WO2020001734A1

WO2020001734A1 - Restoration of a communication node in a communication system

Info

Publication number: WO2020001734A1
Application number: PCT/EP2018/066972
Authority: WO
Inventors: Ashok Kumar Nayak
Original assignee: Nokia Solutions And Networks Oy
Priority date: 2018-06-25
Filing date: 2018-06-25
Publication date: 2020-01-02

Abstract

An apparatus comprising at least one processor, and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: receiving a request for communication between a first node and a second node; and in response to determining a failure of a proxy call-state control function serving the second node, determining whether the second node is communicating using a fixed-line; and when it is determined that the second node is communicating using a fixed-line, initiating proxy call-state control function restoration.

Description

RESTORATION OF A COMMUNICATION NODE IN A COMMUNICATION SYSTEM

Field

This disclosure relates to communications, and more particularly to an apparatus, method and computer program in a communication system. More particularly the present invention relates to restoration of a communication node in a communication system.

Background

A communication system can be seen as a facility that enables communication between two or more devices such as user terminals, machine-like terminals, base stations and/or other nodes by providing communication channels for carrying information between the communicating devices. A communication system can be provided for example by means of a communication network and one or more compatible communication devices. The communication may comprise, for example, communication of data for carrying data for voice, electronic mail (email), text message, multimedia and/or content data communications and so on. Non-limiting examples of sen/ices provided include two-way or multi-way calls, data communication or multimedia services and access to a data network system, such as the Internet.

In a wireless system at least a part of communications occurs over wireless interfaces. Examples of wireless systems include public land mobile networks (PLMN), satellite based communication systems and different wireless local networks, for example wireless local area networks (WLAN). A local area wireless networking technology allowing devices to connect to a data network is known by the tradename WiFi (or Wi-Fi). WiFi is often used synonymously with WLAN. The wireless systems can be divided into cells, and are therefore often referred to as cellular systems. A base station provides at least one cell.

A user can access a communication system by means of an appropriate communication device or terminal capable of communicating with a base station. Hence nodes like base stations are often referred to as access points. A communication device of a user is often referred to as user equipment (UE). A communication device is provided with an appropriate signal receiving and transmitting apparatus for enabling communications, for example enabling communications with the base station and/or communications directly with other user devices. The communication device can communicate on appropriate channels, e.g. listen to a channel on which a station, for example a base station of a cell, transmits.

A communication system and associated devices typically operate in accordance with a given standard or specification which sets out what the various entities associated with the system are permitted to do and how that should be achieved. Communication protocols and/or parameters which shall be used for the connection are also typically defined. Non-limiting examples of standardised radio access technologies include GSM (Global System for Mobile), EDGE (Enhanced Data for GSM Evolution) Radio Access Networks (GERAN), Universal Terrestrial Radio Access Networks (UTRAN) and evolved UTRAN (E-UTRAN). An example communication system architecture is the long-term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) radio-access technology. The LTE is standardized by the third Generation Partnership Project (3GPP). The LTE employs the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access and a further development thereof which is sometimes referred to as LTE Advanced (LTE- A). Since introduction of fourth generation (4G) services increasing interest has been paid to the next, or fifth generation (5G) standard.

A communication system may contain a mixture of both mobile subscribers and fixed-line subscribers. A fixed-line subscriber may comprise, for example, a broadband and/or landline subscriber connected to the communication network via a wire or cable (for example a fibre-optic cable). A fixed-line typically terminates at a location such as a home or office.

Statement of invention

According to a first aspect there is provided an apparatus comprising at least one processor, and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: receiving a request for communication between a first node and a second node; and in response to determining a failure of a proxy call-state control function serving the second node, determining whether the second node is communicating using a fixed-line; and when it is determined that the second node is communicating using a fixed-line, initiating proxy call-state control function restoration.

According to an example the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform determining the failure of the proxy call-state control function by determining failure of the proxy call-state control function to respond to a connection request message from the apparatus.

According to an example the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform sending a message to a back-up proxy call-state control function when initiating the proxy call-state control function restoration.

According to an example, the message to the back-up proxy call-state control function is configured to be forwarded to the second node, for enabling the second node to perform registration with the back-up proxy call-state control function.

According to an example the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform registering the apparatus with the second node in response to receiving a confirmation message at the apparatus, the confirmation message received from the back-up proxy call-state control function.

