SS7 Host Link Failure Detection and Recovery

Overview

The SS7 host link failure detection feature is disabled by default. To enable this feature you must send the System Configuration message to the matrix card with the SS7 card slot number. This message also configures the SS7 host link failure timer.

SS7 Host Link Failure Detection Logic

If the SS7 card does not receive any message from the host after four consecutive Poll messages, the CSP initiates a timer. If the CSP does not receive an message from the host before the timer expires, the CSP determines that a host link failure has occurred. The System Configuration message configures the Failure Confirmation Timer (Data[2,3]).

If no message has been received from the SS7 local host for the period of the timer (four Poll messages), a minor alarm (with card alarm type Host Link Error detected) is sent to the matrix host. See Alarm (0x00B9), API Reference. The SS7 host link failure timer then starts.

If the host link failure timer expires, the SS7 host link failure handling logic applies.

If the SS7 card receives any message from the local host before the host link failure timer expires, the host link is considered recovered. The host link failure timer is then stopped and an Alarm Cleared for the host link error is sent to the matrix host.

SS7 Host Link Failure Handling Logic

When the SS7 card detects a local host link failure, a major card alarm (with alarm type Ethernet link failure) is sent to the matrix host. See Alarm (0x00B9), API Reference. If the SS7 card is in a redundant mode, the card sends an internal message to inform its mate about the host link failure status. This status change will be reflected in the next Poll message the SS7 mate sends to its host. Depending on the SS7 card status, one of the following applies:

• If the card is single, and the host has been configured to take the subsystem out-of-service when a host link failure is detected, then all the subsystems that reside on the SS7 local host are taken out-of-service. The SCCP/TCAP subsystem out-of-service procedure applies. See SCCP/TCAP Subsystem Out-of-Service Procedure.

• Assuming redundant SS7 cards are in the active/standby mode, if the active SS7 card loses the TCP connection to its local host, the SS7 card checks whether its mate standby SS7 card is in host link failure condition. If the standby card is still connected to its local host, the card expects the local host to initiate a switchover by sending a Become Active request to the standby card. No further action is taken by the SS7 card. If the host does not initiate the switchover within 10 seconds or the standby card loses its host link connection within 10 seconds, the SS7 takes the local subsystems out-of-service. The SCCP/ TCAP subsystem out-of-service procedure applies. See SCCP/TCAP Subsystem Out-of-Service Procedure (2-49).

• Assuming redundant SS7 cards are in the active/standby mode, if both active and standby SS7 cards lose the TCP connections to their local host(s), all the subsystems that have been configured to reside on the SS7 local host is taken out-of-service. This assumes also that the SS7 card has been configured to take subsystems out-of-service in the event of a host link failure. The SCCP/TCAP subsystem out-of-service procedure applies.

• Assuming the redundant SS7 cards are in the synchronizing mode, if the active SS7 card loses the TCP connection to its local host then all the subsystems residing on the SS7 local host are taken out-of-service. This assumes also that the SS7 card has been configured to take subsystems out-of-service in the event of a host link failure. The SCCP/TCAP Subsystem out-of-service procedure applies.

• If the redundant SS7 cards are in the active/standby mode or synchronizing mode, and the standby SS7 card loses the TCP connection to its local host, no further action is taken.

Matrix Host Link Failure Recovery

After the matrix host link is recovered, the subsystem on the host is presumed to be in its previously-configured state. If the subsystem was in-service before the host link failure, SCCP goes through local subsystem in-service procedure as if it receives N_STATE request from host.

SS7 Host Link Failure Recovery

If the SS7 card receives any message from its local host, then the SS7 host link is considered recovered. If the SS7 card was in host link failure condition, Alarm Cleared with alarm types, Ethernet Host Link Error Detected (0x10) and Ethernet Link Failure (0x22) is sent to the matrix host. See Alarm Cleared 0x00C1, API Reference.

If the SS7 card is in the redundant mode, it informs its mate SS7 card about the host link recovery. This is reflected in the next Poll message from both SS7 cards if polling is enabled.

If the subsystem was brought in-service by the host before the host link failure, SCCP goes through the local subsystem in-service procedure as if it receives N_STATE request from host.