This paper analyzes the fault tolerance of a class of double-loop networks referred to as forwardloop backward-hop #FLBH#, in which each node is connected via unidirectional links to the node one hop in front of it and to the node S hops in back of it for some S. A new measure of fault tolerance is described, along with techniques based on Markovchains to calculate upper and lower bounds on the fault tolerance of this network topology quickly and e#ciently. The results of these calculations provide a more precise description of network fault tolerance than has been achieved with previously published techniques. 1 Introduction Loop- or ring-based topologies have become increasingly important for local and metropolitan area networks. Loops support simple and e#cient protocols. Their symmetry reduces the diversityof hardware components needed, and their use of unidirectional point-to-point links allows e#cient #ber implementations. Many network architectures have been designed with loo...