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Networsk-on-chip (NoCs) have emerged as a promising on-chip interconnect for future multi/many-core architectures as NoCs are able to scale communication links with the growing number of cores. State-of-the-art NoC designs rely mainly on a static network configuration using fixed routing algorithms and buffer placements. These approaches are not effective in dealing with hard-to-predict system behavior, for instance due to user behavior or varying workloads, since in order for static NoCs to cover these scenarios, they would have to be designed for worst case scenarios. In this paper, we address these problems with a runtime adaptive network-on-chip (AdNoC). Focusing on the architecture-level adaptation, we present an adaptive route allocation algorithm which provides a required level of QoS (guaranteed bandwidth) coupled with an adaptive buffer assignment scheme which reassigns buffer blocks on-demand. Furthermore, the adaptivity requires a comprehensive, hardly intrusive, runtime observability infrastructure, i.e., using monitoring components, in order to gather data on the system state. The area overhead introduced by the adaptive scheme can be traded off against the flexibility gained. Moreover, the area overhead is also reduced by resource multiplexing due to the on-demand buffer assignment at each output port (we achieved on an average 42% buffer saving in our experiments). We demonstrate the advantage by using various digital media applications and compare our approach to the state-of-the-art static NoC architectures e.g., Xpipe, QNoC, and Æthereal.