Session initiation

A session is the logical connection between a logical unit (such as a terminal) and a VTAM® application program (such as IMS). A session must be established before data can be transmitted between a logical unit and IMS.

Session initiation is requested in one of five ways:

  • The terminal operator enters the LOGON sequence. VTAM verifies the command and passes the request to IMS.
    The terminal operator can identify IMS in the logon sequence with the following names:
    • The APPLID name, if the terminal operator is requesting a session with a specific IMS.
    • The MNPS ACB name, if the terminal operator is requesting a session with an XRF with MNPS system.
    • The USERVAR, if the terminal operator is requesting a session with an XRF with USERVAR system.
    • The generic resource name, if the terminal operator is requesting a session with a generic resource group.
  • The z/OS® VTAM network operator requests session initiation on behalf of the logical unit by using the VTAM VARY command with the LOGON option. VTAM processes the request and passes it to IMS.
  • VTAM passes a logon request to IMS for each logical unit that is defined to VTAM as belonging to IMS.
  • The IMS master terminal operator requests session initiation for a logical unit by entering the IMS /OPNDST command.
  • ETO autologon initiates the session and supplies the appropriate user data, based on user descriptors and exit routines.

For LU 6.2, session initiation occurs automatically as conversations are allocated. The IMS MTO can request delivery of queued LU 6.2 output by issuing the /ALLOCATE command. When using LU 6.2, remember that the IMS application name (which matches the LU name) for LU 6.2 conversations is different than the LU name IMS has for non-LU 6.2 types, and that the /START DC and /START APPC commands work independently.

Regardless of how session initiation is requested, identical processing occurs when IMS receives the request.