The next stage in the evolutionary process saw linear networks give way to rings as the criticality of high-speed network transport became obvious and the capabilities of the SONET/SDH overhead are realized. You will recall that the K bytes give SONET and SDH the capability to monitor the health of the network connections between multiplexers, the multiplexers themselves, and in the event of a failure to provide automatic protection switching as a way of surviving a cable cut. As the SONET/SDH overhead became better understood, service providers began to explore alternative network architectures that could take best advantage of the capabilities of that overhead. Two architectures in particular emerged: the two- and four-fiber ring.