TECHNICAL FIELD

[ 0001 ] The present invention generally relates to Application Server restoration in an IP Multimedia Subsystem; and, more specifically, the invention relates to achieving call continuity after an Application Server failover and fall-back.

BACKGROUND

[ 0002 ] 3 GPP TS 23.380 V13.4.0 discloses the IP Multimedia Subsystem (IMS) Restoration Procedures and, in particular, section 6 discloses the IMS Restoration Procedures for service interruption of a Service Call Continuity (SCC) Application Server (AS), hereinafter SCC-AS .

[ 0003] Basically, this document discloses that when a registered SCC-AS, i.e. SCC- ASl where a user equipment (UE) is registered, fails or becomes unreachable, a new available SCC-AS, i.e. SCC-AS2, may be selected by the Serving Call Session Control Function (S-CSCF) currently serving the UE. This document also discloses how required data is recovered by the SCC-AS2 from the HSS to be able to continue processing the call.

[ 0004 ] In some cases, however, there is a preference to recover traffic via a specific SCC-AS; for example, a SCC-AS may be defined as primary and the rest as secondary, or a prioritized list of selectable SCC-Ass could be defined. In this situation, if the preferred SCC-AS, e.g. SCC-ASl, fails a new available SCC-AS, e.g. SCC-AS2, is selected as described above but, as soon as the SCC-ASl is up and running again, the preferred SCC-ASl should be reselected again. 3 GPP TS 23.380 V13.4.0 does not disclose how to recover the traffic via the preferred SCC-AS 1.

[ 0005] For example, when a Single Radio Voice Call Continuity (SRVCC) Handover takes place for a UE-A originated call (as well as for a UE-A terminated call), a number of exemplary cases are commented in the following with different results depending on the availability status of the preferred SCC-AS. [ 0006] A first exemplary case is where the preferred SCC-AS, i.e. SCC-AS1, is reachable during the SRVCC Handover, hereinafter SRVCC-HO, as illustrated in Fig. 1. In this case, once the UE-A is registered via Access Transfer Control Function (ATCF), S-CSCF and SCC-AS 1, the SCC-AS 1 transmits toward the S-CSCF, and the latter toward the ATCF, access transfer information comprising the Access Transfer Update - Session Transfer Identifier (ATU-STI) of the SCC-AS 1, i.e. the ATU-STIl, and the C- MSISDN of the UE-A. The ATCF, or both S-CSCF and ATCF, stores the received ATU-STIl .

[ 0007 ] In this situation, SRVCC-HOs for UE-A originated calls are successful because the ATU-STI stored at the ATCF, in this case the ATU-STIl, identifies the SCC-AS 1 and is used during the SRVCC-HO. A similar behaviour can be observed during SRVCC-HOs for UE-A terminated calls.

[ 0008 ] A second exemplary case is where the preferred SCC-AS, i.e. SCC-AS 1, is unreachable during registration of the UE-A, as illustrated in Fig. 2. In this case, the S- CSCF selects a new available SCC-AS, i.e. SCC-AS2, and once the UE-A is registered via ATCF, S-CSCF and SCC-AS2, the SCC-AS2 transmits toward the S-CSCF, and the latter toward the ATCF, access transfer information comprising the ATU-STI of the SCC-AS2, i.e. the ATU-STI2, and the C-MSISDN of the UE-A. The ATCF, or both S- CSCF and ATCF, stores the received ATU-STI2. That is, the S-CSCF starts using SCC- AS2 for UE-A originating and terminating calls and the ATCF uses the ATU-STI2.

[ 0009] In this situation, SRVCC-HOs for UE-A originated calls are also successful because the ATU-STI stored at the ATCF, in this case the ATU-STI2, identifies the SCC-AS2 and is used during the SRVCC-HO. A similar behaviour can be observed during SRVCC-HOs for UE-A terminated calls. [ 0010 ] A third exemplary case, which aims the present invention, is where the preferred SCC-AS, i.e. SCC-AS 1, becomes unreachable during registration of the UE-A, as illustrated in Fig. 3. In this case, the S-CSCF selects a new available SCC-AS, i.e. SCC-AS2, and once the UE-A is registered via ATCF, S-CSCF and SCC-AS2, the SCC- AS2 transmits toward the S-CSCF, and the latter toward the ATCF, access transfer information comprising the ATU-STI of the SCC-AS2, i.e. the ATU-STI2, and the C- MSISDN of the UE-A. The ATCF, or both S-CSCF and ATCF, stores the received ATU-STI2. That is, the S-CSCF starts using SCC-AS2 for UE-A originating and terminating calls and the ATCF uses the ATU-STI2.

