CurrentAnalysis Operators 100G DWDM Systems

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Advisory Report

Operators Deploy 100G DWDM Systemswith an Eye towards further CapacityEnhancement How Best to Grow?May 14, 2013

Summary

Even as they prepare to deploy their rst 100 Gbps optical channel DWDM solutions (whichare often 100 Gbps transponder/muxponder additions to existing DWDM systems), networkoperators are seeking to ensure that their new infrastructures will satisfy the demands of thecoming years. Tey are nding that demand for capacity, which is currently surging, will continueto increase pace as new markets are opened, high-bandwidth applications proliferate and videobecomes the dominant form of communication. Tus, they need to determine how to grow theirnew systems beyond the 8 bps total capacity offered by traditional 100G DWDM technology.

Tis report provides an overview of the choices operators have in maximizing the capacity of theiroptical networks. It reviews the technology employed in todays typical 100G DWDM system,analyzes the methods available for increasing the capacity of these systems, and concludes with sug-gestions of which of those technologies are available in the near-term, since operators need to havecondence in the capacity enhancement of the systems they are about to deploy.

Current Perspective

When network operators deploy their initial 100 Gbps optical channel DWDM solutions, theyoften enhance their existing DWDM systems with 100 Gbps transponder/muxponder additions,or they deploy new DWDM systems dedicated to 100 Gbps optical channels. In this report, bothtypes of systems will be termed 100G DWDM systems. Network operators have two capacity

objectives as they select and deploy their initial 100G DWDM systems. Teir rst objective ofsatisfying todays traffi c demand is met by the roughly 8 bps (80 wavelengths x 100 Gbps perwavelength) of capacity of the current 100G DWDM systems. However, the operators are alsoconcerned that this capacity may not meet the traffi c demand for very long, so the second objectiveis to meet future traffi c demand that follows a rapid traffi c increase. For example, a July 2012 IEEEIndustry Connections Bandwidth Assessment found that the traffi c in the network core appears tobe increasing at a 58% compound annual growth rate, resulting in a ten-fold increase in traffi c bythe end of 2018.

If service provider revenues followed the increase of traffi c, then network operators could simply de-ploy additional DWDM systems to accommodate the increased traffi c. However, competition hasconstrained service provider pricing, resulting in revenues that cannot keep up with traffi c-drivennetwork costs. In order to support this phenomenon of rapidly escalating traffi c without suffi cient

revenue, network operators are pressed to squeeze every bit-per-second out of legacy networks, andto ensure that new (greeneld) networks meet demand increases over very long timeframes to avoidthe high cost of network overlays. No wonder operators are considering, even now, how to growtheir new systems beyond the capacity offered by traditional 100G DWDM technology.

Unfortunately, an increase of optical channel data rates beyond 100 Gbps requires an associatedincrease in the systems optical signal-to-noise ratio (OSNR), which would, in turn, likely requirecostly network modications, such as closer spacing of amplication sites or the addition of

Rick Talbot

CurrentAnal ysi s

Senior Analyst,

Transport and Routing

Infrastructure


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Operators Deploy 100G DWDM Systems with an Eye towards further Capacity Enhancement

How Best to Grow?(Continued)

optical regeneration (both costly endeavors), even if technological tweaks could marginally improve OSNR. Further,DWDM system designers need to decouple the client interfaces from the line interfaces. Current systems employ IU-Grid channelized line cards that do not take advantage of the divergence of client interface rates (legacy and post 100G)

and line rates. Tis divergence is driven by the different paces of development of integrated electronics and integratedoptics, as well as separate client and line side standards. In the future, developers will focus on total available capacity(AC) and reach provided by a specic line system rather than transponder rates.

Traditional 100G DWDM Systems

Current Capacity and Reach: Equipment providers typically claim a DWDM system capacity of 8 bps with aregenerator span of up to 3,000 km, using 100 Gbps line cards for each of the 80 optical channels. Tis limit is basedon ideal conditions that typically cannot be met in the eld, with practical limits being lower. For example, in a realistic(live) ber span of 1,600 km with 17 intermediate amplier sites, a DWDM system might only support 70 wave-lengths (total 7 bps, not 8). Te system may have an 80 wavelength capability, but to support the full 80 wavelengths,the actual system would require additional intermediate ampliers (at more than 17 sites), or would require intermedi-ate regeneration.

