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.
