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SS7 Polling and Host Link Failure

Overview

Polling has to be enabled to enable SS7 host link failure detection. The polling scheme allows the host to control polling from the SS7 card. That is, the SS7 card sends Poll messages to its local host by default, and the host can stop and start polling as well as change the polling interval.

After receiving a Poll Interval message from its local host, the SS7 card stops the polling or changes the poll interval accordingly. The SS7 card starts to send a Poll message to the local host after processing a Poll Request from the SS7 local host. When using SS7 redundancy, these messages are sent to the active SS7 card. The SS7 card propagates the message to its standby card. If these messages are sent to the standby card, a NACK (0x14) is sent back.

Active/Standby Mode

When SS7 is running in the active/standby mode, if a host link failure occurs on one SS7 card, the mate SS7 card is still connected to the local host. The mate SS7 card sends a Poll message to the host indicating the host link failure. While an SS7 card pair is in the active/standby mode, if the active SS7 card loses its connection to the local host and the standby SS7 card is still connected to the local host, the host software is assumed to send a Become Active message to the standby SS7 card to initiate an SS7 switchover. The host is aware of the active SS7 host link status from the Poll message from the standby card. If the host does not initiate the switchover within 10 seconds, the SS7 card will take the local subsystems out of service. If the host does not initiate the switchover within 10 seconds because the standby card has also lost its host link connection within that 10 seconds, the SS7 card will take the local subsystems out of service. The SCCP/TCAP subsystem out of service procedure applies.

See Signaling System Number Seven.
Negative acknowledgement to a message.
See Transaction Capability Application Part.