Technological Tools in Use: o achieve the capacity and reach of todays 100G DWDM systems, advances innumerous technologies to increase OSNR were required. Coherent detection provided a 2dB OSNR improvement, andsoft-decision forward error correction (SD-FEC) provided 2.6dB net coding gain (NCG) increase over legacy hard-decision forward error correction (HD-FEC). Polarization multiplexing provided a two-fold spectral effi ciency (SE)increase with minimal OSNR penalty. Broadband electronic chromatic dispersion (CD) compensation allowed for theremoval of high-attenuation compensation ber and the reduction of non-linear penalties caused by overcoming thehigh loss of dispersion compensation ber.

Prospects for Current 100G DWDM System Improvements: Te technological tools to improve capacity andreach for DWDM systems deployed on the current infrastructure (erbium-doped ber ampliers, EDFAs, at currentintermediate amplier sites) have been used to their greatest effect. Te industry should expect no further signicantimprovement in the capacity and reach for the current DWDM systems. Tere are no impending equivalents of coher-ent detection or polarization multiplexing; current SDFEC algorithms already provide up to 11.6dB of the theoretical

12.9dB NCG; and future incremental advances, taken together, will result in less than a 2dB of OSNR improvement,well short of next-generation, higher SE modulation format requirements. In addition, these attempts to improve thecapacity and reach for the current DWDM systems push the cost of the system up with diminishing returns.

Potential Sources of DWDM System Capacity Improvements:Any potential capacity improvements forDWDM systems will be achieved through either increasing the line card rate or by increasing the capacity supportedby the DWDM line system. Te designers of current 100G DWDM systems focused on increasing the line card rate.Tey employed the previously mentioned technologies to overcome the more stringent OSNR requirements driven bythe data rate increases; they could address capacity expansion independent of the line system, so the support of thesetechnologies by the next-generation line cards was the primary concern.

However, now that the technologies used in the line cards to equip DWDM systems with 100G capability have beenexercised, developers need to bring the line system to the forefront, focusing on the ampliers that utilize a portion(band) of the intrinsic bandwidth provided by the ber. Discussions about discrete line cards will give way to discus-

sions of maximizing AC.

Options to Increase Capacity

Continue Conventional Approach: Te conventional approach to increasing the capacity of DWDM systemshas been to increase the bit rate of the signal placed on each wavelength, using the existing ber with Erbium EDFAtechnology. Having maximized DWDM capacity with PM-QPSK modulation, developers could turn to more complex


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modulation formats to increase the systems SE. For instance, a PM-16QAM format would double the informationcapacity of the optical channel to 200 Gbps, resulting in a system capacity of 16 bps. A 256QAM format would bringthe system capacity to 32 bps (80 x 400 Gbps wavelengths).

However, increasingly complex modulation formats increase the number of amplitude and/or phase states that the sys-tem detector must distinguish for each received light pulse, resulting in a higher OSNR requirement. Figure 1 depictsthe cost of expanding conventional (with coherent detection, polarization multiplexing, SD-FEC and EDFA) ber-paircapacity past 8 bps via higher-level modulation formats. Te model assumes that the conventional system can support80 x 100G wavelengths on a 3,000 km regenerator span.

In the gure, the conventional PM-QPSK format provides 100 Gbps per wavelength and supports the 3,000 km span,so no intermediate regeneration is required. Te exclusion of intermediate regeneration is critical because of its highcost; the cost of a single line card pair (a regenerator for one wavelength) now equals that of four wide-band ampliers,making regeneration the dominant budget factor. Te use of PM-8QAM to transmit 150 Gbps per wavelength raisesthe system capacity to 12 bps, but requires an OSNR that reduces the reach by over a third (1,900 km), necessitatingone regenerator (per wavelength). If PM-16QAM (a popular format in recent trials) is employed to transmit 200 Gbpsper wavelength, the system capacity is raised to 16 bps, but the required OSNR cuts the reach almost in half, lower-ing it to 1,000 km, and necessitating two regenerators. Te PM-256QAM format required to achieve 32 bps reducesthe reach to less than 50 km. Clearly, the cost of expanding ber-pair capacity past 8 bps via higher-level modulationtechniques increases so sharply that it rapidly becomes impractical.

Employ Sub-Carrier Groups:Some vendors are introducing DWDM systems that group a number of optical sub-carriers to create a composite line side signal of the desired capacity and manage the group as a single optical channel.Such a group of sub-carriers has been popularly termed super-channel. Tis grouping of sub-carriers, by itself, does

not increase the capacity of the DWDM system, which is still constrained by its total allocated spectrum. For example,the currently available 500G super-channel system occupies the same 250 GHz of spectrum as would ve separate100G systems.

