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FEATURE 100G NETWORKS
networks and used high-quality optical fibre. Unfortunately, thanks to the dotcom bubble, there was a glut of unused optical fibre already in the ground, and nobody had the money or inclination to lay new fibre to attain higher speeds. Some fresh thinking was needed.
Expert coalition Ciena pulled together experts from across the company to tackle the problem. Te team included engineers from the company’s high- capacity microwave development as well as specialists in optical devices and high-speed electronic circuit design. Tis was the first sign that the industry would
turn away from simple ‘blinking light’ coding schemes, in favour of forward-error correction (FEC) using electronic algorithms to compensate for fibre impairments, and complex modulation schemes that encoded more than one bit of data per symbol transmitted down the optical fibre. Tese were proven technologies in other applications, but they had not previously been applied to optical transmission. Compensating for optical impairments using electronics generated a new technical problem: the
required digital sampling and processing rates pushed the electronics to its limits. Early implementations of 100G used dual-polarisation quadrature phase-shiſt keying (DP-QPSK), which encodes two bits of data per symbol transmitted down the fibre on each of two polarisations of light, to reduce the symbol rate by a factor of four. Te symbol rate was further reduced by using two subcarriers (wavelengths) in a 50GHz grid slot, which could be treated as a single unit. Tus each subcarrier operated at 12.5Gbaud (not including error correction overhead), which reduced the demand on the electronics. However, this approach also doubled the number of key optical and electronic components, which increased the cost of the solution. So it is not surprising that, as electronics advanced with Moore’s Law, the industry moved to single-channel PM-QPSK approach, which has now become the de facto industry standard. It’s worth keeping this history in mind as we
consider how the optical industry is going to make the next step up in transmission speed. Carriers will certainly need transmission speeds greater than 100G to support increasing traffic, particularly in their fast-growing metro networks. Vendors are keen to show that their equipment will support the evolution to higher speeds, and field trial reports are now coming in thick and fast. UK incumbent BT, US operator Verizon, and Australia’s Telstra, are among the carriers that have trialled higher speeds this year, while France Telecom-Orange has already installed the first live 400G connection, which is being used by research and education network RENATER. Vendors are currently proposing to use a pair of
200-Gbps subcarriers to reach 400G. In March 2013, BT carried out a series of trials with Ciena, demonstrating 200G, 400G, and an 800G transmission over its core network. Te 200G signal used a modulation scheme called 16-QAM (quadrature amplitude modulation), which encodes eight bits of data per symbol, and used this to build higher-speed superchannnels – groups of subcarriers that can be treated as a single operational unit. Te BT trial was performed on an optical fibre that had proven unsuitable for 10-Gbps operation, says Ciena.
Applying intelligence ‘One of the biggest changes when we moved towards coherent is that we can embed more intelligence into the technology,’ said Helen Xenos, director, product and technology marketing at Ciena. Tat enables the programmable silicon in the transmitter to support multiple modulation formats. Te trial with BT used the same hardware that currently
supports the operator’s 100G long-haul traffic, along with pre-production soſtware to configure Ciena’s Wavelogic3 coherent optical processor. Te vendor plans to offer a line card that will allow carriers to choose between DP-QPSK and 16-QAM modulation formats. Alcatel-Lucent also used a pair of 200-Gpbs
subcarriers in the deployment of the 400G connection between Paris and Lyon for France Telcom Orange and RENATER. Te vendor says its photonic service engine also features a digital signal processor in the transmitter that, among other benefits, enables carriers to select among such modulation formats as DP-BPSK, DP-QPSK and 16-QAM modulation formats as well as waveform shaping to improve spectral efficiency. Waveform shaping allows the subcarriers to be squeezed closer together.
Carriers will certainly need transmission speeds greater than 100G to support increasing traffic
Some carriers are also showing an interest in
using 200 Gbps channels for metro/regional applications. If they can have twice the transmission capacity from the same equipment for a minimal additional cost, then why wouldn’t they jump at the chance? Verizon recently completed a field trial using Ciena’s 200G technologies on its network between New York and Boston, a distance of around 260 miles. Te trial doubles spectral efficiency while reducing the cost per bit when compared with 100G long-haul technology, Verizon says. While Verizon says the price point isn’t right to see this technology widely deployed in metro applications, where it would have to compete with 10-Gbps pluggable modules, it will make sense on certain routes. Ultimately, Verizon would like to see pluggable 100G modules available to use in the metro. For now, 200G seems to be the limit for
single-channel transmission, however. Higher- order modulation schemes – like 16-QAM, 32-QAM or 64-QAM – boost the number of bits per symbol, but this has to be traded off against reach. When amplitude becomes one of the transmission variables, the signal becomes more sensitive to noise. Nonlinear effects such as phase noise are also more pronounced, and those are
Issue 1 • Autumn 2013 FIBRE SYSTEMS 15
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