Texas Instruments 990 User Manual page 217

Throughput
B.4.l Turnaround Time (TAT)
The turnaround time (TAT) at a secondary station differs from one
secondary to the next. TAT is the time component of a secondary
system that is used to formulate, queue, and begin the
transmission of the response to a poll. This TAT can be
considered at both the data link level and the levels above the
data link. For example, the data link level TAT is relatively
small in comparison to the line transmission time of a poll with
response. But at the upper levels, the TAT is much larger. For
example, to send a 260-byte message to a PM550 and get a I-byte
response message back requires a TAT of about 1.92 seconds at the
PM550 software levels above the data link level. This time must
be considered, along with the line data rate, by an applications
task at the primary that communicates with a PM550 secondary
station.
Another component of TAT is the time between polls. After a
message is sent to a secondary station, it must be delivered to
its final destination. This delivery requires the passing of the
message from queue to queue. At each queue "stop" in this
movement, some processing is performed on the header information
that is a part of every message. After the message arrives at
its destination, a response may be formed and written to the
communications system. This response is now processed back
through the output queues, header information is attached as
required, and, finally, the message is queued awaiting
transmission. However, transmission cannot occur until the
secondary receives a poll from the primary. At that time the
message can be sent with the response to the poll. If another
message had arrived in the queue earlier, it would have been sent
first (that is, another poll must come in before the next message
in the output queue can be sent).
Determining the response times to traffic generated at all levels
is a complex task and, generally, of little or no use to the
user. The user is primarily concerned with how long it takes to
send a message (or series of messages) and how long it usually
takes to get a reply to a message or a series of messages sent to
a secondary station. This information can be determined by using
the statistical functions referred to as artificial data.
Artificial data can be generated to a TX5 secondary station; the
results provide the user with information on response times that
can be expected in the actual operating environment. This
information is the result of simulation performed by the
artificial data task.
2270526-9701 B-5