Adjacent link stations are type 2.1 physical units (or type 4 or type 5 PUs that appear as type 2.1 PUs) through which independent LUs can access the network. There is no hierarchical relationship between an adjacent link station (PU) and an independent LU. For more information about adjacent link stations and how independent LUs can use them to access the network, see Using CDRSC definition statements for independent LUs, and Multiple connections between a type 2.1 node and a subarea node.