VTAM part

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The part of an SNA network controlled by VTAM® is called its domain. LUs are normally defined as part of the domain during VTAM definition and are then activated by start procedures or VTAM operator commands. The VTAM application program, which is itself an LU, then establishes sessions with one or more other LUs (on its own initiative or as the result of session-initiation requests by other LUs). A session is a logical connection between two LUs that allows an orderly series of communications between the LUs. Once a session is established, the program requests VTAM to perform data-transfer operations. In addition to managing the domain and building channel programs, VTAM performs such services as input and output data buffering, automatic scheduling of application program exit routines, and sequence numbering of outbound requests. VTAM requests the operating system to execute channel programs that it has built; the channel programs result in communication with channel-attached communication controllers or cluster controllers, which in turn forward the information toward its ultimate destination. For processors with a communication adapter, the channel programs can be used to communicate directly with terminals, in addition to cluster controllers and communication controllers.