100 Gigabit: The Coming Metro Opportunity

A Position Paper by Gazettabyte

October 2011

Contents

Summary

Status of 100 Gigabit deployments

100 Gigabit networking drivers• Network congestion• Data centre, PoP interconnects and 100 Gigabit services

Technology options for 100 Gigabit• The IEEE 100 Gigabit Ethernet interfaces• The 10x10 MSA• Coherent transmission• Non-coherent transmission

The coming battle for the metro• 100 Gigabit metro transport

• Direct-detection point-to-point• Direct-detection WDM• Coherent metro

• Developments to watch

How the metro technologies will play out• Cost performance• Pluggable form factor • Power consumption and compactness• Cost reduction roadmap and standards

Position statement

Conclusions

Acknowledgements

Gazettabyte © Roy Rubenstein

SummaryOne hundred Gigabit-per-second (100Gbps) optical networking technology is staking its claim in the network. Like 10Gbps, first introduced a decade ago, 100Gbps can expect to be a bedrock technology for years to come.

The technology is already carrying live traffic in the network core, albeit so far in a modest number of deployments.

The industry has already settled on coherent technology for long-haul (>1,000km) optical transmission. In contrast, a variety of technologies will be used for the emerging 100Gbps cost-conscious metro market. These include direct-detection as well as coherent technology. Indeed several 100Gbps direct-detection and coherent choices for the metro have already been announced.

The 100Gbps metro market is an attractive opportunity for equipment, optical module, optical component and chip makers though it will be challenging and fiercely competitive. For any proposed 100Gbps metro technology to be adopted widely, it must meet several important requirements:

• Have an attractive cost-performance • Have a relatively low power consumption and be compact - ideally pluggable. • Have a product roadmap that maintains the cost-performance advantage.

Direct detection has an important lead on coherent technology in terms of cost and pluggability. This gives the technology a window of opportunity before coherent comes down in price and becomes compact enough to fit within a CFP. Coherent will eventually become the predominant 100Gbps technology for the metro. But coherent designs are relatively complex and developments in telecom take time. If that proves the case, then direct detection can expect to secure an important role in the metro.

Status of 100 Gigabit deployments

The first 100Gbps optical transmission link was deployed by Verizon Communications in its European network in December 2009. Linking Frankfurt to Paris, the 900km route was already carrying more than 30, 10Gbps light paths.

Since then Ciena (which acquired Nortel's Metro Ethernet Networks division that supplied the first 100Gbps system) has deployed 100Gbps systems for a variety of customers including service providers - fixed and mobile, cable operators and large research organisations.

Alcatel-Lucent, which launched its 100Gbps system in June 2010, says it has sold 1,000, 100Gbps lines, with 800 already deployed. And in September 2011, Infinera announced its DTN-X platform based on a 5x100Gbps photonic integrated circuit that will transport data over a 500Gbps super-channel. The DTN-X will become available in the first half of 2012 and is expected to reduce the cost of 100Gbps long-haul transport.

Industry analysts are largely upbeat about the 100Gbps market. Ovum, Infonetics Research and LightCounting all expect strong demand for 100Gbps in the coming years, with volumes ramping from 2013-14. The one note of caution is sounded by market research firm, Telecom Pragmatics, which warns of industry hyperbole and that volume deployments of 100Gbps could be a decade away.

Infonetics' view is China could even accelerate the uptake of 100Gbps before 2013. China is planning a huge rollout of fibre-to-the-x (FTTx) and the three main operators - China Telecom, China Unicom and China Mobile - are all interested in 100Gbps given the inevitable traffic growth in their core networks.

China Telecom and China Unicom have strong requirements for 100Gbps dense wavelength division multiplexing (DWDM) systems and plan field trials by the year end or early 2012, while China Mobile will skip 40Gbps and go straight to 100Gbps, says ZTE.

Huawei says it has already conducted a trial with China Telecom and that some of the operator's provincial branches have conducted their own 100Gbps field trials. The Chinese vendor stresses there will be 100Gbps commercial deployment in the coming months, highlighting the China's Yangtze River Delta region in particular.

