When two logical units in the network want to exchange
data, VTAM® establishes a communication
connection between the two LUs. The logical connection between the
LUs is called a session in SNA. Figure 1 shows
APPL1, running on LU 6.2A, in session with APPL2, running on LU 6.2C.
Figure 1. Session between
APPL1 and APPL2
When two LUs communicate over a session, the protocols
used on the session correspond to the type of LUs communicating. Consequently,
protocols on the session can be described as an LU session type.
The following list gives the characteristics of the LU
session types. LU types 1 through 4 and LU type 7 describe a host
program supporting a device with limited function. LU types 6.1 and
6.2 describe program-to-program protocols.
- LU 0
- These protocols are not defined by SNA. The LUs that are communicating
must implement a set of protocols. For example, the protocols that VTAM uses to support non-SNA 3270
binary synchronous communication (BSC) terminals are LU 0 protocols.
- LU 1
- LU 1 protocols provide access to nondisplay I/O devices such as
printers and keyboard printer terminals.
- LU 2
- LU 2 protocols provide access to display terminals with the IBM® 3270 data stream.
- LU 3
- LU 3 protocols provide access to printers with a subset of the IBM 3270 data stream.
- LU 4
- LU 4 protocols provide access to terminals that are similar to
LU 1 terminals but have more functions.
- LU 6.1
- LU 6.1 protocols provide communication access to other application
programs. These are the original protocols SNA defined for communication
between application programs. Only application programs in a host
processor can use LU 6.1 protocols.
- LU 6.2
- LU 6.2 protocols support sessions between two application programs in a
distributed data processing environment. LU 6.2 provides a connection
between its transaction programs and network resources. These protocols
can be implemented by application programs running on non-host systems
and by programmable hardware devices in the network. (This manual,
however, is concerned only with LU 6.2 application programs that are
in the host and that can issue VTAM APPCCMD
macroinstructions.)
- LU 7
- LU 7 protocols provide access to a single display station.
VTAM application
programs can function as many different LU types simultaneously. For
example, when an application program exchanges data with a terminal
that supports only LU type 2, the session between the two logical
units uses only LU type 2 protocols; during that session, the application
program is an LU type 2. When the application program exchanges data
with another application program and LU protocols of 6.1 or 6.2 are
used, the LU is one of those types during that session.
An application program can function as a type 6.2 LU and
can still communicate as any of the other SNA LU types. It can have
sessions with printers or terminals and still request LU 6.2 services
from VTAM when it communicates
with another LU 6.2 application program.
Refer to SNA Technical Overview for more
details on SNA protocols for non-LU 6.2 application programs.