Communication Performance - Siemens SIMATIC S7-400H System Manual

Communication

14.7 Communication performance

14.7 Communication performance
Compared to a fault-tolerant CPU in stand-alone mode or standard CPU, the communication
performance (response time or data throughput) of a fault-tolerant system operating in
redundant mode is significantly lower.
The aim of this description is to provide you with criteria which allow you to assess the
effects of the various communication mechanisms on communication performance.
You get information on the latest connection statistics of all CPU connections in the
"Connection statistics" tab of the "Module state" tab dialog.
Definition of communication load
Communication load is the sum of requests per second issued to the CPU by the
communication mechanisms, plus the requests and messages issued by the CPU.
Higher communication load increases the response time of the CPU, meaning the CPU
takes more time to respond to a request (such as a read job) or to output requests and
messages.
Operating range
In every automation system there is a linear operating range in which an increase in
communication load will also lead to an increase in data throughput. This then results in
reasonable response times which are acceptable for the automation task at hand.
A further increase in communication load will push data throughput into the saturation range.
Under certain conditions, the automation system therefore may no longer be capable of
processing the request volume within the response time demanded. Data throughput
reaches its maximum, and the reaction time rises exponentially; see the figures below.
Data throughput may even be reduced somewhat due to additional internal loads inside the
device.
Figure 14-20 Communication load as a variable of data throughput (basic profile)
254
System Manual, 03/2012, A5E00267695-11
S7-400H