Mapping Technological Trajectories as Patent Citation Networks. An Application to Data Communication Standards

We use patent citation networks to study the dynamics of technical change in Ethernet, a standard for data communication. We first consider the evolution of the broad technological system of Local Area Networks (LANs) and we identify the most significant inventions within the specific subfield of Ethernet. Then we analyse the structure of connectivity of patent citation networks to reconstruct the main technological trajectories in this subfield. Our results suggest that these technological trajectories are characterized by the presence of a number of interconnected technological environments which cluster following an engineering logic. These clusters include the most significant patents related to 'milestones inventions' in the evolution of the LAN technology. Thus, our analysis of the structure of connectivity in patent citation networks seems appropriate to capture both the cumulativeness associated to the development of a specific technology and the significant discontinuities that punctuate the trajectory, as well as the technological interrelatedness characterising large technical systems. An extensive literature exists on innovation in 'large technological systems' (Bunn and David, 1988). This literature outlines how the dynamics of technical change is affected by the role played by network effects (Katz and Shapiro, 1994), technological interrelatedness (Antonelli, 1993), and the need for compatibility standards (David and Greenstein, 1990). In particular, it has been pointed out that a proper understanding of the evolution of these systems requires an appreciation of the technical bottlenecks arising from the need of integrating different interacting components. In large technical systems innovation and change at the component level create unbalances between the different parts of the system. These unbalances act as 'focussing devices' (Rosenberg, 1976) or 'heuristics' (Dosi, 1982) for inventing activities. This paper examines the patterns of technical change in large technical systems as captured by patent citations networks. One of the most common approaches for evaluating the importance of (patented) innovations is to 'weigh' them by using patent citations. Indeed, several studies have generally found that patent citations provide a reasonable 'proxy' for their technological significance, as they generally appear to be highly correlated with other measures of the value of innovations such as the assessments by technology experts (Jaffe and Trajtenberg, 2002). In this paper we argue that, in the specific case of technical systems, this approach may have drawbacks. First, innovation in technical systems is 'distributed' in the sense that it takes place at the level of individual components across the system. However, single components …