The field trial, which was carried out with German incumbent Deutsche Telekom last December, was part of the 100 Gigabit Ethernet Transport (100GET) initiative, a €65 million EU-funded project involving 30 academic and industrial partners (see EU projects explore 100G alternatives).
The main highlights of the field trial were a transmission rate of 112 Gbit/s on a single wavelength over 600 km of installed fibre. En route the signal passed through multiple reconfigurable optical add-drop multiplexers (ROADMs), a couple of fixed OADMs, and unequally distributed amplifiers. The fibre was also carrying a mix of 10 Gbit/s and 40 Gbit/s live traffic (presumably low-priority traffic!) with 50 GHz spacings between channels.
Optical was front and centre in this field trial, with the main purpose being to show robust optical transmission over long-haul distances using a commercial platform, in this case the Marconi MHL 3000 WDM platform.
The vendor has not conquered the electronics side of the equation, instead opting to transmit PRBS sequences that were multiplexed to achieve the higher data rate. To be fair, however, standard components for 100Gbit/s framing and forward and error correction (FEC) are not yet available — although that didn't stop Ciena from showing that it could multiplex ten 10 Gigabit Ethernet channels into a single 100 Gbit/s data stream for its 100 Gbit/s demo last year (see Ciena demos 'true' 100G transmission).
Like other developers of 100 Gbit/s systems, Ericsson opted for dual-polarization quadrature phase-shift keying (DP-QPSK) — the modulation scheme that the Optical Internetworking Forum (OIF) is aiming to standardize.
However, unlike Nortel and Nokia Siemens Networks, which used coherent detection in their systems, Ericsson opted for direct detection. This method, although not so sensitive to the optical signal, is easier and cheaper to implement because it doesn't require advanced digital signal processing (DSP). So far only one chip maker, Fujitsu Microelectronics Europe (FME) is claiming to have developed sufficiently fast DSP technology.
To make direct detection possible over a long distance, Ericsson inserted a fast polarization tracker into the link. This device compensates for accumulated polarization mode dispersion (PMD) so it doesn't get to the receiver. Ericsson, in collaboration with the University of Paderborn, will be presenting a paper on the fast polarization tracking technology at OFC/NFOEC next week.
"There are multiple ways you can do this [100G]," explained Rodolfo Di Muro, optical product marketing manager, business unit networks, for Ericsson in Italy. "You can compensate for PMD in the line, so you don't have to do it in the receiver, or you can let it go to the receiver, and then you need complicated DSP in the receiver."
Di Muro is also keen to point out that 600 km is not a limitation on distance, but simply the physical distance between the two cities where Ericsson had permission to install its equipment. "We are confident that we can go to 1200 km because this was the result of our lab demo," he said.