Service providers say that first deployments of 100Gbps metro will coincide with long-haul deployments although volume deployments of 100Gbps metro will come later. Equally, while the content service providers that operate "mega-data centres" are very interested in 100Gbps, the technology is still too expensive when compared with 10Gbps. But once 100Gbps technology becomes available at a competitive price-point compared to 10Gbps, the transition to 100Gbps will likely occur quickly.

100 Gigabit networking driversService providers' adoption of 100Gbps optical networking technology has been to expand capacity on key congested network routes. But operators are looking at using the technology more strategically as they plan their next-generation backbones using 100Gbps and future, faster speeds - 200Gbps, 400Gbps and 1 Terabit-per-second (Tbps) - based on coherent detection technology.

There is also a demand by content service providers to connect their data centres across the metro. And while multiple 10Gbps services are used now, adopting 100Gbps will bring operational benefits. These 100Gbps drivers are now discussed.

Network congestionIP traffic continues to grow yearly. The causes for the traffic growth are well documented: fixed broadband subscriber growth, expanding use of video, and the rise of smartphones that benefit from new mobile broadband technologies such as HSPA and LTE. Video is by far the most data-intensive content format and is forcing operators to reconsider how they distribute and cache such content in their networks.

These drivers will continue and traffic growth will remain exponential although it will slow. Cisco Systems' visual networking index has reduced the yearly fixed Internet traffic growth forecast to 32% per annum through to 2015.

Service providers use 10Gbps predominantly to carry the growing traffic in their IP backbones. Certain operators such as AT&T have already upgraded their backbone to 40Gbps. For particular congested routes operators have added 100Gbps light paths.

For now 100Gbps optical transport technology is in its infancy. Systems vendors' 100Gbps systems are still significantly more expensive than 10x10Gbps. This should not be surprising. System vendors' systems are first-generation designs and they have sold relatively few units. In contrast, 10Gbps

technology has had over a decade of cost reduction and been sold in considerable volumes. So much so that 10Gbps continues to come down in price, presenting a moving cost-target for 100Gbps to match.

Operators will pay a premium for 100Gbps if it solves network pinch-points since it is cheaper to adopt 100Gbps than light a new fibre with the associated outside plant costs. This is how Verizon has used 100Gbps technology to date.

But Verizon is also planning its next-generation network, upgrading the backbone to 100Gbps over the next two years.

BT's core network, which carries the bulk of its IP, business and voice traffic, consists of 106 nodes: 20 in the inner core are fully meshed, and an outer 86-node core. The core network interfaces to some 1,000 Points-of-Presence (PoPs). The UK incumbent says it will deploy 40Gbps in the core before moving rapidly to 100Gbps. BT expects such deployments to start year-end 2011.

Data centre, PoP interconnects and 100 Gigabit servicesEnterprises and content service providers view 100Gbps technology as a way to enhance connectivity to their businesses and data centres to achieve greater operational efficiencies.

What is used today to transport services is one or more 10Gbps links. It is more cost effective at present to use Nx10Gbps services than adopt a 100Gbps interface due to their relative costs. The bulk of the services for the next 3-5 years will remain at 10Gbps, for example 10 Gigabit Ethernet and 8 Gbps Fibre Channel storage traffic.

But managers of data centres want to reduce the number of 10Gbps links they have to manage by adopting 100Gbps links. Large content service providers such as Google and MSN already have multiple 10Gig links. They also have secured or even own fibre. By moving to 100Gbps interfaces they improve equipment port density and free up space as well as reduce the number of ports they must manage. There is also greater capacity efficiencies using a 100Gbps link than multiple aggregated 10Gbps ones.

Such data centre requirements should be viewed as distinct from service providers' own core and metro network capacity upgrades using 100Gbps, although clearly enterprise and content service provider 100Gbps connectivity requests will impact the telcos' 100Gbps plans.

Operators upgrade their core networks first to 100Gbps and upgrade the metro at a later stage, once the technology comes down in price for the more cost-conscious metro market and once enterprises' requests for 100Gbps services grow in volume.

