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In the third instalment of our multimode fibre series, Keely Portway looks at how some of the latest standards demonstrate multimode's continuing relevance

In the third instalment of our multimode fibre series, Keely Portway looks at how some of the latest standards demonstrate multimode's continuing relevance

Multimode fibre is by no means a new technology. Used in the telecoms sector since the early deployments of the 1980s, it works by simply carrying light over a number of different paths, or modes.
 
The laser-optimised OM3 and OM4 multimode fibres were developed towards the end of the 1990s. It was these which were instrumental in the development and standardisation of the kind of wide band multimode fibre (OM5) that is used today, with transmitters based on vertical-cavity surface-emitting lasers (VCSELs).
 
VCSELs bring their own advantages, helping to reduce the overall cost of the system, and providing ease of manufacturability and integration, as well as reliability, testability and power efficiency. Mabud Choudhury, standards manager at OFS Optics said: ‘Yes, multimode fibre and VCSEL technology are both relatively established. But, there is a reason that people keep using them, and they will certainly continue to do so.’
 
Short and sweet
Multimode fibre is most widely associated with short-haul, and is particularly prevalent in enterprise and data centre markets. It has a larger core size than single-mode, can guide multiple modes and more easily capture light from a transceiver.
 
‘While the cost of multi-mode fibre itself is arguably higher,’ said Choudhury, ‘Cost savings are made in the long run due to the lower cost connectivity, installation, lower power consumption and ongoing maintenance and usability.’
 
What’s more, multimode fibre has not ‘rested on its laurels’, as it were, and continues to support evolving network speeds in these markets. It has largely maintained its ability to do this, thanks to a combination of fibre and component development, taking advantage of advances in next generation technology.
 
The latest industry trends include the migration towards cloud-based, data centre network architectures, driven by cost savings and revenue-generating benefits for network operators. Such benefits include improved operational efficiency, decreased capital costs, differentiated services and faster service delivery. These benefits rely on interoperable networking solutions that can deliver scalable, high performance, open and automated network hardware and software solutions.
 
Choudhury has witnessed this trend for interoperability have a positive impact on the multimode fibre market. ‘The entire data communications industry benefits from interoperable, multi-vendor industry standards,’ he said. ‘It increasingly benefits vendors, customers and the industry as whole with every generation. No mean feat when you consider that for many, interoperability means that you are working alongside some of your toughest competitors.’
 
New standards are continually being developed for interoperability work, and for next generation multimode fibre, co-ordinated by associations such as Telecommunications Industry Association (TIA); International Electrotechnical Commission (IEC) component/fibre standards; American National Standards Institute (ANSI)/TIA; International Organization for Standardization (ISO) IEC structured cabling standards; Institute of Electrical and Electronics Engineers (IEEE) and InterNational Committee for Information Technology Standards (INCITS) and Fibre Channel application standards.
 
In addition, the Shortwave Wavelength Division Multiplexing Multisource Agreement Alliance (SWDM MSA) is an industry consortium specifically dedicated to defining optical specifications and promoting the adoption of interoperable SWDM optical transceivers.
 
The high life
The alliance believes that SWDM allows data centres to upgrade to higher data rates, while still deploying duplex multimode fibre. This, in turn, can help to avoid expensive infrastructure upgrades. In the SWDM MSA’s view, SWDM can provide an alternative solution for 4Gb/s, and higher connectivity over duplex multimode. Some of the many advantages of SWDM listed by the alliance include reach, which is up to 440m for 40G and 150m for 100G; power dissipation, which can be as low as 1.5W; the ability to simply tap for network security appliances; and full digital data multiplexing, including Tx power monitoring.

In addition, the consortium extols the virtues of SWDM for operational simplicity. Data centre technicians are comfortable with individual Tx and Rx fibres, and WDM is a familiar concept to them. But it reasons that bi-directional solutions require technicians to connect together fibres where both sides transmit light, which can be counter-intuitive. This level of complexity is not a problem in the SWDM solution.
 
The alliance developed and released specifications for 40G SWDM4 and 100G SWDM4, and commercial 40GSWDM4 and 100G SWDM4 products were available by the end of 2017. The key value proposition of SWDM4, which utilises four wavelengths on 30nm spacing of 850nm, 880nm, 910nm and 940nm, is to use much lower cost duplex fibre cabling and connectivity, instead of parallel fibre cabling and connectivity. The 100G bi-directional small form factor pluggable (BiDi) uses two wavelengths, 857nm and 908nm to also extend the value of duplex cabling and connectivity infrastructure, and commercial 100G BiDi products became available at the start of 2018.
 
The 400G BiDi MSA was announced in July that year to define ‘optical data link specifications and promoting adoption of interoperable 100Gb/s and 400Gb/s optical transceivers for 100m link distance based on a dual wavelength bidirectional transmission technology in multi-mode optical fibre.’ The reach objectives are 70m OM3, 100m OM4 and 150m OM5. The specifications for 400G- BD4.2 were available two months later.
‘Shorter reaches have become very relevant,’ said Choudhury. ‘100m has always been a short-reach. The next generation of standards demonstrate just how relevant multimode fibre and VCSEL transmitters also remain, and will continue to be. Our recent paper on standards provides more in-depth information, which serves to highlight the continued importance and relevance of this technology.’
 
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