OM5: Is This a New Era of Wideband? | Corning

by Cindy Ryborz (Marketing Manager DC EMEA bei Corning Optical Communications) and Doug Coleman (Manager of Technology and Standards bei Corning Optical Communications)
Appearing in "Lanline Magazin", August 2019

In October 2014, the Telecommunications Industry Association (TIA) initiated a work group to develop guidance for a wideband multimode fiber (WB MMF) 50/125 µm fiber standard to support short wavelength division multiplexing (SWDM) transmission. The TIA-492AAAE detailed fiber standard was published in June 2016. The designation OM5 was approved by the ISO/IEC JT1/SC25 in October 2016.  However, to date there have been very few OM5 deployments. Why is this? And why was OM5 created in the first place? What does the future hold; will OM5 ever catch on?  

In local area networks (LANs) and in data centers (DCs) MMF remains the dominant fiber type used because it enables the lowest link cost (defined as the cost of the connectivity, and optical transceivers) for short distances. To understand the circumstances around why OM5 was born, it is necessary to understand a few things about optical transceivers, and the standards that govern them.  

We need to understand the distinction between standards-compliant transceivers, and proprietary /MSA transceivers.  Standards-compliant transceivers in an Ethernet context work with optical transmit, receive guidance and have been ratified as part of an IEEE 802.3 Ethernet standard.  Proprietary / MSA transceivers on the other hand are not part of the IEEE standard – either because the proposed physical media dependent (PMD) technology did not get enough member votes to be included in the standard, or because the transceiver utilizes a technology that was never offered to become part of an open industry standard. Various manufacturers have developed MSA transceivers (Multi-Source Agreement Transceivers) after establishing special, mutual agreements to ensure the products’ constant compatibility. Thus, they fulfill de facto standards, but do not meet official industry standards such as IEEE. The distinction between IEEE standards-compliant transceivers and proprietary transceivers is important because of the multiple transceiver types available in the market, with many proprietary designs.

In the Ethernet realm from 1G up to 400G, all standards-compliant multimode transceivers have one thing in common: they utilize vertical-cavity surface-emitting lasers (VCSELs) which operate at the 850 nm wavelength. When VCSELs first became commercially available, they were designed to produce light at 850 nm, which was one of the default MMF specified operating wavelengths. Since the VCSELs were operating at 850 nm, subsequent enhancements in MMF fiber design and manufacturing focused on optimizing the fiber bandwidth at 850 nm. For example, when OM4 fiber was introduced, it offered significant bandwidth improvements over OM3 at 850 nm, with OM4 offering 4700 MHz•km of effective modal bandwidth (EMB) at 850 nm, compared to the 2000 MHz•km provided by OM3.

Commercial 400G VCSEL transceivers for 400GBASE-SR8 and 400GBASE-SR4.2 are expected to be available between 3rd quarter 2019 to 1st quarter 2020. The extension of VCSEL’s to 400G with multimode fiber clearly demonstrates the synergistic relationship to provide low-cost optical connectivity and electronic solutions. 

The second thing to know about multimode transceivers is the concept of parallel transmission, which some people refer to as parallel optics. For Ethernet speeds of 1G, 10G, 25G, and 50G the multimode transceivers utilize two fibers, with one fiber carrying the transmit signal and one fiber carrying the receive signal. This is often known as serial transmission, and since these are 2-fiber devices, the connector interface into the transceiver is the LC duplex connector. However, with the adoption of the 40G 802.3ba Ethernet standard in 2010, the concept of parallel optics was introduced. In the case of the 40GBASE-SR4 transceiver, there are four fibers in parallel, each transmitting 10G per fiber, and another four fibers each receiving 10G per fiber. Therefore, these transceivers require eight fibers, and as a result, the multifiber MTP connector is the defined connector interface into the transceiver. A major feature of parallel optics transceivers such as the 40GBASE-SR4 is that since individual fibers are each carrying a 10G signal, a single 40G MTP port on a switch can be broken out to four LC duplex 10GBASE-SR ports, which typically results in significant per-port power cost savings and higher switch port density. With this type of breakout, a line card with 32 x 40G ports can be broken out to 128 x 10G channels. For network managers who need port breakout functionality and need 40G or 100G distances beyond the 150 m which the 40GBASE-SR4 and 100GBASE-SR4 transceivers support, a proprietary extended reach transceiver, the eSR4, (40G-eSR4 – 300/400m OM3/OM4 and 100G eSR4 170/300m) (OM3/OM4) has also been introduced. In addition to power cost savings, cooling cost savings can be expected. This is an important fact regarding sustainability of investments and has been proven not only for 40G, but also for 100/200/400G-SR4 variants.

