Acreo demonstrates long-haul optical transport at 40 Gbit/s
Field test shows 40 Gbit/s long-haul transmission ready for deployment.
anders.berntson@acreo.se
Acreo has lately performed a number of transmission experiments at 40 Gbit/s through installed fibers between Kista and Hudiksvall. We have reported field transmission of a 40 Gbit/s signal up to 874 km. New advanced modulation formats for increased non-linear tolerance have been verified and state-of-the-art commercial systems operating at 40 Gbit/s have been evaluated. Partly the work was done in co-operation with TeliaSonera and within the European project IP NOBEL. At the ECOC in Stockholm in September Acreo together with Mintera presented the world’s first live long haul 40 Gbit/s field trail.
Strong growth of IP traffic
If the steady growth of IP traffic continues, currently it is 50%-100% annually; it will not be long before the optical networks will have to be upgraded to carry larger traffic volumes. The traffic growth is fuelled by the increase of broadband access connections, in particular based on DSL and cable, but the number of fiber access connections is also increasing rapidly. The rollout of residential broadband will continue and in the coming years the traffic growth may be increased by the new emerging applications such as IP-TV, Video on demand and HDTV.
The revenues grow slower than the traffic, and taking into account an anticipated reduced demand of network services apart form IP, the overall revenues from core and metro networks may even fall.
Improved cost efficiency is vital
With increasing traffic and declining revenues it is simple mathematics to conclude that improved cost efficiency is vital for any operator of a fixed network. To support this evolution next generation of optical transmission equipment will transport more information at a lower price per bit than today.
Transmission at 40 Gbit/s offers smaller footprint, greater flexibility, lower power consumption and easier management than current state of the art systems operating at 10 Gbit/s. These advantages will inevitably lead to lower cost. For a number of years the investments in new transmission equipment have been very small. A possible strategy to make 40 Gbit/s take off in such business climate would be to focus on solutions with a small threshold investment. One such alternative would be if existing 10 Gbit/s infrastructure could be upgraded smoothly to 40 Gbit/s by adding 40G wavelengths one by one as the traffic grows. Preferably the 40 Gbit/s wavelengths should be added without interrupting the traffic on the already working 10 Gbit/s wavelengths. However, this last year the telecom sector appears to have recovered from the downturn after the ”IT bubble”, many of the operators and equipment manufacturers have shown profit every quarter for more than a year. This has increased the optimism and probably also the willingness of manufacturers to develop and release new products, as well as the operators’ willingness to invest in the transport network on the long-term horizon.
Important transmission issues
Transmission at 40 Gbit/s also introduces a number of issues and problems related to fiber transmission itself needing to be solved. Important transmission issues that have been assessed by Acreo in field transmission experiments are dispersion, nonlinearities and polarisation mode dispersion. The experiments have been performed in the 410 km link between Kista and Hudiksvall, see figure above.
Optical signals with 100 GHz frequency spacing are combined in an optical multiplexer at the transmitting node and separated again in the receiving node by an optical demultiplexer. The loss and the dispersion of the transmission fiber are compensated for by amplifiers and dispersion compensating fibers (DCFs) in five intermediate nodes. DCFs are costly and only used up to a minimum. The residual dispersion in each direction of Acreo’s link corresponds approximately to the dispersion of 50 km of standard transmission fiber. The design is a standard design used for commercial systems operating at 10 Gbit/s and it represents a typical infrastructure of the backbone production networks. The link is an ideal platform for studying and verifying solutions for upgrading 10 Gbit/s transmission infrastructure to 40 Gbit/s.
Important problems that have to be addressed when upgrading to 40 Gbit/s are:
- Filtering characteristics and channel spacing. The channel spacing needed to perform the mux/demux operations is what determines the spectral efficiency. A question of particular concern for the upgrading 10 Gbit/s links to 40 Gbit/s is that if existing 10 Gbit/s channels are not to be interrupted, the 40 Gbit/s channel would have to be passed through the same mux/demux as the 10 Gbit/s channels. A possible solution is to use a modulation format with a narrow spectrum, duo-binary or carrier-suppressed return-to-zero (CSRZ).
- Chromatic dispersion. The dispersion tolerance of a 40 Gbit/s transmission link corresponds to just a few kilometres of standard fiber. Therefore, the compensation of chromatic dispersion has to be upgraded. One attractive solution would be to use a fiber-grating based tuneable dispersion compensator per channel.
- Polarisation mode dispersion (PMD). PMD is the random birefringence of the causes the information to disperse. The problem increases with the bit-rate and is particularly severe in dispersion shifted fibers. PMD can be counteracted in the optical domain by using specific PMD compensators but the devices are complex. An alternative more cost-effective solution would be to use an electrical equaliser that simultaneously compensates for other signal impairments.
- Power budget. A 40 G receiver is inherently less sensitive than a 10 Gbit/s receiver and the maximum launch power is limited by nonlinearities as for 10 Gbit/s. One way to increase the loss budget would be to use a modulation format tolerant to non-linear effects, e.g. alternate-phase return-to-zero (APRZ) or a format with improved receiver sensitivity, e.g. differential phase shift keying in combination with a balanced receiver.
During this year we reported the first field transmission experiment using the APRZ modulation format. In these experiments, performed through 540 km of standard fiber, the APRZ modulation format showed increased non-linear tolerance in addition to what could be obtained by optimising the amount of pre-compensation. The experimental set-up is schematically shown in the figure below.
A commercial system from Mintera Corp. was evaluated in an experiment through 820 km of standard fiber. In this experiment, using the CSRZ modulation format, the 40 Gbit/s signal was transmitted in through a fiber that simultaneously carried 8 wavelengths at 2.5G, schematically shown in the figure above. The beauty of this experiment is that the signals at 40 Gbit/s and 2.5G were combined and separated using the standard multiplexer and demultiplexer respectively, of the Ericsson DWDM system. Later the transmission distance was increased to 874 km when the end terminal in Kista was moved to the ECOC exhibition in Älvsjö.
The transmission quality can be improved by using very short pulses but at the expense of broadening the signal spectrum. In WDM transmission a broad spectrum makes it more difficult to combine signals without increasing the channel spacing. In an experiment using RZ modulation, 3 ps pulses with error-free performance was demonstrated over the full 820 km without forward error correction.
40 Gbit market
On the market there are currently two vendors actively marketing 40 Gbit/s transport products, Mintera and Stratalight, both are based in the US. Many other big vendors most likely have a 40 Gbit/s roadmap with products ready or almost ready for release. For the time being these big vendors choose to push first of all for their 10 Gbit/s solutions. However, the situation may change rapidly if one of the big system vendors decides to change strategy and actively start to market 40 Gbit/s. We believe that 40 Gbit/s is ready for deployment and that commercial breakthrough is close.