Net advantage

Some complex networks are resilient. Cut out one of the hubs in the Internet’s backbone, for example, and information will find alternate routes through other hubs. In some cases, however, cutting connections can make a network so inefficient as to be unusable, researchers have now shown.

Eduardo López of the Los Alamos (N.M.) National Laboratory and his collaborators looked at a variety of abstract networks and randomly cut links between nodes. Even when up to 60 percent of the links had been severed, the networks usually stayed connected—meaning that for any two nodes, a path still existed between them. But as links were cut, the average distances between nodes got longer.

For disruptions in certain kinds of networks, such as those modeling interconnected roads or the spread of a disease epidemic, a threshold is quickly reached beyond which getting from one node to another becomes impractical, the team writes in the Nov. 2 Physical Review Letters.

For disease epidemics, therefore, measures such as quarantines or travel restrictions can slow contagion enough that the epidemic would peter out. In the case of the Internet, data should still be able to reach its destination unless all the principal hubs are cut off, López says. “The Internet will continue to be resilient unless the failures occur in a very targeted way.”

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