SNAX/APN Application Programming Manual

The SNALU Interface
SNAX/APN Application Programming Manual—420111-001
4-10
Session Phases
With SNAX/APN, an application on a Tandem system can act as either a primary logical
unit (PLU) or a secondary logical unit (SLU):
•
When acting as a PLU, the application can establish and control LU-LU sessions
with any application or any SNA device connected to an SNA terminal node
controller (physical unit [PU] type 1) or an SNA cluster controller (PU type-2).
•
When acting as an SLU, the application can enter into LU-LU sessions with SNA
application subsystems on the remote system such as IMS and CICS. In the
SNAX/APN environment, the Tandem system appears to the remote system as a
type 2.1 node.
When establishing an LU-LU session, the two LUs agree upon a set of operating criteria
by means of an SNA BIND request. These include such concepts as pacing, chaining,
bracketing, request and response modes, and contention resolution. The two LUs must
adhere to the agreed-upon rules throughout the course of the session; otherwise, it
becomes impossible to maintain communication.
SNALU Application as PLU
As a PLU, the application essentially controls all aspects of the LU-LU session:
•
It formats and transmits the particular BIND request, which both initiates the session
and specifies the rules governing the session.
•
It controls the flow of information to and from the device by way of SNA requests
and control indicators.
•
It may eventually terminate the session.
SNALU Application as SLU
As an SLU, the application essentially functions under the control of the remote PLU
program:
•
It accepts an incoming BIND request, which both initiates the session and specifies
the rules governing the session.
•
It exchanges information with the remote PLU using SNA requests and control
indicators in accordance with the session rules.
•
It accepts and sends an UNBIND request, which terminates the session.
Session Phases
An SNA LU-LU session can be logically divided into three major phases:
1. Session Initiation
An LU-LU session is established when the PLU sends an SNA BIND request to the
SLU and the SLU accepts it (sends back a positive response).