 500 meters
  • 100GBase-SR2: 100-Gbit/sec transmission over two lanes (4 fibers total) for operation over multimode optical fiber cabling with reach up to at least 100 meters

Of note concerning the IEEE 802.3cd specification is that the task force did not include an iteration that calls for duplex multimode operation of 100-Gbit/sec per fiber using short-wave WDM. That transmission would have used wideband multimode fiber (WBMMF), which recently obtained the nomenclature Om5. In a web seminar hosted by Cabling Installation & Maintenance on October 27, OFS’s John Kamino presented an update on WBMMF standardization. Within that presentation Kamino explained that the 802.3cd task force did not approve the inclusion of WBMMF in the standard. He expressed the opinion that, although there are no current plans to include WBMMF in an IEEE standard, there is likely to be some form of this technology developed in the market.

We will continue to follow standards and technology developments, and will keep you updated.

Patrick McLaughlin is our chief editor.

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