OM5 — hype or the future of multimode?

OM5 multimode fibre (MMF) was introduced with fanfare in 2016, but to date has seen very few deployments. Why is this? Why was OM5 created in the first place? What does the future hold for OM5 and will it ever catch on?

MMF remains the dominant fibre type deployed in local area networks (LANs) and data centres (DCs) because it almost always delivers the lowest link cost (defined as the cost of fibre, connectivity and optical transceivers) for short distances. Today, higher-bandwidth fibres such as OM3 and OM4 are more prevalent, with OM5 recently joining the list of available fibre options. 

To understand the circumstances around why OM5 was created, it is necessary to understand a few things about optical transceivers and the standards which govern them.

Standards-compliant transceivers vs proprietary transceivers

We need to understand the distinction between standards-compliant transceivers and proprietary transceivers. When we refer to standards-compliant transceivers in an Ethernet context, we’re talking about optical transmit and receive guidance which has been ratified as part of an IEEE 802.3 Ethernet standard.

Proprietary transceivers on the other hand are transceivers whose guidance is not part of the IEEE standard, either because the proposed physical media dependent (PMD) technology did not garner enough member votes to be included in the standard or because the transceiver uses a technology that is not part of an open industry standard. The distinction between IEEE standards-compliant transceivers and proprietary transceivers is important because the last few years have seen a proliferation of transceiver types available in the market, many of which are proprietary designs.

Parallel optics

The second thing to know about multimode transceivers is the concept of parallel transmission, also referred to as parallel optics. For Ethernet speeds of 1G, 10G and 25G, the multimode transceivers use two fibres with one fibre carrying the transmit signal and one fibre carrying the receive signal. This is often known as serial transmission, and since these are 2-fibre 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, we have four fibres in parallel, each transmitting 10G per fibre at the 850 nm wavelength, and another four fibres each receiving 10G per fibre. These transceivers require eight fibres for a single channel, and as a result, the multifibre MTP connector is the defined connector interface into the transceiver.

A major feature of parallel optics transceivers (eg, the 40GBASE-SR4 or eSR4) is that since individual fibres each carry 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.

Figure 1: 40GBASE-SR4 8-fibre parallel transmission.

How is OM5 different?

As mentioned before, the last few years have seen a large number of proprietary transceiver types come onto the market, starting with the 40G BiDi transceiver. The BiDi transceiver is a 2-fibre device, with bidirectional transmission over each fibre; each fibre carries both a transmit and receive signal, operating at different wavelengths (850 and 900 nm). Since the BiDi transceiver requires only two fibres, it was designed to provide a migration path up to 40G where OM3 or OM4 duplex fibre connectivity was already installed, such that additional MTP connectivity would not be required. The BiDi transceivers have proven to be a good solution for 40G switch uplinks.

Entering this fray is another transceiver transmission technology: the short wavelength division multiplexing (SWDM) transceiver. SWDM differs in that it operates over four wavelengths per fibre across the range from 850 to 940 nm, with one fibre dedicated for transmit and one fibre dedicated for receive. As with BiDi, the SWDM transceiver is designed to give network managers, with an installed base of OM3/OM4 duplex connectivity, another path to migrate to 40G without having to deploy additional fibre. OM3/OM4 fibre only have bandwidth specified at 850 nm. When OM5 was introduced in 2016, it had bandwidth specified at both 850 nm and 953 nm to accommodate the SWDM transmission window up to 940 nm.

Figure 2: 40G SWDM transmission (4x10G/wavelength).

The benefits of OM5 fibre

Due to the increasing demand for higher network speeds, let’s evaluate the distance capabilities for both 40G and 100G, based on published transceiver manufacturer specifications with standard connectivity.

		40G transceivers					100G transceivers
Fibre Type		40GBASE-SR4	eSR4	BiDi	SWDM		100GBASE-SR4	eSR4	BiDi	SWDM
OM3		100 m	300 m	100 m	240 m		70 m	200 m	70 m	75 m
OM4		150 m	400 m	150 m	350 m		100 m	300 m	100 m	100 m
OM5		150 m	400 m	200 m	440 m		100 m	300 m	150 m	150 m

Table 1: Transmission distances (in metres) per fibre type and transceiver type.

Note 1: Distances represent guidance published by the transceiver manufacturers; some switch vendors could provide different guidance.

What do we observe here?

• Using either SR4 or eSR4 transceivers (both operating solely at the 850 nm wavelength) there are distance benefits for OM4 over OM3, but no distance benefit for OM5 over OM4. Both OM4 and OM5 meet the same bandwidth spec at 850 nm.
• At 40G, for the multiple wavelength transceivers, BiDi and SWDM offer a distance benefit for OM5 over OM4. However, the OM4 distances of 150 m for BiDi and 350 m for SWDM are sufficient for the vast majority of MMF applications. Published industry data reports that up to 95% of OM3/OM4 links in the data centre run 100 m or less.
• At 100G, an OM5 distance benefit exists for both BiDi and SWDM transceivers. OM5 provides up to 150 m of reach, as compared to the 100 m reach provided by OM4. The longest overall reach of 300 m is provided by the eSR4 transceiver with either OM4 or OM5 fibre.

To determine the right usage for OM5, data centre operators need to understand a number of factors related to network speed, required transmission distance and the transceiver technology being used. For example, if you intend to use standards-compliant transceivers, then you will be using an SR4 type transceiver where OM5 provides no value over OM4. Or if you require port breakout capability, then you will be using an SR4 or eSR4 type transceiver where again, OM5 provides no value over OM4.

If you intend to use BiDi or SWDM transceivers, then the network speed and the required transmission distance become deciding factors. In a 40G world, most network managers will not have many (if any) links beyond 150 m so OM4 will accommodate most needs. However, if you plan to migrate to 100G and have a significant number of links beyond 100 m then in this scenario, OM5 does have a use case as it provides an additional 50 m of reach over OM4.

Given that few enterprise networks have MMF links beyond 100 m and there have been extremely few deployments of 100G in enterprise LAN or data centre networks, this explains the slow adoption of OM5 so far.

Future fibre deployment trends

As 100G deployments in enterprise LAN or data centre networks become more prevalent, OM5 could become attractive if a reach up to 150 m is needed. Deployments of OM5 do provide some value for network managers who deploy 100G networks using BiDi or SWDM transceivers and who have links between 100 and 150 m.

One thing is certain — the lowest link cost for the distance needed will win.

Image credit: ©stock.adobe.com/au/ipopba