ITU Preps SDM Standard to Boost Capacity of Fibre Optic Cables

The International Telecommunication Union (ITU) has published a new technical report that could form the basis of a roadmap toward setting standards for the adoption of Space-Division Multiplexing (SDM) transmission into fibre optic broadband cables, which could in turn deliver a big data capacity boost for some future links.

Most fibre optic cables tend to harness Wavelength-Division Multiplexing (WDM), which put simply is a technology that simultaneously combines (multiplexes) a number of optical signals onto a single optical fibre cable by using the different colours (wavelengths) of laser light to carry data. There’s also Time-Division Multiplexing (TDM), which can help to carry data by interleaving channels temporally.

However, one way to build on top of all that would be to complement it with Space-Division Multiplexing (SDM), where the cable can utilize multiple spatial channels too (i.e. space can be used to communicate information too) – SDM essentially uses the transverse dimension of the fibre to separate the channels.

The problem is that today’s cables are approaching what’s known as the “Shannon limit“, which is the estimated maximum rate at which they can transmit data reliably. SDM is thus seen as one way to push against that, although until now reaching an industry consensus on related standards has been tricky (i.e. doing this in a cost-effective way isn’t easy as SDM, in its most conventional sense, can simply mean more fibres in a cable).

Just to be clear, the new technical report from the ITU’s Study Group 15 is not yet a standard, but it is the first technical document on SDM technology published with industry consensus by any standards’ development organisation and defines, as well as analyses, the potential properties of both single and multi-core SDM optical fibres.

For example, the Single-core SDM optical fibre designs include cables with reduced coating diameter fibre (RCDF), reduced cladding diameter fibre (RCF), and few-mode fibre (FMF).

ITU Statement

Long-haul submarine and terrestrial backbone networks, data centre interconnection, and intra-data-centre links could all benefit significantly from SDM optical fibre and cable technologies.

New design concepts for ultra-high-density cables could help to overcome space limitations for cables or ducts, align rising density requirements with cost effectiveness, and reduce environmental impact.

The investigation of anticipated SDM applications also considered connectorization, splicing, and breakout technologies, with the work analysing current technical progress and assessing the evolution required for future standardization of those technologies.

At this point we should be clear that ordinary consumers don’t need to be worrying about any of this because the fibre optic cables being put into the ground today should be able to carry enough broadband data capacity to keep homes fed for a generation. But future deployments of major international or subsea links could potentially benefit from adopting an SDM approach – particularly as we enter the Petabit era for individual links. The ITU’s work is an important stepping stone on that road.