terfaces may serve as a potential hindering factor for OSS/BSS market horizontalization. OSS/BSS software provided by a software vendor often needs to be integrated with a large number of heterogeneous subsystems. If the interface implementation efforts are high, we assume that the number of companies capable of providing necessary integration decreases, and hence the OSS/BSS market horizontalization may be delayed. An interface in a software system can be defined as a stack of protocols and associated data formats that govern a communication between software subsys- items. Protocols in the stack may have different versions; two interfaces comprised of distinct versions of the same protocols are referred to as variations of the interface. Given an interface to be supported, human resources are required not only to implement or configure the software providing the interface, but also afterwards - for maintenance, for reconfiguration of a standardized interface, or in order to O. Mazhelis (X) Information Technology Research Institute, University of Jyväskylä, P.O. Box 35, 40014 Jyväskylä, Finland e-mail: Oleksiy.Mazhelis@jyu.fr P. Tyrväinen and O. Mazhelis (eds.), Vertical Software Industry Evolution, Contributions to Management Science, DOI 10.1007/978-3-7908-2352-3_12 C Springer-Verlag Berlin Heidelberg 2009 O. Mazhelis and P. Tyrväinen perform new integration projects involving the interface. Therefore, human resources are needed for each interface for the entire lifetime of that interface. Let us consider the efforts devoted by the software vendors for interface imple- mentation activities. Due to different interfaces used, two types of CSPs are considered: incumbent CSPs and commencing operators: the incumbent CSPs are likely to have a large number of heterogeneous subsystems, often with proprietary interfaces, whereas commencing operators are expected to deal with streamlined systems having a limited set of standardized interfaces. The assumption is that the "older" the operator, the larger the number of hete