10 General (G)MSC server-MGW Procedures

23.2053GPPBearer-independent circuit-switched core networkRelease 17Stage 2TS

10.1 MGW Unavailable

The (G)MSC server recognises that the MGW is unavailable in the following 4 cases:

1. The signalling connection is unavailable

Figure 10.1: Signalling connection failure

2. The MGW indicates the failure condition to all connected (G)MSC servers

Figure 10.2: MGW indicates the Failure/Maintenance locking

The failure indication indicates that the MGW will soon go out of service and that no new connections should be established using this MGW. The MGW 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 (G)MSC server recognises that the MGW is not functioning correctly, e.g. because there is no reply on periodic sending of Audits. The periodic sending of Audits by (G)MSC server should go on.

4. The MGW indicates the maintenance locking condition to all concerned (G)MSC servers.

The maintenance locking indication indicates that the MGW is locked for new calls and that no new connections shall be established using this MGW. The MGW 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

In all of the above cases the (G)MSC server shall prevent the usage of the MGW until the MGW has recovered or the communication with the MGW is restored. The (G)MSC server shall prohibit the surrounding network from seizing circuits connected to the unavailable TDM access by sending blocking messages.

10.2 MGW Available

The (G)MSC server discovers that the MGW is available when it receives an MGW Communication Up message or an MGW Restoration message. If the (G)MSC does not wish to sustain an association with the MGW, the response sent to the MGW may indicate an alternative MGCId signalling address, in which case the MGW shall not consider itself registered and should preferably attempt to re-register with this alternative MGC before any further alternate MGCs. Otherwise, the response shall not indicate any alternative MGCId signalling address.

When the (G)MSC server discovers that the MGW is available the following shall occur:

1. Signalling recovery

The MGW indicates to all connected (G)MSC servers that the signalling connection is restored. Any changes to the physical termination state during the loss of communication shall be reported by the MGW using Termination Out Of Service (10.7) or Termination Restoration (10.8), otherwise the (G)MSC Server can assume that Terminations service state has not changed. To avoid unsuccessful calls for physical terminations which went out of service during the loss of communication but have not yet been reported by the MGW the MSC Server may Audit the physical terminations before it uses them.

NOTE: Auditing in this case will cause duplicate signalling.

Figure 10.3: Communication goes up

2. MGW restoration/maintenance unlocking indication.

The MGW indicates to all connected (G)MSC servers that normal operation has resumed. Changes of the physical termination state during the connection break shall be reported by the MGW using Termination Out Of Service (10.7) and Termination restoration (10.8) procedures, otherwise the (G)MSC Server can assume that Terminations service state has not changed. To avoid unsuccessful calls for physical terminations which went out of service during the loss of communication but have not yet been reported by the MGW the (G)MSC Server may Audit the physical terminations before it uses them.

NOTE: Auditing in this case will cause duplicate signalling.

Figure 10.4: MGW indicates recovery from a failure/or maintenance unlocking

NOTE: This procedure may be used after recovery from a signalling failure.

3. The (G)MSC server recognises that the MGW is now functioning correctly, e.g. because there is a reply on periodic sending of Audits.

After this the (G)MSC server can use the MGW. If the corresponding devices of the surrounding network are blocked, unblocked messages shall be sent to the nodes concerned for devices that are in service: the (G)MSC Server has determined the actual state of the physical devices either by receiving Termination Restoration procedure (10.8) or by performing an Audit..

If none of 1,2, and 3 happens the (G)MSC server can initiate the (G)MSC Server Ordered Re-register procedure.

10.3 MGW Recovery

If the MGW recovers from a failure, is maintenance unlocked, or it has been restarted, it registers to its known (G)MSC servers using the MGW Restoration procedure or the MGW Registration procedure. The MGW can indicate whether the Service has been restored or whether it has restarted with a cold or warm boot. If the (G)MSC does not wish to sustain an association with the MGW, the response sent to the MGW may indicate an alternative MGCId signalling addres, in which case the MGW shall not consider itself registered and should preferably attempt to re-register with this alternative MGC before any further alternate MGCs. Otherwise, the response shall not indicate any alternative MGCId signalling address.

