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Signaling information
is passed over the signaling links which are called signal units. Signal Units
are continuously transmitted in both directions on any link that is in service.
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SS7 uses 3 different
types of signal units:
o
Message Signal Units
(MSU) :
§ All signaling associated with
call setup and teardown, database query and response, and SS7 management
requires the use of MSUs
o
Links Status Signal
Units (LSSU):
§ communicate information about the signaling link between the nodes on
either end of the link. This information is contained in the status field of
the signal unit. They signal the initiation of link alignment, quality
of received traffic, and status of processors at either end of the link.
o
Fill In Signal Units
(FISU):
§ do not carry any information; they simply occupy the link when there are
no LSSUs or MSUs. FISUs support the monitoring of link traffic because they
undergo error checking. They can also be used to acknowledge the receipt of
messages using backward sequence number (BSN)
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