Operators will eventually have requests from their customers for 100Gbps services. At present 10GbE services are sufficient for most enterprises' telecom requirements such as VPNs, phone and data requirements. But over time this will grow and once enterprises require several such 10 GbE services, a 100Gbps service becomes an option. Level 3 Communications expects it will take some five years before 100Gbps service requests from enterprise become commonplace.

In summary, such demand drivers will happen but for the next few years 100Gbps demand in the metro will be limited. But there are developments that point to future healthy demand:

• IP core router interfaces: Whereas the bulk of 40Gbps deployments have been to carry multiplexed 10Gbps traffic rather than expensive 40Gbps SONET/SDH interfaces on routers, the advent of 100 Gigabit Ethernet (GbE) interfaces suggest that IP router interfaces could play a more important role in the growth of 100Gbps. Operators have had several more years since the advent of 40Gbps to consolidate their various networks onto IP. Such IP router interfaces will also be how enterprises and data centres move their 100Gbps traffic off site.

• The number of data centres and PoPs in the metro will grow. Service providers will have more PoPs and locations in the metros as traditional services are consolidated on the one IP network. These distributed locations will be connected using a more agile, mesh architecture.

• With the uptake of cloud and virtualisation, data centre resources will increasingly be shared between sites. This will place new traffic demands on the metro network.

• New services such as 16 Gigabit Fibre Channel will require larger pipes than a 10Gbps wavelength. The first 16Gbps Fibre Channel optical transceivers are appearing from Avago Technologies and Finisar.

Technology options for 100 Gigabit

Exhibit 1 shows the different technology options available for 100Gbps and the metro in particular. These can be separated into client interfaces that touch the metro edge, and line side interfaces that address metro core, regional and long-haul. What is not shown are the very short reach, 100m, IEEE 100Gigabit Ethernet (GbE) based on multimode fibre.

Exhibit 1: 100Gbps technologies for the metro

The IEEE 100 Gigabit Ethernet interfaces

The main 100GbE interfaces as defined by the IEEE 802.3ba standard are the 100GBASE-LR4 for 10km transmission and the 100GBASE-ER4 for 40km. Both interfaces use a 10x10Gbps electrical interface and a 4x25Gbps optical interface.

The IEEE interfaces are point-to-point interfaces that provide a single 100GbE connection. Thus while they use WDM technology to transmit the four optical channels that carry the 100Gbps signal, they are "grey" optics and not DWDM.

The LR4/ ER4 are designed for point-to-point links, to connect two switches or to connect an IP router to DWDM equipment which take the 100GbE and transport it over line-side optics using Optical Transport Network (OTN) framing. But for certain applications the 10 and 40Gbps reaches can be used to connect equipment between two data centres and as such are a starting point for 100Gbps metro connectivity.

The IEEE 802.3ba standard was officially released in June 2010.

The 10x10 MSAThe 10x10 MSA is an industry initiative to deliver a cheaper 100GbE interface than the IEEE defined ones. Content service providers, most notably Google, want an optical interface with only a 2km reach and that is significantly cheaper than the first generation 10km 100BASE-LR4. The 10x10 2km MSA, as the name implies, uses a 10x10Gbps electrical interface and a 10x10Gbps optical architecture. The 10x10 MSA takes advantage of 10Gbps optics to compete on cost with the 4x25Gbps IEEE standard.

But the 10x10 MSA is not a universally welcome initiative. Whereas for 10GbE the electrical interface evolved from a parallel implementation to a serial one, the optical interface remained the same. As a result the design evolved from the initial 300-pin module to an SFP+ pluggable form factor. Now, with the advent of the 10x10 MSA, there is a risk of industry fragmentation as the IEEE and 10x10 MSA use different optical interfaces: 10x10Gig and 4x25Gig respectively.

In August 2011 the 10x10 MSA's members expanded the specification to include several additions, as detailed in Table 1.

Additional 10x10 MSA Comment

10x10 MSA 10km Same reach as the 100GBASE-LR4

4x100G 40km 100GHz channel per 10G lane or 4THz of spectrum in total for 4x100Gbps. A reach of 100-200km is possible with amplification.

8x100G 40km 50GHz channel per 10G lane or 4THz of spectrum in total for 8x100Gbps. A reach of 100-200km is possible with amplification.

