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Space: The next frontier in optical networks

Massive connection point distribution and optical fibre cable densification is occurring in access and data centre networks. Each connection point needs an optical fibre, so the number of fibre strands needed to deliver network connectivity is spiralling upwards, while space and physical pathways to route these fibres is fixed or rapidly being consumed.  

Consequently, fitting as many fibres into as small a cable as possible is key, but doing this requires highly complex engineering. In this article, we will explore the factors driving ever-increasing optical fibre counts and consider two highly spatially-efficient cable designs that help operators use their physical space more effectively to meet the connectivity distribution and densification challenge. 
Access networks 
Access networks are the final link delivering bandwidth directly to the subscriber, and consumer demand is driving bandwidth growth at predicted per annum rates of:
• 47 per cent in mobile networks 
• 24 per cent in IP networks 
• 34 per cent in machine-to-machine connections 
In response, network operators are implementing optical fibre networks that terminate closer to their subscribers – at the home, business, antenna or street cabinet. In many cases the fibre infrastructure has terminations serving both fixed-line and mobile customers and, increasingly, more machine-tomachine communications associated with the internet of things (IoT). This is known as the converged network. 
These new access networks are characterised by many connections distributed across regional districts over links of up to 20km. The proliferation of connection endpoints and the need to distribute those endpoints across an extensive regional district, is driving high fibre counts in the cable feeder pathways that connect subscribers back to local data centres, exchanges and head-ends. At the same time, the physical space available in these feeder pathways remains fixed, or is being consumed by network overlays. This creates pressure on the existing infrastructure and a need to maximise the density of optical fibre within cables along these pathways. 

Data centre networks 

Inside data centres, spine-and-leaf architectures are greatly increasing the number of optical connections into the millions. There is a preference to retain point-to-point connectivity between data centres for switch-to-switch connectivity. Therefore, the large number of connections in the data centre is driving the need for high-fibre-count interconnect cables between data centres. These Data Centre Interconnect (DCI) cables can contain 3,000 optical fibres or more. 

Concurrently, a trend towards decentralisation is distributing data centres to locations closer to the user interface. The result is a distributed data centre architecture connected by high-fibre-count DCI cables. As in access networks, DCI applications are moving towards increasing fibre counts and the distribution of endpoint connections; but again, physical space within the existing data centre infrastructure is limited. Indeed, the very-high-fibre-count cables required (3,000+ fibres) must fit into ducts measuring ø50mm (diameter) that are commonplace between data centres. 

Spatial efficiency requires complex optical cable engineering

The key challenge currently facing optical cable engineers is to fit as many optical fibres into as small a cable as possible. Yet, competing optical and mechanical performance parameters and industry design and installation standards leave a very small window for the ideal cable solution. However, two spatially-efficient cable constructions which strike this balance are high-density micro cables and extreme-density ribbon cables.  

Micro cables are up to 60 per cent smaller and 70 per cent lighter than traditional stranded loose tube cables. As optical fibre counts have risen over time, cable diameters have been driven down through concerted industry effort. Today, the largest micro cable contains 432 fibres with an outer diameter of 10.8mm, which is just 0.5mm bigger than the smallest standard loose tube cable containing 72 fibres with an outer diameter of 10.3mm. This represents 5.5 times greater fibre density (measured in fibres per square millimetre). This example of optical cable miniaturisation is actually enabled by optical fibre miniaturisation. Traditional Recommendation ITU-T G.652 single-mode fibres feature a light-carrying core of 9.2µm, surrounded by a glass cladding to keep light from escaping, that brings the glass strand outer diameter to 125µm and a final acrylate coating layer to protect the glass, that brings the final fibre diameter to approximately 250µm. In recent years, manufacturers have produced G.652 fibres with a thinner coating that reduces the overall fibre diameter to 200µm. The 125µm cladding diameter is maintained for splicing compatibility purposes, but the 33 per cent reduction in cross-sectional area means more fibres can be packed into each buffer tube for more fibre capacity in a cable, or duct.

