6.1 Non-Call Related Procedures
23.3343GPPIP Multimedia Subsystem (IMS) Application Level Gateway (IMS-ALG) - IMS Access Gateway (IMS-AGW) interface: Procedures descriptionsRelease 17TS
6.1.1 General
The non-call related procedures are based on corresponding procedures of 3GPP TS 23.205 [8] where the IMS-ALG takes the place of the MSC server and the IMS-AGW takes the place of the MGW.
6.1.2 IMS-AGW Unavailable
The IMS-ALG server recognises that the IMS-AGW is unavailable in the following 4 cases:
1. The signalling connection is unavailable
Figure 6.1.2.1: Signalling connection failure
2. The IMS-AGW indicates the failure or the maintenance locking condition to all connected IMS-ALG servers
Figure 6.1.2.2: IMS-AGW indicates the Failure/Maintenance locking
The failure or maintenance locking indication indicates that the IMS-AGW is locked for new calls or will soon go out of service and that no new connections should be established using this IMS-AGW. The IMS-AGW can choose between the "graceful" and the "forced" method. In the graceful method the connections are cleared when the corresponding calls are disconnected. In the forced method all connection are cleared immediately.
3. The IMS-ALG recognises that the IMS-AGW is not functioning correctly, e.g. because there is no reply on periodic sending of Audits. The periodic sending of Audits by IMS-ALG should go on.
In all of the above cases the IMS-ALG shall prevent the usage of the IMS-AGW until the IMS-AGW has recovered or the communication with the IMS-AGW is restored.
6.1.3 IMS-AGW Available
The IMS-ALG discovers that the IMS-AGW is available when it receives an IMS-AGW Communication Up message or an IMS-AGW Restoration message. If the IMS-ALG does not wish to sustain an association with the IMS-AGW, the response sent to the IMS-AGW may indicate an alternative IMS-ALG signalling address, in which case the IMS-AGW shall not consider itself registered and should preferably attempt to re-register with this alternative IMS-ALG before any further alternate IMS-ALGs. Otherwise, the response shall not indicate any alternative IMS-ALG signalling address.
When the IMS-ALG discovers that the IMS-AGW is available the following shall occur:
1. Signalling recovery
The IMS-AGW indicates to all connected IMS-ALGs that the signalling connection is restored.
Figure 6.1.3.1: Communication goes up
2. IMS-AGW restoration/maintenance unlocking indication.
The IMS-AGW indicates to all connected IMS-ALGs that normal operation has resumed.
Figure 6.1.3.2: IMS-AGW indicates recovery from a failure/or maintenance unlocking
NOTE: This procedure may be used after recovery from a signalling failure.
3. The IMS-ALG recognises that the IMS-AGW is now functioning correctly, e.g. because there is a reply on periodic sending of Audits.
After this the IMS-ALG can use the IMS-AGW.
If none of 1, 2, and 3 happens the IMS-ALG server can initiate the IMS-ALG Ordered Re-register procedure.
6.1.4 IMS-AGW Recovery
If the IMS-AGW recovers from a failure, is maintenance unlocked, or it has been restarted, it registers to its known IMS-ALGs using the IMS-AGW Restoration procedure or the IMS-AGW Registration procedure. The IMS-AGW can indicate whether the Service has been restored or whether it has restarted with a cold or warm boot. If the IMS-ALG does not wish to sustain an association with the IMS-AGW, the response sent to the IMS-AGW may indicate an alternative IMS-ALG signalling address, in which case the IMS-AGW shall not consider itself registered and should preferably attempt to re-register with this alternative IMS-ALG before any further alternate IMS-ALGs. Otherwise, the response shall not indicate any alternative IMS-ALG signalling address.
Figure 6.1.4.1: IMS-AGW Restoration
Figure 6.1.4.2 IMS-AGW Registration
After the recovery the IMS-ALG can use the IMS-AGW.
6.1.5 IMS-ALG Recovery
6.1.5.1 General
If an IMS-AGW-unavailable condition is provoked by a failure/recovery action, the IMS-ALG recovery sequence will, from an information flow point of view, look like IMS-AGW unavailable and then IMS-AGW available. If an IMS-AGW-unavailable condition is not provoked, the IMS-ALG recovery sequence will look like IMS-AGW available.
After the information flow, the terminations affected by the recovery action are released.
6.1.5.2 IMS-ALG Restoration
NOTE: Normal release procedure may also be initiated.
Figure 6.1.5.2.1: IMS-ALG Restoration
After the recovery action is complete and it is possible to signal to the IMS-AGW the IMS-ALG starts a timer Tw. If recovery indications are not received (IMS-AGW Communication Up or IMS-AGW Restoration) from the IMS-AGW during Tw an Audit is sent. If the IMS-ALG receives a recovery indication or IMS-AGW communication up indication, it shall acknowledge the indication before the IMS-ALG Restoration may be sent or the release procedure is initiated.
6.1.6 IMS-AGW Re-register
When the IMS-ALG requests an IMS-AGW to perform a registration (see clauses 6.1.3, 6.1.4 and 6.1.7), the IMS-AGW performs a re-registration to the IMS-ALG which is defined in the IMS-ALG address.
Figure 6.1.6.1: Re-registration of an IMS-AGW
6.1.7 IMS-AGW Re-registration Ordered by IMS-ALG
If the IMS-ALG knows that communication is possible, but the IMS-AGW has not registered, the IMS-ALG can order re-registration of the IMS-AGW.
