Abstract: Protocols developed during the last years for Wireless Sensor Networks (WSNs) are mainly focused on energy-consumption optimization and autonomous mechanisms (e.g. self-organization, self-configuration, etc). Nevertheless with new WSN applications appear new QoS requirements such as time constraints. Real-time applications require the packets to be delivered before a known time bound which depends on the application requirements. We particularly focus on applications which consist in alarms that are sent to the sink node. We propose Real-Time X-layer Protocol (RTXP) a real-time communication protocol. This protocol integrates both MAC and routing layers. Our proposition aims at guaranteeing an end-to-end constraint delay while keeping good performances on other parameters, such as energy consumption. For this purpose the protocol relies on a hop-count-based Virtual Coordinate System (VCS) which classifies nodes having the same hop-count from the sink, allows forwarder selection, and gives to the nodes an unique identifier in a 2-hop neighborhood allowing deterministic medium access. In this document we describe the protocol mechanisms and properties. A theoretical comparison of our proposition with a state-of-the-art protocol is given. Intensive simulation results confirm the theoretical predictions. We also discuss the performances of the protocol under the assumption of unreliable radio links.