Access to LUs can be controlled through their identification. For SNA physical units on switched lines, use SNA ID verification. The IDs of SNA physical units on switched lines are built by VTAM® from the PU definition statements. After a physical unit dials into or answers a dial-out request from an application program, an XID (exchange ID) command is sent to the physical unit. The physical unit responds with its station ID, which is sent to VTAM for verification against the IDs defined to VTAM. If the station ID is valid, VTAM sends its SSCP ID to the physical unit in the ACTPU command. If the station ID is not valid, VTAM disconnects the physical unit. When the identifications have been exchanged, normal physical connection procedures continue. This exchange of IDs provides a means for limiting access to the network.