Specifying terminal support

The following table summarizes the keywords you can use to change the priority or terminal type of various devices.

All terminals that IMS supports can be used in an XRF complex. The terminal characteristics determine the type of support a terminal can have in an XRF complex, with class-1 having the best support and class-3 having no support.

Table 1. Specifying terminal support
Device Class BACKUP keyword on TERMINAL macro or ETO logon descriptor

SLUTYPE1 (3286)
SLUTYPE2 (3278,3279,…)
SLUTYPEP (4700,8100,
SERIES 1,…)
FINANCE 3601

Eligible for
class 1,2,3

For class-1 service,
  BACKUP=(1-7,YES).
For class-2 service,
  BACKUP=(1-7,NO).
For class-3 service,
  BACKUP=NO.

LUTYPE6 (ISC)
MSC (VTAM®) NTO (leased line)
3270 (VTAM) SPOOL line groups
Locally attached devices

Eligible for
class 2,3

For class-2 service,
  BACKUP=(1-7,NO).
For class-3 service,
  BACKUP=NO.

LU 6.2

Eligible for
class 3

Not Applicable

The primary and the secondary master terminals are special types of terminals that exist simultaneously on both the active and alternate IMS systems. They are not treated as class-1 or class-2 terminals. The master terminals on the alternate IMS are not disrupted by a takeover. After takeover, the master terminal on the alternate IMS processes the undelivered messages from the active IMS's master terminal.

The BACKUP keyword on several macros in the system definition statement or ETO logon descriptor specifies the actual support a terminal is to receive. Your installation determines the switching priority of the terminals, with level 1 being the lowest priority and level 7 the highest. Although IMS orders VTAM requests by priority, the requests might be completed in any order because of unpredictable factors in the network, such as pacing, line speeds, and congestion.

If BACKUP=(1-7,YES), the terminal is automatically taken over without losing session control. If the terminal is connected to an XRF-capable VTAM, this constitutes class-1 support.

For both class-1 and class-2 terminals, the default switching priority is 4.

If BACKUP=NO, this terminal must be restarted manually after takeover. This forces class-3 support.

If options on the BACKUP parameter are set incorrectly, IMS resets the incorrect option at session initialization. For example, if you specify BACKUP=(1-7,YES) for an ISC terminal, IMS will reset YES to NO because the highest class of support that an ISC terminal can receive is class-2.

XRF with USERVAR and the BACKUP parameter

For XRF systems with USERVAR, BACKUP=(1-7,YES) also tells the alternate IMS to maintain backup sessions for all class-1 terminals. The maximum number of terminal backup sessions that can be established on the alternate IMS is specified in the NCP BUILD statement with the BACKUP=n option. If the (n+1) class-1 terminal user tries to log onto the active IMS, the logon is denied.

If BACKUP=(1-7,NO), this terminal is automatically restarted after takeover, but no BACKUP session is established. This forces class-2 support.