Singlemode fibre optic cable is more and more considered as a viableÂ solution for 10Gb/s LAN and data centre applications on the basis of price.Â Carsten Fehr at Draka/Prysmian Group argues that, despite the increasedÂ cable cost, multimode fibre is often the most economical solution.
The need and desire forÂ more bandwidth and fasterÂ communication is rapidlyÂ growing across all types ofÂ communication networks.Â To support these demands in EthernetÂ networks, such as data centres, the IEEEÂ developed and ratified the standardÂ IEEE802.3ba in June 2010. This extendsÂ Ethernet speeds from 10Gb/s to 40Gb/sÂ and 100Gb/s across single or multimodeÂ fibre optics.
The IEEE standard sets different link lengthsÂ for singlemode and multimode fibres.Â Multimode optical fibre will support bothÂ 40 and 100Gb/s speeds over lengths of atÂ least 150m. Singlemode fibre will supportÂ 100Gb/s link lengths of 10 and 40km and aÂ 40Gb/s link length of up to 10km.Â To supporters of twin fibreÂ technology, even if the potential linkÂ distance of 10km is not required, usingÂ two singlemode 40GBase-LR4 fibresÂ to create a network may appear toÂ be a viable solution. However, todayâ€™sÂ network manager must not only planÂ to upgrade to a 40 Gigabit EthernetÂ environment, they must also understandÂ the implications that transformation willÂ have on the eventual network migrationÂ to 100 Gigabit Ethernet and beyond.Â Whether a singlemode solution is theÂ best option under these circumstances isÂ questionable as the following examinationÂ of systems costs shows.
Light signals are typically transmittedÂ along a fibre with a silica glass core thatÂ confines the incident light beam to theÂ inside through total internal reflection.
Attenuation is an important factorÂ limiting the transmission of a digitalÂ signal across large distances.Â When theÂ first fibre optic fibre appeared in 1970,Â its optical attenuation (reduction in theÂ intensity of the light beam with respect toÂ distance travelled) was 20dB/km, whichÂ is far beyond the level of attenuation ofÂ todayâ€™s fibres.
The effects that contribute toÂ attenuation depend on the opticalÂ wavelength of the transmitted signal.Â The wavelength bands, or windows, thatÂ exist where these effects are weakest areÂ most favourable for transmission. It wasÂ the optical resonance of the hydroxideÂ molecules in the glass, in particular, thatÂ made the wavelength window of 850nmÂ favourable and earned it the moniker theÂ â€˜first optical windowâ€™.
As manufacturing technologyÂ advanced, process improvements thenÂ opened up further windows, of which theÂ â€˜second optical windowâ€™ of 1,310nm hasÂ become the most significant.
When manufacturers implementedÂ the first commercial fibre opticÂ transmission paths in the 1980s, thereÂ were two main problems: the couplingÂ of sufficient light into the optical fibre;Â and the limitation of bandwidth due toÂ dispersion, which is the spreading ofÂ the optical pulses as they travel alongÂ the fibre.
Increasing the size of the fibreâ€™sÂ light guiding core considerablyÂ simplifies the light coupling and cablingÂ technology by transmitting light in aÂ number of wavelengths (modes). TheseÂ wavelengths, however, move at differentÂ speeds along the fibre distorting theÂ signal. A graded index fibre core can beÂ used to overcome this modal dispersion.Â This works by radially varying theÂ refractive index of the core to evenÂ out the speed of travel of the differentÂ modes. However, its synchronising effectÂ is limited.
A further reduction in dispersion isÂ only possible by restricting operation toÂ a singlemode, which requires very smallÂ core dimensions, less than 10Î¼m, whichÂ in turn makes light coupling difficult.
The result of this dichotomy is thatÂ two distinct types of optical fibres haveÂ evolved. Singlemode fibres, with theirÂ smaller core, have come to dominateÂ long distance applications, while theÂ larger cored multimode fibres areÂ more beneficial over shorter distances.Â As a consequence, multimode fibreÂ is commonly used for inter- or intrabuildingÂ applications while singlemodeÂ fibres are commonly used forÂ communication links over 1km, forÂ example for running a telecoms cableÂ from one continent to another.
Historically, singlemode fibresÂ transmitted infrared laser light, andÂ multimode fibres transmitted infraredÂ light from light emitting diodes (LEDs),Â which kept system costs low. Today, LEDsÂ have been largely superseded by verticalÂ cavity surface emitting lasers (VCSELs)Â which couple well to multimode fibreÂ and offer improved speed, power andÂ spectral properties, at a similar cost.
