BRIAN SMITH ROADM NETWORKS
Brian Smith , who is responsible for transport product and technology strategy in Lumentum’s office of the CTO, explains how contentionless MxN wavelength selective switches (WSS) will provide superior optical performance and lower cost in next-generation CDC ROADM networks. W ith traffic growing exponentially, and spectral efficiency and baud rate growth not with limited fibre availability, is to extend the optically amplified bandwidth to include both the C and L bands. While this only provides a 2x increase in bandwidth, it does reduce the amount of fibre required and delays the need for additional fibre overlay by a few years. rate coherent transmission for lower cost per bit and where effective, implement multi-carrier transceivers to expand the bandwidth of channels (Figure 1b). With a fixed 50GHz channel grid, the number of active channels per node grows exponentially in lock step with the increase in node bandwidth (green line in Figure 2). Alternatively, where channel EFFICIENTLY SUPPORTING AGGRESSIVE NETWORK CAPACITY GROWTH IN NEXT GENERATION CDC ROADM NETWORKS
keeping pace, coherent transceivers must aggressively expand in bandwidth to provide an effective means of supporting capacity growth. ROADM transport networks must be ‘future- proofed’ to support these evolving coherent interfaces without constraints on bandwidth or power. Optical networks have approximately doubled in capacity every few years over recent decades but we are approaching a fundamental limit on the amount of information that can be packed into an optical fibre. Improvements in spectral efficiency are limited as we approach the Shannon limit and baud rate alone cannot keep pace with the rate of growth required. Given that we cannot rely on the combination of spectral efficiency improvements and baud rate increases to meet traffic growth requirements, the only way to support growth for a given reach will be to increase the total amplified bandwidth per node at nominally the same rate as traffic is growing. Overlaying additional fibre pairs per route is a practical approach that can provide multiple increments of C-band capacity growth and this is already happening today. An alternative approach, desirable for some operators
EFFICIENT CAPACITY GROWTH Today’s ROADM networks are typically based around a fixed 50GHz ITU grid regardless of whether the optical network hardware supports flexible grid channels or not. This is largely due to the fact that baud rates for deployed core network traffic have not exceeded 32GBaud. A simple strategy for increasing capacity would be to maintain a 50GHz grid structure, fill fibres to capacity at the best achievable spectral efficiency and deploy overlay fibres as capacity grows (Figure 1a). Alternatively, we could take advantage of higher baud
bandwidth is increased progressively through increasing baud rate and/or number of carriers, the number of active channels per node grows considerably more gradually and relatively linearly (blue line in Figure 2). Clearly, the use of increasing channel bandwidth results in fewer active channels within a node compared to the case of fixed 50GHz bandwidth channels, and in this example, up to 60% less over the life of the network. This is important from a ROADM infrastructure perspective.
Figure 1. Options for dividing amplified bandwidth into optical channels
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ISSUE 14 | Q3 2018
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