Several vendors have experimented with DSP pulse-shaping in the transmitter and enhanced DSP processing inthe receiver to achieve closer spacing of the sub-carriers within the group. In one such trial, the vendor used a largenumber of PM-QPSK-modulated sub-carriers (including 13 in a 350 GHz sub-carrier group and seven in a 200 GHz




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group) to achieve a 1 bps optical channel. However, these hero experiments are generally conducted under special-ized conditions such as employing special low-loss ber, neglecting the use of operational margin for the system, andemploying signal processing outside the system (in a separate computer). A real-life system would not replicate such

high performance in the next few years. Any near-term transmission of tightly packed wavelengths (or sub-carriers) isonly expected to yield a marginal capacity improvement of 20% to 40% without signicant shortening of system reach.Further, sub-carriers that are not based on the 50 GHz IU grid would require that the operator replace its existingROADMs with new ex-grid ROADMs. Tus, the capacity gains provided by todays sub-carrier technology is likelyto be insuffi cient to meet an operators long-range capacity challenge.

Increase Line System Bandwidth: One of the reasons that the conventional approach focused on improving linecard technology to increase DWDM system capacity, rather than increasing the systems bandwidth, was that amplierswere relatively expensive, so operators did not want to replace them. However, the coherent technology used in 100Gline cards has increased the line card cost to the point of reversing the cost differential between line cards and ampliers.Further development of line card technology, including increasing the line SE and/or adding the additional processorsto reduce optical sub-carrier spacing, will inevitably result in increased line card cost. It is time to investigate increasingthe bandwidth of the line system to increase the systems capacity.

Te number of wavelengths that can be placed on a ber (and therefore its capacity) is directly proportional to theamount of spectrum available to the DWDM system. DWDM systems do not use the entire optical spectrum of theber, but rather the specic bands of spectrum that provide the best propagation characteristics for the optical signal.Figure 2 shows the gain-bandwidths of several different line systems (ampliers and bers).

C-Band EDFAs and standard SiO2 ber provide 35nm of spectrum to support approximately 80 wavelengths. Tisubiquitous architecture serves as a reference point long standing as the lowest overall day-one cost solution, a benetthat is lost as end-of-life capacity targets exceed 8 bps. Prior to the availability of 100G systems, when the 10G sys-tems faced exhaustion some vendors extended capacity by offering L-Band EDFAs to supplement the existing C-Bandampliers at each site and amplify wavelengths within the new spectral band. Teoretically, this addition of amplierscould also be used for the S-Band, but both solutions incur the cost of the new ampliers, the cost of splitting theoptical signal into the separate bands for amplication and recombining on the ber, and the cost and complexity ofmanaging the parallel optical links with their own transmission characteristics.

Another method of increasing the system bandwidth is to employ wide-band Raman ampliers on the standard SiO2

ber to provide 100nm of spectrum which supports 240 50 GHz wavelengths. Raman amplication expands systemcapacity without requiring high SE modulation formats that shorten the systems reach and without requiring special-ized pulse shaping. In the future, an operator could also further increase the spectrum to 170nm, supporting 412wavelengths, by employing eO2 ber with Raman ampliers.




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Conclusions

No further signicant improvements can be expected in the capacity and reach offered by the technologies employed intraditional (EDFA-based) 100G DWDM systems. New technologies, such as modulation formats that offer higher SEor closely spaced wavelengths can increase capacity, but at the expense of system reach. Modulation formats with greaterSE than that provided by PM-16QAM appear to support prohibitively short reaches because of their excessive noisesensitivity.

Te fundamental capacity of DWDM systems can be increased, without the reach penalty, by expanding the opticalspectrum available to the DWDM system. Raman amplication is a proven technology that provides this extensionin the optical spectrum. By employing Raman amplication to expand system capacity while using current 100G linecard technology (Figure 3), an operator could delay introducing potentially expensive new technologies that would haveincurred reach penalties. Tose reach penalties could, in turn, have required adding amplier sites (which still mightnot have provided suffi cient OSNR) or additional regeneration. A wideband line system also appears to be the onlycurrently viable method to achieve ultra-high capacities of larger than 16 bps, where regeneration frequency becomesuntenable.

Recommended Actions

Recommended Vendor Actions

Optical network vendors need to include in product roadmaps for their 100G solutions steps for adding capacity torelieve network operator concerns of system exhaust in a few years. Te current 100G solutions meet existing capacityrequirements, but core network traffi c is growing rapidly, potentially requiring a ten-fold capacity increase over the nextsix years. Since changing systems (or vendors) can be expensive and complex, operators want to be assured that theirselected 100G system will grow to accommodate the network growth.