[ 0011 ] At this stage, the SCC-AS 1 becomes reachable again and, if there is a preference to use the SCC-AS 1, the S-CSCF will select the SCC-AS 1 for the next initial call setup. Upon arrival, an initial call setup message, e.g. a Session Initiation Protocol (SIP) Invite message, is progressed from the S-CSCF toward the SCC-AS 1 and, from the SCC-AS 1 , the initial call setup message may be progressed toward a terminating UE, if this is the case. A corresponding acknowledge, e.g. a SIP 200 OK, is returned back through the entities involved. [ 0012 ] In this situation, SRVCC-HOs for UE-A originated calls are unsuccessful because the ATU-STI currently stored at the ATCF is in this case the ATU-STI2, which identifies the SCC-AS2 and is used during the SRVCC-HO. The ATCF then transmits toward the SCC-A2 an SRVCC-HO setup message such as e.g. SIP Invite message and, since the SCC-AS2 is not aware of the initial call setup message that the S-CSCF transmitted to the SCC-AS 1, the SCC-AS2 returns back to the ATCF an unsuccessful result, e.g. SIP error 400, and the SRVCC-HO fails.

SUMMARY

[ 0013] The present invention is aimed to at least minimize the above drawbacks and provides for a new method of handling IMS Restoration Procedures for service interruption of an SCC-AS, as well as network nodes cooperating to carry out such method.

[ 0014 ] In particular, this method is useful to facilitate a SRVCC-HO for UE originated and terminating calls.

[ 0015] In accordance with a first aspect of the present invention, there is provided a new method of handling IMS Restoration Procedures for service interruption of an SCC- AS, the method executed at a network node implementing the ATCF, i.e. ATCF node.

[ 0016] This method comprises receiving, at the ATCF node from a selected SCC- AS via a S-CSCF node, access transfer information with an SCC-AS identifier, which identifies the selected SCC-AS, and a UE identifier, which identifies a UE, storing at the ATCF node the received SCC-AS identifier, receiving at the ATCF node an initial call setup message, and transmitting, from the ATCF node toward the selected SCC-AS via the S-CSCF node, the received initial call setup message along with the stored SCC-AS identifier [0017] In an embodiment, the SCC-AS identifier may correspond to an Access Transfer Update - Session Transfer Identifier (ATU-STI) of the SCC-AS. In an embodiment, the UE identifier may correspond to a Correlation Mobile Station ISDN number, C-MSISDN. In an embodiment, the initial call setup message may correspond to a Session Initiation Protocol, SIP, Invite message. In an embodiment, this method may further comprise storing at the ATCF node the received UE identifier and, where this is the case, the received initial call setup message may be transmitted along with the stored UE identifier, which identifies the UE.

[0018] In accordance with a second aspect of the present invention, there is provided a new method of handling IMS Restoration Procedures for service interruption of an SCC-AS, the method executed at a network node implementing the S-CSCF, i.e. S-CSCF node.

[0019] This method comprises receiving, at the S-CSCF node from a selected SCC- AS, an access transfer information that comprises an SCC-AS identifier, which identifies the selected SCC-AS, and a UE identifier, which identifies a UE; transmitting, from the S-CSCF node toward an ATCF node, the received access transfer information with the received SCC-AS identifier and the received UE identifier; receiving, at the S-CSCF node from an ATCF node, an initial call setup message along with an access transfer information that comprises an SCC-AS identifier and a UE identifier; and transmitting, from the S-CSCF node toward the selected SCC-AS, the received initial call setup message with the received SCC-AS identifier.

[0020 ] In an embodiment, the SCC-AS identifier may correspond to an ATU-STI of the SCC-AS. In an embodiment, the UE identifier may correspond to a C-MSISDN. In an embodiment, the initial call setup message may correspond to a SIP Invite message. In an embodiment, the received initial call setup message may be transmitted along with the received UE identifier. [ 0021 ] In an embodiment, receiving the access transfer information may be responsive to, and this method may further comprise receiving, at the S-CSCF node, a SIP Register message originated from a UE; selecting, at the S-CSCF node, an available SCC-AS; and transmitting, from the S-CSCF node toward the selected SCC-AS, the SIP Register message.

[ 0022 ] In accordance with a third aspect of the present invention, there is provided a new method of handling IMS Restoration Procedures for service interruption of an SCC- AS, the method executed at a network node implementing the SCC-AS, i.e. SCC-AS node. [ 0023] This method comprises receiving, at the SCC-AS node from an S-CSCF node, an initial call setup message along with an access transfer information that comprises an SCC-AS identifier and a UE identifier; determining, at the SCC-AS node, a mismatch between the received SCC-AS identifier and its own SCC-AS identifier; and transmitting, from the SCC-AS node toward the S-CSCF node, a new access transfer information that comprises the own SCC-AS identifier of the SCC-AS node.

[ 0024 ] In an embodiment, the SCC-AS identifier may correspond to an ATU-STI of the SCC-AS node. In an embodiment, the UE identifier may correspond to a C- MSISDN. In an embodiment, the initial call setup message may correspond to a SIP Invite message. In an embodiment, the new access transfer information may also comprise the received UE identifier.

[ 0025] Participating in the above methods and exemplary embodiments there are provided an ATCF node, an S-CSCF node and an SCC-AS node.

[ 0026] In accordance with a fourth aspect of the present invention, there is provided a new ATCF node participating in IMS Restoration Procedures for service interruption of a SCC-AS.