Figure 10.5/1: MGW Restoration

Figure 10.5/2 MGW Registration

After the recovery the (G)MSC server can use the MGW. After a MGW warm boot or service restored, the (G)MSC Server can assume the physical terminations state has not changed. Changes of the physical termination state during the connection break shall be reported by the MGW using Termination Out Of Service (10.7) and Termination restoration (10.8) procedures. To avoid unsuccessful calls for physical terminations which went out of service during the loss of communication but have not yet been reported by the MGW the MSC Server may Audit the physical terminations before it uses them.

NOTE: Auditing in this case will cause duplicate signalling.

After receiving a MGW Register (with cold boot) the (G)MSC Server shall audit the termination state of all physical terminations (10.9) if it has not been specifically informed of the actual service state since receiving the MGW service change on ROOT (Register) via independent service change commands (10.7 or 10.8). The (G)MSC Server shall thereby determine the actual Service State of devices. If the corresponding devices of the surrounding network are blocked, unblocked messages are sent to the nodes concerned for all devices that are in service.

10.4 (G)MSC server Recovery

10.4.1 General

If an MGW-unavailable condition is provoked by a failure/recovery action, the (G)MSC server recovery sequence will, from an information flow point of view, look like MGW unavailable and then MGW available. If an MGW-unavailable condition is not provoked, the (G)MSC server recovery sequence will look like MGW available.

After the information flow, the terminations affected by the recovery action are released.

10.4.2 (G)MSC Server Restoration

Figure 10.6: (G)MSC Server Restoration

NOTE: Normal release procedure may also be initiated.

After the recovery action is complete and it is possible to signal to the MGW the (G)MSC server starts a timer Tw. If recovery indications are not received (MGW Communication Up or MGW Restoration) from the MGW during Tw an Audit is sent. If the (G)MSC server receives a recovery indication or MGW communication up indication, it shall acknowledge the indication before the (G)MSC Server Restoration may be sent or the release procedure is initiated.

10.5 MGW Re-register

When the (G)MSC requests an MGW to perform a registration (see clause 10.2, 10.3 and 10.6), the MGW performs a re-registration to the (G)MSC which is defined in the (G)MSC address. If the (G)MSC Server is uncertain of the termination service state of physical terminations after the Re-Register it shall use the Audit Procedure (10.9). Changes of the physical termination state during the connection break shall be reported by the MGW using Termination Out Of Service (10.7) and Termination restoration (10.8) procedures.

Figure 10.7: Re-registration of an MGW

10.6 MGW Re-registration Ordered by (G)MSC server

If the (G)MSC server knows that communication is possible, but the MGW has not registered, the (G)MSC server can order re-registration of the MGW.

Figure 10.8: Re-registration ordered by the (G)MSC server

If the re-registration request is accepted the MGW uses the MGW Re-register procedure to register with the (G)MSC server.

10.7 Removal from Service of a Physical Termination

The MGW indicates the removal from service of a physical termination using the Termination Out-of-Service procedure. In this procedure the MGW indicates which termination is to be removed from service and whether the "graceful" or "forced" method will be used. In the graceful method a possible connection is cleared when the corresponding call is disconnected. In the forced method the possible connection is cleared immediately.

Figure 10.9: Removal from service of a Physical Termination

The (G)MSC server shall prevent the use of the Termination(s) concerned until the physical termination is restored to service.

10.8 Restoration to Service of a Physical Termination

If the physical termination is restored to service, the MGW shall report it to the (G)MSC server(s) using the Termination Restoration procedure.

Figure 10.10: Restoration to service of a Physical Termination

The (G)MSC server can use the physical termination when the termination has been restored to service. If the corresponding devices of the surrounding network are blocked, the (G)MSC server sends an unblocked message to each node concerned.

