International audienceWe study the minimization of ADMs (Add-Drop Multiplexers) in Optical WDM Networks with Bidirectional Ring topology considering symmetric shortest path routing and all-to-all unitary requests. We insist on the statement of the problem, which had not been clearly stated before in the bidirectional case. Optimal solutions had not been found up to date. In particular, we study the case C = 2 and C = 3 (giving either optimal constructions or near-optimal solutions) and the case C = k(k+1)/2 (giving optimal decompositions for specific congruence classes of N). We state a general Lower Bound for all the values of C and N, and we improve this Lower Bound for C=2 and C=3 (when N=4t+3). We also include some comments about the simulation of the problem using Linear Programming