elastic optical networking Archives

Optical networking: The next 10 years

Feature - Part 2: Optical networking R&D

Predicting the future is a foolhardy endeavour, at best one can make educated guesses.

Ioannis Tomkos is better placed than most to comment on the future course of optical networking. Tomkos, a Fellow of the OSA and the IET at the Athens Information Technology Centre (AIT), is involved in several European research projects that are tackling head-on the challenges set to keep optical engineers busy for the next decade.

“We are reaching the total capacity limit of deployed single-mode, single-core fibre,” says Tomkos. “We can’t just scale capacity because there are limits now to the capacity of point-to-point connections.”

Source: Infinera

The industry consensus is to develop flexible optical networking techniques that make best use of the existing deployed fibre. These techniques include using spectral super-channels, moving to a flexible grid, and introducing ‘sliceable’ transponders whose total capacity can be split and sent to different locations based on the traffic requirements.

Once these flexible networking techniques have exhausted the last Hertz of a fibre’s C-band, additional spectral bands of the fibre will likely be exploited such as the L-band and S-band.

After that, spatial-division multiplexing (SDM) of transmission systems will be used, first using already deployed single-mode fibre and then new types of optical transmission systems that use SDM within the same optical fibre. For this, operators will need to put novel fibre in the ground that have multiple modes and multiple cores.

SDM systems will bring about change not only with the fibre and terminal end points, but also the amplification and optical switching along the transmission path. SDM optical switching will be more complex but it also promises huge capacities and overall dollar-per-bit cost savings.   

Tomkos is heading three European research projects - FOX-C, ASTRON & INSPACE.

FOX-C involves adding and dropping all-optically sub-channels from different types of spectral super-channels. ASTRON is undertaing the development of a one terabit transceiver photonic integrated circuit (PIC). The third, INSPACE, will undertake the development of new optical switch architectures for SDM-based networks.

Tomkos’s research group is also a partner in three other EU projects. One of them - dubbed ACINO - involves a consortium developing a software-defined networking (SDN) controller that oversees sliceable transponders.

These projects are now detailed.

FOX-C

Spectral super-channels are used to create high bit-rate signals - 400 Gigabit and greater - by combining a number of sub-channels. Combining sub-channels is necessary since existing electronics can’t create such high bit rates using a single carrier.

Infinera points out that a 1.2 Terabit-per-second (Tbps) signal implemented using a single carrier would require 462.5 GHz of spectrum while the accompanying electronics to achieve the 384 Gigabaud (Gbaud) symbol rate would require a sub-10nm CMOS process, a technology at least five years away.

In contrast, implementing the 1.2 Tbps signal using 12 sub-channels, each at 100 Gigabit-per-second (Gbps), occupies the same 462.5 GHz of spectrum but could be done with existing 32 Gbaud electronics. However, instead of one laser and four modulators for the single-carrier case, 12 lasers and 48 modulators are needed for the 1.2 Tbps super-channel (see diagram, top).

Operators are already deploying super-channels on existing networking routes. For example, certain 400 Gbps links use two sub-channels, each a single carrier modulated using polarisation-multiplexed, 16 quadrature amplitude modulation (PM-16-QAM).

Meanwhile, CenturyLink was the first operator, in the second quarter of 2012, to deploy a 500 Gbps super-channel using Infinera’s PIC. Infinera’s 500 Gigabit uses 10 sub-channels, each carrying a 50 Gbps signal modulated using polarisation-multiplexed, quadrature phase-shift keying (PM-QPSK).

There are two types of super-channels, says Tomkos:

Those that use non-overlapping sub-channels implemented using what is called Nyquist multiplexing.
And those with overlapping sub-channels using orthogonal frequency division multiplexing (OFDM).

Existing transport systems from the optical vendors use non-overlapping super-channels and Optical Transport Networking (OTN) at the electrical layer for processing, switching and grooming of the signals, says Tomkos: “With FOX-C, we are developing techniques to add/ drop sub-channels out of the super-channel without going into the electronic domain.” 

Accordingly, the FOX-C project is developing transceivers that implement both types of super-channel, using non-overlapping and overlapping sub-channels, to explore their merits. The project is also developing techniques to enable all-optical adding and dropping of sub-channels from these super-channel types.

