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418 VM/HSC 6.0 System Programmer’s Guide
1st ed., 6/30/04 - 312579601
Functioning of Host-to-Host Communications
The method hierarchy that is established is, from highest to lowest, VTAM, LMU, CDS.
Initially, all methods are set to CDS. Appropriate entries can be placed in the PARMLIB
statement to set the communications method at HSC initialization. If an error in
communications occurs during communications with the current communication method,
a method switch is performed.
The system may switch from a method of VTAM to LMU, selecting the first LMU
available from a list of defined LMUs. If the current method is LMU, the system attempts
to find the next LMU available from the list, or switches from LMU to CDS. If a list of
LMUs has not been defined by the PARMLIB statement, or if no LMUs are available, the
system switches directly from VTAM to CDS.
Note: After a downward switch has occurred, you must issue an operator command to
perform an upward switch (for example, from CDS to LMU, CDS to VTAM, or LMU to
VTAM).
Figure 8 on page 79 illustrates how various combinations of communication methods can
exist at the same time between multiple hosts.
Designation of Communication parameters
When specifying communication parameters in the operator command, the parameters can
designate different actions depending upon whether or not the host specified in the
command is the host that is executing the command.
When the command is defining parameters for the host executing the command, the
method parameter defines the highest method in the hierarchy allowed to be defined on
that host.
If a command defining parameters for another host specifies a higher method, the current
method for this other host is set to the executing host’s limit, if possible. To change a
host’s method hierarchy limit, the command must be issued on that host, except during
HSC initialization when the method is set from a PARMLIB control statement.
When the command is defining parameters for the host executing the command, the
LMUpath parameter defines those LMUs this host is allowed to use for sending messages.
For two hosts to communicate via an LMU, there must be matching entries in their
respective LMUpath lists.
Careful analysis should be performed when defining the communication method in the
PARMLIB control statement because of how the system handles the switching.