Due to the development of several new technologies concerning high speed networks, one can get PC clusters for intensive computing at a low price. Parallel programming environments must take into account the heterogeneity of workstation networks into their communication kernels. New technologies must be easily integrated into such environments. The LANDA-HSN project has been designed for an efficient use of the available resources (network, processor, memory). This paper describes the mechanisms and the implementation of its communication kernel on SMP hosts and through interconnection networks. The contribution of this paper is to propose a flexible and efficient communication kernel.