 Segment Y 2000 1,371.8 908.8 866.1 1,114.6 2,985.3 747.2 1,391.1 2,105.2 612.0 704.1 1,759.4 729.6 1,917.0 Y 2002 1,480.9 1,041.5 796.9 1,676.8 1,547.4 941.7 977.8 1,929.4 595.5 595.1 1,876.9 699.3 1,535.7 Y 2005 2,126.5 1,701.6 1,021.8 2,363.2 777.4 1,235.1 1,078.4 1,689.0 630.4 747.0 2,040.6 771.4 1,368.8 Y 2006 2,209.0 1,441.4 861.6 2,777.7 1,042.5 1,827.3 801.0 1,657.3 716.3 1,006.2 1,880.7 703.6 1,259.6 dHHI% 149.2 -65.1 144.6 -42.4 -21.3 -34.3 L. Frank et al. in 2006. Also, the table clearly shows that in almost every OSS/BSS product segment, the number of vendors has decreased. We speculate that one main reason for this decline is the mergers and acquisitions carried out during the observed time period. Table 4 depicts the development in the market concentration of the OSS/BSS segments as measured by the changes in the HHI index values. The findings are mostly in line with the analysis of the number of companies and our hypothesis regarding the phase in the industry lifecycle model. In seven segments, the HHI index value has increased, up to 149.2% in the Offline Billing segment and in six segments it has decreased. The segments of "Fraud Management and Revenue Assurance" and of "Online Charging" show major negative values in the change of the HHI index. This could be explained by these comparatively new product segments being in a different evolutionary stage within the OSS/BSS industry. To summarize, while the number of companies is falling, the industry becomes more concentrated. This indicates a shift from the growth stage to the maturity stage in terms of Table 1. In order to analyze the vertical integration of the OSS/BSS companies, they were first classified into categories applying the following rules (see Fig. 1 of Chap. 4 for the market segments, vertical and horizontal layers). A company was treated as carrying out a niche strategy in case the vendor received revenue from only one of the thirteen segments. A company executed a vertical strategy if it had revenues from two or more segments in di