[ 0027 ] This ATCF node is configured to receive, from a selected SCC-AS via an S- CSCF node and via a receiver of the ATCF node, access transfer information with an SCC-AS identifier, which identifies the selected SCC-AS, and a UE identifier, which identifies a UE; the ATCF node is configured to store a plurality of SCC-AS identifiers identifying a respective plurality of SCC-ASs; and the ATCF node is configured to receive, via the receiver, an initial call setup message, and transmit, toward the selected SCC-AS via a transmitter of the ATCF node and via the S-CSCF node, the received initial call setup message along with the stored SCC-AS identifier, which identifies the selected SCC-AS. [ 0028 ] In an embodiment, the SCC-AS identifier may correspond to an ATU-STI of the SCC-AS. In an embodiment, the UE identifier may correspond to a C-MSISDN. In an embodiment, the initial call setup message may correspond to a SIP Invite message. In an embodiment, the ATCF node may be configured to store a plurality of UE identifiers respectively associated with the plurality of SCC-AS identifiers and, where this is the case, the received initial call setup message may be transmitted along with the stored UE identifier, which identifies the UE.

[ 0029] In accordance with an embodiment, the ATCF node may comprise at least one processor, and at least one memory that stores processor-executable instructions. In this ATCF node, the at least one processor interfaces with the at least one memory to execute the processor-executable instructions, so that the ATCF node is operable to perform the actions disclosed above.

[ 0030 ] In accordance with a fifth aspect of the present invention, there is provided a new S-CSCF node participating in IMS Restoration Procedures for service interruption of a SCC-AS. [ 0031 ] This S-CSCF node is configured to receive, from a selected SCC-AS node via a receiver of the S-CSCF node, an access transfer information that comprises an SCC-AS identifier, which identifies the selected SCC-AS, and a UE identifier, which identifies a UE, and transmit, toward an ATCF node via a transmitter of the S-CSCF node, the received access transfer information with the received SCC-AS identifier and the received UE identifier.

[ 0032 ] This S-CSCF node is also configured to receive, from the ATCF node via the receiver, an initial call setup message along with an access transfer information that comprises an SCC-AS identifier and a UE identifier, and transmit, toward the selected SCC-AS via the transmitter, the received initial call setup message with the received SCC-AS identifier. [0033] In an embodiment, the SCC-AS identifier may correspond to an ATU-STI of the SCC-AS. In an embodiment, the UE identifier may correspond to a C-MSISDN. In an embodiment, the initial call setup message may correspond to a SIP Invite message. In an embodiment, the S-CSCF node may be configured to store a plurality of UE identifiers respectively associated with the plurality of SCC-AS identifiers and, where this is the case, the received initial call setup message may be transmitted along with the stored UE identifier.

[0034] In an embodiment, receiving the access transfer information may be responsive to, and this S-CSCF node may further be configured to receive, via the receiver of the S-CSCF node, a SIP Register message originated from a UE; select an available SCC-AS; and transmit, toward the selected SCC-AS via the transmitter of the S-CSCF node, the SIP Register message.

[0035] In accordance with an embodiment, the S-CSCF node may comprise at least one processor, and at least one memory that stores processor-executable instructions. In this S-CSCF node, the at least one processor interfaces with the at least one memory to execute the processor-executable instructions, so that the S-CSCF node is operable to perform the actions disclosed above.

[0036] In accordance with a sixth aspect of the present invention, there is provided a new SCC-AS node participating in IMS Restoration Procedures for service interruption of a SCC-AS.

[0037] This SCC-AS node is configured to receive, from an S-CSCF node via a receiver, an initial call setup message along with an access transfer information that comprises an SCC-AS identifier and a UE identifier; this SCC-AS node is also configured to determine a mismatch between the received SCC-AS identifier and its own SCC-AS identifier, and transmit, toward the S-CSCF node via a transmitter, a new access transfer information that comprises the own SCC-AS identifier of the SCC-AS node.

[0038] In an embodiment, the SCC-AS node may be configured to receive, from an S-CSCF node via the receiver, a SIP Register message; and transmit, toward the S- CSCF node via the transmitter, an access transfer information comprising an own SCC- AS identifier, which identifies the SCC-AS node, and a UE identifier, which identifies a UE.

[ 0039] In an embodiment, the SCC-AS identifier may correspond to an ATU-STI of the SCC-AS node. In an embodiment, the UE identifier may correspond to a C- MSISDN. In an embodiment, the initial call setup message may correspond to a SIP Invite message. In an embodiment, the new access transfer information may also comprise the received UE identifier. In an embodiment, the SCC-AS node may be configured to store registration and/or call related data, e.g. C-MSISDN, for each UE served by the SCC-AS node. [ 0040 ] In accordance with an embodiment, the SCC-AS node may comprise at least one processor, and at least one memory that stores processor-executable instructions. In this SCC-AS node, the at least one processor interfaces with the at least one memory to execute the processor-executable instructions, so that the SCC-AS node is operable to perform the actions disclosed above. [ 0041 ] On the other hand, the invention may be practised by one or more computer programs, in accordance with a seventh aspect of the invention, each computer program being loadable into an internal memory of a computer with a processor, and comprising executable code adapted to carry out the above method steps. In particular, this executable code may be recorded in a carrier readable in the computer. [ 0042 ] As used throughout the present specification, the word "comprising" does not exclude the presence of other elements or steps than those listed and the words "a" or "an" preceding an element do not exclude the presence of a plurality of such elements. Further, any reference signs do not limit the scope of the claims, the invention may be at least in part implemented by means of both hardware and software, and several "means" or "units" may be represented by the same item of hardware.

