OSI/TS Configuration and Management Manual

Configuring the OSI/TS Subsystem
OSI/TS Configuration and Management Manual—424831-001
3-15
Traffic and Protocol Overhead Attributes
Checksums are needed only when a line is unreliable. When considering the use of
checksums, you need to weigh the overhead incurred against the value of checksums to
detect errors. Since each of these three attributes generates a checksum on a different
part of the PDUs, you should set all three of these attributes to ON if your subsystem
requires the use of checksums.
Message-Size and PDU-Size Attributes
It is important to select the TPDU size and the X25AM packet size so that they
accommodate the message sizes of your application processes and of the network.
These can have a dramatic effect on performance. You should set these sizes large
enough to minimize the number of short packets or TPDUs sent. The smaller the
number of short packets, including acknowledgment packets and TPDUs, the lower the
total protocol overhead. If possible, you should block as much data into one single I/O
as possible. In general, the larger the ratio between the user message size and TPDU
size, the higher the CPU overhead and the lower the throughput.
If performance is of primary concern, and if the quality of the network allows it, the
TPDUSIZE attribute should be set to as large a value as possible (maximum 8K, or 8192
bytes, except for class 0), especially if the data traffic consists of large bulk transfers.
However, setting TPDUSIZE to the maximum value does not mean that the maximum
size is always used. The transport protocol accepts the lesser of two values configured
for the two ends of a connection. In addition, the protocol only sends as much data as it
can fit in a TPDU it does not pad the TPDU with filler bytes.
You can set the X.25 packet size using the SYSGEN PACKETSIZE attribute, and can
modify it using DSC. You can set the maximum TPDU size using SCF commands on
the TPDUSIZE attribute.
Window-Size Attributes
You can specify window size for the Transport Layer (using OSI/TS) and for the
Network Layer and Data Link Layer (using X25AM).
At the Transport Layer, the L4WINDOW attribute defines the initial credit allocation to
the remote transport entity, and defines the number of DT-TPDUs that can be sent
without acknowledgment. This attribute has a direct impact on response time and the
use of buffer space. The value you set for L4WINDOW affects response time when
there are multiple TPDU messages being sent. If the number of TPDUs in the message
is larger than the L4WINDOW value, the message must wait until the acknowledgment
is received before the rest of the TPDUs can be sent. The value of this attribute also
affects the use of buffer space, since each output TPDU must be retained until it is
acknowledged, in case it has to be retransmitted. You should, therefore, consider buffer
size and its impact on memory utilization.
Increasing the L4WINDOW attribute can increase the throughput and reduce CPU
overhead. However, if connections are primarily unidirectional (one side only sends and
the other only receives), or if there are many connections active, or if TPDUSIZE is set
to a large value, increasing L4WINDOW may not be useful. In general, a large
TPDUSIZE value has a more positive effect on performance than a large L4WINDOW
value.