However, its
absence signifies trouble with the transport (the physical link). The main purpose of
providing this as a function is to provide a proactive, rather than reactive, detection
mechanism for link failures.
In other words, instead of finding out that a link is out of service when there is data
to send, the protocol and all of the network entities use the FISU as a link integrity
measurement. As long as the entity is able to receive and process the FISU, the link is
operating correctly. If the entity is no longer receiving FISUs, there is something wrong
with the link.
The links themselves are always transmitting something. If there is no signaling
data to be delivered, then FISUs are transmitted over the idle links. The entities are
always checking the health of the link even when there is no data being sent.
This proactive approach to maintaining transport reliability is seen at level 3 of the SS7
protocol as well. When a link fails, there are automatic procedures to ensure the orderly
routing of signaling data around the failed routes or links. This improves the availability
of signaling routes by ensuring packets are rerouted through good routes when other
routes and links fail.
All of this is automatic, of course, and many times when a link fails, it is brought back
into service before anyone even has time to react. These processes have to be emulated
in an IP environment, to continue to ensure the availability and reliability of the signaling
network.
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