Recent development in information and communication technology has enabled to study networks of social interactions of unprecedented size. Such systems include email or phone networks and e-communities. In contrast to the traditional, questionnaire-based investigations, in these cases a natural quantitative measure of the strength of the interactions is present (like the frequency or duration of calls) leading to weighted network representations. One important observation is that this strength of the interactions varies over many orders of magnitude. A natural conclusion is that the weights play important roles both in the evolution of the networks and in the dynamics of the processes on them. Based on simple rules borrowed from sociology we construct a model, where the emergence of the community structure is a consequence of the interplay between topology and weights. We show that the model reflects well the observations made on a huge call network. References: [1] J.M. Kumpula, J.-P. Onnela, J. Saramäki, K. Kaski, J. Kertész: Emergence of communities in weighted networks, Phys. Rev. Lett. 99, 228701 (2007) [2] J.-P. Onnela, J. Saramäki, J. Hyvonen, G. Szabó, D. Lazer, K. Kaski, J. Kertész, A.-L. Barabási: Structure and tie strengths in mobile communication networks, PNAS 104, 7332-7336 (2007) [3] J.-P. Onnela, J. Saramäki, J. Hyvonen, G. Szabó, M. Argollo de Menezes, K. Kaski, A.-L. Barabási, J. Kertész: Analysis of a large-scale weighted network of one-to-one human communication, New J. Phys. 9, 179 (2007)