In elastic optical networks, a connection may occupy a frequency slot that spreads over adjacent frequency slot units (FSUs). Such connection (called -slot connection) also must be realized in optical nodes used in these networks. The paper considers two architectures of the three-stage switching fabric that can be used in such network nodes. The switching fabric applies space switching in the first and the third stages and wavelength switching in the second stage [the space-wavelength-space (S-W-S) switching fabric]. For both architectures, denoted by SWS1 and SWS2, we derived and proved the strict-sense nonblocking conditions when -slot connections are set up. The number of center stage wavelength switches is calculated and evaluated. The switching fabrics are compared with the strict-sense nonblocking wavelength-space-wavelength (W-S-W) switching fabric. It is shown that the number of center stage switches does not depend on the number of FSUs available in input and output fibers but only on the maximum number of FSUs, which can be used by one connection and the number of input/output fibers in one input/output space switch. When the number of FSUs in one connection is, for instance, limited to 5, only 11 center stage switches are needed for nonblocking operation in one of the architectures. Moreover, for switching fabrics with a greater number of maximum FSUs in one connection and a greater number of FSUs in one input or output fiber, the S-W-S switching fabrics can be practically realized, unlike the W-S-W switching fabric.
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