OSI/AS Configuration and Management Manual

Performance Considerations
OSI/AS Configuration and Management Manual—424119-001
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Window-Size Attributes
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.
Window-Size Attributes
You can specify window size for the Transport Layer (using OSI/TS), and 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 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.
Transport class 1 is a special case because any value greater than 1 can lead to a
deadlock situation. This can occur when the sending transport entity waits for an
acknowledgment of each DT-TPDU before sending the next one. This is an obvious case
in which both sides of the connection need to be aware of what the other side is
expecting.
If you are using X25AM, there are two window attributes you can set. L3WINDOW
specifies the number of packets that can be outstanding before an acknowledgment is
required before any more packets are sent. L2WINDOW specifies the number of Data
Link Layer frames that can be outstanding before an acknowledgment is required before
any more frames are sent. These two attributes are similar to L4WINDOW; they must be
set to accommodate the size of the TPDUs, the packet size selected, and the number of
packets and frames being used to send an application message.
Protocol-Overhead Attributes
There is protocol overhead associated with each layer of an OSI system. Although the
size of OSI addresses, for example, contributes to overhead, the typical user does not
have much control over this. When designing your OSI/AS subsystem, you should
consider the tradeoff between having the required address uniqueness within a given
environment and the desirability of keeping addresses short to reduce the amount of data
being sent on the line. On X.25 networks this may not be too critical, since addresses are
only contained in the connection TPDUs, unless there are a lot of connections being
initiated during a session. On LANs this may very well be critical, since addresses are
contained in each network message.
The EXTENDEDFORMAT attribute also has an affect on line utilization. This attribute
specifies that the credit and sequence numbers of the TPDUs are to be expressed in
extended format. Extended format adds three bytes to each TPDU. In systems with a
high volume of traffic, these additional bytes could become significant, especially if
multiplexing is being used on the line.