Weakly versus strongly multihop space-division optical networks

Transparent multihop optical networks suffer from the accumulation from node to node of crosstalk and amplified spontaneous emission noise, which may severely degrade the quality of received signals. It is thus important to keep the number of intermediate hops as low as possible. This paper compares two single-wavelength cell-switching space-division optical networks that employ deflection routing. The first has a well-known Manhattan street (MS) distributed topology. The mean internodal distance of this network is approximately the square root of the number of nodes. We term this network as strongly multihop. The second has a centralized star topology: the star is a multistage space-division photonic switch with limited buffers. Deflected cells delivered to the wrong user are transparently rerouted to the star. This network is intrinsically single-hop and gradually becomes multihop because of deflections. We term this network as weakly multihop. As the carried traffic increases, the link load increases much more rapidly in the strongly multihop topology, and so do both the crosstalk level per hop and the number of hops caused by deflections. For the same carried traffic, the accumulated crosstalk and spontaneous emission levels in a well-designed star-based network are much lower than in a strongly multihop network. Hence, lower packet error rates and lower delay jitter are expected for the centralized network. Moreover, for both networks, a simple frequency sweeping technique is shown to substantially reduce the dominant signal-crosstalk beat, thus allowing network operation with switch crosstalk factors as low as -20 dB.