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Today's optical wavelength division multiplexing backbone networks need to support traffic demands with very diverse capacity requirements. Recent studies have shown how to design an optical transport network that supports mixed line rates (MLR), where the wavelength channels of the optical paths (i.e., lightpaths) can have a variety of capacities (10/40/100 Gbps). Some preliminary work on the design of MLR optical networks has already appeared, but survivability, which is a key concern in optical network design, is a nascent topic in MLR networks. This study investigates the problem of protection in MLR optical networks: in particular, we study how to design a cost-effective transparent MLR network that provides dedicated protection at the lightpath level. We propose three mechanisms: MLR-at-p-lightpath protection (MLR-p), MLR-at-lightpath protection (MLR-l), and MLR-with-backup-flow-grooming protection (MLR-g). The design problem is solved by two different approaches: (1) a two-step approach that formulates part of the problem as an integer linear program and (2) a heuristic approach. Our results show that, by appropriate assignment of rates to lightpaths, MLR networks can provide protection for diverse traffic demands with much lower transponder cost compared to single-line-rate networks.