But the 10x10 MSA raises several issues in addition to industry fragmentation:

• The 10x10 MSA 2km and 10km options are at best a temporary option even if they are adopted for the next few years. The two interface variants may cost half the LR4 but once the LR4 adopts a 4x25Gbps electrical interface, the power-hungry gearbox IC will be removed. The gearbox IC translates between 10 lanes of 10Gbps to 4x25Gbps, the electrical input for the optical channels. This will reduce significantly the power consumption and design cost of the LR4 and will lead to the interface fitting into the smaller CFP2 and CFP4 pluggable form factors.

• The 10x10 MSA will continue to use the 10x10Gbps optical architecture. As a result, if it is to be compatible with the future 4x25Gbps electrical interface, it will need to introduce an inverse gearbox IC (4x25G to 10x10G). Adding an IC reverses the trend all optical module makers want and will also add cost to the 10x10 design while the IEEE interfaces come down in price.

• The 4x100Gbps and 8x100Gbps interfaces are a more interesting development that will extend the 10x10 MSA in distance and capacity that the current IEEE standards cannot match. The WDM design uses channel spacings for each of the 10Gbps wavelengths at 50GHz (8x100Gbps) and 100GHz (4x100Gbps). These are the channel spacings 100Gbps data can fit into. Thus the 10x10 MSA is by far the least spectrally efficient of the various WDM 100Gbps solutions. That said, for data centre interconnect, the 4x100Gbps and 8x100Gbps solutions promise a low-cost implementation which with amplification and dispersion compensated fibre, could provide 100 to 200km links.

Coherent transmission

Coherent detection has become the technology of choice for long-haul transmission at 100Gbps and is seen as the basis for future faster speeds of 200Gbps, 400Gbps and 1Tbps.

For 100Gbps, the coherent technology promoted by the Optical Internetworking Forum (OIF) and supported by operators and optical system vendors is polarisation-multiplexing, quadrature phase-shift keying (PM-QPSK). This is also referred to as dual-polarisation, QPSK (DP-QPSK). PM-QPSK coherent delivers key optical transmission advantages.

• A coherent receiver uses a local oscillator in the form of a tunable laser to recover the received signal and preserves the phase information after photo-detection. This phase information can be used by a digital signal processor (DSP) to improve the chromatic and polarisation mode dispersion compensation performance. The result is robust optical transmission and an outside fibre plant that does not require dispersion-compensation fibre loops.

• System cost benefits hugely from being able to use 28bps electronics and optics to transmit a 100Gbps signal. And CMOS-based chips such as the DSP ASIC that perform dispersion compensation can be coupled to the optics when operating at 28Gbps.

• Industry standardisation on DP-QPSK allows the optical component industry to focus on one solution. The standardisation promotes increasing levels of photonic integration and the DSP-based ASIC that drive down the cost and size of 100Gbps line side designs.

For now coherent designs are relatively large - one per line card - and fit into a 7x5-inch module. The first-generations designs also have a power consumption of the order of 100W.

If coherent is suited to long-haul technology, why is it being mentioned in the context of metro? It is because certain operators and system vendors are strong proponents of coherent technology and believe it will also be needed to meet metro requirements. Others are more open, and will consider non-coherent requirements that promise cost, size and power advantages compared to coherent as long as such alternatives meets their networking needs.

So far at least two vendors - Ciena and Transmode - have announced coherent solutions tailored for the metro.

Non-coherent transmission

Non-coherent or direct-detection transmission have a simpler receiver structure but at the expense of performance when compared to coherent. With direct detection, the signal is recovered using timing information from the transmitted bits. And the detection is based on the square-law: the received photo-detector current is related to the square of the strength of the detected light. Here, unlike coherent, phase information is not preserved after the photo-detection impacting dispersion compensation performance. But direct detection requires a simpler receiver design and is thus cheaper to build, is more compact and can be fitted in a pluggable optical module.

For 40Gbps transmission several direct-detection technologies have been developed in addition to standard on-off keying and coherent: optical duo-binary, differential phase-shift keying (DPSK) and differential quadrature phase-shift keying (DQPSK).

