1.3 LUs and Sessions

SNA Components Used for LUA Communications, shows the SNA components used for LUA communications.

SNA Components Used for LUA Communications

An LUA application uses an LU of type 0-3 that communicates with the host system by means of the SNAP-IX node. There are three sessions between the SNAP-IX node and the host node, as follows:

• The physical unit-system services control point (PU-SSCP) session, between the PU 2.1 and the host's system services control point (SSCP); this is used for controlling the PU.

• The SSCP session, between the SNAP-IX LU and the SSCP; this is used for controlling the LU.

• The LU session, between the SNAP-IX LU and the host LU; this is used for data transfer between the LU and the host application.

The LUA application programming interface enables applications to send and receive data on the SSCP session and on the LU session. It does not provide access to the PU-SSCP session. An LUA application can send data on this session using the Management Services (MS) verb TRANSFER_MS_DATA; for more information, refer to the SNAP-IX MS Programmer's Guide.

For Windows operating systems, TRANSFER_MS_DATA is provided as part of the Common Service Verb (CSV) API; for more information, refer to the SNAP-IX CSV Programmer's Guide.

Each of the LU sessions provides two priorities of messages: normal and expedited. Expedited flow messages take precedence over other messages waiting to be transmitted on the same session. There are four different flows on which a message can be sent or received:

• SSCP session, expedited flow

• LU session, expedited flow

• SSCP session, normal flow

• LU session, normal flow

The LU session normal flow carries application data; the other flows are used for control messages and start-up.

The SNAP-IX implementation of LUA does not enable applications to send data on the SSCP expedited flow, and will not return data to an application on this flow.