FEATURE MID-REACH
The long and the short of it
Architects of the largest data centres have set optical engineers a challenge: to create cheaper 100 Gigabit interfaces that span up to 2km. Roy Rubenstein reports
E
xisting 100 Gig interfaces have reaches that are either too long and costly or too short – and web companies’ demands have stirred a flurry of industry activity,
with four optical module initiatives announced since the year’s start. Cheaper 100 Gig mid-reach interfaces also promise to benefit telecoms, with wireless being one application already identified. Optical module designers must reconcile two
contradictory trends: data centres are getting larger, inevitably lengthening the links between systems, yet optical reach gets shorter with increasing channel speed. Te move from 10 to 100 Gig interfaces increases
the lane speed from 10 to 25Gbit/s; 100 Gig uses either four 25 Gig links over parallel fibre, or four 25 Gig light paths on a single fibre with wavelength division multiplexing (WDM). As a result, the reach of 10 and 40 Gig multimode interfaces is 300m, or 400m using OM4 fibre, but for the 100 Gigabit 100GBASE-SR10 and 100GBASE-SR4 multimode standards, the reach plummets to 100m (150m on OM4 fibre). ‘And 100 metres is not enough for most data centres,’ said Arlon Martin, senior director for marketing at Mellanox Technologies. Te next available 100 Gig interface option
specified by the IEEE is the single mode 100GBASE-LR4 standard, but its 10km reach is overkill for most data centre applications. Te LR4 is also expensive, costing seven times that of the 100GBASE-SR10. Certain larger data centre operators have chosen to forgo multimode fibre altogether. Microsoſt, for one, has said it will deploy single-mode fibre exclusively in its data centres. Such cloud players want a 100 Gig single-mode interface that is cost-competitive with existing multimode solutions, yet covers this enormous middle ground between 100m and 10km. ‘Te largest data centre operators will tell you that less than 1km, less than 500m, is their sweet
28 FIBRE SYSTEMS Issue 5 • Autumn 2014
spot,’ said Martin Hull, director of product management at switch vendor Arista Networks. What is being discussed is not so much exact point-to-point data centre distances but a module’s optical link budget, the amount of loss that can be accommodated for the sent signal to be received. A signal may pass through one or more fibre patch panels in transit, introducing additional attenuation. For 10km links, the optical link budget is around 6dB, for 2km it is more like 4dB. ‘Some companies are at the cutting edge of
technology, building very large data centres that have to have access to these reaches,’ said Rafik Ward, Finisar’s vice president of marketing. But there are also a large number of enterprises that continue to operate smaller data centres and for them, existing IEEE multimode standards and reaches meet their needs. Finisar estimates that the longer-reach interfaces
– up to 2km – will account for 20 per cent of all the interfaces in the future.
@fibresystemsmag |
www.fibre-systems.com
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