Several standard-development projects will extend multimode’s capabilities. We take a close look at how SWDM and OM5 can fit in.
Multimode optical fiber has proven itself to be dependable and versatile in high-speed data transmission for data center networks. Some forward-looking planners have made the case that singlemode is the future in data center optical networking, and that may be the case. If it is, then it’s likely the mid- to long-term future, because indications are that multimode will continue to serve users’ needs for generations of networking to come.
This article will provide examples of standards-, technical, and opinion-based information from technical experts pointing to multimode optical fiber’s value.
Tony Irujo, sales engineer for optical fiber with OFS, explains, “For many years multimode fiber has served the industry well as a cost-effective solution for short reach. Today OM4 is a workhorse multimode fiber, capable of 10-Gigabit reach to about 600 meters and 40/100-Gigabit transmission in the 300-meter range, depending on the transceivers used.”
He further explains that OM3 and OM4 fiber “support transmission using a single wavelength, primarily at 850 nanometers. To achieve higher data rates, you can add fibers; these additional fibers allow you to aggregate to higher speeds” through parallel optics. For example, one version of 40-Gbit/sec Ethernet calls for the use of eight multimode fibers, in which four fibers each transmit 10 Gbits/sec, and four others each receive 10 Gbits/sec. The concept can extend to 100 Gbits, with each of four fibers transmitting 25 Gbits/sec and the other four each receiving 25G.
“Parallel or multifiber transmission is a good way to increase data rates, but just to a point,” Irujo points out. “When you start talking about 32 fibers to support 400G, it gets unwieldy.”
Transceivers that offer short-wavelength division multiplexing (SWDM) operation employing four wavelengths offer longer reach over OM5 fiber than over OM4 or OM3. Source: CommScope.
Enter SWDM
An alternative to higher-fiber-count multimode constructions is the use of OM5 fiber and transceivers that facilitate wavelength-division multiplexing (WDM). “OM5 allows you to transmit multiple wavelengths on one fiber,” OFS’s Irujo explains. WDM is “a technology that has been used with singlemode fiber for many years. It’s called SWDM—short-wavelength division multiplexing. OM5 is designed for performance not just at 850, but across the range from 850 to 953 nm. SWDM on OM5 fiber can provide duplex 100G links and allow for 400G operation using the same eight-fiber technology that’s currently in use for 40 and 100G links,” he notes.
The commercial success of OM5 fiber and SWDM technologies is yet to be determined, and they have some vocal detractors. Regardless, the technological capabilities are not in dispute, and proponents of OM5/SWDM have a voice as well.
Christian Urricariet, senior director of global marketing for Finisar, is one such voice. He explains that for many enterprise users, employing LC-connector-based SWDM for 40 or 100G networking is an attractive alternative to either singlemode-based optics or parallel, MPO-based multimode. He explains that for a 10G network employing LC connectors, moving to MPO-based, parallel-optic 40 or 100G “is not only capex-intensive, but also may increase opex given the unfamiliarity of the IT networking personnel with the handling and cleaning of the MPO connector … Ideally, enterprises would like to upgrade their data centers to 40G/100G Ethernet without changing their existing duplex multimode fiber infrastructure being used for 10G Ethernet. And many would like to maintain the same supported link distances as 10G Ethernet.
“SWDM addresses this market need,” Urricariet continues. “The four-wavelength implementation of SWDM is called SWDM4, and these four wavelengths are multiplexed/demultiplexed inside the QSFP transceiver into a pair of multimode fibers—one fiber in each direction, i.e. a standard duplex interface.”
Finisar’s 40G SWDM4 QSFP+ transceivers support up to 300-meter transmission on OM3 and 400-meter transmission on OM4 legacy multimode. The transceiver supports up to 500 meters on OM5. Finisar’s 100G SWDM4 QSFP+ transceivers support up to 100 meters on OM3 and 150 meters on OM4 legacy multimode. It supports up to 180 meters on OM5.
Making the case
“Besides extending the reach, an additional benefit of upgrading to OM5 multimode fiber is futureproofing the fiber infrastructure for 200G/400G/800G multimode interfaces suitable for the data center, which may take advantage of SWDM technology,” Urricariet adds.
