  The three-stage Clos networks remain the most popular solution to many
practical switching systems to date. The aim of this paper is to show that the
modular structure of Clos networks is invariant with respect to the
technological changes. Due to the wavelength routing property of
arrayed-waveguide gratings (AWGs), non-blocking and contention-free
wavelength-division-multiplexing (WDM) switches require that two calls carried
by the same wavelength must be connected by separated links; otherwise, they
must be carried by different wavelengths. Thus, in addition to the non-blocking
condition, the challenge of the design of AWG-based multistage switching
networks is to scale down the wavelength granularity and to reduce the
conversion range of tunable wavelength converters (TWCs). We devise a logic
scheme to partition the WDM switch network into wavelength autonomous cells,
and show that the wavelength scalability problem can be solved by recursively
reusing similar, but smaller, set of wavelengths in different cells.
Furthermore, we prove that the rearrangeably non-blocking (RNB) condition and
route assignments in these AWG-based three-stage networks are consistent with
that of classical Clos networks. Thus, the optimal AWG-based non-blocking Clos
networks also can achieve 100% utilization when all input and output wavelength
channels are busy.
