Abstract: Congestion is an essential problem in wireless sensor networks. It occurs when the offered traffic load exceeds available capacity at any point in a network. Congestion causes overall channel quality to degrade and drop rates to rise. Furthermore, redundant transmissions are always adopted to guarantee reliable data delivery, which may deteriorate congestion since they bring about more contention and in return hampers the reliability. In this paper, a framework is proposed to avoid, detect and mitigate congestion effectively. Compared with previous works, this work doesn't control congestion at the cost of desired reliability, instead, both goals can be achieved. There are mainly two parts in this framework: congestion aware traffic allocation (COTA) and congestion detection and mitigation (CODEM). COTA is used in multipath routing to balance traffic around the whole network. COTA uses some heuristic information to analyze the potential congestion region and avoid traversing these regions. Once congestion happens inevitably, CODEM is presented to detect congestion using accurate metric and reallocate traffic among multiple paths to mitigate congestion. The multi-frequency characteristic of CC2420 radio is used to minimize contention between control packets and data packets. Comprehensive simulations validate the distinguished performance in several aspects of the framework.