PowerNet 3270 Reference Manual
Connect, Inc.
4-2
February 1998
Mainframe 3270 installations perceive the PowerNet server as a terminal cluster
controller. The server is connected to a front-end processor (FEP) running a
Network Control Program (NCP), and the FEP is channel-attached to the Host
running a communications control program, known as the Virtual Terminal Access
Method (VTAM). VTAM supplies access to mainframe regions and application
programs within those regions.
In the IBM SNA environment, the combination of NCP and VTAM provide a set
of host network services known as the System Services Control Point (SSCP).
Initially, the SSCP establishes two sessions with the server, one for Control Point
(CP) exchanges, and another for Physical Unit (PU) exchanges. These command
and control sessions use the first two addresses (0 and 1) of the 256 Logical Unit
(LU) addresses available within each PU.
NCP and VTAM make use of configuration tables to determine what services are
available to the terminal. While the NCP is deciding what to do with a terminal
LU requesting activation, the LU is unowned. Depending on the configuration
tables, the LU is then connected to VTAM and in session with the SSCP, which
expects the terminal to select a host region. Following region selection, the
terminal BINDs to a Host application, or transaction, which is known as the
Primary LU (PLU).
On most hosts, the first host transaction is a security program requiring user name
and password input. Following security clearance the terminal is generally able to
select the desired application transaction.
Session States
The state of the LU session and the data within the PS are both available to the
Dialog Language Script programmer. The LU states are summarized as follows:
inactive
Link is down.
unown
Waiting for a session.
sscp
In session with VTAM.
plu
In session with host.
busy
In session, host is busy.
Initially, a session on an active link is in the
unowned
state. The state changes to
sscp
after VTAM recognizes the terminal. In this state, the PS is normally
unformatted. Depending on the host configuration, entry of a region name is
required at this point. Once a region has been assigned and a host transaction is
attached to the terminal, the state changes to
plu
.
