SNA Overview 443
Dependent and Independent LUs
In your SNA travels, you will probably run across logical-unit terms other than SLU and PLU.
Logical units can be either dependent on or independent of an SSCP. The protocols the LU uses
to initiate a session is what makes it dependent or not dependent on an SSCP. In Figure 12-8,
PU 2 needs to communicate with an SSCP.
Figure 12-8
Dependent and Independent LUs
Boundary Nodes
Before discussing boundary nodes, the types of path information units (PIUs) used to
communicate between physical units should be covered. Communication between subarea
nodes is usually done with these PIUs. PIUs can be segmented for transmission between
subarea nodes. The adjacent link station reassembles the PIU. This causes a lot of overhead and
should be avoided.
There are two types of FID frames: format identifier 4 (FID4) and format identifier 2 (FID2).
·
FID4 are the frame types used to communicate between subarea nodes only: FEP to FEP,
mainframe to mainframe, and mainframe to FEP. The address field in the PIU FID4
consists of two parts:
-- Subarea number--The subarea component identifies the subarea to which a
NAU belongs, similar to a subnet in IP.
IBM 3174
PC
Mainframe
PU 5
PU 4
Dependent LU
"I am dependent on the
SSCP to initiate a
session!"
PU 2
IBM 3174
PU 4
PC
PC
LU 6.2 can be dependent
or independent
PU 2.1 node
"I am not dependent on
an SSCP to initiate a session!"
87200333.book Page 443 Wednesday, August 22, 2001 2:53 PM