Duct miniaturisation

Micro cables are installed in microducts, which deliver spatial-efficiency in two key scenarios: 

Overbuild – when installing additional cables into an already occupied duct, a microduct override provides more optical fibres than an equivalent-sized loose tube cable. A 96-fibre loose tube cable measures approximately 12mm, whereas a 12/10mm (outer diameter of 12mm, inner diameter of 10mm) microduct accommodates a 288-fibre high-density micro cable (with an outer diameter of 8mm) for three times more fibres. 

New Build – when new duct infrastructure is necessary, multi-path microducts provide greater capacity in the same footprint as traditional conduits. A common 40/33mm duct is approximately the same size as a 7 x 10/8mm multipath microduct bundle (ø ~33mm) and whilst both cost the same to deploy initially, the microduct bundle offers six extra dig-free pathways over the traditional conduit for fast, inexpensive future capacity upgrades. 

Extreme-density ribbon cables

In a ribbon cable, the standard 12 coloured optical fibres are encapsulated in an array, or ribbon. Multiple ribbons are stacked to achieve fibre counts up to 3,456 fibres in a single cable. Until recently, ribbon cables offered a maximum of 1,728 fibres with an outer diameter of 32mm. But today, a new generation of extremedensity ribbon cable offers twice as many fibres in the same cable size, for twice the density in a ø50mm (2-inch) duct.

Splicing en-masse

As optical fibre counts hit multiple thousands, splice time becomes a key concern, as to splice 3,456 fibres individually would take over 100 hours. Fortunately, ribbon cables enable mass fusion splicing, where 12 fibres are fused in a single step, reducing total splice time by as much as 80 per cent. This means a 3,456-fibre cable can be spliced in less than 20 hours, but this efficiency can be enjoyed on any scale, as mass fusion splicing is faster than single fusion splicing at any fibre count of 12 or higher. 

Many extreme-density ribbon cable designs feature a ‘net design’ ribbon, whereby the optical fibres are only intermittently connected, so that they collapse upon one another to achieve the required packing density. This structure is less robust than proven solid ribbons, and individual fibres can separate during handling. Some installers prefer to turn net design ribbons into solid ribbons using glue before splicing. This process, known as ‘ribbonisation’, can add significant time and cost to an installation. 
Moreover, due to the packing density in such designs microbend attenuation is a real concern. Consequently, many extreme-density ribbon cables feature Recommendation ITU-T G.657 optical fibre for extra bend resistance that feature a smaller (often 8.6µm) modefield diameter (MFD). This can cause test and measurement compatibility issues when attempting to splice to legacy G.652.D fibres with an MFD of 9.2µm; bidirectional testing is required, leading to increased testing and troubleshooting time, a significant issue for high-fibre-count 3,000+ fibre cables. 
However, choosing an extreme-density ribbon cable with a proven, solid ribbon would eliminate the need to pre-ribbonise, yielding up to 30 per cent faster installation than other high-fibre-count cables. These cables use fibre which provides G.657. A1 bend performance with a backwards compatible G.652.D-rated 9.2µm MFD. This gives an operator 3,456 fibres in a 50mm (2-inch) duct, without sacrificing infrastructure robustness, installation efficiency or backwardscompatibility. 
As connection point distribution and densification occurs in access and data centre networks, more optical fibres are deployed along more pathways. But the physical duct space in which optical cables are deployed is fixed or rapidly being consumed. In response, the optical fibre cable industry has delivered two highly spatially-efficient cable solutions that help meet this distribution and densification challenge. High-density micro cables provide scalable, high-fibre-count capacity where space is most scarce; while extreme-density ribbon cables, with thousands of individual fibre strands, provide fast splice installation and the highest fibre counts available in a single cable. 

Roshene McCool is optical fibre market and technology development manager; and Matthew Guinan is outside plant cable market and technology development manager at Corning Incorporated


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