This selects whether this SONET/SDH interface derives its transmit timing by recovering the clock from the incoming received signal (loop) or by using the device's own internal timing reference (internal). Administrators must configure timing sources carefully across a SONET/SDH network's topology, since only one clock source should typically be the ultimate master to avoid timing slips throughout the network. Setting this object to loop on an interface that should simply follow the timing of its upstream neighbor keeps this node synchronized to that upstream clock reference instead of introducing its own.