We propose a novel traffic engineering (TE) technique that can readily handle impairments so as to satisfy quality-of-signal (QoS) requirements in alloptical networks. The existence of impairment limits the maximum transmission distance of optical signals given the need to satisfy QoS requirements. The problem is that impairment-aware route computation must consider many parameters and so is rather complicated. We, therefore, propose the new TEconstraint of “logical distance” which covers several impairment factors converted into distance. It reflects the degree of impairment created by external conditions, and a transmission distance constraint can be checked easily by summing up each element's distance. Logical distance makes it simple and effective to judge whether a computed route satisfies the desired QoS or not. We confirm the feasibility of our proposal by developing a TE system for a real optical network.