Session types

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

Diagram of the session between APPL1 and APPL2, in which APPL1 is running on LU 6.2A and APPL2 is running on LU 6.2C.

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.