ANALYSIS & OPINION: FIBRE & CABLE


OUT WITH THE OLD?


IS LEGACY MULTIMODE FIBRE READY FOR FUTURE BANDWIDTH NEEDS? KEVIN LENGLÉ


M


ultimode fibre is widely associated with short-haul, and is particularly prevalent in enterprise and data centre markets. Tis 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 product lines that can leverage the full potential of multimode fibres and increase their bandwidth capacity. As a relevant alternative to recabling, these


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 in 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 in 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 in the campus. A standard requirement is oſten 10Gb/s over the links between floors or buildings constituting the cable mesh in campus networks. A large majority of these multimode fibres do


MULTIMODE IS BY NO MEANS NEW, IT HAS BEEN USED IN THE TELECOMS SECTOR SINCE THE EARLY DEPLOYMENTS OF THE 1980S


26 FiBRE SYSTEMS n Issue 30 n Winter 2021


not support throughput of more than 10Gb/s over campus-wide links. Tis 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.


Te higher the transmited throughput is, the greater the distortion is. For a given throughput, it reduces the distance that can be reliably atained between transmiter and receiver. Tis problem was discussed a few years ago


in 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 (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 oſten a difficult task. It depends on the distances involved and involves removing and replacing the fibres. An audit of the cabling infrastructure is


necessary 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. Tis 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


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