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Kevin Lenglé asks, is legacy multimode fibre ready for future bandwidth needs?

Multimode fibre is widely associated with short-haul, and is particularly prevalent in enterprise and data centre markets. This article dispels some myths about multimode fibre and its intrinsic limited bandwidth. 

In recent years, new optical technologies have emerged and many players are launching new product lines that can leverage the full potential of multimode fibres and increase their bandwidth capacity. 

As a relevant alternative to recabling, these new technologies can solve many problems in a practical, economical and ecological way. In particular, it presents a number of opportunities to the LAN market, in terms of future proofing existing campus cabling infrastructure.

Multimode fibre is a proven technology to provide high-speed connections between very short distances. It is particularly popular within data centres with OM3/OM4/OM5 multimode fibre type. It is by no means a new technology, it has been used in the telecoms sector since the early deployments of the 1980s, notably within local area networks. 

But, what about first generation OM1 and OM2 multimode fibres deployed massively within campus cabling infrastructure? Are these capable of meeting the growing demand for high throughput on these networks?

A perfect 10

Bandwidth-intensive applications and latency-aware traffic types are becoming ubiquitous in LAN. As a result, it is necessary to be able to reliably transport these data streams with a high quality of service within the campus. A standard requirement is often 10Gb/s over the links between floors or buildings constituting the cable mesh within campus networks. 

A large majority of these multimode fibres do not support throughput of more than 10Gb/s over campus-wide links. This is largely due to the optical design of this fibre type. Contrary to the active components of the network, for which upgrades can be carried out relatively easily, the optical cabling infrastructure has a performance envelope that generally could not be increased without physically changing the cable that carries the information, such as deploying new generation cables (OM5 or SMF).  

Multimode possesses intrinsic performance limitations which are inherent to its physical properties. More specifically, for multimode optical fibres widely installed in LANs, such as OM1 (62.5/125μm) and OM2 (50/125μm), high-speed transmission is limited as a result of modal dispersion, which is a distortion mechanism occurring in all multimode fibres. The higher the transmitted throughput is, the greater the distortion is. For a given throughput, it reduces the distance that can be reliably attained between transmitter and receiver. 

This problem was discussed a few years ago within IEEE and MSA groups. Designed for backward compatibility with OM1 and OM2 fibres, an alternative type of 10Gb/s interface had been developed within the IEEE 802.3aq standard, referred to as LRM for long reach multimode. By using electronic dispersion compensation signal processing technology combined with specific light launching conditions into the fibre using mode conditioning patch cords, it is possible to achieve a reach of 220m. But this does not cover all today’s LAN needs in terms of bandwidth.

Trials and tribulations

A typical solution that prevents throughput and distance limitations due to modal dispersion in currently deployed multimode fibres would therefore be to replace them with new-generation multimode fibre, or with a single mode fibre that does not experience modal dispersion. However, redeploying the fibre is oftentimes a difficult task. It depends upon the distances involved and involves removing and replacing the fibres. 

An audit of the cabling infrastructure is necessary in order to verify the availability of the cable ducts as well as their condition. Depending on the case, microtrenching or civil engineering may be required to deploy new cable. This kind of operation could be very intrusive. 

But optical technologies have evolved. Following the light-launching principle used in this IEEE LRM standard (offset launching), the best-case scenario would be to perform a mode-conditioner, enabling a launching condition that accurately excites only the fundamental centre mode into the multimode fibre core. 

In this way, the light transmission within the fibre would no longer induce modal dispersion, as only one mode would propagate, a kind of quasi-single-mode transmission over the multimode fibre. Nowadays, there are technologies that enable flexible and complete light shaping through a succession of very complex optical lenses. Such technologies shape a laser beam from a transceiver in such a way that each mode within the fibre can be independently and precisely excited.  

This kind of technology is used as a way to increase the transmission capacity of existing multimode fibres by performing centre launching on legacy multimode fibres. By overcoming speed and distance limitations, bit rates like 10/25/40/100Gb/s can be carried over an existing legacy multimode infrastructure of several kilometers, allowing for a progressive and flexible evolution of the legacy network optical infrastructure, compatible with any type of 62.5/125μm or 50/125μm multimode fibre. 

Getting physical

In addition, being that such technologies consist solely of optical elements, the solutions are passive (no energy consumption, no configuration, no monitoring) and operate at the level of the physical layer, which makes them transparent to the wavelength, the communication protocol and modulation format being used. 

Beyond the increase in bandwidth, it is now possible to perform the single mode/multimode media converter function in a totally passive way thanks to this modal approach. Indeed, this function can be found in many cases of use in the LAN, especially in a context of campus extension. It is therefore possible today to achieve the transition without any active layer. As a complement, this modal approach also makes multimode fibres compatible with single mode wavelength-division multiplexing (WDM) technologies, enabling a gradual and flexible increase in network capacity towards very high-speed broadband and thereby ensuring a durable cabling infrastructure.

In conclusion, the limited capacity of LAN cabling infrastructure is a prevalent issue and complex wiring cases are not uncommon. You do not have to remove and replace the old cable by default. You now have the possibility to upgrade legacy multimode fibres. 

These modal adapting passive technological approaches have already proven their effectiveness across numerous industrial, military, university and hospital campuses around the world. As a complement to traditional cabling, this new trend to transform multimode into single mode fibre makes it possible to recycle the existing cabling infrastructure, one more step towards green IT, with proven benefits for end customers and ICT professionals.   

Kevin Lenglé is product line manager at Cailabs

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