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In the second case, each unit controls half of the total
available transactions. (The Intel NetStructure SIU520
SS7 signaling gateway supports up to 16,384 transac-
tions. A dual-resilient system can consequently handle up
to 32,768 simultaneous transactions.) Each transaction is
processed for its entire duration by the SIU that
processed the first TCAP message. The user application
must therefore direct all messages for a transaction to
the same SIU, and load balance outgoing dialogues
between the two units. An incoming TCAP dialogue mes-
sage other than BEGIN or QUERY is handled by the SIU
that processed the first TCAP message for that dialogue
received from the SS7 network. When an SIU receives a
TCAP message that belongs to a transaction that was
initiated on the other unit, it will pass this message to its
peer SIU over the RSI connection. This is shown in
Figure 14. Failure of an SIU reduces the number of avail-
able transactions to one-half.
In a dual-resilient transaction-based SIU system running
the SS7 stack up to SCCP on the SIU, and TCAP (and
above) on the application host, each host controls a fixed
number of transactions. Each transaction is processed
for its entire duration by the application host that
processed the first TCAP message. Upon failure of one
SIU unit, the TCAP capacity and ongoing transaction are
totally unaffected.
The architectural decision taken on where the TCAP
module is running also has consequences on the level of
application resiliency and total system capacity. These
consequences are explained in more details in “Failure of
Application”, pg. 15.
Application Note Building Fault-tolerant SS7 Systems Using the Intel
®
NetStructure™ SIU520 SS7 Signaling Gateway
14
Figure 14. Message Flow on a Dual-resilient System Running the SS7 Stack up to TCAP
SS
7
SS
7
SS7
Inter-chassis
communication
(RSI)
Transmit message for transaction handled by TCAP B
MTP
MTP
SCCP
SCCP
TCAP
instance = 0
TCAP
instance = 1
Message received for transaction handled by TCAP B
SIUA
SIUB
Application
Ethernet
Adjacent
Signaling
Point