Exchange/SNA Manual

Appendix G Using Exchange/SNA in LU
Configurations
104700 Tandem Computers Incorporated G–1
This appendix lists the new LU configurations and operations for Exchange/SNA.
This appendix presents the following information:
Single-LU Configuration
Comparison of Single- and Multiple-LU Operations
Single-LU
Configuration
The new AUTOLOGON feature requires support for single-LU configurations because
the JES2 AUTOLOGON feature for remote workstations issues a BIND request for a
single LU only. All users of AUTOLOGON will be operating in a single-LU
configuration. The following discussion describes the differences in the operation of
Exchange/SNA between a single-LU configuration and a multiple-LU configuration.
Additionally, instructions are given for how to use Exchange/SNA in a single-LU
configuration.
Comparison of Single-LU
and Multiple-LU Operation
There is one major difference between the operation of Exchange/SNA in a
multiple-LU configuration and in a single-LU configuration: how Exchange/SNA
responds to a BDS from the host when the requested subdevice has not been opened
(either a RECEIVE command has not been issued for that subdevice or the application
program has not opened that subdevice).
Multiple-LU Operation
In a multiple-LU configuration, Exchange/SNA does not respond to a BDS received
from the host until the specified subdevice has been opened. The LU-LU session on
which the begin destination selection (BDS) arrived is blocked until the response to the
BDS is sent. In other words, the LU-LU session is unavailable until a RECEIVE
command is issued for the required subdevice or an application program opens the
required subdevice.
Why a Single-LU Configuration Must Operate Differently
In a multiple-LU configuration, there is no need to respond to a BDS until the
subdevice has been opened, because there are other LU-LU sessions that can be used
while only one LU-LU session is blocked. However, in a single-LU configuration it is
necessary to respond to a BDS before the subdevice has been opened because once the
single-LU session is blocked no other communication with the host is possible. This
can easily result in both Exchange/SNA and the host waiting for each other. For
example, if a console command is issued from the Exchange/SNA command
interpreter while the LU-LU session is blocked, a deadlock state is entered. The host is
waiting for a response to the BDS, and Exchange/SNA is waiting for an LU-LU
session on which to send the console command.
Single-LU Operation
In a single-LU configuration, Exchange/SNA responds immediately to a BDS received
from the host. If the subdevice specified in the BDS has already been opened,
Exchange/SNA sends a positive response, as it does in a multiple-LU configuration. If
the subdevice is not open, Exchange/SNA sends a negative response. If the BIU that