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To prepare for an SSCP-LU session, the SSCP must first establish a session with the PU that controls the LU. This type of session is an SSCP-PU session. It enables the exchange of requests and responses pertaining to startup and shutdown of the configuration or the individual PU, and to the recovery of operations after a device or link failure. After VTAM® establishes the SSCP-PU session, the SSCP might attempt to establish a session with any active LU associated with that PU. The SSCP-PU session is established on a nonswitched line as soon as the PU is activated. On a switched line, the session is established following a dial-in or dial-out operation. For channel-attached SNA devices, the session is established when physical connection is established. In VTAM, the SSCP establishes the SSCP-PU session; a VTAM application program itself does not take any direct action to establish that session.