An overview of the architecture
The LAS is required only to initialize a TAS for each user who is logging on from a VTAM terminal.
A TAS is required to support each session, whether it is an EXCP or a VTAM session. Each TAS uses the SAF interface to directly interact with the external security manager and allocates the ISPF environment during session initialization.
A TAS can be initialized in the following ways:
VTAM terminal users log on from their terminals.
A START command is issued for a VTAM terminal session.
A START command is issued for an EXCP terminal session.
The following figure outlines how the LAS and TAS address spaces interact. The following sections discuss the functions of each address space and the limitations of the architecture:
How the MainView Alternate Access architecture establishes VTAM or EXCP sessions