A wireless sensor network comprising - a first headnode - a first subnode adapted to communicate with the first headnode, thus making the first headnode and the first subnode members of a first cluster - a second headnode and - a second subnode adapted to communicate with the second headnode, thus making the second headnode and the second subnode members of a second cluster; characterized in that - the first headnode is adapted to select a first frequency for use in wireless communications within the first cluster - the first subnode is adapted to communicate with the first headnode on said first frequency, using a time slotted channel access scheme to allow also other nodes to communicate with the first headnode on said first frequency - the second headnode is adapted to find out the first frequency selected by the first headnode, and to select a second frequency - different from said first frequency - for use in wireless communications within the second cluster - the second subnode is adapted to communicate with the second headnode on said second frequency, using a time slotted channel access scheme to allow also other nodes to communicate with the second headnode on said second frequency, - for communicating information from the first cluster to the second cluster, the first headnode is adapted to find out the second frequency selected by the second headnode and to communicate with the second headnode on said second frequency, using the same time slotted channel access scheme as said second subnode, and- the first headnode and the second headnode are additionally adapted to repeatedly transmit network beacon signals (331, 332) on a third frequency, which is different from said first frequency and said second frequency. - a first headnode - a first subnode adapted to communicate with the first headnode, thus making the first headnode and the first subnode members of a first cluster - a second headnode and - a second subnode adapted to communicate with the second headnode, thus making the second headnode and the second subnode members of a second cluster; characterized in that - the first headnode is adapted to select a first frequency for use in wireless communications within the first cluster - the first subnode is adapted to communicate with the first headnode on said first frequency, using a time slotted channel access scheme to allow also other nodes to communicate with the first headnode on said first frequency - the second headnode is adapted to find out the first frequency selected by the first headnode, and to select a second frequency - different from said first frequency - for use in wireless communications within the second cluster - the second subnode is adapted to communicate with the second headnode on said second frequency, using a time slotted channel access scheme to allow also other nodes to communicate with the second headnode on said second frequency, - for communicating information from the first cluster to the second cluster, the first headnode is adapted to find out the second frequency selected by the second headnode and to communicate with the second headnode on said second frequency, using the same time slotted channel access scheme as said second subnode, and- the first headnode and the second headnode are additionally adapted to repeatedly transmit network beacon signals (331, 332) on a third frequency, which is different from said first frequency and said second frequency.