On product form tandem structures

This paper is written in honour to A. Hordijk. It establishes product form results for a generic and instructive multi-class tandem queue with blocking, to which A. Hordijk has directly and indirectly contributed. First, a sufficient and necessary product form characterization is provided. Next, three special cases are briefly presented. These illustrate the possibility of product forms despite finite capacity constraints (blocking), unproportional processor sharing mechanisms and resource contentions (such as for access control). The results are partially new and of interest for present-day applications. In essence these rely upon the pioneering work by A. Hordijk.