For economic reasons sensor networks are often implemented with resource constrained micro-controllers and low-end radio transceivers. Consequently, communication is inherently unreliable and especially multi-hop communication suffers severely from packet losses. Transmission protocols that rely on implicit acknowledges for multi-hop communication are energy efficient but require symmetric communication links to work properly. In this paper we introduce IMPACT, a family of transmission protocols that rely on implicit acknowledges and employ a cross layer approach to handle asymmetric links.