With Nyquist-WDM super-channels, the sub-channels are adjacent to each other but are non-overlapping such that dropping or adding a sub-channel is straightforward. Today’s 25 GHz wide filters can separate a sub-channel and insert another in the empty slot.

The FOX-C project will use much finer filtering: 12.5GHz, 6.25GHz, 3.125GHz and even finer resolutions, where there is no fixed grid to adhere to. “We are developing ultra-high resolution filtering technology to do this all-optical add/drop for Nyquist multiplexed sub-channels without any performance degradation,” says Tomkos. The FOX-C filter can achieve a record resolution of 0.8GHz.

OFDM is more complicated since each sub-channel interacts with its neighbours. “If you take out one, you disturb the neighbouring ones, and you introduce severe performance degradation,” says Tomkos. To tackle this, the FOX-C project is using an all-optical interferometer.

“Using the all-optical interferometer introduces constructive and destructive interference among the OFDM sub-channels and the sub-channel or channels we want to drop and add,” says Tomkos. “By properly controlling the interferometer, we are able to perform add/ drop functions without performance degradation.”

ASTRON

The second project, ASTRON, is developing a one terabit super-channel PIC. The hybrid integration platform uses planar lightwave circuit (PLC) technology based on a glass substrate to which are added the actives: modulator arrays and the photo-detectors in indium phosphide. “We have kept the lasers outside the PIC mostly due to budgetary constraints, but there is no problem to include them also in the PIC,” says Tomkos. The one terabit super-channel will use eight sub-channels, occupying a total spectrum of 200 GHz. 

The PLC acts as the integration platform onto which the actives are placed. “We use 3D waveguide inscription inside the glass using high-power lasers and flip-chip bonding to couple the actives to the passives inside the PIC,” says Tomkos. 

The modulation arrays and the passives have already been made, and the project members have mastered how to create 3D waveguides in the glass to enable the active-passive alignment.

“We are in the process of finalising the technique for doing the hybrid integration and putting everything together,” says Tomkos.

The physical layer PIC is complemented by developments in advanced software-defined digital signal processing (DSP) and forward error correction (FEC) modules implemented on FPGAs to enhance the transmission performance of the transceiver. The working one terabit PIC, expected from October, will then be used for experimentation in transmission testbeds.   

INSPACE

Spatial-division multiplexing promises new efficiencies in that instead of individual transponders and amplifiers per fibre, arrays of transponders and amplifiers can be used, spread across all the spatial super-channels. Not only does the approach promise far higher overall capacities but also lower cost.   

The introduction of bundled single-mode fibres, as well as new fibers that transmit over several modes and cores within such SDM systems complicates the optical switching. The channels will be less used for point-to-point transmission due to the huge capacities involved, and there will be a need to process and switch spatial sub-channels from the spatial super-channels. “We are developing a wavelength-selective switch that also operates at the spatial dimension,” says Tomkos.

Already it is clear there will be two main SDM switching types.

The first, simpler case involves spatial sub-channels that do not overlap with each other so that individual sub-channels can be dropped and added. This is the case using fibre with a few cores only, sufficiently spaced apart that they are effectively isolated from each other. Existing cable where a bundle of single-mode, single-core fibres are used for SDM also fits this category.  The switching for these fibre configurations is dubbed independent switching.

The second SDM switch type, known as joint switching, uses fibre with multiple cores that are closely spaced, and few core multi-mode fibre. In these cases, individual sub-channels cannot be added or dropped and processed independently as their overlap causes crosstalk. “Here you switch the entire spatially-multiplexed super-channel as a whole, and to do so you can use a single wavelength-selective switch making the overall network more cost effective,” says Tomkos. 

Only after dropping the entire super-channel can signal processing techniques such as multiple input/multiple output (MIMO), a signal processing technique already used for cellular, be used in the electronic domain to access individual sub-channels.       

The goal of the INSPACE project is to develop a new generation of wavelength-selective switches (WSSes) that operate at the spatial dimension.

“The true value of SDM is in its capability to reduce the cost of transport through spatial integration of network elements: fibers, amplifiers, transceivers and nodes,” says Tomkos. If by performing independent switching of several SDM signals using several switches, no cost-per-bit savings result. But by using joint switching for all the SDM signals with the one switch, the hope is for significant cost reductions, he says.

The team has already implemented the first SDM switches one year into the project.