BRIEF DESCRIPTION OF THE DRAWINGS

[ 0043] The features, objects and advantages of the invention will become apparent by reading this description in conjunction with the accompanying drawings, in which: [ 0044 ] FIG. 1 represents a basic sequence of actions to perform an SRVCC-HO for UE-A originating calls, where the preferred SCC-AS is reachable, in accordance with an exemplary first embodiment.

[ 0045] FIG. 2 represents a basic sequence of actions to perform an SRVCC-HO for UE-A originating calls, where the preferred SCC-AS is not reachable, in accordance with an exemplary second embodiment.

[ 0046] FIG. 3 represents a basic sequence of actions to perform an SRVCC-HO for UE-A originating calls, where the preferred SCC-AS becomes reachable again, in accordance with an exemplary third embodiment. [ 0047 ] FIG. 4 represents a basic sequence of actions to perform an SRVCC-HO for UE-A originating calls, where the preferred SCC-AS becomes reachable again, in accordance with an embodiment of the invention.

[ 0048 ] FIG. 5 shows a basic component structure of an ATCF node in accordance with an embodiment. [ 0049] FIG. 6 shows a basic component structure of an S-CSCF node in accordance with an embodiment.

[ 0050 ] FIG. 7 shows a basic component structure of an SCC-AS node in accordance with an embodiment.

[ 0051 ] FIG. 8 illustrates an exemplary sequence of actions carried out by the ATCF node in accordance with one or more embodiments.

[ 0052 ] FIG. 9 illustrates an exemplary sequence of actions carried out by the S- CSCF node in accordance with one or more embodiments.

[ 0053] FIG. 10 illustrates an exemplary sequence of actions carried out by the SCC- AS node in accordance with one or more embodiments. [ 0054 ] FIG. 11 shows a basic component structure of an ATCF node in accordance with another embodiment. [ 0055] FIG. 12 shows a basic component structure of an S-CSCF node accordance with another embodiment.

[ 0056] FIG. 13 shows a basic component structure of an SCC-AS node accordance with another embodiment. DETAILED DESCRIPTION

[ 0057 ] The following describes currently preferred embodiments for a method of handling IMS Restoration Procedures for service interruption of an SCC-AS, as well as embodiments for the network nodes cooperating to carry out such method.

[ 0058 ] In particular, embodiments of this method are useful to facilitate SRVCC- HOs for UE-A originated and terminating calls, and to ensure that an identifier of the SCC-AS, like e.g. the ATU-STI, stored at the ATCF is always correct, i.e. it always identifies the SCC-AS used during a call setup. This allows that even though the S-CSCF may select a new SCC-AS, e.g. a preferred SCC-AS recovered from a failure, the call will be properly routed towards such new SCC-AS.

[ 0059] Fig. 4 illustrates a sequence of actions that may be followed in accordance with embodiments of the invention. In this scenario, the preferred SCC-AS, i.e. SCC- AS 1, becomes unreachable during registration of the UE-A, as also illustrated in Fig. 3 and commented above in the background section. In this exemplary case illustrated in Fig. 4, the S-CSCF 3 receives during step S-405 a SIP Register message originated from a UE-A 1.

[ 0060 ] The S-CSCF 3 may transmit the SIP Register message toward a preferred SCC-AS, i.e. SCC-AS 1 4, during step S-410 and determine that the preferred SCC-AS 1 is unreachable. Then, the S-CSCF selects a new available SCC-AS, i.e. SCC-AS2 5, and transmits during step S-415 the SIP Register message toward the selected SCC-AS2. The registration is completed by transmitting, from the SCC-AS2 toward the UE-A 1 though network nodes, a 200 OK message during step S-420.

[ 0061 ] Once the UE-A 1 is registered via ATCF 2, S-CSCF 3 and SCC-AS2 5, the SCC-AS2 5 transmits during step S-425 toward the S-CSCF 3, and the latter transmits during step S-430 toward the ATCF 2, access transfer information comprising the ATU- STI of the SCC-AS2, i.e. the ATU-STI2, and the C-MSISDN of the UE-A. The ATCF 2, or both S-CSCF 3 and ATCF 2, stores the received ATU-STI2. That is, the S-CSCF starts using SCC-AS2 for UE-A originating and terminating calls and the ATCF uses the ATU-STI2, as for the background third case commented above with reference to Fig. 3. [ 0062 ] At a certain point in time, the SCC-ASl becomes reachable and running again, so that the S-CSCF 3 can start using the SCC-ASl 4 for a next initial call setup.

