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FIBRE OPTIC CABLING
Independent of IEEE standards , several MSA ’ s ( Multisource agreements ) have been produced by leading manufacturers in parallel . One such MSA that supports single mode at 100G , is known as 100G PSM4 MSA ( 100G Parallel Single Mode 4 lanes ). This has become popular due to its wide manufacturer support and was early to market in 2014 . It has been deployed mainly as 100GBASE-PSM4 . Using widely adopted QSFP28 form factor transceivers , and it ’ s also flexible in that 100G can be split into 4x25G breakout channels . PSM4 has been a proving ground for silicon photonics ; an evolving technology that ’ s bringing the cost of single mode optics for the data centre down . Silicon photonics in itself is a big topic , but is not expected to be a disruptive technology for several years as it competes with established Indium Phosphide ( InP ) & VCEL optics .
While QSFP28 has been the predominant form factor for 100G deployment , last year another MSA brought forward its successor – the QSFP-DD ( Quad small form factor pluggable – double density ) which will help to accelerate the adoption of 200G and 400G . In March this year the specification was announced . This double density module will employ eight lanes that operate up to 25Gbps NRZ modulation or 50Gbps PAM4 modulation providing 200Gbps or 400Gbps aggregate . The new form factor will in summary support :
• 3m of passive direct attached copper cables ( DAC ),
• 100m over multi-mode fibres ( MMF )
• 500m over single-mode fibres ( SMF )
• Backwards compatibility with QSFP transceivers from 40G to 200G
For transceivers and active copper or active optical cable assemblies , this includes those defined by Ethernet , Fibre Channel , and InfiniBand .
Looking after legacy While it ’ s looking like single mode fibre will dominate hyper scale and new cloud data centres , the majority of legacy data centres in the UK are equipped with VCSEL based 850nm short range transceivers . 10G SFP + is the dominant form factor connected through duplex LC OM3 or OM4 fibre via cassettes , and combined onto 12 fibre MTP trunks . These provide mostly 10G connections and also a lot of copper 1Gig connections in the HDA ( Horizontal Distribution Area ). This existing infrastructure can migrate using parallel optics to 40Gig or 100G over short distances , 100m OM3 / 150m OM4 , which is adequate for most data centres .
Transceiver modules aggregate multiple 10G signal paths onto one 40G or 100Gconnection . This has increased the popularity of QSFP +, CXP and other form factors . However the number of fibre MTP trunks , especially if re-configuring from a 3-tier to 2-tier architecture is expensive as each 100G link requires one MTP fibre trunk , using either 20 fibres ( 100GBASE-SR10 ) or 8 fibres ( 100GBASE-SR4 ). In the case of 100GBASE-SR4 using 25G per fibre , only 100m reach is possible for OM4 . Breaking out from 100G also requires MTP to LC breakout cables , which also adds expense . The option of migrating further to 400G for which some data centres are already provisioning is hence practically unrealistic using single wavelength VCEL lasers .
Last June we saw the publication of ANSI / TIA 492-AAAE standard for wideband multi-mode fibre ( WBMMF ) or OM5 . OM5 standardised fibre is 50microns just like OM4 and is backwards compatible , but designed to carry multiple short wavelength signals ( 850 to 953 nm ) that can be aggregated for high bandwidth applications such as short wave division multiplexing ( SWDM ). This could enable a data centre to retain its installed OM3 / OM4 duplex fibre infrastructure or supplement with OM5 where longer reach is required . In 2015 the SWDM alliance was created to promote this technology .
With SWDM4 ( SWDM + four pass-bands i . e . four signal paths on a single multi-mode fibre ), an optical transceiver with a standard QSFP28 interface can drive 100Gbps over a legacy single pair of multi-mode fibres . However , OM3 and OM4 fibre will incur a loss penalty * at the upper pass band , restricting reach . OM5 provides greater reach , typically 150m compared with 70 / 100m for standard OM4 / OM5 . A possible drawback of SWDM over duplex MM fibre is that it can ’ t readily be broken out , e . g . to 10G or 25G in the same way as parallel optics . (* Not all OM3 / OM4 fibres are born equal . It ’ s worth checking the specification e . g . Effective Modal Bandwidth EMB with the supplier if contemplating running SWDM optics with it ).
SWDM4 VCSEL optics are expected to be available this year , but there are already existing proprietary WBMMF solutions on the market . Cisco ’ s 40G BiDi transceiver solution ( 40GBASE-SR-BiDi ) allows re-use of duplex multimode fibre for a 40G connection over OM4 . The BiDi transceiver employs two wavelengths ( 850nm & 900nm ) transmitting in opposite directions over each fibre .
Looking ahead it ’ s anticipated that with tighter , faster VCSEL optics , and with PAM modulation it will be possible to achieve 200G over multi-mode . Some are sceptical whether the cost of 100G SWDM4 optical transceivers and associated OM5 fibre cable will prove to be a compelling business decision . This remains to be seen , but avoiding the business disruption in switching out existing fibre infrastructure must make SWDM worth looking at for legacy data centres . One thing for sure is as more data migrates onto a single circuit , each circuit becomes increasingly mission critical . n
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