Exchange/SNA Manual

Exchange/SNA Overview
Introduction
104700 Tandem Computers Incorporated 1–3
Exchange/SNA
Overview
Exchange/SNA emulates IBM 3770 SNA MLU terminals. The purpose of
Exchange/SNA is the same as the purpose of the IBM terminals being emulated—to
provide batch communications for remote job entry applications with an IBM host
system. Like the IBM terminals, Exchange/SNA can be used to submit jobs to a
central host system and to receive output from the host. Additionally, Exchange/SNA
can be used to transfer files between the Tandem system and the host system.
Subdevices Exchange/SNA is a software emulation of the IBM 3770 SNA MLU terminals and does
not require that the Tandem system support the same physical devices as the IBM
terminals support. Instead, Exchange/SNA provides logical subdevices that
correspond to the physical devices supported by the IBM terminals. Exchange/SNA
provides subdevices that correspond to card readers, printers, and card punches for
sending and receiving data to and from the host. Exchange/SNA also provides a
console subdevice for sending commands to the host RJE subsystem and receiving the
output of those commands.
Exchange/SNA does not provide subdevices that correspond to diskette storage units.
However, many of the functions of the diskette storage units, such as their use in
offline operations and in the exchange of information between remote sites, can be
performed on Tandem systems outside of Exchange/SNA and do not require a
corresponding subdevice in Exchange/SNA.
When you use Exchange/SNA to communicate with the host system, you specify a
subdevice over which the communication occurs, and you associate the specified
subdevice with a file on the Tandem system. The associated file contains the data to be
sent to the host or receives the data sent from the host. This file can be any type of file
supported on the Tandem system, such as a printer, a process, or a disk file. By letting
you associate subdevices with any type of file, Exchange/SNA does not restrict input
and output operations to the physical device types used in the IBM terminals.
Communications
Capabilities
Like the IBM 3770 SNA MLU terminals, Exchange/SNA uses the SNA multiple logical
unit protocol and supports up to six concurrent, independent data streams with the
host system. Each data stream uses a separate subdevice and a separate LU-LU
session. All the data streams are transmitted over a single SNA data communications
line to the host system. The actual number of concurrent data streams is limited by the
number of subdevices you define and the number of logical units you define for the
data communications line, whichever is less.
Within Exchange/SNA, the subdevices do not correspond to specific logical units.
Rather, the logical units represent a pool of resources (logical data links, in effect) that
are assigned by Exchange/SNA to the subdevices as needed.
Exchange/SNA handles the high-level protocols that manage the communications
between Exchange/SNA and the host RJE subsystem. The low-level protocols, such as
SDLC, are handled by the SNAX/XF or SNAX/CDF access method, which are the
Tandem implementations of SNA. SNAX/CDF uses EnvoyACP/XF for the lowest-
level protocols.