“SWDM technology also provides additional operational advantages with respect to proprietary bidirectional, or BiDi, solutions addressing the same market need,” he says. “The first advantage is supporting longer link lengths. The second is making network monitoring much simpler, since SWDM4 transceivers can be used with standard network taps and with standard transceivers on the monitoring equipment, unlike BiDitransceivers.”
He concludes by emphasizing that SWDM is “not a single-vendor, proprietary solution. A group of companies including optical transceiver suppliers, fiber and cabling vendors as well as system OEMs have formed the SWDM Alliance and the SWDM MSA [multisource agreement]. Their goal is to promote the use of SWDM technology on duplex multimode fiber, as well as to ensure optical interoperability among the different vendors’ SWDM product offerings.”
Although Urricariet’s emphasis is on the use of SWDM with duplex fiber cabling, SWDM and parallel-optics can coexist—and standard-development organizations are leveraging that fact. In an early-September post to the CommScope Blog titled “400G Ethernet is About to Get a Boost,” Paul Kolesar, an engineering fellow in the company’s connectivity solutions division, provides some detail.
Standard-recognized
“A task force within the IEEE on project 802.3cm was formed in March 2018 to fulfill two new multimode objectives for 400-Gbit/sec Ethernet operation,” Kolesar explained. 400GBase-SR is an 8-pair, 1-wavelength multimode solution supporting reach of 70/100/100 meters over OM3/4/5. And 400GBase-SR4.2 is a 4-pair, 2-wavelength multimode solution supporting reaches of 70/100/150 meters over OM3/4/5.
“400GBase-SR8 is the first IEEE fiber interface to use 8 pairs of fibers,” he said. “Its 8-fiber pair interface will have two variants. One will use the 24-fiber MPO, configured as two rows of 12 fibers. The second interface will use a single-row MPO 16.
“In comparison, 400GBase-SR4.2 is the first instance of an IEEE 802.3 solution that employs both multiple pairs of fibers and multiple wavelengths. It will operate over the same type of cabling used to support 40GBase-SR4, 100GBase-SR4 and 200GBase-SR4. This is also the first Ethernet standard to use two short wavelengths to enable doubling of the multimode fiber capacity from 50 Gbits/sec to 100 Gbits/sec per fiber. It will do so using bidirectional propagation on each fiber, with each wavelength traveling in opposite directions. As such, each active position at the transceiver is both a transmitter and a receiver.”
Later in the post, Kolesar informs, “400GBase-SR4.2 joins a stable of four other existing transceiver types that also use short wavelength multiplexing. Two of these are … BiDi solutions … Two more are the 40G-SWDM4 and 100G-SWDM4 MSAs that employ four wavelengths. Each type enjoys longer reach over OM5, owing to OM5’s SWDM optimization that ensures essentially equal performance for all wavelengths from 850 nm to 953 nm.
“Wavelength division multiplexing has long been a staple of singlemode transmission, continuing to this day. Now the use of short wavelength multiplexing has become an important addition to add capacity to multimode fiber that is being employed to reduce the number of fiber pairs. And while using two wavelengths will soon serve to cut the number of pairs in half between 400GBase-SR8 and 400GBase-SR4.2, using two more wavelengths opens possibilities for single-pair operation at 200 Gbits/sec and for four-pair operation at 800 Gbits/sec—all without requiring an increase in lane rate or a reduction in reach. As with today’s wavelength multiplexed solutions, OM5 will continue to offer improved support as this future unfurls.”
Andy Jimenez, vice president of technology for Anixter, comments, “Where the 802.3cm standard will make the biggest difference is in global webscale data centers and perhaps some of the largest enterprise data centers. Customer adoption of off-premise solutions has changed the way data center network infrastructure is architected. While data transmission speed still dominates, low latency has also proven to be critical to designing a data center for high performance, flexibility and scalability.
“400-Gbit/sec backbone circuits in the data center facilitate the migration of 10- to 40-Gbit/sec connectivity to the server. More capacity is required to deliver network traffic from the distribution and access layers of the data center network to the core.”
Network planners who see multiple-hundred-gigabit transmission speeds sometime in their future have several possible paths to get there. And as proponents of SWDM and OM5 technology are quick to point out, multimode fiber is alive and well as one of those paths.
