Optical networking: The next 10 years
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.
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.
- 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).
A FOX-C approach to flexible optical switching
Flexible switching of high-capacity traffic carried over ’super-channel' dense-wavelength division multiplexing wavelengths is the goal of the European Commission Seventh Framework Programme (FP7) research project.
The €3.6M FOX-C (Flexible optical cross-connect nodes) will develop a flexible spectrum reconfigurable optical add/drop multiplexer (ROADM) for 400Gbps and one Terabit optical transmission. The ROADM will be designed not only to switch super-channels but also the carrier constituent components.
Companies involved in the project include operator France Telecom and optical component player Finisar. However, no major European system vendor is taking part in the FOX-C project although W-Onesys, a small system vendor from Spain, is participating.
“We want to transfer to the optical layer the switching capability”
Erwan Pincemin, FT-Orange
“It is becoming more difficult to increase the spectral efficiency of such networks,” says Erwan Pincemin, senior expert in fibre optic transmission at France Telecom-Orange. “We want to increase the advantages of the network by adding flexibility in the management of the wavelengths in order to adapt the network as services evolve.”
FOX-C will increase the data rate carried by each wavelength to achieve a moderate increase in spectral efficiency. Pincemin says such modulation schemes as orthogonal frequency division multiplexing (OFDM) and Nyquist WDM will be explored. But the main goal is to develop flexible switching based on an energy efficient and cost effective ROADM design.
The ROADM’s filtering will be able to add and drop 10 and 100 Gigabit sub-channels or 400 Gigabit and 1 Terabit super-channels. By using the developed filter to switch optically at speeds as low as 10 Gigabit, the aim is to avoid having to do the switching electrically with its associated cost and power consumption overhead. “We want to transfer to the optical layer the switching capability,” says Pincemin.
While the ROADM design is part of the project’s goals, what is already envisaged is a two-stage pass-through-and-select architecture. The first stage, for coarse switching, will process the super-channels and will be followed by finer filtering to extract (drop) and insert (add) individual lower-rate tributaries.
The project started in Oct 2012 and will span three years. The resulting system testing will take place at France-Telecom Orange's Lab in Lannion, France.
Project players
The project’s technical leader is the Athens Institute of Technology (AIT), headed by Prof. Ioannis Tomkos, while the administrator leader is the Greek company Optronics Technologies.
Finisar will provide the two-stage optical switch while France Telecom-Orange will test the resulting ROADM and will build the multi-band OFDM transmitter and receiver to evaluate the design.
Athens Institute of Technology will work with Finisar on the technical aspects and in particular a flexible networking architecture study. The Hebrew University is working with Finisar on the design and the building of the ultra-selective adaptive optical filter, and has expertise is free-space optical systems. The Spanish firm, W-Onesys, is a system integration specialist and will also work with Finisar to integrate its wavelength-selective switch for the ROADM. Other project players include Aston University, Tyndall National Institute and the Karlsruhe Institute of Technology.
No major European system vendor is taking part in the FOX-C project. According to Pincemin this is regrettable although he points out that the equipment players are involved in other EC FP7 projects addressing flexible networking.
He believes that their priorities are elsewhere and that the FOX-C project may be deemed as too forward looking and risky. “They want to have a clear return on investment on their research,” says Pincemin.
Briefing: Flexible elastic-bandwidth networks
Vendors and service providers are implementing the first examples of flexible, elastic-bandwidth networks. Infinera and Microsoft detailed one such network at the Layer123 Terabit Optical and Data Networking conference held earlier this year.
Optical networking expert Ioannis Tomkos of the Athens Information Technology Center explains what is flexible, elastic bandwidth.
Part 1: Flexible elastic bandwidth
"We cannot design anymore optical networks assuming that the available fibre capacity is abundant"
Prof. Tomkos
Several developments are driving the evolution of optical networking. One is the incessant demand for bandwidth to cope with the 30+% annual growth in IP traffic. Another is the changing nature of the traffic due to new services such as video, mobile broadband and cloud computing.
"The characteristics of traffic are changing: A higher peak-to-average ratio during the day, more symmetric traffic, and the need to support higher quality-of-service traffic than in the past," says Professor Ioannis Tomkos of the Athens Information Technology Center.
"The growth of internet traffic will require core network interfaces to migrate from the current 10, 40 and 100Gbps to 1 Terabit by 2018-2020"
Operators want a more flexible infrastructure that can adapt to meet these changes, hence their interest in flexible elastic-bandwidth networks. The operators also want to grow bandwidth as required while making best use of the fibre's spectrum. They also require more advanced control plane technology to restore the network elegantly and promptly following a fault, and to simplify the provisioning of bandwidth.
