Planning for VTAM ownership of class-1 terminals

When determining where to place the ownership of the class-1 terminals, consider what causes the least operator effort, the least disruption to class-1 terminal users, and the least strain on the CPC.

Before you make the decision, consider some of the implications of VTAM® failures on ownership. First, review some information about VTAM that has special meaning for XRF.

Ownership is significant when a terminal user logs on to IMS or logs off from IMS. Each VTAM has one system services control point (SSCP); it is responsible for session initiation and session termination for terminals that it owns. Therefore, if the owning VTAM fails, some functions of VTAM are no longer available. Service on the class-1 terminals that are logged on to IMS does continue. But when a user of a class-1 terminal within its domain tries to log on, VTAM cannot honor the request. Upon restarting VTAM , the network operator must also reestablish the ownership of the terminals. Alternatively, the network operator can transfer ownership to another VTAM .

At a takeover, the ownership of class-1 terminals does not shift to the VTAM of the alternate IMS system. If you assign ownership to the active VTAM and the active IMS system fails, the class-1 terminals switch to the alternate IMS system, but the VTAM of the failed IMS system retains ownership. The VTAM of the failed IMS system retains ownership even if the failed IMS system returns to service, but functions only as the alternate IMS system.

In making the decision about terminal ownership, consider three options. The owner of the class-1 terminals can be:

• The VTAM in the active system
• The VTAM in the alternate system
• A VTAM in a Communication Management Configuration (CMC)

You should assign the ownership of the class-1 terminals to a third VTAM in a CMC. Having the owning VTAM in a CPC that is outside the XRF complex offers the following advantages:

• It reduces contention on the active system during normal operations.
• It reduces contention on the alternate system during the takeover.
• It prevents failures in the XRF complex from terminating the VTAM that owns the class-1 terminals.
• It allows terminals that are connected to the NCP on SNA switched (that is, dial-up) lines to be class-1 terminals.

If you do not have a CMC, assign the ownership of the class-1 terminals to either VTAM in the complex, and do not change the ownership after a takeover unless you are restarting VTAM.