For 100Gbps transmission there are already direct-detection solutions being proposed for the metro. This include on-off keying, point-to-point up to 80km and optical duo-binary that can implement WDM solutions for 200-500km and up to 800km. These initial non-coherent 100Gbps solutions will be discussed in the next section.

The coming battle for the metro

The metro network has unique requirements that differ from long-haul. For long-haul, fibre spectrum is a precious commodity and exploiting backbone capacity to the full is key. For metro, maximising capacity is less of an issue. What is important is being able to carry and drop a mixture of services. At present this is mainly 10Gbps services but metro networks will also have to support 40 and 100Gbps services.

100 Gigabit metro transport

The metro can be divided into metro access, metro core and metro regional.

Definitions vary. For example in the US networking distances are typically longer. Long-haul in Europe such as BT's core backbone has a span of up to 1000km though most links are far shorter. Verizon or AT&T's US backbone must span 3000km for coast-to-coast communication.

The same applies for metro: a metro regional network in Europe equates to a large metro core in the US. Table 2 characterises the main metro network characteristics.

Table 2: Metro networks segmentation Source: MultiPhy

It is clear from Table 2 that there are a mix of metro reaches and requirements.

Even though the 100Gbps market has yet to begin, several announcements have been made that indicate how the market will evolve. These can be split into three approaches as shown in Exhibit 2.

Exhibit 2: Direct detection and coherent technologies for the metro

Network Metro edge Metro core Metro regional

Reach <120km 120-600km 600-1000km

Channel spacing 100GHz 50/100GHz 50GHz

Lambdas 16-40 40-80 80

Topologies Ring Ring/ mesh Mesh/ point-to-point

Direct-detection point-to-point

Direct-detection solutions link equipment between buildings and between sites including connecting data centres or PoPs in a metro. Distances range up to 80km and are a single link such that only one 100Gbps pipe is used across the fibre. This assumes that the enterprise/ data centre operator/ service provider has ample fibre and that there is no need for greater capacity than a single 100Gbps link for now. And if the solution is pluggable, upgrading the link to WDM when needed becomes straightforward.

The advantage of point-to-point single-channel links is that the 'grey' interfaces are the most compact and least costly of all the 100Gbps metro solutions.

It should also be noted that there are 40Gbps point-to-point metro options. These include the IEEE 40GBASE-FR (2km serial) and the 40GBASE-LR4 (10km). This report is focussed on 100Gbps but it is important to note that 40Gbps has yet to be deployed in the metro. The point-to-point 40Gbps solutions will play a minimal role in the metro but there are line-side 40Gbps solutions that do span the metro reaches. And while 40Gbps now faces competition for 100Gbps, 40Gbps will be a valuable and competitive option alongside 10Gbps in the metro for the next 5-10 years.

The announced point-to-point solutions include the IEEE 100GBASE-LR4/ ER4 (10, 40km), 10x10 MSA (2 and 10km) and MultiPhy's announced design that will enable CFP modules with an 80km reach.

Direct-detection WDM

There are also three direct-detection technologies for metro at 40Gbps: optical duo-binary, DPSK and DQPSK. These options compete with 10Gbps and 100Gbps, and the technology has already benefited from more than one design iteration, benefitting from 40Gbps' deployment for long-haul.

While 40Gbps offers the same benefits as 100Gbps over 10Gbps in terms of expanding capacity and reducing the ports that need to be managed, 100Gbps will deliver these benefits on a greater scale. But to displace 40Gbps as an option in the metro, 100Gbps must cost less than 2x 40Gbps (40Gbps > 0.5x100Gbps cost).

Direct-detection WDM technology has several advantages over point-to-point. The technology supports 40 and 80 wavelengths over the C-band. It also is able to support reaches such as 200km, 500km even 800km. Thus the technology spans metro core and metro regional spans.

Direct-detection WDM designs fit in a pluggable CFP form factor and have a cost that already competes with 10x10Gbps optics.

The first 100Gbps direct-detection WDM options have now been detailed. ADVA Optical Networking has a direct-detect card for its FSP 3000 platform while MultiPhy has a chip that enables on-off keying and direct-detection CFP modules. The performance of both companies' optical duo-binary solutions are detailed in Table 3.