[ 0063] In this embodiment illustrated in Fig. 4, the initial call setup message, e.g. SIP Invite message, is received at the ATCF 2 during step S-440 and transmitted during step S-445 from the ATCF 2 toward the S-CSCF 3 along with the access transfer information comprising the ATU-STI2 of the SCC-AS2 5, which was stored at the ATCF 2, and the C-MSISDN.

[ 0064 ] The initial call setup message, received from the ATCF with the ATU-STI2 of the SCC-AS2 5 and the C-MSISDN, is progressed during step S-450 from the S- CSCF 3 toward the SCC-ASl 4 and, from the SCC-ASl, the initial call setup message may be progressed during step S-455 toward a terminating UE-B, if this is the case. A corresponding acknowledge, e.g. a SIP 200 OK, is returned back through the entities involved during step S-460.

[ 0065] Apart from that, as Fig. 4 illustrates, the SCC-ASl 4 determines during step S-465 a mismatch between the received ATU-STI2 and its own ATU-STI, which in this exemplary embodiment is ATU-STIl . Then, the SCC-ASl 4 transmits during step S-470 toward the S-CSCF 3, and the latter transmits during step S-475 toward the ATCF 2, a new access transfer information now comprising the ATU-STI of the SCC-ASl 4, i.e. the ATU-STIl, and the C-MSISDN. The ATCF 2, or both ATCF 2 and S-CSCF 3, stores the received ATU-STIl and C-MSISDN. [ 0066] In this situation, when an SRVCC-HO setup message is received during step S-485 at the ATCF 2, e.g. a SIP Invite message for UE-A originated calls, the ATCF uses the stored ATU-STI, now the ATU-STIl identifying the SCC-ASl, and transmits during step S-490 toward the SCC-ASl 4 the SRVCC-HO setup message, e.g. the SIP Invite message. [ 0067 ] Now, in contrast with the embodiment illustrated in Fig. 3 and discussed above, the SCC-AS receiving the SRVCC-HO setup message, i.e. SCC-AS 1, is the same as the one that received the initial call setup message from the S-CSCF 3.

[ 0068 ] Then, as Fig. 4 illustrates, apart from communicating during steps S-495 and S-496 with a terminating UE-B, if this is the case, a corresponding successful acknowledge, i.e. a SIP 200 OK, is returned back during step S-497, from the SCC-AS 1 4 through the entities involved, and the SRVCC-HO is considered successful.

[ 0069] For the sake of simplicity, a network node implementing the ATCF may further be referred to as an ATCF. In some embodiments, a Proxy Call Session Control Function (P-CSCF) may play the same role as the ATCF. Likewise, a network node implementing an SCC-AS may further be referred to as an SCC-AS, a network node implementing a P-CSCF may further be referred to as a P-CSCF and a network node implementing an S-CSCF may further be referred to as an S-CSCF.

[ 0070 ] Embodiments of the above method, i.e. method of handling IMS Restoration Procedures for service interruption of a SCC-AS, when executed at a network node implementing the ATCF, i.e. ATCF node, are disclosed in the following with reference to Fig. 8 and any ones of them may be combined in any manner without restrictions.

[ 0071 ] In an embodiment, the above method comprises receiving S-810, at the ATCF node from a selected SCC-AS via a S-CSCF node, access transfer information with an SCC-AS identifier, such as e.g. an ATU-STI, which identifies the selected SCC- AS, and a UE identifier, such as e.g. a C-MSISDN, which identifies a UE, and storing S- 820 at the ATCF node the received SCC-AS identifier, e.g. ATU-STI, and optionally the received UE identifier, e.g. C-MSISDN.

[ 0072 ] In an embodiment, the above method comprises receiving S-830 at the ATCF node an initial call setup message, e.g. a SIP Invite message, and transmitting S- 840, from the ATCF node toward the selected SCC-AS via the S-CSCF node, the received initial call setup message along with the stored SCC-AS identifier, e.g. ATU- STI, which identifies the selected SCC-AS, and optionally with the stored UE identifier, e.g. C-MSISDN, which identifies the UE. [ 0073] Embodiments of the above method, i.e. method of handling IMS Restoration Procedures for service interruption of a SCC-AS, when executed at a network node implementing the S-CSCF, i.e. S-CSCF node, are disclosed in the following with reference to Fig. 9 and any ones of them may be combined in any manner without restrictions.

[ 0074 ] In an embodiment, the above method comprises receiving S-910, at the S- CSCF node, a SIP Register message originated from a UE, selecting S-920, at the S- CSCF node, an available SCC-AS and transmitting S-930, from the S-CSCF node toward the selected SCC-AS, the SIP Register message. [ 0075] In an embodiment, the above method comprises receiving S-940, at the S- CSCF node from a selected SCC-AS, an access transfer information that comprises an SCC-AS identifier, such as e.g. an ATU-STI, which identifies the selected SCC-AS, and a UE identifier, such as e.g. a C-MSISDN, which identifies a UE, and transmitting S- 950, from the S-CSCF node toward an ATCF node the received access transfer information with the received SCC-AS identifier, e.g. ATU-STI, and the received UE identifier, e.g. C-MSISDN.

