In this paper, we make an analysis of the main problems in wireless sensor networks focusing in the absence of standard network architectures for this technology. We also study other problems like network lifetime or high packet loss rate. For this, we propose our own cross-layer architecture with protocol integration. After some experiments with real sensor networks, we concentrate on role assignment, exposing some of the role assignment algorithms available in the literature. Finally, we propose our own heterogeneous network, in which sensors play different roles. Copyright 2007 ACM.