  Error tolerance and attack vulnerability are two common and important
properties of complex networks, which are usually used to evaluate the
robustness of a network. Recently, much work has been devoted to determining
the network design with optimal robustness. However, little attention has been
paid to the problem of how to improve the robustness of existing networks. In
this paper, we present a new parameter alpha, called enforcing parameter, to
guide the process of enhancing the robustness of scale-free networks by
gradually adding new links. Intuitively, alpha < 0 means the nodes with lower
degrees are selected preferentially while the nodes with higher degrees will be
more probably selected when alpha > 0. It is shown both theoretically and
experimentally that when alpha < 0 the attack survivability of the network can
be enforced apparently. Then we propose new strategies to enhance the network
robustness. Through extensive experiments and comparisons, we conclude that
establishing new links between nodes with low degrees can drastically enforce
the attack survivability of scale-free networks while having little impact on
the error tolerance.
