| 摘要 |
The switch units of a distributed switching system are connected by a central switch for advancing connections between the switch units and also by inter-switch unit trunks. To save switch unit resources, trunk services such as scanning and signaling are not provided to the inter-switch unit trunks while the central switch is available for advancing connections. When the central switch becomes unavailable for advancing connections, as detected by control units in the switch units, the inter-switch unit trunks are connected to the trunk serving circuitry of the switch units and connections are advanced between switch units over the inter-switch unit trunks. Serving the inter-switch units trunks while the central switch is unavailable requires additional switch unit resources but provides for continuity of service which would to otherwise be lost due to the unavailability of the central switch.
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