According to an example the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform storing identities of a plurality of proxy call-state control functions, the back- up proxy call state control function comprising one of the plurality of proxy call-state control functions.

According to an example the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform determining whether the second node is communicating using a fixed line by determining location information.

According to an example the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform obtaining the location information from a registration message from the first node, the registration message comprised in the request for communication between the first node and the second node.

According to an example the message to the back-up proxy call-state control function comprises a NOTIFY message, the NOTIFY message comprising a shortened contact element or sub-element.

According to an example the second node serving a user equipment, the request for communication comprising a request for communication between the first node and the user equipment, the communication being via the second node.

According to an example the second node comprises a private branch exchange node.

According to a second aspect there is provided an apparatus comprising means for performing: receiving a request for communication between a first node and a second node; and in response to determining a failure of a proxy call-state control function serving the second node, determining whether the second node is communicating using a fixed-line; and when it is determined that the second node is communicating using a fixed-line, initiating proxy call-state control function restoration.

According to an example the means is for performing determining the failure of the proxy call-state control function by determining failure of the proxy call-state control function to respond to a connection request message from the apparatus. According to an example the means is for performing sending a message to a back-up proxy call-state control function when initiating the proxy call-state control function restoration.

According to an example the means is for performing configuring the message to the back-up proxy call-state control function to be forwarded to the second node, for enabling the second node to perform registration with the back-up proxy call-state control function

According to an example the means is for performing registering the apparatus with the second node in response to receiving a confirmation message at the apparatus, the confirmation message received from the back-up proxy call-state control function. According to an example the means is for performing storing identities of a plurality of proxy call-state control functions, the back-up proxy call state control function comprising one of the plurality of proxy call-state control functions.

According to an example the means is for performing determining whether the second node is communicating using a fixed line by determining location information.

According to an example the means is for performing obtaining the location information from a registration message from the first node, the registration message comprised in the request for communication between the first node and the second node. According to an example the message to the back-up proxy call-state control function comprises a NOTIFY message, the NOTIFY message comprising a shortened contact element or sub-element.

According to an example the second node is serving a user equipment, the request for communication comprising a request for communication between the first node and the user equipment, the communication being via the second node.

According to a third aspect there is provided a method comprising: receiving at an apparatus a request for communication between a first node and a second node; and in response to determining a failure of a proxy call-state control function serving the second node, determining whether the second node is communicating using a fixed-line; and when it is determined that the second node is communicating using a fixed-line, initiating proxy call-state control function restoration.

According to an example, the method comprises determining the failure of the proxy call-state control function by determining failure of the proxy call-state control function to respond to a connection request message from the apparatus.

According to an example, the method comprises sending a message to a backup proxy call-state control function when initiating the proxy call-state control function restoration. According to an example, the method comprises configuring the message to the back-up proxy call-state control function for forwarding to the second node, for enabling the second node to perform registration with the back-up proxy call-state control function. According to an example, the method comprises registering the apparatus with the second node in response to receiving a confirmation message at the apparatus, the confirmation message received from the back-up proxy call-state control function.

According to an example, the method comprises storing identities of a plurality of proxy call-state control functions, the back-up proxy call state control function comprising one of the plurality of proxy call-state control functions.

According to an example, the method comprises determining whether the second node is communicating using a fixed line by determining location information.

According to an example, the method comprises obtaining the location information from a registration message from the first node, the registration message comprised in the request for communication between the first node and the second node.

According to an example, the message to the back-up proxy call-state control function comprises a NOTIFY message, the NOTIFY message comprising a shortened contact element or sub-element. According to an example, the second node is serving a user equipment, the request for communication comprising a request for communication between the first node and the user equipment, the communication being via the second node.

According to an example, the second node comprises a private branch exchange node. According to a fourth aspect there is provided a non-transitory computer readable medium comprising program instructions for causing an apparatus to perform at least the following: receiving at an apparatus a request for communication between a first node and a second node; and in response to determining a failure of a proxy call-state control function serving the second node, determining whether the second node is communicating using a fixed-line; and when it is determined that the second node is communicating using a fixed-line, initiating proxy call-state control function restoration.

According to a fifth aspect there is provided a non-transitory computer readable medium comprising program instructions stored thereon for performing at least the following: receiving at an apparatus a request for communication between a first node and a second node; and in response to determining a failure of a proxy call-state control function serving the second node, determining whether the second node is communicating using a fixed-line; and when it is determined that the second node is communicating using a fixed-line, initiating proxy call-state control function restoration.