ACINO

The ACINO project is headed by the Italian Centre of Research and Telecommunication Experimentations for Networked communities (Create-net), and also involves Telefonica I+D, ADVA Optical Networking and Tomkos’s group.

The project, which began in February, is developing an SDN controller and use sliceable transponders to deliver different types of application flows over the optical network. 

To explain the sliceable transponder concept, Tomkos uses the example of a future 10 terabit transponder implemented using 20 or 40 sub-channels. All these sub-channels can be combined to deliver the total 10 Tbps capacity between two points, but in a flexible network, the likelihood is that flows will be variable. If, for example, demand changes such that only one terabit is needed between the two points, suddenly 90 percent of the overall capacity is wasted. Using a sliceable transponder, the sub-channels can be reconfigured dynamically to form different capacity containers, depending on traffic demand. Using the transponder in combination with WSSes, the different sub-channel groupings can be sent to different end points, as required.

Combining such transponders with the SDN controller, ACINO will enable high-capacity links to be set up and dismantled on demand and according to the different application requirements. One application flow example is large data storage back-ups scheduled at certain times between an enterprise’s sites, another is backhauling wireless traffic from 5G networks.

Tomkos stresses that the key development of ACINO is not sliceable transponders but the SDN controller and the application awareness that the overall solution will offer.

The roadmap

So how does Tomkos expect optical networking to evolve over the next 10-plus years?

The next five years will see further development of flexible optical networking that makes best use of the existing infrastructure using spectral super-channels, a flexible grid and sliceable software-defined flexible transponders.

From 2020-2025, more of the fibre’s spectral bands will be used, coupled with first use of SDM. SDM could start even sooner by using existing single-core, single-mode fibres and combining them to create an SDM fibre bundle.

But for the other versions of SDM, new fibre must be deployed in the network and that is something that operators will find difficult to accept. This may be possible for certain greenfield deployments or for data centre interconnects, he says.

Only after 2025 does Tomkos expect next-generation SDM systems using higher capacity fibre with a high core and mode count, or even hybrid systems that use both low and high core-count fibre with advanced MIMO processing, to become more widely deployed in backbone networks.

For part 1, click one

by Michael

ECOC reflections: final part

Gazettabyte asked several attendees at the recent ECOC show, held in Cannes, to comment on key developments and trends they noted, as well as the issues they will track in the coming year.

Dr. Ioannis Tomkos, Fellow of OSA & Fellow of IET, Athens Information Technology Center (AIT)

With ECOC 2014 celebrating its 40th anniversary, the technical programme committee did its best to mark the occasion. For example, at the anniversary symposium, notable speakers presented the history of optical communications. Actual breakthroughs discussed during the conference sessions were limited, however.

Ioannis Tomkos

It appears that after 2008 to 2012, a period of significant advancements, the industry is now more mainstream, and significant shifts in technologies are limited. It is clear that the original focus four decades ago on novel photonics technologies is long gone. Instead, there is more and more of a focus on high-speed electronics, signal processing algorithms, and networking. These have little to do with photonics even if they greatly improve the overall efficient operation of optical communication systems and networks.

Coherent detection technology is making its way in metro with commercial offerings becoming available, while in academia it is also discussed as a possible solution for future access network applications where long-reach, very-high power budgets and high-bit rates per customer are required. However, this will only happen if someone can come up with cost-effective implementations.

Advanced modulation formats and the associated digital signal processing are now well established for ultra-high capacity spectral-efficient transmission. The focus in now on forward-error-correction codes and their efficient implementations to deliver the required differentiation and competitive advantage of one offering versus another. This explains why so many of the relevant sessions and talks were so well attended.

There were several dedicated sessions covering flexible/ elastic optical networking. It was also mentioned in the plenary session by operator Orange. It looks like a field that started only fives years ago is maturing and people are now convinced about the significant short-term commercial potential of related solutions. Regarding latest research efforts in this field, people have realised that flexible networking using spectral super-channels will offer the most benefit if it becomes possible to access the contents of the super-channels at intermediate network locations/ nodes. To achieve that, besides traditional traffic grooming approaches such as those based on OTN, there were also several ground-breaking presentations proposing all-optical techniques to add/ drop sub-channels out of the super-channel.

