11.1.10. Connection Between Subsystems (Islands) 
If an OpenScape 4000 system breaks down in emergency mode into several
autonomous islands, how can the islands then communicate with each other?
An autonomous island is defined as a survivability unit and all the access
points/OpenScape 4000 SoftGates over which the unit currently has active
control.
Everything else remains the same within each individual island:
- Calls between subscribers in the same access point/OpenScape 4000 SoftGate are switched within the access point/OpenScape 4000 SoftGate.
- Calls between subscribers in different access points/OpenScape 4000 SoftGates of an island are switched via IP connections between the access points/OpenScape 4000 SoftGates.
- Incoming/outgoing trunk calls are conducted via the island’s trunk interfaces.
- Incoming/outgoing calls in networked systems are conducted via the island’s tie trunk interfaces.
What happens to calls for subscribers on another island?
All survivability units have the same complete host system database.
Therefore, every island knows every subscriber in the entire configuration,
including all subscribers on other islands. Access points/OpenScape 4000
SoftGates outside the island are configured but cannot be reached. All
boards and their subscribers, trunks and tie trunks are therefore known
and belong to a set hierarchy (UNACH).
Although the IP infrastructure between some islands may still be intact,
no inter-island calls between access points/OpenScape 4000 SoftGates
are switched via IP.
Trunk interfaces can still be used for inter-island communication.
It is therefore still possible to reach a subscriber who is known but
not directly available in another island over CO. The “Alternate Routing
on Error“ feature supports rule-based call number modification in emergency
mode.
This feature guarantees Basic Call connectivity for traffic between
the islands.
For this feature to work, the trunks on the islands must use different
access codes. If all trunks use the same access code, directed routing
must be performed by the carrier using the complete number dialed (CENTREX).
If the islands are integrated in an extensive OpenScape 4000 network
with QSig trunks, the LCR configuration must include normal and emergency
mode because the system has only one configuration. Even in this scenario,
Basic Call connectivity is guaranteed for incoming, system-wide traffic
to an access point/OpenScape 4000 SoftGate which results in a transit
connection. In order to support network-wide features, the islands should
be configured as virtual nodes.
Payload survivability does not work in emergency mode as the islands
involved no longer have a shared control unit.