[ 0076] In an embodiment, the above method comprises receiving S-960, at the S- CSCF node from an ATCF node an initial call setup message, e.g. SIP Invite message, along with an access transfer information that comprises an SCC-AS identifier, e.g. ATU-STI, and a UE identifier, e.g. C-MSISDN, and transmitting S-970, from the S- CSCF node toward the selected SCC-AS, the received initial call setup message with the received SCC-AS identifier, e.g. ATU-STI, and optionally the received UE identifier, e.g. C-MSISDN.

[ 0077 ] Embodiments of the above method, i.e. method of handling IMS Restoration Procedures for service interruption of a SCC-AS, when executed at a network node implementing the SCC-AS, i.e. SCC-AS node, are disclosed in the following with reference to Fig. 10 and any ones of them may be combined in any manner without restrictions.

[ 0078 ] In an embodiment, the above method comprises receiving S-1030, at the SCC-AS node from an S-CSCF node, an initial call setup message, e.g. SIP Invite message, along with an access transfer information that comprises an SCC-AS identifier, e.g. ATU-STI, and a UE identifier, e.g. C-MSISDN, determining S-1040, at the SCC- AS node, a mismatch between the received SCC-AS identifier, e.g. received ATU-STI, and its own SCC-AS identifier, e.g. own ATU-STI, and transmitting S-1050, from the SCC-AS node toward the S-CSCF node, a new access transfer information that comprises the own SCC-AS identifier, e.g. own ATU-STI, of the SCC-AS node and optionally the received UE identifier, e.g. C-MSISDN.

[ 0079] Participating in the above methods and exemplary embodiments of IMS Restoration Procedures for service interruption of a SCC-AS there is provided a network node implementing an ATCF, hereinafter ATCF node. Embodiments of this ATCF node are illustrated in Fig. 5 and Fig. 11.

[ 0080 ] In both embodiments illustrated in Fig. 5 and Fig. 11, the ATCF node 2 is configured to receive, from a selected SCC-AS 4 via an S-CSCF node 3 and via a receiver 530 of the ATCF node, access transfer information with an SCC-AS identifier, which identifies a selected SCC-AS, and a UE identifier, which identifies a UE; the ATCF node 2 is configured to store a plurality of SCC-AS identifiers identifying a respective plurality of SCC-ASs; and the ATCF node 2 is configured to receive, via the receiver 530, an initial call setup message, and transmit, toward the selected SCC-AS 4 via a transmitter 540 of the ATCF node and via the S-CSCF node 3, the received initial call setup message along with the stored SCC-AS identifier, which identifies the selected SCC-AS.

[ 0081 ] In accordance with an embodiment, the ATCF node 2 may comprise, as shown in Fig. 5 and Fig. 11, an SCC-AS handler 524 configured to receive, from a selected SCC-AS 4 via S-CSCF node 3 and via receiver 530 of the ATCF node 2, access transfer information with an SCC-AS identifier, such as e.g. an ATU-STI, which identifies a selected SCC-AS, and a UE identifier, such as e.g. a C-MSISDN, which identifies a UE; a data memory configured to store a plurality of SCC-AS identifiers, e.g. ATU-STIs, identifying a respective plurality of SCC-ASs and to optionally store a plurality of UE identifiers, e.g. C-MSISDNs, respectively associated with the plurality of SCC-AS identifiers; and a call handler 527 configured to receive, via the receiver 530, an initial call setup message, e.g. a SIP Invite message, and transmit, toward the selected SCC-AS 4 via transmitter 540 of the ATCF node 2 and via the S-CSCF node 3, the received initial call setup message along with the stored SCC-AS identifier, e.g. ATU- STI, which identifies the selected SCC-AS, and the UE identifier, e.g. C-MSISDN, which identifies the UE. [ 0082 ] In the embodiment illustrated in Fig. 5, the ATCF node 2 may comprise a data memory 518 configured to store the plurality of SCC-AS identifiers and to optionally store the plurality of UE identifiers; whereas in the embodiment illustrated in Fig. 11, any one of the SCC-AS handler 524 and the call handler 527 may be configured to store the plurality of SCC-AS identifiers and to optionally store the plurality of UE identifiers.

[ 0083] In accordance with an embodiment, the ATCF node 2 may comprise, as shown in Fig. 5, at least one processor 520, and at least one memory 510 that stores processor-executable instructions 514. In this ATCF node 2, the at least one processor interfaces with the at least one memory to execute the processor-executable instructions, so that the ATCF node 2 is operable to perform the actions disclosed above.