Table 3: Direct-detection DWDM offerings for the metro

Additional direct-detection designs can be expected to be launched in the coming months and years that may use alternative modulations schemes to achieve different reach and cost/ performance points.

Coherent metro

Coherent metro solutions make use of the momentum coherent technology is gaining for 100Gbps long-haul while tailoring the design for metro reach and cost requirements.

Coherent offers the best performance in terms of reach and spectral efficiency. The technology can operate robustly alongside existing 10Gbps and 40Gbps light paths on the same fibre. But the advanced performance comes at a price: coherent designs are expensive, have a relatively high power consumption and are bulky.

The first long-haul coherent module designs will fit into a 5x7-inch MSA and are not pluggable. Opnext has announced such a 100Gbps module. For metro, designs are currently line card-based. Transmode has announced a 750km coherent system tailored for the metro, while Ciena has two coherent metro cards - for 300km and 600km. The various technologies are shown in Exhibit 3

Exhibit 3: 100Gbps technologies for metro

Company ADVA Optical Networking MultiPhy

Product Line card using a CFP module Receiver chip for a CFP module

Reach 500km 800km

100Gbps Channel width 200GHz, will be 100GHz 100GHz

Modulation Optical duo-binary Optical duo-binary

Developments to watch

Several networking developments should be noted as they will impact the technology requirements of metro and hence the prospects of these metro technology options:

• Service providers' PoPs will grow in number in the network and become increasingly like data centres as operators embrace IT. In turn data centres will become more distributed in the metro.

• The largest data centres being proposed have 1 million sq. ft of floor space. Such huge IT centres will introduce new networking demands within the data centre and inevitably to and from the site. Power consumption of data centres has also become a key design consideration.

• Metro networks will move to mesh architectures to connect increasing numbers of sites and meet their connectivity demands. Metro networks will embrace ROADMs to do this. This will have two effects on optical transmission: the number of ROADM filter stages the light path signal will have to go through will increase, and path distances will effectively increase due to the mesh and the move away from defined point-to-point connections. Both will make greater demands on metro optical transmission.

• Discussion of higher speeds than 100Gbps is premature given the status and challenges 100Gbps deployments face. Yet system vendors are already developing in the lab programmable line interfaces that will trade modulation schemes with transmission distances. Given the industry focus on coherent for 100Gbps and higher speeds, this favours coherent technology as the long term choice in the core and metro. That said, it will take at least 10 years before such programmable interfaces become mainstream and impact the metro in particular.

How the metro technologies will likely play outThere are several key criteria that can be used to assess any metro transport offering:

• Cost-performance • Pluggable form factor• Power consumption and compactness • Cost reduction roadmap and standards

Each issue is now discussed.

Cost-Performance

Cost-performance is the most complex of the criteria to assess. The metrics when the switchover to 100Gbps becomes mainstream is when cost of 100Gbps is lower than 10x the equivalent 10Gbps interface cost, or when 100Gbps costs less than 2x 40Gbps. That said, one system vendor says that operators are already requesting DWDM systems where the 100Gbps line interface costs between 6x-8x 10Gbps.

Performance is usually assessed based on such parameters as reach, spectral efficiency, power consumption, size and pluggability.

Optical robustness is also important as performance is diminished when a light path passes through multiple ROADM filter stages. Other issues that can affect optical link performance include the quality of the metro fibre or whether the 100Gbps signals is alongside existing 10Gbps and 40Gbps light paths.

Exhibit 4 plots estimated cost-performance of the different metro technologies where performance as defined by reach divided by spectral efficiency. For example, Transmode's reach/ spectral efficiency is known but power and cost are assumptions for coherent designs in general.

Exhibit 4: Estimated cost-performance of 100Gbps metro technologies

Pluggable form factor

Having the design in a pluggable form factor is important for several reasons:

• It enables the device to be addressed at a variety of platforms such as DWDM platforms, carrier Ethernet switch routers (CESR), IP routers and data centre Ethernet switches.

• Having a 100Gbps metro design in a CFP pluggable form factor means that the operator can switch in other 100Gbps CFP optics as required.