Vendors should explicitly note the reach limitations for their capacity expansion solutions to avoid uncomfortable ex-


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planations later when operators seek to apply these solutions to their networks. High SE modulation formats and pulseshaping that closely packs wavelengths provide several alternatives for increasing system capacity, but they also limitsystem reach, sometimes to the point of impracticality. Rather than refer to hero experiments that provide a false sense

of system capabilities, vendors should provide concrete capacity versus reach benchmarks for roadmap enhancements togive operators a true sense of system capabilities.

Vendors should investigate broadening the available optical spectrum on the ber rather than simply trying to increasecapacity within the C-Band. Efforts to squeeze signicant additional capacity out of the traditional spectrum is becom-ing expensive, while producing diminishing returns. Granted, operators do not like change, but the cost of trying towring additional capacity out of the currently used spectrum will likely be even greater.

Vendors who are proposing high SE modulation formats and pulse shaping that closely packs wavelengths to increasesystem capacity should also consider combining their solutions with Raman amplication to increase the reach of theirsolutions. Raman amplication is recognized for its capability of improving OSNR, thereby improving reach, whichis a major concern for the new high-capacity systems. Operator resistance to this new (for them) technology will bereduced as they will likely be deploying new ex-grid ROADMs, anyway.

Vendors who currently offer Raman amplication to accommodate excessively long amplier or regenerator spansshould consider addressing the technological and operational requirements for Raman amplication up-front to presentthe technology as mainstream compatible. Tough some vendors have embraced it as a valuable tool for meeting therequirements of challenging eld routes, operators will need to view Raman as a mainstream compatible technologybefore deploying it.

Vendors should focus their development and messaging on AC provided by a specic line system rather than tran-sponder rates when increasing the capacity of their systems. Client interface rates are now being standardized separatelyfrom the DWDM line rates due to separate technological developments such as silicon-optical integration (used inclient connections) versus photonic integration (used on the line side) and the client-side switching increasingly beingused in 100G DWDM systems that aggregates client inputs onto the DWDM line signals. AC design frees developersto maximize the system capacity without being restricted to certain client rates.

Recommended User Actions

Network operators should understand that, after the introduction of 100G DWDM technology, ber capacity will nolonger inherently expand proportionally with the optical channel capacity. Tat is, a 400G system will not provide fourtimes the ber capacity of a 100G system at the equivalent 100G route length. Developers have employed technologyfor the 100G DWDM systems that is nearing the maximum information transfer capability of the 35nm of opticalspectrum supported by EDFAs. System capacity will increase, but not in proportion to the optical channel rate, and notwithout likely reach penalties.

Operators should press their vendors for concrete roadmaps for expanding capacity of their current 100G systems.odays 100G solutions may meet current capacity requirements, but core network traffi c is growing rapidly, potentiallyrequiring a ten-fold capacity increase by the end of this decade. Vendors are conducting numerous hero experimentsthat promise impressive capacity increases, but there is no guarantee of how much of this high transmission perfor-mance will translate into real product features. Operators will want to ensure that their soon-to-be-deployed 100Gsystem will not leave them stranded in a few years.

Operators need to press their vendors to address the system reach implications of the technologies they employ inexpanding system capacity. Both high SE modulation formats and pulse shaping that closely packs wavelengths ad-versely affect system reach, and hero experiments with these technologies that demonstrate suffi cient span lengths areunlikely to translate into products with similar performance in the short term. Operators can seldom afford to expandtheir capacity at the expense of additional regeneration or numerous additional amplication sites.

Operators should inquire of vendors if, and how, they are seeking to extend the use of the optical spectrum beyond


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the 35nm spectrum of the C-Band. Fiber transmits light in a much broader spectrum, so developers could fundamen-tally increase the capacity of their DWDM systems by using this additional spectrum.

Operators should refresh their familiarity with Raman amplication so that they can consider some of the benets

of the technology. Multiple vendors offer Raman to provide transport over excessively long amplier or regeneratorspans, and the technology is practically ubiquitous in submarine unrepeatered systems. Now the technology promisesto increase DWDM system capacity. Once operators learn the proper design, installation and operations methods ofRaman technology, they should become more comfortable with it, as have many operators that have deployed Ramansystems with success. Tere should now be enough users of Raman amplication to provide valid operator perspectiveon the technology.

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CurrentAnalysis Operators 100G DWDM Systems