[ 0084 ] For example, in the embodiment illustrated in Fig. 5, the ATCF node 2 may comprise at least one processor 520 and at least one memory 510, both in communication with each other, with the SCC-AS handler 524, the call handler 527, the receiver 530 and the transmitter 540, and with other elements or units of the ATCF node 2. The at least one memory 510 may comprise volatile and/or non- volatile memory. In particular, the at least one memory 510 may have a computer program 514 and data 518 stored therein. The computer program 514 may be loaded in the at least one memory 510 from a computer program product 550, such as any non-transitory computer readable medium, in which the computer program is stored. The data 518 may comprise a plurality of SCC-AS identifiers, e.g. ATU-STIs, identifying a respective plurality of SCC-ASs and optionally a plurality of UE identifiers, e.g. C-MSISDNs, respectively associated with the plurality of SCC-AS identifiers. The at least one processor 520 may be configured to carry out the functions of the SCC-AS handler 524 and the call handler 527. [ 0085] Participating in the above methods and exemplary embodiments of IMS Restoration Procedures for service interruption of a SCC-AS there is also provided a network node implementing an S-CSCF, hereinafter S-CSCF node. Embodiments of this S-CSCF node are illustrated in Fig. 6 and Fig. 12.

[ 0086] In both embodiments illustrated in Fig. 6 and Fig. 12, the S-CSCF node 3 is configured to receive, from a selected SCC-AS node 4 via receiver 630 of the S-CSCF node 3, an access transfer information that comprises an SCC-AS identifier, which identifies the selected SCC-AS, and a UE identifier, which identifies a UE, and transmit, toward an ATCF node 2 via transmitter 640 of the S-CSCF node 3, the received access transfer information with the received SCC-AS identifier and the received UE identifier. This S-CSCF node 3 is also configured to receive, from the ATCF node 2 via the receiver 630, an initial call setup message along with an access transfer information that comprises an SCC-AS identifier and a UE identifier, and transmit, toward the selected SCC-AS 4 via the transmitter 640, the received initial call setup message with the received SCC-AS identifier.

[ 0087 ] In accordance with an embodiment, the S-CSCF node 3 may comprise, as shown in Fig. 6 and Fig. 12, an SCC-AS handler 627 configured to receive, from the selected SCC-AS node 4 via the receiver 630 of the S-CSCF node 3, an access transfer information that comprises an SCC-AS identifier, e.g. ATU-STI, which identifies the selected SCC-AS 4 and a UE identifier, e.g. C-MSISDN, which identifies a UE, and transmit, toward an ATCF node 2 via the transmitter 640 of the S-CSCF node 3, the received access transfer information with the received SCC-AS identifier, e.g. ATU-STI, and the received UE identifier, e.g. C-MSISDN.

[ 0088 ] In this embodiment, the S-CSCF node 3 may also comprise, as shown in Fig. 6 and Fig. 12, a call handler 624 configured to receive, from an ATCF node 2 via the receiver 630, an initial call setup message, e.g. SIP Invite message, along with an access transfer information that comprises an SCC-AS identifier, e.g. ATU-STI, and a UE identifier, e.g. C-MSISDN, and transmit, toward the selected SCC-AS 4 via the transmitter 640, the received initial call setup message with the received SCC-AS identifier, e.g. ATU-STI, and optionally the received UE identifier, e.g. C-MSISDN.

[ 0089] In a further embodiment, the call handler 624 shown in Fig. 6 and Fig. 12 may be configured to receive, via the receiver 630, a SIP Register message originated from a UE-A 1, select an available SCC-AS and transmit, toward the selected SCC-AS 4 via transmitter 640, the SIP Register message;

[ 0090 ] In the embodiment illustrated in Fig. 6, the S-CSCF node 3 may also comprise a data memory 618 configured to store a plurality of SCC-AS identifiers, e.g. ATU-STIs, identifying a respective plurality of SCC-ASs and to optionally store a plurality of UE identifiers, e.g. C-MSISDNs, respectively associated with the plurality of SCC-AS identifiers; whereas in the embodiment illustrated in Fig. 12, any one of the SCC-AS handler 627 and the call handler 624 may be configured to store the plurality of SCC-AS identifiers and to optionally store the plurality of UE identifiers. [ 0091 ] In accordance with a further embodiment, the S-CSCF node 3 may comprise, as shown in Fig. 6, at least one processor 620, and at least one memory 610 that stores processor-executable instructions 614. In this S-CSCF node 3, the at least one processor interfaces with the at least one memory to execute the processor-executable instructions, so that the S-CSCF node 3 is operable to perform the actions disclosed above.

[ 0092 ] For example, in the embodiment illustrated in Fig. 6, the S-CSCF node 3 may comprise at least one processor 620 and at least one memory 610, both in communication with each other, with the call handler 624, the SCC-AS handler 627, the receiver 630 and the transmitter 640, and with other elements or units of the S-CSCF node 3. The at least one memory 610 may comprise volatile and/or non- volatile memory. In particular, the at least one memory 610 may have a computer program 614 and data 618 stored therein. The computer program 614 may be loaded in the at least one memory 610 from a computer program product 650, such as any non-transitory computer readable medium, in which the computer program is stored. The data 618 may comprise a plurality of SCC-AS identifiers, e.g. ATU-STIs, identifying a respective plurality of SCC-ASs and optionally a plurality of UE identifiers, e.g. C-MSISDNs, respectively associated with the plurality of SCC-AS identifiers. The at least one processor 620 may be configured to carry out the functions of the call handler 624 and the SCC-AS handler 627.