• Pluggable optical modules have a product roadmap which the industry works to. For the CFP, these will include the smaller CFP2 and further out the CFP4. By embracing common MSAs the industry benefits from economies of scale.

Power consumption and compactnessOnce a design fits within a pluggable form factor, a certain power consumption and equipment face plate interface density are implied.

Putting a 100Gbps direct-detect design in a CFP is a notable achievement. But it is already viewed as a starting point as vendors design future systems that have four and even eight 100Gbps ports per line card.

Such designs are also dependent on progress in other system technologies such as the move from 10Gbps electrical interfaces to 25Gbps, and advances in backplane technology. There is also a relationship between client side and line side interfaces in terms of input/ output traffic of systems. And it is easier for client side interfaces such as the 100GbE LR4 to be squeezed into a next-smallest pluggable than the more complex direct detection or coherent systems.

What can be said is that direct-detection designs already fit into a pluggable while coherent will first appear in a non-pluggable 5x7-inch MSA. Also their respective power consumptions are 24W and of the order or 100W. Power consumption is a big issue for operator and data centre managers and will only grow in importance over time.

So for now direct detection has a notable advantage. But coherent designs will move to a pluggable form factor in the next three years and that will be key milestone for the technology.

Cost-reduction roadmap and standardsMoving to a smaller form factor is an important part of the cost reduction roadmap. But standards also play a role.

The only 100 Gigabit standards body is the IEEE which has defined the various 100 Gigabit Ethernet standards. For the line side, there is no official standards body although the work of the OIF has created a defacto standard with 100Gbps coherent having been adopted by the industry for long-haul.

Does long-haul need standards? After all vendors differentiate their long-haul transmission based on their proprietary designs and they control both ends of a DWDM line.

The answer is no but standards and defacto standards play a key role in cost reduction. By a critical mass of companies backing a standard, an MSA or a technology defined by an industry organisation like the OIF, companies can focus their precious R&D dollars on developing a standard set of components.

The industry as a whole benefits from the cost reduction of one technology over time. This is not what happened at 40Gbps with optical duo-binary. DPSK, DQPSK and then coherent technologies being developed.

Coherent technology for the metro benefits from the industry backing of coherent for long-haul. While the direct-detect solutions benefit from the IEEE 100GbE standards and all the work being done to develop the upcoming CFP2 and CFP4 modules.

Operators favour standards. For cable MSOs and especially content service providers, adopting a standards-backed technology is less of an issue. Any proposed 100Gbps metro technology should bear that in mind.

The issues regarding cost-reduction roadmaps for the various metro technologies are as follows:

• Direct-detection point-to-point: These are the lowest cost designs. The 10x10 MSA has limited scope to further reduce cost while the move to 4x25Gbps electrical interfaces will benefit the IEEE and other direct detect designs.

• Direct-detection WDM: The advent of CFP-based direct-detect WDM modules has create a new cost-performance for the metro that is already cost competitive with 10Gbps technology. Existing designs will move to smaller form factors benefitting from IEEE client-side interface developments.

• Coherent: Best performance in terms of reach and spectral efficiency. But coherent is not a pluggable design, yet. Coherent is also costly and has a high-power consumption. It will take timefor costs to come down.

The advent of direct-detection 100Gbps technology has introduced a new cost-performance point for the metro. Indeed by 2012 it will already be within the 10x10Gbps cost of XFP tunable laser modules as shown in Exhibit 5. The IEEE 100GbE interfaces and coherent, meanwhile, have still got work to do to get closer to the equivalent 10x 10Gbps cost points.

Exhibit 5: Relative cost points of various 40 and 100Gbps compared to the equivalent 10x10Gbps costs.

Position statement

Of the three main 100Gbps technologies for metro, direct-detection point-to-point will be of most interest for enterprises and content service providers to link closely spaced data centres and sites. Service providers will consider the technology but for specific requirements only.

The bulk of the metro requirements - core and regional - will be addressed using direct-detection WDM and coherent. Direct-detection WDM is arriving at a competitive price point and in a pluggable CFP, which is attractive. But direct-detection vendors must demonstrate robust optical performance if it is to be considered by service providers.

