With the adjacent link station (ALS) selection function, VTAM® can dynamically determine or select the ALS to use for an independent LU that serves as the destination logical unit (DLU) in a session. ALS selection is called only if the DLU is the SLU. The DLU cannot be a local dependent LU or an application program residing in this VTAM.

If you code your session management exit routine to support the ALS selection function, VTAM invokes the function when it receives an INIT request or a DSRLST request. This function is invoked for DSRLST routing only if DSRLST is specified in the exit options and enabled during the begin function. In the origin logical unit (OLU) domain, the ALS selection is the first function invoked for the session. In other domains, VTAM invokes the function following the session management alias selection function that determines the DLU real name and network ID. VTAM invokes the function for INIT OTHER CD if the new PLU is not located in this domain and the SLU can potentially be an independent LU.

The session management exit provides a list of potential adjacent link stations an independent LU can use for this session. The ALS list is passed to the exit in the ALS list information vector. An ALS determines the boundary function link an independent LU uses to connect to the subarea network. By selecting an ALS, the exit can dynamically determine whether the session is to use a boundary function connection from this SSCP, or route to another SSCP to complete the connection.

The ALS selection function also allows you to specify an ALS for a session that is not in the current ALS list. The ALS name information vector returns a PU name to use for the connection. The ALS (the PU name) specified in the vector does not have to be in the list passed to the exit. In fact, you can code the ALS selection function such that the ALS specified in the name vector overrides the current ALS list. If the specified PU name is not in the current ALS list associated with the LU, you can set an indicator in the ALS name information vector to add the PU name to the current list.

You can use the ALS selection function to select a different ALS for each session where the independent LU is the DLU. This enables a DLU to have multiple concurrent session paths from the network. By default, VTAM invokes the ALS selection function whenever the SLU resource is a cross-domain resource (CDRSC) that is considered to be an independent LU served by this SSCP. If you want to perform ALS specification and selection without any prior indication that the CDRSC represents an independent LU, you can specify that ALS selection is called for all CDRSCs that could be independent LUs. This eliminates the requirement to predefine an ALS for the resource. If your installation does not need this feature, but instead defines an ALS for each independent LU (or otherwise indicates that this CDRSC is for an independent LU), you can specify that ALS selection is allowed only when the DLU is considered to be an independent LU. This can reduce the number of invocations of the exit. Whether the ALS selection function is invoked for all CDRSCs that could be independent LUs, or only for DLUs considered to be independent LUs, is specified in the exit options and enabled during the begin function.

After the ALS is selected, through either the current ALS list or the session management exit, session setup continues. If the chosen PU becomes unavailable after setup begins, VTAM does not attempt to select another ALS for the session. The session must be restarted.