QBD processes provided a useful tool in the study of stochastic models, such as queueing systems, manufacturing systems and computer networks. Chapter 3 of Neuts  gave a complete picture of a level independent QBD process. Latouche and Ramaswami  gave a logarithmic reduction algorithm for the rate matrix. Ramaswami  gave an excellent overview for the QBD processes. Bright and Taylor  provided effective algorithms for computation of level dependent QBD processes. Naoumov  proposed an algebraically matrix-multiplicative approach to study the stationary probability vector of a QBD process. Ramaswami and Taylor  discussed useful properties of the rate operator in a level dependent QBD processes with a countable number of phases. Li and Cao  gave two types of RG-factorizations, which lead to a unified and complete framework to deal with stationary (or transient) solution to performance of various QBD processes. Li and Liu  provided a sensitivity analysisfor a perturbed QBD process. Nielsen and Ramaswami  and Li and Lin  studied QBD processes with continuous phases.