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In case (A), since the F-links all exist in the same link set,
messages may be sent from the adjacent SSP/SCP to
either SIUA or SIUB. If an SIU receives a message from
the SS7 network for a circuit or transaction that it does
not control, this receive message is passed automatically
to the other SIU for processing via the TCP/IP LAN.
Connection to an Adjacent Mated STP Pair
There are two ways of connecting a pair of SIUs to a
mated STP pair. In the first method, the straight link
configuration, each SIU is configured with two link sets:
one link set containing STP-facing links, plus the
inter-SIU link set. The straight link configuration consists
in connecting SIUA to STPA and SIUB to STPB, as
shown in Figure 7.
The second method, the crossed link configuration,
consists of adding crossed link connections between
SIUA and STPB and between SIUB and STPA to the
previous mode. In a crossed link configuration, each SIU
is configured with three linksets: two link sets containing
the links towards each STP, plus the inter-SIU link set
(see Figure 8). The configuration of the DPC will contain
the link set IDs of the two link sets connected to the
STPs. Load sharing can be enabled to take advantage of
all the system resources. In such a configuration, the
inter-SIU link set is not used for traffic failover, but only for
synchronization of network management messages.
Consequently, a single link within this link set is fairly
enough, giving an increased bandwidth for network-
facing links.
The inter-SIU link must be carefully dimensioned as it will
have to support the outgoing traffic of the SIU that would
have lost its entire network-facing links, as shown in
Figure 9. Again, a common practice is to allocate a fourth
of the total number of network-facing links in the inter-
SIU link set.
Application Note Building Fault-tolerant SS7 Systems Using the Intel
®
NetStructure™ SIU520 SS7 Signaling Gateway
8
Figure 7. Dual-resilient SIUs Connected to a Mated STP Pair in a Straight Link Configuration
Figure 8. Dual-resilient SIUs Connected to a Mated STP Pair in a Crossed Link Configuration
SSP/SCP
Inter-SIU
Link Set
Single Point Code
STPA
STPB
Link Set id 1
Link Set id 2
A-Links
SIUA
SIUB
Link Set id 0
SSP/SCP
Inter-SIU
Link Set
Single Point Code
STPA
STPB
Link Set id 1
Link Set id 2
SIUA
SIUB
Link Set id 0