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16 - 10 Gb/s tunable XFPs: perfomance analysis for the SURFnet7 Next-Generation Ethernet layer

Roeland Nuijts (SURFnet)

XFPs are currently the smallest commercially available form factor pluggable that can support tunable 10Gb/s DWDM transmission and these devices have gained considerable momentum in the marketplace. Component vendor JDSU announced it shipped 25,000 tunable XFPs from end of 2009 and through 2010. Analysts predicted in 2011 that shipment of tunable XFPs would triple in 2011, if compared to 2010. Over the past three years, SURFnet has developed its new vision for the SURFnet7 network, a Carrier Ethernet network, which largely depends on DWDM XFPs because of their attractiveness in terms of small size, low power consumption, low cost and sparing. In summary, the performance of these components is sufficient to be used on the existing SURFnet DWDM layer. This paper reports on the design of the new SURFnet7 network using tunable DWDM XFPs for the 10Gb/s connections as alien wavelengths.
First, we show the results of a numerical evaluation of the existing SURFnet6 DWDM network with tunable XFPs. The XFPs that we simulated comprised a tunable laser with a wavelength locker, externally modulated by an MZ (Mach-Zehnder) modulator and an APD-based receiver. We tested XFPs from three different vendors. The XFPs from one of the vendors were equipped with FEC. The typical power consumption was only 2.0W, in comparison, typical power consumption of the 300-pin
equivalent of 10W. The national SURFnet6 DWDM network comprises 2200km of transmission fiber and consists of five rings interconnected by ROADMs. The optical layer can transport up to 88 full duplex 10/40/100 Gb/s lambdas on a single optical fiber pair. The XFPs that we used had
negative chirp which provides the widest eye opening for an accumulated dispersion of about +800 ps/nm which is closely matched to the under-compensation in the SURFnet DWDM network. The calculation results show that the performance of the tunable XFPs is sufficient to deploy on the
existing SURFnet DWDM network.
Next, we present experimental results of transmission performance, in terms of the measured OSNR that is required to achieve a BER of 10-12 of these XFPs on the existing SURFnet6 DWDM infrastructure. Specifications show that these XFPs provide an OSNR penalty of less than 2.0dB in a dispersion window from -400ps/nm to +1600ps/nm. However, since the SURFnet DWDM network is dispersion compensated, total dispersion of the optical 10Gb/s path of each in the NGE network are between -200ps/nm and +1100ps/nm. Because of this, all transmission penalties in terms of OSNR were well below 2.0dB. We also performed measurements of BER as a function of OSNR for a decision threshold level of 30% (instead of the factory standard setting of 50%) and show that the ROSNR performance can be improved by up to 5dB by using this decision threshold setting. In the transmission experiments, several different combinations of transmitters and receivers were characterized to determine mix and match behavior and variations in transmitter and receiver performance with and without transmission fiber. From the measurement results, we observed a small variation in ROSNR performance over the different combinations of transmitters and receivers in the XFPs and over a transmission distance of (average) 400km of only about 1dB. The small variation in ROSNR performance for the different Tx/Rx combinations clearly shows that the manufacturability of these devices has greatly improved over the years. This data clearly shows that these XFPs can be used on the existing SURFnet DWDM layer and provide the required system BER performance. We also show measurement results of ROSNR for XFP devices with built in FEC for the routes with large span losses.
Finally, in order to determine the system margin, we also examined the OSNR that is actually delivered by the DWDM transmission system at the receivers and compare them to the ROSNR values of the XFPs in order to determine the system margin in terms of OSNR.

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