VTAM facilities used for ISC connections

For ISC connections that are supported by VTAM®, VTAM controls the physical transmission of data between IMS and a logical unit.

Restriction: The VTAM facilities discussed here do not apply to ISC TCP/IP connections.

Both VTAM applications (such as IMS) and other subsystems can be viewed as VTAM logical units. A logical unit is an addressable resource, such as an application program or a subsystem. A logical unit can also be a component of a general-purpose terminal system.

The VTAM concepts and facilities used by IMS that are particularly relevant to ISC include:
  • Connection, disconnection, and establishing logon mode.
  • Messages and responses.
  • Request definite response 1 (RQD1) or request definite response 2 (RQD2) and the associated definite responses (DR1 or DR2). Note that definite response 1 and definite response 2 have been separated and redefined for LU 6.1 protocols. RQD2 requests and DR2 responses are now known as sync-point requests and responses and are functionally independent of those responses associated with DR1.
  • Sequencing and chaining.
  • Orderly session disconnection from stop bracket initiation (SBI) and bracket initiation stopped (BIS) commands.
  • Facilities for ensuring orderly communication, including the use of brackets and change-direction indicators.
  • Sequence number recovery.
  • Receiving input and sending messages.
  • Conditional bracket termination.
  • Extended Error Recovery Procedure (EERP).
  • Use of parallel sessions between ISC nodes.
  • Support for both negotiable and nonnegotiable session bind parameters.

IMS also supports the class-of-service (COS) and session-outage-notification (SON) facilities.

Related reading: For more information about the communication concepts and facilities that govern data transmission between a VTAM application program (such as IMS) and another subsystem, see z/OS® Communications Server: SNA Programming.

The ISC system programmer and system analyst must be familiar with these concepts and facilities in order to design and implement a communications interface between IMS and a remote subsystem using SNA LU 6.1 protocols.