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partner SIU across the inter-SIU link set and sent out to
the adjacent signaling point by the partner unit. The inter-
SIU link set also provides the capability of message
retrieval and retransmission when a changeover operation
occurs between the two units.
■
For circuit-switched applications, the circuit groups are
configured on both units, letting the application select
which SIU controls each group. In normal operation,
the control of circuit groups is distributed between both
the SIUA and SIUB. In the event of failure of a unit
(or for maintenance), the application can move control
of each circuit group from one SIU to the other.
■
For transaction-based applications, the transactions are
shared equally between the two units.
Routing Architectures of a Dual-resilient SIU
System
The routing options (i.e., straight connection to the DPC
vs. indirect connection via a pair of STPs) described in
this section will vary based on the actual network architec-
ture that is being supported for a particular application.
Connection to a Single Adjacent Signaling Point
Figure 6 shows two possible routing alternatives for SIUA
routing to an adjacent SSP or SCP. Messages issued
from SIUA are sent to the destination SSP or SCP using
local SS7 links if available. If these fail, transmit messages
are relayed through SIUB over the inter-SIU links. In this
case, the DPC is also the adjacent signaling point.
Although Figure 6 shows an SIU pair connected to a sin-
gle adjacent signaling point, the pair may be connected
to multiple destinations.
The number of links allocated in the inter-SIU link set has
to be carefully calculated. Since these links will be used
for outgoing traffic only, they can be used at a higher
capacity than network-facing links. Moreover, since only
half of the traffic can potentially be routed through that
link set, a common rule is to allocate a fourth of the total
number of network-facing links in the inter-SIU link set. In
a fully populated pair of SIU520 (12 links per unit; 24 links
total), 16 links will be typically allocated in the network-
facing link set and four links will be allocated in the inter-
SIU link set.
Routes to destinations are configured such that there is a
primary link set (the link set from the SIU to the DPC) and
a secondary link set (the inter-SIU link set) which is used
only when the primary link set has failed.
Building Fault-tolerant SS7 Systems Using the Intel
®
NetStructure™ SIU520 SS7 Signaling Gateway Application Note
7
SSP/SCP
F-Links
Link Set id 1
A) Normal routing case
Inter-SIU
Link Set
Link Set id 0
SSP/SCP
B) Routing under network link failure
Inter-SIU
Link Set
SIUA
SIUB
SIUA
SIUB
Single Point Code
Single Point Code
Link Set id 0
Link Set id 1
Figure 6. Transmit Routing to a Single Destination