IBM LU6.2
Logical Unit 6.2 is an IBM-originated communications protocol specification dating from 1974, and is part of IBM's Systems Network Architecture.
A device-independent SNA protocol, it is used for peer-to-peer communications between two systems, for example, between a computer and a device (e.g. terminal or printer), or between computers. LU6.2 is used by many of IBM's products, including Common Programming Interface for Communications Intersystem Communications (CICS ISC), and Information Management System.
Some examples of a non-IBM product which implemented the SNA stack including LU6.2 are: Microsoft Host Integration Server, and Netware for SAA.
APPC is the protocol used with LU6.2 architecture.
LU6.2-compliant devices operate as peers within the network and can perform multiple simultaneous transactions over the network. LU6.2 devices can also detect and correct errors. The LU6.2 definition provides a common API for communicating with and controlling compliant devices. Although the concepts were the same on all platforms, the actual API implementation often varied on each IBM platform which implemented it. Other vendors also implemented LU6.2 in their own products and with their own APIs. IBM later defined the "Common Programming Interface - Communications" (CPIC) API which would eventually become widely implemented. CPIC allowed for the authoring of multi-platform code. APPC is often used to refer to the LU6.2 architecture or to specific LU6.2 features.
See also: LU0, LU2 and LU3 of the IBM Systems Network Architecture (SNA).