Evaluating internal blocking in noncontentionless flex-grid ROADMs [invited]

In heterogeneous flex-grid networks, optical connections for different line rates, spectral widths, and modulations can coexist in the same infrastructure. Optical connections are transparently routed by reconfigurable add/drop multiplexers (ROADMs). Flex-grid ROADMs are enabled by state-of-the-art evolution from original wavelength-selective switches (WSSs), designed for fixed-grid applications, to WSSs that operate in flexible grids. Broadcast-and-select (B&S) flex-grid ROADMs are subject to the same internal blocking as their fixed-grid counterparts, and this internal blocking can also be reduced by augmenting the number of add/drop transponder banks. In this paper, we dimension the number of transponder banks C required in flex-grid ROADMs to make the internal blocking negligible. To this end, we present a graph transformation that takes a network topology based on B&S ROADMs and converts it into an equivalent network composed of contentionless ROADMs. Then, any conventional (non-contention-aware) algorithm can be applied to the transformed topology to make lightpath allocations. Our results show that often C = 2, and in some cases C = 1, transponder banks are enough to eliminate internal contention in practice. Interestingly, these results are similar to the ones obtained in the fixed-grid case. This is not an obvious conclusion, since spectrum fragmentation in flex-grid networks may amplify internal blocking. Results have been obtained using the Net2Plan 0.2.3 open-source tool. Algorithms' source code and data are available in the Net2Plan public repository for reuse and validation.