SNAX/HLS Configuration and Control Manual

Step 2. Planning SNAX Requirements
Planning the SNAX/HLS Environment
104705 Tandem Computers Incorporated 2–5
Planning Communication
Links
To plan communication links, it is important to understand the capabilities of both
modems and devices, because SNAX/XF line definitions must be consistent with
device or host features and with the telephonic capabilities of the communications
channel.
If the device or host is close to the Tandem system, you should examine the
possibilities of using relatively inexpensive modem eliminators.
When communicating with a device, the SNAX/XF line acts as the primary, and the
device is the secondary. You should carefully evaluate the possibilities of two-way-
simultaneous (TWS) communication (also known as full-duplex communication)
because of its extended capabilities. Link and local device address assignments should
be carefully managed.
When communicating with a gateway, the SNAX/XF line acts as the secondary, and
the host (for example, the IBM 3725) is the primary. TWS is recommended. This
maximizes performance because both Tandem and IBM systems can send frames
simultaneously.
In a gateway environment, you should give careful consideration to the use of
multiple SDLC links. When feasible, multiple links are preferable because of the
opportunities for tuning (links can be placed on separate controllers, on different
CPUs, and so on), and greater fault tolerance in the face of device problems or human
error. In particular, note that two lines running at 9.6 kbps behave better than one line
running at 19.2 kbps.
Planning Configuration When defining the line and PU for SNAX/XF, you should specify the RECSIZE
parameter to optimize performance subject to device or application constraints. The
LU RECSIZE parameter is not significant for SNALU applications. The basic
parameter needed is the maximum RU size that will be used by applications using the
line and PU. If the large message option (LMO) PROFILE attribute is used, it
corresponds to the buffer size used.
BIND messages specify the MAX-RU sizes for messages flowing in both directions. A
special code (%H00) is used to signify unlimited-length messages. SNAX/XF
interprets this value as 4,096, an interpretation that might cause problems because it
might not agree with values used by the partner. Therefore, you should avoid BIND
images with unlimited RU sizes. The RECSIZE parameter for the PU is used within
SNAX/XF to control segmentation, a process invisible to the SNAX/HLS application.