Progress made so far on long-haul high-capacity space-division-multiplexed systems, as reported in a tutorial, invited talks and some contributed presentations, is amazing, yet the potential for wide-scale deployment of such technology was discussed by many as being at least a decade away. Certainly, this research generates a lot of interesting know-how but the impact in the industry might come with a long delay, after flexible networking and terabit transmission becomes mainstream.

Much attention was also given at ECOC to the application of optical communications in data centre networks, from data-centre interconnection to chip-to-chip links. There were many dedicated sessions and all were well attended.

Besides short-term work on high-bit-rate transceivers, there is also much effort towards novel silicon photonic integration approaches for realising optical interconnects, space-division-multiplexing approaches that for sure will first find their way in data centres, and even efforts related with the application of optical switching in data centres.

At the networking sessions, the buzz was around software-defined networking (SDN) and network functions virtualisation (NFV) now at the top of the “hype-cycle”. Both technologies have great potential to disrupt the industry structure, but scientific breakthroughs are obviously limited.

As for my interests going forward, I intend to look for more developments in the field of mobile traffic front-haul/ back-haul for the emerging 5G networks, as well as optical networking solutions for data centres since I feel that both markets present significant growth opportunities for the optical communications/ networking industry and the ECOC scientific community.

Dr. Jörg-Peter Elbers, vice president advanced technology, CTO Office, ADVA Optical Networking

The top topics at ECOC 2014 for me were elastic networks covering flexible grid, super-channels and selectable higher-order modulation; transport SDN; 100-Gigabit-plus data centre interconnects; mobile back- and front-hauling; and next-generation access networks.

For elastic networks, an optical layer with a flexible wavelength grid has become the de-facto standard. Investigations on the transceiver side are not just focussed on increasing the spectral efficiency, but also at increasing the symbol rate as a prospect for lowering the number of carriers for 400-Gigabit-plus super-channels and cost while maintaining the reach.

Jörg-Peter Elbers

As we approach the Shannon limit, spectral efficiency gains are becoming limited. More papers were focussed on multi-core and/or few-mode fibres as a way to increase fibre capacity.

Transport SDN work is focussing on multi-tenancy network operation and multi-layer/ multi-domain network optimisation as the main use cases. Due to a lack of a standard for north-bound interfaces and a commonly agreed information model, many published papers are relying on vendor-specific implementations and proprietary protocol extensions.

Direct detect technologies for 400 Gigabit data centre interconnects are a hot topic in the IEEE and the industry. Consequently, there were a multitude of presentations, discussions and demonstrations on this topic with non-return-to-zero (NRZ), pulse amplitude modulation (PAM) and discrete multi-tone (DMT) being considered as the main modulation options. 100 Gigabit per wavelength is a desirable target for 400 Gig interconnects, to limit the overall number of parallel wavelengths. The obtainable optical performance on long links, specifically between geographically-dispersed data centres, though, may require staying at 50 Gig wavelengths.

In mobile back- and front-hauling, people increasingly recognise the timing challenges associated with LTE-Advanced networks and are looking for WDM-based networks as solutions. In the next-generation access space, components and solutions around NG-PON2 and its evolution gained most interest. Low-cost tunable lasers are a prerequisite and several companies are working on such solutions with some of them presenting results at the conference.

Questions around the use of SDN and NFV in optical networks beyond transport SDN point to the access and aggregation networks as a primary application area. The capability to programme the forwarding behaviour of the networks, and place and chain software network functions where they best fit, is seen as a way of lowering operational costs, increasing network efficiency and providing service agility and elasticity.

What did I learn at the show/ conference? There is a lot of development in optical components, leading to innovation cycles not always compatible with those of routers and switches. In turn, the cost, density and power consumption of short-reach interconnects is continually improving and these performance metrics are all lower than what can be achieved with line interfaces. This raises the question whether separating the photonic layer equipment from the electronic switching and routing equipment is not a better approach than building integrated multi-layer god-boxes.

There were no notable new trends or surprises at ECOC this year. Most of the presented work continued and elaborated on topics already identified.

As for what we will track closely in the coming year, all of the above developments are of interesting. Inter-data centre connectivity, WDM-PON and open programmable optical core networks are three to mention in particular.

