Data communication EXEC parameters for the control region

Using EXEC parameters, you can override definitions for data communication that were initially set during IMS system definition.

Three kinds of parameters in the IMS procedure are related to data communication:

Overrides of predefined buffer sizes
Overrides and options particular to Message Format Service (MFS)
Performance options

Buffer sizes for data communications

By monitoring communication traffic you might decide to increase buffer space. Use the parameters of the IMS procedure shown below to allocate virtual storage and override the buffer pool sizes predefined in system definition.

Table 1. Overriding buffer pool sizes with data communication parameters
IMS procedure parameter	System definition specification
IMS procedure parameter	Macro	Parameter
FBP¹	BUFPOOLS	FORMAT(size1)
QBUF²	MSGQUEUE	BUFFERS
RECA	COMM	RECANY
¹ Specify a value representing the number of 1024-byte blocks. ² By increasing the number of buffers (used for the message queue data sets), you can reduce the frequency of I/Os.

Overrides of predefined parameters

Monitoring communication traffic often results in a decision to increase buffer space or the number of buffers. The following execution parameters are provided to override the size and number of some of the buffer pools predefined at system definition:

FBP: Overrides the size specified on the FORMAT(size1) keyword of the BUFPOOLS macro.
QBUF: Message queue buffer. Adjusts the size of the message queue buffer pool. QBUF enables you to override the number of blocks to hold messages in the control program's storage. By increasing the number of buffers used for the message queue data sets, you can reduce the frequency of I/Os. The default number of buffers is specified in the MSGQUEUE macro.
EMHL: Specifies the EMH buffer length for Fast Path transactions.
RECASZ: Specifies the size of the Receive Any buffer.
RECA: Overrides the number of Receive Any buffers predefined in system definition by the RECANY keyword of the COMM macro.
SAV: Specifies the allowed maximum number of concurrently active device I/Os.

ETO parameters

Use the following parameters to define ETO options:

ALOT: Specifies the amount of time allotted before a terminal in session is automatically logged off if no user successfully signs on.
ASOT: Specifies the amount of time allotted before a signed-on user is automatically signed off if no input or output activity occurs.
LHTS: Specifies the number of slots for the CNT/LNB/RCNT hash table.
NHTS: Specifies the number of slots for the VTCB hash table.
UHTS: Specifies the number of slots for the SPQB hash table.
DLQT: Specifies the number of days allotted before a queue containing data that has not been allocated is classified as a potential dead-letter queue.
ETO: Specifies whether terminals and queues that are not defined to IMS are to be supported.
DSCT: Specifies the suffix of the descriptor member DFSDSCTy.

Performance options

Use the following parameters to define performance options:

VAUT: Specifies the use of VTAM-Authorized Path.
NLXB: Specifies that parallel sessions are added during system startup.
FESTIM: Overrides the timeout value for Front End Switching; this value is predefined during system definition.

MFS options

When you use MFS, you can adjust the capacity of the system by adjusting the availability of format blocks. Use the FRE parameter to specify the maximum number of active blocks in the message format buffer pool. You need to coordinate this number with the value specified for the FBP pool size parameter.

Performance options for VTAM systems

The VAUT parameter offers a performance option to systems using VTAM®. You can override the null value generated in the IMS procedure and specify the use of VTAM Authorized Path.

The NLXB parameter allows you to add parallel sessions during system startup. The number you specify is added to the number defined in the SESSION keyword in the definition of the MSC physical link (MSPLINK).

The FESTIM parameter overrides the timeout value for front-end switching that is predefined in system definition. You specify the parameter on the COMM macro with a value from 1 second to 300 seconds.