The design of large-scale networked control systems urges an efficient usage of available resources, such as communication, energy and computation. Recent results indicate substantial benefits of event-based control compared to conventional designs, when these resources are sparse. This paper considers multiple entities of heterogeneous control systems that are coupled through a common communication medium. Each control system may decide upon its available information, whether a state update shall be transmitted to the controller over a contention-based medium. The objective is to design an optimal decentralized control and transmission scheme that minimizes the aggregate quadratic cost function. A state aggregation technique is used to derive a decentralized event-triggering scheme, which is asymptotically optimal as the number of control units increases. Numerical simulations give a comparison of the optimal centralized, time-triggered and event-triggered schemes and corroborate the efficiency of the proposed design method.