[ 0093] Participating in the above methods and exemplary embodiments of IMS Restoration Procedures for service interruption of a SCC-AS there is also provided a network node implementing an SCC-AS, hereinafter SCC-AS node. Embodiments of this SCC-AS node are illustrated in Fig. 7 and Fig. 13.

[ 0094 ] In both embodiments illustrated in Fig. 7 and Fig. 13, the SCC-AS node 4 is configured to receive, from an S-CSCF node 3 via a receiver 730 of the SCC-AS node, an initial call setup message along with an access transfer information that comprises an SCC-AS identifier and a UE identifier; this SCC-AS node 4 is also configured to determine a mismatch between the received SCC-AS identifier and its own SCC-AS identifier, and transmit, toward the S-CSCF node 3 via a transmitter 740 of the SCC-AS node, a new access transfer information that comprises the own SCC-AS identifier of the SCC-AS node.

[ 0095] In accordance with an embodiment, the SCC-AS node 4 may comprise, as shown in Fig. 7 and Fig. 13, a call handler 724 configured to receive, from an S-CSCF node 3 via receiver 730 of the SCC-AS node, an initial call setup message, e.g. SIP Invite message, along with an access transfer information that comprises an SCC-AS identifier, e.g. ATU-STI, and a UE identifier, e.g. C-MSISDN, and an SCC-AS handler 727 configured to determine a mismatch between the received SCC-AS identifier, e.g. ATU-STI, and its own SCC-AS identifier, e.g. own ATU-STI, and transmit, toward the S-CSCF node 3 via transmitter 740 of the SCC-AS node, a new access transfer information that comprises the own SCC-AS identifier, e.g. own ATU-STI, of the SCC- AS node 4 and, optionally, the received UE identifier, e.g. C-MSISDN.

[ 0096] In an embodiment, the call handler 724 may be configured to receive, from an S-CSCF node 3 via the receiver 730, a SIP Register message; and the SCC-AS handler 727 may be configured to transmit, toward the S-CSCF node 3 via the transmitter 740, an access transfer information comprising an own SCC-AS identifier, e.g. own ATU-STI, identifying the SCC-AS node 4 and a UE identifier, e.g. C- MSISDN, identifying a UE.

[ 0097 ] In the embodiment illustrated in Fig. 7 the SCC-AS node 4 may also comprise a data memory 718 configured to store registration and/or call related data, e.g. C-MSISDN, for each UE served by the SCC-AS node; whereas in the embodiment illustrated in Fig. 13, any one of the SCC-AS handler 727 and the call handler 724 may be configured to store the registration and/or call related data, e.g. C-MSISDN, for each UE served by the SCC-AS node.

[0098] In accordance with a further embodiment, the SCC-AS node 4 may comprise, as shown in Fig. 7, at least one processor 720, and at least one memory 710 that stores processor-executable instructions 714. In this SCC-AS node 4, the at least one processor interfaces with the at least one memory to execute the processor- executable instructions, so that the SCC-AS node 4 is operable to perform the actions disclosed above.

[0099] For example, in the embodiment illustrated in Fig. 7, the SCC-AS node 4 may comprise at least one processor 720 and at least one memory 710, both in communication with each other, with the call handler 724, the SCC-AS handler 727, the receiver 730 and the transmitter 740, and with other elements or units of the SCC-AS node 4. The at least one memory 710 may comprise volatile and/or non- volatile memory. In particular, the at least one memory 710 may have a computer program 714 and data 718 stored therein. The computer program 714 may be loaded in the at least one memory 710 from a computer program product 750, such as any non-transitory computer readable medium, in which the computer program is stored. The data 718 may comprise registration and/or call related data, e.g. C-MSISDN, for each UE served by the SCC- AS node. The at least one processor 720 may be configured to carry out the functions of the call handler 724 and the SCC-AS handler 727.

[0100] In embodiments of the invention for terminating calls, a terminating SCC- AS node may transmit its own SCC-AS identifier, e.g. own ATU-STI, along with an outgoing call setup message, e.g. SIP Invite message, toward a terminating ATCF node, and the terminating ATCF node may store the SCC-AS identifier, e.g. ATU-STI, received with the incoming call setup message, e.g. SIP Invite message.

[0101] The invention may also be practised by one or more computer programs, each one loadable into an internal memory of a computer with input and output units as well as with a processor. This computer program comprises to this end executable code adapted to carry out the above method steps when running in the computer. In particular, the executable code may be recorded in a carrier readable means in a computer. [ 0102 ] The invention is described above in connection with various embodiments that are intended to be illustrative and non-restrictive. It is expected that those of ordinary skill in this art may modify these embodiments. The scope of the invention is defined by the claims in conjunction with the description and drawings, and all modifications that fall within the scope of the claims are intended to be included therein.