What is TCP Keep Alive, how does it work and what should you watch
out for in a connection via ISDN?
In TCP connections messages are sent
cyclically between the connection partners to establish whether the
partner station can still be reached. If the partner station cannot
be reached for a certain time ("Keep Alive"), the TCP connection
between the stations is cleared down. This is particularly
important for server applications (passive connection setup) to be
able to release occupied resources again if the connection is
interrupted. In CPs for SIMATIC S7 you can set the "Keep Alive"
cycle. In Cps for SIMATIC S5 the "Keep Alive" cycle time is 120
seconds by default when switched on.
Note for ISDN connections:
For cost reasons some ISDN
routers allow the ISDN connection to be cleared down in the case of
inactivity, i.e. when no data is transferred for a specific period
of time. If, however, "Keep Alive" messages continue to be sent,
then the ISDN connection will never be cleared down. This can lead
to high telephone costs. It is recommended to deactivate the "Keep
Alive" mechanism for each ISDN connection.