d telecom operator software market. Vertical Integration due to Software Systems' Modularity Oleksiy Mazhelis and Pasi Tyrväinen In their product portfolio, OSS/BSS software vendors make products pre-integrated with each other as well as with other related OSS/BSS products. Prior to the full- scale development and integration, it is vital for these vendors to assess which products are reasonable to integrate. Such integration of software may be aimed at achieving different goals, such as: Reducing software integration efforts. Software vendors aim at maximizing their profits, whereas their customers (i.e., CSPs) aim at minimizing their costs by avoiding the integration job. As a result, to approach a win-win scenario, vendors provide already integrated products or subsystems (pre-integrated modules). Sharing similar knowledge base. When reusing earlier experience in implementing new products or subsystems, the vendors strengthen the quality of the implementa- tion, as compared with the software implemented from the scratch. Other goals, such as sharing the same contact (e.g., technical evaluator) in the customer organization, or the like. Besides the arguments above, integration is often used in order to facilitate innova- tions in software. According to previous studies on complex biological systems, improving individually one of the interconnected modules in such systems is unlikely to bring benefits at the system level. The reason for this is the propagation of the changes in the system - changes to one module bring about chains of changes in dependent modules, and the improvements in performance of one module are likely to be outweighed by decreased performance of some of the dependent modules (Altenberg 1995) which usually results in a near-zero effect on the system level. Hence, given a system of several modules, some of which are interdependent, O. Mazhelis (X) Information Technology Research Institute, University of Jyväskylä, P.O. Box 35, 40014 Jyväskylä, Finland e-mail: Oleksiy.Mazhelis@jyu.fr P. Tyrväi