Finisar WSS WHITE PAPER: Balancing Performance, Flexibility, and Scalability in Optical Networks

The availability of Wavelength Selective Switches (WSS) supporting 100 Gb/s and 400 Gb/s data rates enables network operators to significantly increase bandwidth capacity in DWDM optical networks with substantial CAPEX and OPEX savings. Moving to such higher data rates, however, requires a shift from the continuing trend of implementing narrower optical channel spacing given that data rates beyond 100 Gb/s cannot fit within a 50 GHz channel….

Download Finisar’s latest WSS white paper from our website (see blue downloads box): Balancing Performance, Flexibility, and Scalability in Optical Networks

ECOC Tradeshow Recap and the Importance of Flexgrid™

Just got back from the ECOC tradeshow, or as I joked at our customer event on Tuesday night, the ECDC trade show – “European Colorless Directionless Contentionless” Conference.

Despite losing my suitcase and having to rush-out to buy some of the latest Italian fashions (can you say slim fit?), it was a good industry show.

Finisar was no stranger to the ROADM architecture theme with our new Flexgrid™ technology demo. Flexgrid is a WSS software feature we believe will be critically important to carriers in their deployments of ROADMs in the future.

As described in our recent press release, Flexgrid™ WSS technology enables dynamic control of channel center frequency and channel bandwidth within a WSS, from 50 GHz to 200 GHz in 12.5 GHz steps, with no penalty on any aspect of WSS performance. Flexgrid™ draws upon the inherent flexibility and performance of Finisar’s Liquid Crystal on Silicon (LCoS) optical engine which we believe will address carrier demands for flexible bandwidth-capable ROADMs in next-generation networks. LCoS technology enables a WSS to be very flexible with many features including the ability to optimize (or contour) the channel shape of each individual wavelength and provide configurable dispersion compensation for best transmission performance.

Flexgrid becomes very interesting when one looks to the next Ethernet data rate as the following example shows.

If you assume the next data rate beyond 100G will be 400G, then:

Obviously 400G will require an advanced modulation format, so if you assume:
• Modulators and electronics are limited to 30G symbols/s
• 16 QAM give 4 bits/symbol
• Two polarizations x two wavelengths gives another factor of four
• FEC takes away 20%
• 30x4x2x2= 480Gb/s. Take away 20% = 400Gb/s Ethernet

Based on this 16-QAM, 30GHz, dual polarization modulation format:
• 60GHz is needed for the signal
• Add 10GHz channel boundaries on each side of the signal
• Allow 3-5GHz between signals (including laser drift)

This means the bandwidth required to transport a 400GE signal is somewhere around 85GHz.
One way to transport this would be to use a 100GHz grid since 85GHz would fit within the 100GHz channel. However, moving to 100GHz channel spacing to accommodate the 400G channel means that the carrying efficiency of the fibre actually drops for other (lower bandwidth) signals which would normally be carried on 50GHz channels. Since signal heterogeneity is likely to be a feature of future networks, and carriers are looking to maximize fiber carrying capacity (and hence minimize cost/Mbit/km), then it is clearly not an option to just return to a 100GHz grid.

Furthermore, the actives associated with transporting 400GE (say using a 16-QAM dual polarization modulation format) are likely to be expensive. Thus, we will spend a very significant amount on optics to increase the throughput on a given fiber in a way in which spectral bandwidth is not optimized. However, if a network operator has already deployed a ROADM using Flexgrid™ technology, they could preset that specific wavelength to 87.5GHz granularity using a very simple software command. Voila, you have a spectrally efficient 400GE wavelength that increases the spectral efficiency of fiber by a factor of 2.3 (130% increase as opposed to 100% increase using 100 GHz channels) as well as providing efficient bandwidth allocation for other types of traffic and hence maximizing the carrying capacity of a given fibre route. This should ultimately also make the cost associated with upgrading to 400GE significantly less expensive for a carrier.

People ask why should we worry about deploying ROADMs for 400GE? As an industry, we are just starting to deliver 100GE. Well, as Glenn Wellbrock at Verizon stated “we like to deploy our ROADM equipment for 10 years”. If you assume that 400GE will likely start to ship in the next 5 years (pretty likely considering that the standard efforts for 100GE started in 2006 and now 4 years later, we are shipping 100GE), then not deploying Flexgrid would be very short-sighted. It would mean that in 5 years time, a carrier would have to rip out old ROADMs to support 400GE – a very expensive proposition.

Any comments are welcome!