This divergence between single andÂ multimode fibres was sanctioned withÂ the introduction of multimode OM3Â fibre, which was specifically optimised forÂ VCSEL operation in the 850nm window.Â The deployment of 850nm fibre systemsÂ represents a significant saving overÂ singlemode fibres for 10 Gigabit EthernetÂ users. So much so, that the cost of aÂ singlemode link, including the transceiverÂ either side of the fibre optic, soonÂ exceeded that of the multimode optionÂ by a three digit Sterling sum.
Today, users have a multitude of fibreÂ optic network options from which toÂ choose. Compatibility is essentiallyÂ guaranteed by standardised transceivers,Â which comply fully with physical mediumÂ dependent sub-layers (PMDs), whichÂ define the details of transmission andÂ reception of individual bits on a physicalÂ medium. These â€˜plug and playâ€™ solutionsÂ allow the most economical transceiverÂ for a transmission distance and speed toÂ be selected.
There is no need to carry out aÂ major cost analysis for proof of theÂ price differential; the prices of cablingÂ and transceivers are readily availableÂ on the Internet. A quick Google searchÂ of prices will show that the cost of aÂ transceiver jumps considerably in lineÂ with the range of the PMD standard forÂ any system supplier.
No matter which system brand youÂ chose, the result will always be the same:
The difference in costs for activeÂ multimode and singlemode components,Â such as transceivers, becomes even moreÂ significant on large installations suchÂ as a data centre with, perhaps, 10,000Â cable links. Here, even a small saving inÂ component cost can result in a significantÂ overall saving.
To futureproof an installation to ensureÂ its long term operation installers shouldÂ consider using a solution which supportsÂ system upgrades and ever quickerÂ transceivers in the most flexible manner.Â It should also be backwards compatibleÂ to allow the new systems to continueÂ to operate with the existing systemÂ otherwise it is not possible to upgradeÂ individual sections of the system withoutÂ the entire active network having to beÂ revised. This is the case for all users withÂ 1GBase-SX and 100Base-SX systems inÂ their network, neither of which is capableÂ of operating on singlemode fibre.
The examples above demonstrateÂ that if the cost of the network isÂ important, there is no alternative toÂ multimode fibre for LAN and data centreÂ backbones. True, the range of multimodeÂ optical fibres will certainly shrink withÂ higher data rates; as a result, the onlyÂ solution for long range cabling willÂ remain the singlemode fibre. However,Â to use this as a reason to fit out anÂ entire optical fibre network would be anÂ expensive mistake.
Some users are already planning forÂ 40 Gigabit Ethernet and beyond. TheÂ majority of these applications areÂ switch-to-switch backbone installationsÂ comprising OM3 cabling with two fibresÂ per section. The introduction of 40Â Gigabit Ethernet does not give suchÂ backbones higher data densities since theyÂ continue to run 10Gb/s on two fibres.
In this instance the existingÂ infrastructure can still be used byÂ recabling. However, the amount ofÂ subsequent cabling of at least OM4Â quality required is immense. The solutionÂ also costs roughly the same as the passiveÂ expansion with singlemode optical fibreÂ but offers another upgrade option:Â 100GBase-SR4 with 4x25Gb/s on eightÂ multimode optical fibres, which is in lineÂ with the advice in IEEE 802.3.
Once installed it is actually possibleÂ to enhance a system by upgrading theÂ transceiver to 100 Gigabit Ethernet.Â At this network level, OM4 canÂ be recommended as a flexible andÂ reasonably priced cabling solution.Â As part of an independently plannedÂ cabling strategy, OM4 offers the optionÂ of reducing, and thereby simplifying theÂ cabling runs without compromising theÂ universality of the network.
With the next generation of networksÂ moving towards 100 Gigabit Ethernet,Â new multimode fibre assessment criteriaÂ are emerging. Todayâ€™s 10Gb per fibreÂ pair technology focuses on the effectiveÂ modal bandwidth (EMB), which isÂ determined by the differential modeÂ delay (DMD).
For a standard OM4 fibre, a higherÂ EMB would improve the workableÂ fibre distance. This benefit is restrictedÂ to VCSELs with small to moderateÂ spectral widths. For laser transceiversÂ with increased spectral widths found onÂ next generation networks, chromaticÂ dispersion is responsible for restrictingÂ transmission lengths. The chromaticÂ dispersion of the multimode optical fibresÂ therefore takes on new significance.
In contrast to previous assumptions,Â the ideal DMD profile of a multimodeÂ optical fibre for use with VCSELs shouldÂ not have perfectly synchronised modes.Â Instead it should deliberately aim toÂ introduce modal dispersion to theÂ signal to compensate for the chromaticÂ dispersion. Such an OM4+ fibre isÂ commercially available and can achieveÂ ranges of up to 200m at 40/100 GigabitÂ Ethernet with selected transceivers asÂ part of an engineered solution.
Source: NCN Magazine, UK