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This paper presents both theoretical and experimental studies carried on wavelength-division multiplexing (WDM) networks with arbitrary (mesh) topology that provide optical circuits with differentiated reliability (DiR). Reliability is obtained by means of a modified shared path protection (SPP) switching scheme-here referred to as SPP-DiR. As explained in the paper, SPP-DiR networks provide multiple degrees of circuit reliability that satisfy client-specific reliability requirements in a cost-effective way. The theoretical study first defines the problem of optimally designing SPP-DiR WDM networks. It then presents a time-efficient suboptimal algorithm that determines the routing and the reliability degree of each demand in the given traffic matrix by applying both the conventional SPP and the SPP-DiR scheme. When compared to dedicated path protection switching, results obtained for the pan-European network using the proposed algorithm indicate cost reductions of about 16% when SPP is applied, and up to 34% when SPP-DiR is applied. The experimental study describes the Ω testbed-a WDM optical circuit-switched mesh network with an IP control plane-which is believed to be the first testbed ever built that makes use of the SPP-DiR scheme. Experimental results performed on the Ω testbed report restoration times of the optical circuits-disrupted by a fiber fault-that are few tens of milliseconds.