For the first ECOC reflections, click here

by Michael

The evolution of optical networking

An upcoming issue of the Proceeedings of the IEEE will be dedicated solely to the topic of optical networking. This, says the lead editor, Professor Ioannis Tomkos at the Athens Information Technology Center, is a first in the journal's 100-year history. The issue, entitled The Evolution of Optical Networking, will be published in either April or May and will have a dozen invited papers.

One topic that will change the way we think about optical networks is flexible or elastic optical networks.

Professor Ioannis Tomkos

"If I have to pick one topic that will change the way we think about optical networks, it is flexible or elastic optical networks, and the associated technologies," says Tomkos.

A conventional dense wavelength division multiplexing (DWDM) network has fixed wavelengths. For long-haul optical transmission each wavelength has a fixed bit rate - 10, 40 or 100 Gigabit-per-second (Gbps), a fixed modulation format, and typically occupies a 50GHz channel. "Such a network is very rigid," says Tomkos. "It cannot respond easily to changes in the network's traffic patterns."

This arrangement has come about, says Tomkos, because the assumption has always been that fibre bandwidth is abundant. "But at the moment we are only a factor of two away from reaching the Shannon limit [in terms of spectral efficiency bits/s/Hz) so we are going to hit the fibre capacity wall by 2018-2020," he warns.

The maximum theoretically predicted spectral efficiency for an optical communication system based on standard single-mode fibres is about 9bits/s/Hz per polarisation for typical long-haul system reaches of 500km without regeneration, says Tomkos. "At the moment the most advanced hero experiments demonstrated in labs have achieved a spectral efficiency of about 4-6bits/s/Hz," he says. This equates to a total transmission capacity close to 100 Terabits-per-second (Tbps). After that, deploying more fibre will be the only way to further scale networks.

Accordingly, new thinking is required.

Two approaches are being proposed. One is to treat the optical network in the same way as the air interface in cellular networks: spectrum is scarce and must be used effectively.

"We are running close to fundamental limits, that's why the optical spectrum of available deployed standard single mode fibers should be utilized more efficiently from now on as is the case with wireless spectrum," says Tomkos.

How optical communication is following in the footsteps of wireless.

The second technique - spatial multiplexing - looks to extend fibre capacity well beyond what can be achieved using the first approach alone. Such an option would need to deploy new fibre types that support multiple cores or multi-mode transmission.

Flexible spectrum

"We have to start thinking about techniques used in wireless networks to be adopted in optical networks," says Tomkos (See text box). With a flexible network, the thinking is to move from the 50GHz fixed grid, down to 12.50GHz, then 6.25GHz or 1.50GHz or even eliminate the ITU grid entirely, he says. Such an approach is dubbed flexible spectrum or a gridless network.

With such an approach, the optical transponders can tune the bit rate and the modulation format according to the reach and capacity requirements. The ROADMs or, more aptly, the wavelength-selective switches (WSSes) on which they are based, also have to support such gridless operation.

WSS vendors Finisar and Nistica already support such a flexible spectrum approach, while JDS Uniphase has just announced it is readying its first products. Meanwhile US operator Verizon is cheerleading the industry to support gridless. "I'm sure Verizon is going to make this happen, as it did at 100 Gigabit," says Tomkos.

Spatial multiplexing

The simplest way to implement spatial multiplexing is to use several fibres in parallel. However, this is not cost-effective. Instead, what is being proposed is to create multi-core fibres - fibres that have more than one core - seven, 19 or more cores in an hexagonal arrangement, down which light can be transmitted. "That will increase the fibre's capacity by a factor of ten of 20," says Tomkos.

Another consideration is to move from single-mode to multi-mode fibre that will support the transmission of multiple modes, as many as several hundred.

The issue with multi-mode fibre is its very high modal dispersion which limits its bandwidth-distance product. "Now with improved techniques from signal processing like MIMO [multiple-input, multiple out] processing, OFDM [orthogonal frequency division multiplexing] to more advanced optical technologies, you can think that all these multiple modes in the fibre can be used potentially as independent channels," says Tomkos. "Therefore you can potentially multiply your fibre capacity by 100x or 200x."

The Proceedings of the IEEE issue will have a paper on flexible networking by NEC Labs, USA, and a second, on the ultimate capacity limits in optical communications, authored by Bell Labs.

Further reading:

MODE-GAP EU Seventh Framework project, click here.

by Michael