IEEE 802.15.4 is considered as the de facto standard for wireless sensor networks (WSN). However, the increased probability of collision at heavy load caused by the inherent behavior of CSMA/CA degrades the performance of WSN in terms of throughput and energy consumption. This led some to propose other MAC layer solutions to tackle these problems. Most of them are TDMA-based which provides deterministic medium access. This makes them not scalable in general for large scale WSN. Contrary to that trend, we retain CSMA/CA's simplicity and good scalability properties and propose CoSenS, a collecting and sending burst scheme, that is implemented on the top of CSMA/CA. The idea of CoSenS is that a router does not retransmit packets as they arrive. Instead, it collects data from its children and other neighbor routers during a period of time that we call waiting period (WP), queue them until the expiration of that period and then sends them into a burst during a period of time that we call transmission period (TP). Simulation results show that this scheme greatly enhances throughput, end to end delay and transmission success rate (reliability). In addition, the scheme exhibits a nice property of self-adaptability of transmission periods between neighbor routers.