The last few years have seen a large number of proprietary / MSA transceiver types come onto the market, starting with the 40G BiDi and 100G BiDi transceivers as proven solutions. Also, there is the short wavelength division multiplexing (SWDM) transceiver. Similar to BiDi for a 40G or 100G transmission, the SWDM transceiver only requires a 2-fiber LC duplex connection, SWDM differs in that it operates over four wavelengths per fiber across the range from 850 to 940 nm. 

Four available transmission wavelengths present an interesting question for the industry: how could the peak performance of this transceiver with an operating wavelength up to 940 nm be quantified, given that OM3/OM4 fiber bandwidth is typically specified at 850 nm? WB MMF is effectively a type of OM4 fiber, as the WB MMF still has to meet the OM4 bandwidth criteria of EMB ≥4700 MHz•km at 850 nm. However, the WB MMF also has bandwidth specified at 953 nm. The EMB specification at 953 nm is ≥2470 MHz•km.

This is a 100G overview, how OM4 fiber compares to OM5:

Since OM5 fiber exists and is being priced at a premium over OM4, it occurs that it must provide some benefit. Obviously, OM5 is beneficial towards OM3 regarding transmission possibilities at 850nm, as OM5 has the same OM4 bandwidth at 850nm.  Multimode fibers were created with the same wavelengths. Within a wavelength range beyond the 850nm multimode transceiver standards, OM5 and OM4 are the better choice. There are multiple options possible, as shown in Table 3.

Analytics show that with growing data rates from 10G to 40G to 100G, 90 to 95 percent of all OM3 and OM4 links work with transmission distances up to 100 m work (Figure 3 and 4). The majority of data centers obviously only requires a transmission distance up to 100 m in order to fulfill all requirements. 

A conclusion would be:

1. OM5 provides no value compared to OM4 when leveraging standards based 850 nm optics.
   
   
2. At 40G, both BiDi and SWDM have a distance benefit for OM5 over OM4, given that these are multiple wavelength transceivers.
   
   
3. At 100G, an OM5 distance benefit exists for both the BiDi and SWDM transceivers, as OM5 provides up to 150 m of reach, as compared to the 100 m reach provided by OM4.
    

So where is the perfect place to use OM5? The Solomon-like answer is: it depends. Especially when it comes to factors of network speed, required transmission distance, and used transceiver technology.

When one intends to use BiDi or SWDM transceivers, the network speed and the required transmission distance become deciding factors. However, if the network is already operating 100G or there are plans to migrate to 100G, and a significant number of links is beyond 100 m, then OM5 can be useful.

So what are the top three questions to ask? 

1.  Is a migration to 100G planned in the near future?
   
   
2.  Is a significant portion of the link distances in the data center longer than 100 m?
   
   
3.  Is the total cost per link a concern?
    

Given that a few network managers have MMF links beyond 100 m, and that there have been extremely few deployments of 100G in enterprise LAN or data center networks, this explains the very slow adoption of OM5 so far. There has simply been no need for it. Also, it is very expensive.

With more 100G deployments, OM5 may become attractive, when a reach up to 150 m is needed. Deployments of OM5 do provide some value for network managers who use 100G networks utilizing BiDi or SWDM transceivers and who have links between 100 and 150 m. As OM4 and OM5 are included in the IEEE standards for 100G-SR2, 200G-SR4, 25G -SR, 50G-SR, and for 400G (SR8, SR4.2) there is a choice to make, which is always good for customers in competitive markets.  Certainly one fact is clear: the lowest link cost for the distance needed will win.