Logical Units

End users and applications access SNA networks through logical units (LUs). Because SNA is a connection-oriented protocol, prior to transferring data the
respective logical units must be connected in a session.

In SNA hierarchical networks, logical units require assistance from system services control points (SSCPs), which exist in type 5 nodes, to activate a session with another logical unit.

The control point assists in establishing the session between the two LUs and does
not take part in the data transfer between the two LUs.

LU types identify sets of SNA functions that support end-user communication. LU-LU sessions can exist only between logical units of the same LU type. For example, an LU type 2 can communicate only with another LU type 2; it cannot communicate with an LU
type 3.

LU Type 1 - An example of the use of LU type 1 is an application program running under IMS and communicating with a 3270 printer.

LU Type 2 - application or devices that use SNA 3270 data stream. An example of the use of LU type 2 is an application program running under IMS and communicating with an IBM 3270 display station

LU TYpe 3 - This is for application programs and printers using the SNA 3270 data stream. An example of the use of LU type 3 is an application program running under CICS/VS and sending data to a 3270 printer.

LY Type 6.2 - This is for transaction programs communicating in a client/server data processing environment. The type 6.2 LU supports multiple concurrent sessions. LU 6.2 can be used for communication between two type 5 nodes, a type 5 node and a type 2.1 node, or two type 2.1 nodes.

LU-LU session initiation generally begins when the session manager in an LU (secondary LU) submits a session-initiation request to the appropriate control point. Using the specified set of session parameters (defined in a mode table), the control point builds a BIND image. The control point transmits the BIND image in a control initiate request
(CINIT request) to the primary logical unit (typically the application LU). The primary logical unit (PLU) is the LU responsible for activating the session. The PLU activates the session by sending a bind session request (BIND request, also called a session-activation request) to the secondary logical unit (SLU). The SLU then returns a BIND response to the PLU. A response unit flows between the session partners and the session is started.  Note that the "server" (PLU) is the one initiate the session with the "client" (SLB).  Contrasting with TCPIP, the client always initiate the session and the server listens for such requests.