FEATURE TERABIT NETWORKS
Capacity conundrum
With providers facing the challenges of keeping up with internet traffic growth, terabit-scale systems are on the rise, Robert Roe writes
very expensive solution to the problem in most cases, so network providers are under pressure to come up with innovative solutions that will increase the capacity of fibre that has already been deployed within the network infrastructure. Existing capacity is beginning to reach the
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ceiling of what can be achieved, according to Uwe Fischer, chief technology officer for Coriant: ‘What you are seeing when you look into system design – today’s state-of-the-art technology – is 100Gb 4QAM coherent using a 50Ghz grid in the system. When you look from a spectral efficiency perspective, the 100Gb 4QAM modulation format is more or less using the complete passband.’ If the spectrum is already fully utilised by
4QAM modulation, one obvious way of squeezing more information through the fibre would be to go to different modulation formats with a higher number of modulation states – 8QAM or 16QAM. However, there is a trade-off: the reach – the distance over which the signal can be transmitted – falls because of increased noise when the system is being operated at its non-linear limit. Tis cannot be compensated for by just adding more power, because the system is already operating at that limit. One approach, therefore, is to develop flexible,
intelligent technology that includes both the higher modulation states and also super-channels, which bundle several carriers into one common signal. In this way, the technology will allow the modulation state appropriate to the range to be selected, while the capacity can be achieved by combining the carriers into a compound signal. Ciena, Infinera, and Coriant have all
demonstrated terabit scale network capabilities using super-channel technology. Infinera has been shipping 500G super-channel line cards for
34 FIBRE SYSTEMS Issue 5 • Autumn 2014
s internet traffic grows year on year, service providers have to deliver more capacity at cheaper rates. Simply putting more fibre into the ground is a
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the past two years, while other DWDM vendors are expected to begin shipping 200G line cards this year, and to roll out super-channel products at some point in the future. Ciena demonstrated this technology in
collaboration with Comcast. Helen Xenos, director of product marketing at Ciena, said: ‘We used a lot of technologies here. Te first one was the WaveLogic coherent technology to do 16QAM modulation – this allows it to carry 200Gbps on a single wavelength in less than 50 GHz of spectrum. Tat more than doubles the capacity of what they were able to do before in their existing, state-of-art 100G network.’ Ciena collaborated with Comcast in early
October 2013, completing a live field trial of a 1Tbps optical transmission spanning nearly 1,000 km across a span of Comcast’s US network connecting Charlotte, NC and Ashburn, VA. Te primary focus of the trial was to show the
distance achieved using super-channel based technology; it also demonstrated the ability to increase the capacity of the network by a factor of 2.5. Te network simultaneously carried customer traffic over 10G, 40G and 100G wavelengths, all coexisting on a mix of flexible and 50GHz-spaced fixed grid channels. Coriant has achieved similar results in a trial it
conducted in Australia. Tis super-channel transmission trial was conducted over a 1,066 km fibre optic ring in South East Queensland, Australia using Coriant’s innovative FlexiGrid technology on NBNCo’s Transit Network. Fischer said: ‘Tis was a super-channel experiment. Here we used 100Gb 4QAM per carrier as a base modulation format and we compiled 10 of these carriers into a one terabit compound signal.’ Infinera not only demonstrated a terabit
capacity but also broke a world record – for fastest
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