Figure 6.1.7.1: Re-registration ordered by the IMS-ALG
If the re-registration request is accepted the IMS-AGW uses the IMS-AGW Re-register procedure to register with the IMS-ALG.
6.1.8 Audit of IMS-AGW
6.1.8.1 Audit of Value
The IMS-ALG may request the IMS-AGW to report the current values assigned to distinct objects in the IMS-AGW. This procedure may be used when a change has occurred in the IMS-ALG such that the IMS-ALG is unsure of the current Service State of Terminations.
Figure 6.1.8.1.1: Audit Value
6.1.8.2 Audit of Capability
The IMS-ALG may request the IMS-AGW to report the capabilities of distinct objects in the MGW.
Figure 6.1.8.2.1: Audit Capability
6.1.9 IMS-AGW Capability Change
The IMS-AGW reports a change of capability of distinct objects in the MGW.
Figure 6.1.9.1: Capability Update
The IMS-ALG may use the Audit Value and/or Audit Capability procedures to obtain further information, about the objects whose capabilities have changed.
6.1.10 IMS-ALG Out of service
Figure 6.1.10.1: IMS-ALG Out of Service
If an IMS-ALG discovers that it wants to go out of service it may start an IMS-ALG Out of Service procedure. The IMS-ALG can indicate whether it requires the context to be cleared immediately (forced) or cleared as the bearer control protocol clears the bearer (Graceful).
6.1.11 IMS-AGW Resource Congestion Handling – Activate
When the IMS-ALG requires that an IMS-AGW congestion notification mechanism be applied in the MGW, the IMS-ALG shall use the IMS-AGW Resource Congestion Handling – Activate procedure towards the IMS-AGW.
Figure 6.1.11.1: IMS-AGW Resource Congestion Handling – Activate
6.1.12 MGW Resource Congestion Handling -Indication
When the IMS-ALG receives a load reduction notification from the IMS-AGW via the IMS-AGW Resource Congestion Handling – Indication procedure, the IMS-ALG tries to reduce the processing load that the IMS-ALG creates on the IMS-AGW. The IMS-AGW shall decide the actual level of traffic reduction.
Figure 6.1.12.1: IMS-AGW Resource Congestion Handling – Indication
6.1.13 Control association monitoring
Monitoring of the H.248 control association may be performed by monitoring the status of the transport link association where the transport protocol provides sufficient coupling to the H.248.1 protocol, i.e. if the transport link association is disconnected when no local H.248.1 protocol connection exists.
An alternative method for the IMS-AGW to detect loss of the IMS-ALG may be achieved by requesting the IMS-AGW to poll the IMS-ALG periodically
Upon registration of an IMS-AGW, the IMS-ALG may use the Inactivity Timeout – Activate procedure towards the IMS-AGW to request the IMS-AGW to monitor incoming messages for periods of silence exceeding the maximum inactivity timer value.
Figure 6.1.13.1: Inactivity Timeout – Activate
Upon receipt of an inactivity timeout notification from the IMS-AGW via the Inactivity Timeout – Indication procedure, the IMS-ALG shall send a reply to the IMS-AGW. If the IMS-ALG has failed, the IMS-AGW will not receive a reply.
Figure 6.1.13.2: Inactivity Timeout – Indication
If no Inactivity Timeout – Indication Ack reply is received, the IMS-AGW shall consider the IMS-ALG to have failed. The IMS-AGW may then attempt to re-contact its controlling IMS-ALG by performing IMS-AGW Communication Up. If not successful, the IMS-AGW may then attempt to register to a new IMS-ALG.
6.1.14 Realm Availability Monitoring
If the IMS-AGW supports IP Realm Availability monitoring, the IMS-ALG may request the monitoring of the available IP Realms by the IMS-AGW; the IMS-AGW shall inform the IMS-ALG of any changes in realm availability.
NOTE: The IMS-ALG can use the AuditValue procedure to determine which IP realms are currently available.
The IMS-ALG may use the Realm Availability – Activate procedure towards the IMS-AGW to request the IMS-AGW to monitor the status of its IP Realms.
Figure 6.1.14.1: Realm Availability – Activate
The IMS-AGW shall inform the IMS-ALG via the Realm Availability – Notification procedure.
Figure 6.1.14.2: Realm Availability – Indication
On being informed of newly available/unavailable realms, IMS-ALG shall take appropriate action (e.g. update its list of available realms etc.).
6.1.15 Failure of IP Port, Interface or Group of Interfaces
This procedure only applies when text encoding is used on the H.248 interface.
The IMS-ALG shall and the IMS-AGW may support the Termination Out-of-Service procedure.
If the IMS AGW suffers a loss of physical IP device(s) that pertain to a whole IP Realm it may report the IP Realm as unavailable (see clause 6.1.14). However, it is possible that a failure affects a physical port or group of ports that forms a subset of the IP Realm and therefore many terminations are affected. In such cases the IMS-AGW may initiate a Termination Out of Service procedure to inform the IMS-ALG that the set of terminations is out of service. This is shown in Figure 6.1.15.1.
Figure 6.1.15.1: Termination Out of Service
On receipt of the Termination Out Of Service the IMS-ALG shall initiate the appropriate actions, e.g. by subtracting the affected terminations and releasing the affected calls.
NOTE: This procedure provides an alternative failure reporting to the IP Bearer Released procedure (which allows reporting the failure of one IP Bearer / termination). The Termination Out-of-Service procedure avoids sending an avalanche of notifications when the failure affects multiple ephemeral terminations.