Logical units

A logical unit is a device or application program by which an end user (an application program, a terminal user, or an input/output mechanism) gains access to a SNA network. Any device or application program that implements LU 6.2 protocols appears as an LU 6.2 in the network. All application programs using SNA sessions, including LU 6.2 application programs, are considered logical units by SNA. To the network, a logical unit is the source of a request coming into the network, although the logical unit might not be the original source. The contents of the request or the information on which the request is based might have originated at a device controlled by the logical unit.

For example, in a 4702 Finance Controller, the logical unit is an application program that handles input and output for one or several finance terminals attached to the controller. Input actually originates at one of the terminals, but the logical unit (the application program) in the 4702 uses the input to create and transmit the request. Similarly, the network sees a logical unit as the destination of a request, but the logical unit might actually pass the data to a device for recording, printing, or displaying.

A VTAM® application program is also a logical unit. VTAM sees an application program as an originator of and destination for requests. Other programs in the host processor can interact with a VTAM application program and use it to communicate with other parts of the network. In such cases, these programs are the application LU's end users.

SNA categorizes LUs into different types depending on the set of SNA functions the LU supports. SNA defines LU types of 1, 2, 3, 4, 6.1, 6.2, and 7. SNA does not define an LU type of 0. However, type 0 is used to describe communication between LUs that do not conform to SNA protocols.

Figure 1 shows an SNA network composed of type 6.2 LUs.

Figure 1. SNA network of LU 6.2s
A SNA network composed of type 6.2 LUs, in which Host 1 is with LU6.2F on AS/400 and Host 2 is with LU6.2E on APPC/PC.