Service providers point out that developments in the metro mean that performance and spectral efficiency are key. As such direct-detection WDM faces stiff competition from coherent technology with its robust performance and 50GHz channel spacing. Operators that are planning their next-generation networks favour coherent and have yet to be convinced by direct-detection.

Other service providers that have IP networks, with platforms such as CESR and none of the legacy networks built up over decades, are more open to direct-detection but only if it meets their reach/ spectral density requirements. Enterprises and content service providers will be most receptive to direct detection's cost and pluggability.

Direct detection faces another challenge: the degree of interest by the system vendors is yet to be proven. The technology has been adopted by metro specialist ADVA Optical Networking and is being promoted by MultiPhy for module makers and system vendors. But Transmode, another metro specialist, has chosen coherent. Meanwhile many of the leading optical system vendors, which have in-house coherent technology, have yet to show their 100Gbps metro hand. As such there is still a question mark regarding the degree to which direct detection will be embraced.

In summary, direct detection has an important lead on coherent technology in terms of cost and pluggability. This gives the technology a window of opportunity before coherent comes down in price and is compact enough to fit into a CFP. Coherent will eventually become the predominant 100Gbps technology for the metro. But coherent designs are relatively complex and developments in telecom take time. If that proves the case, then direct detection can expect to secure an important role in the metro.

ConclusionsOne hundred Gigabit is a fundamental optical networking technology that will be in the network for years to come:

• The industry has consolidated on coherent technology for long haul, unlike 40Gbps when several technology approaches were pursued and which spread the industry's efforts.

• There is already demand for 100Gbps technology, demand which will only grow

There are two main 100Gbps networking requirements: transporting traffic across operators core and soon metro networks, and for enterprise and content service providers' data centre interconnect.

Connecting data centres of large content service providers and enterprises is an important driver of 100Gbps in the metro. Metro developments such as distributed data centres and a growth in PoPs, as well move as the move to mesh networking will further benefit 100Gbps' uptake.

Unlike long-haul transport, metro transport will use a far richer mix of innovative 100Gbps technologies. These including direct-detection schemes and coherent technology. Such choices for metro are already being announced.

The rich choice of metro offerings will benefit the industry, giving operators, content providers and enterprises a variety of 100Gbps price-performance options. But the variety also raises challenges: the diversity of choice makes networking decisions more complicated. And given the choices will continue to evolve over the next 3-4 years, first-mover advantage and early design wins are no guarantee of success.

Any proposed 100Gbps scheme for the metro must deliver acceptable performance at an improved cost point to existing technologies. What is meant by acceptable varies with customer: requirements of content deliver providers with fibre will be quite different to a large service providers that are planning the long-term future of their networks.

The metrics that will dictate the success of 100G metro schemes are:

• Cost-performance (with performance dictated by reach, robustness, spectral efficiency, pluggability and power consumption).

• Product roadmap: This is not a must for deployment but without a cost reduction roadmap any 100Gbps interface risks being swapped out by newer, more compelling solutions.

Coherent technology represents the biggest threat to direct-detection schemes long term: it has a price reduction roadmap, it is the scheme of choice for future faster speed options, and has the performance (spectral efficiency, reach, robustness).

But bringing the coherent's cost down sufficiently for metro will take time. And based on the duration it has taken for 10Gbps and 40Gbps to come down in price, this could make a big difference in the window of opportunity for direct-detection schemes to become established.

The competitive advantage of direct detection is cost-performance and pluggability. The direct-detection proponents will need to be nimble in how they reduce cost to remain a valid competitor. The longer direct detection retains its cost-performance advantage, the more promising and established the technology will become.

Acknowledgements

Gazettabyte would like to thank the following companies and individuals interviewed for this 100Gbps Position Paper:

ADVA Optical Networks, Alcatel-Lucent, Brocade, BT, BTI Systems, Ciena, Cisco Systems, Cyoptics, John D'Ambrosia - chair of IEEE 100Gbps backplane study group, ECI Telecom, Ericsson, Finisar, Huawei, Infinera, Ixia, Juniper Networks, MultiPhy, Nokia Siemens Networks, Oclaro, Opnext, Level 3 Communications, Transmode, Verizon and ZTE.