The growth of internet traffic will require core network interfaces to migrate from the current 10, 40 and 100Gbps to 1 Terabit by 2018-2020, says Tomkos. Such bit-rates must be supported with very high spectral efficiencies, which according to latest demonstrations are only a factor of 2 away of the Shannon's limit. Simply put, optical fibre is rapidly approaching its maximum limit.
"We cannot design anymore optical networks assuming that the available fibre capacity is abundant," says Tomkos. "As is the case in wireless networks where the available wireless spectrum/ bandwidth is a scarce resource, the future optical communication systems and networks should become flexible in order to accommodate more efficiently the envisioned shortage of available bandwidth.”
The attraction of multi-carrier schemes and advanced modulation formats is the prospect of operators modifying capacity in a flexible and elastic way based on varying traffic demands, while maintaining cost-effective transport.
Elastic elements
Optical systems providers now realise they can no longer keep increasing a light path's data rate while expecting the signal to still fit in the standard International Telecommunication Union (ITU) - defined 50GHz band.
It may still be possible to fit a 200 Gigabit-per-second (Gbps) light path in a 50GHz channel but not a 400Gbps or 1 Terabit signal. At 400Gbps, 80GHz is needed and at 1 Terabit it rises to 170GHz, says Tomkos. This requires networks to move away from the standard ITU grid to a flexible-based one, especially if operators want to achieve the highest possible spectral efficiency.
Vendors can increase the data rate of a carrier signal by using more advanced modulation schemes than dual polarisation, quadrature phase-shift keying (DP-QPSK), the defacto 100Gbps standard. Such schemes include amplitude modulation at 16-QAM, 64-QAM and 256-QAM but the greater the amplitude levels used and hence the data rates, the shorter the resulting reach.
Another technique vendors are using to achieve 400Gbps and 1Tbps data rates is to move from a single carrier to multiple carriers or 'super-channels'. Such an approach boosts the data rate by encoding data on more than one carrier and avoids the loss in reach associated with higher order QAM. But this comes at a cost: using multiple carriers consumes more, precious spectrum.
As a result, vendors are looking at schemes to pack the carriers closely together. One is spectral shaping. Tomkos also details the growing interest in such schemes as optical orthogonal frequency division multiplexing (OFDM) and Nyquist WDM. For Nyquist WDM, the subcarriers are spectrally shaped so that they occupy a bandwidth close or equal to the Nyquist limit to avoid inter symbol interference and crosstalk during transmission.
Both approaches have their pros and cons, says Tomkos, but they promise optimum spectral efficiency of 2N bits-per-second-per-Hertz (2N bits/s/Hz), where N is the number of constellation points.
The attraction of these techniques - multi-carrier schemes and advanced modulation formats - is the prospect of operators modifying capacity in a flexible and elastic way based on varying traffic demands, while maintaining cost-effective transport.
"With flexible networks, we are not just talking about the introduction of super-channels, and with it the flexible grid," says Tomkos. "We are also talking about the possibility to change either dynamically."
According to Tomkos, vendors such as Infinera with its 5x100Gbps super-channel photonic integrated circuit (PIC) are making an important first step towards flexible, elastic-bandwidth networks. But for true elastic networks, a flexible grid is needed as is the ability to change the number of carriers on-the-fly.
"Once we have those introduced, in order to get to 1 Terabit, then you can think about playing with such parameters as modulation levels and the number of carriers, to make the bandwidth really elastic, according to the connections' requirements," he says.
Meanwhile, there are still technology advances needed before an elastic-bandwidth network is achieved, such as software-defined transponders and a new advanced control plane.
Tomkos says that operators are now using control plane technology that co-ordinates between layer three and the optical layer to reduce network restoration time from minutes to seconds. Microsoft and Infinera cite that they have gone from tens of minutes down to a few seconds using the more advanced optical infrastructure. "They [Microsoft] are very happy with it," says Tomkos.
But to provision new capacity at the optical layer, operators are talking about requirements in the tens of minutes; something they do not expect will change in the coming years. "Cloud services could speed up this timeframe," says Tomkos.
"There is usually a big lag between what operators and vendors do and what academics do," says Tomkos. "But for the topic of flexible, elastic networking, the lag between academics and the vendors has become very small."
Further reading:
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.