logical unit (SNA) (LU)

The term Logical Unit (LU) is used in storage networks and in the SNA architecture.

1. In storage area networks( SAN), a logical unit (LU) is an instance ofSCSI that executes I/O commands. For example, a SCSI physical disk typically has one logical unit. Tape drives and array controllers can have multiple Logical Units that can be accessed via I/O commands. Logical units are identified by the Logical Unit Number( LUN).
2. In SNA networks, a logical unit is a logical virtual port that provides the user with access to network services. An LU supports at least two sessions: with the host-resident system service control point( SSCP) and with another LU representing, for example, an application program. LUs are classified into types, with LU 0, 1, 2, 3, 4, 5, and 7 communicating with host systems, peripheral devices, and applications. For example, LU 0 is for special devices and programs, LU 1 and LU 3 are for printers, and LU 2 is for 3270 series viewers with different screen sizes. LU 4 is similar to LU 1, but older, and LU 5 is undefined. LU 7 is similar to LU 2, but is for sessions between the /36, /38, AS/400 and 5250 terminals. LU types 6.0, 6.1 and 6.2 can communicate with the same LU type. LU 6.2 is responsible for peer-to-peer communication in SNA networks. LU 6.2 supports general communication between programs in a distributed application environment, Advanced Program to Program Communication( APPC). With the definitions specified in the LU 6.2 protocol, the direct path without FEP via the Cluster Controller is also possible. A prerequisite for the use of LU 6.2 is a PU 2.1 (Physical Unit).