10.9 Audit of MGW

10.9.1 Audit of Value

The (G)MSC server may request the MGW to report the current values assigned to distinct objects in the MGW. Objects, which can be addressed, are listed in 3GPP TS 29.232 [6]. This procedure shall be used when a change has occurred in the (G)MSC server such that the server is unsure of the current Service State of Terminations, such as restart or new trunks configured in the server. For any situation where a change to the Service State occurs in the MGW the (G)MSC server shall expect the Service State to be reported by Termination Restoration (10.8) or Termination Out Of Service (10.7).

Figure 10.11: Audit Value

10.9.2 Audit of Capability

The (G)MSC server may request the MGW to report the capabilities of distinct objects in the MGW. Objects, which can be addressed, are listed in 3GPP TS 29.232 [6].

Figure 10.12: Audit Capability

10.10 MGW Capability Change

The MGW reports a change of capability of distinct objects in the MGW. Objects, which can be addressed, are listed in 3GPP TS 29.232 [6].

Figure 10.13: Capability Update

The (G)MSC server can use the Audit Value and/or Audit Capability procedures to obtain further information, about the objects whose capabilities have changed.

10.11 Void

10.12 (G)MSC Server Out of service

Figure 10.14: (G)MSC Server Out of Service

If a (G)MSC server discovers that it wants to go out of service it starts a (G)MSC Server Out of Service procedure. The (G)MSC server can indicate whether it requires the context to be cleared immediately (forced) or cleared as the bearer control protocol clears the bearer (Graceful). Physical terminations are always cleared when the (G)MSC Server Out of Service indication reaches the MGW.

10.13 MGW Resource Congestion Handling – Activate

When the (G)MSC server requires that a MGW congestion notification mechanism be applied in the MGW, the (G)MSC server shall use the MGW Resource Congestion Handling – Activate procedure towards the MGW.

Figure 10.15: MGW Resource Congestion Handling – Activate

10.14 MGW Resource Congestion Handling -Indication

When the (G)MSC server receives a load reduction notification from the MGW via the MGW Resource Congestion Handling – Indication procedure, the (G)MSC server tries to reduce the processing load that the (G)MSC server creates on the MGW. The MGW shall decide the actual level of traffic reduction.

Figure 10-17: MGW Resource Congestion Handling – Indication

10.15 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 MGW to detect loss of the MGC may be achieved by requesting the MGW to poll the (G)MSC periodically

Upon registration of a MGW, the (G)MSC server may use the Inactivity Timeout – Activate procedure towards the MGW to request the MGW to monitor incoming messages for periods of silence exceeding the maximum inactivity timer value.

Figure 10.15.1: Inactivity Timeout – Activate

Upon receipt of an inactivity timeout notification from the MGW via the Inactivity Timeout -Indication procedure, the (G)MSC server shall send a reply to the MGW. If the MGC has failed, the MGW will not receive a reply.

Figure 10.15.2: Inactivity Timeout – Indication

If no Inactivity Timeout – Indication Ack reply is received, the MGW shall consider the (G)MSC to have failed. The MGW may then attempt to recontact its controlling (G)MSC by performing MGW Communication UP. If not successful, the MGW may then attempt to register to a new (G)MSC.

10.16 Hanging termination detection

Whenever requesting new ephemeral bearer terminations, the (G)MSC shall request the MGW to periodically report termination heartbeat indications to detect hanging context and termination in the MGW that may result e.g. from a loss of communication between the MSC-S and the MGW. This may also be done when requesting TDM terminations, though alternative means exist for the (G)MSC to detect hanging TDM terminations, e.g. via receipt of the error #433 when seizing the termination.

When the (G)MSC server receives a termination heartbeat notification from the MGW via the Termination heartbeat – Indication procedure, the (G)MSC shall return a Termination heartbeat –Indication Ack (without an error) if the context id / termination identity combination exists in the (G)MSC. If it does not exist, the (G)MSC shall return an error and shall correct the mismatch, e.g. by requesting the MGW to subtract the indicated termination and to clear any associated context.

Figure 10.16.1: Termination heartbeat – Indication