According to a sixth aspect there is provided a computer program comprising instructions for causing an apparatus to perform at least the following: receiving at an apparatus a request for communication between a first node and a second node; and in response to determining a failure of a proxy call-state control function serving the second node, determining whether the second node is communicating using a fixed- line; and when it is determined that the second node is communicating using a fixed- line, initiating proxy call-state control function restoration.

According to a seventh aspect there is provided a computer program comprising instructions stored thereon for performing at least the following: receiving at an apparatus a request for communication between a first node and a second node; and in response to determining a failure of a proxy call-state control function serving the second node, determining whether the second node is communicating using a fixed-line; and when it is determined that the second node is communicating using a fixed-line, initiating proxy call-state control function restoration.

Brief description of Figures

The invention will now be described in further detail, by way of example only, with reference to the following examples and accompanying drawings, in which:

Figure 1 shows a schematic example of a communication system where the invention may be implemented;

Figure 2 is a signalling diagram showing communication between entities in an

IMS; Figure 3 is a signalling diagram showing communication between entities in an

IMS;

Figure 4 is a signalling diagram showing communication between entities in an IMS according to an example;

Figure 5 is a flow-chart of a method according to an example;

Figure 6 shows an example of a communication device;

Figure 7 shows an example of a control apparatus.

Detailed description

Before explaining in detail the examples, certain general principles of parts of an example wireless communication system are briefly explained with reference to Figure 1 to assist in understanding the technology underlying the described examples.

One example of a system in which embodiments may be provided is shown in Figure 1 . Figure 1 shows an example of a communication system enabled to offer multimedia sen/ices. In this example this comprises an Internet Protocol (IP) Multimedia network. IP Multimedia (IM) functionalities can be provided by means of an IP Multimedia Core Network (CN) subsystem, or IP Multimedia subsystem (IMS). The IMS includes various network entities for the provision of the multimedia services. Multimedia services may comprise for example text, and/or images or photos, and/or video, and/or audio.

In Figure 1 an IP Multimedia Network 100 is provided for offering IP multimedia services for IP Multimedia Network subscribers. The network subscribers may comprise mobile and/or fixed line subscribers. IP Multimedia (IM) functionalities can be provided by means of a communication system, which may comprise a mobile communication system. A communication system is typically arranged to serve a plurality of user equipment usually. For mobile users this may be via a wireless interface between the user equipment and at least one base station of the communication system. The mobile communication system may logically be divided between a radio access network (RAN) and a core network (CN).

In the system of Figure 1 , base station 131 is arranged to transmit signals to and receive signals from mobile user equipment 130 of a mobile user i.e. a subscriber via a wireless interface between the user equipment and the radio access network. Correspondingly, the mobile user equipment 130 is able to transmit signals to and receive signals from the radio access network via the wireless interface. A fixed-line node is shown schematically at 140. The fixed-line node 140 may comprise for example a land-line termination point or a user equipment or modem or router or the like (such as a broadband modem or router). The fixed-line node 140 may be located in a home or office or the like. Where the fixed-line node 140 comprises a router or the like, then the fixed-line node may communicate with user equipment such as user equipment 141 over a wireline interface.

In the shown arrangement the user equipment 130 may access the IMS network 100 via the access network associated with base station 131 . The fixed-line node may access the IMS network via the access network associated with the fixed- line node 140.

A user equipment may comprise any appropriate user equipment adapted for Internet Protocol (IP) communication to connect to the network 100. For example, a user may access the network 100 by means of a Personal computer (PC), Personal Data Assistant (PDA), mobile station (MS), smart phone and so on.

One skilled in the art is familiar with the features and operation of a typical UE. Thus, it is sufficient to note that the user may use a UE for tasks such as for making and receiving phone calls, for receiving and sending data from and to the network and for experiencing multimedia content or otherwise using multimedia services. A UE may include an antenna for wirelessly receiving and transmitting signals from and to base stations of a mobile communication network. A UE may also be provided with a display for displaying images and other graphical information for the user of the UE. Camera means may be provided for capturing still or video images. Speaker means are also typically provided. The operation of a UE may be controlled by means of an appropriate user interface such as control buttons, voice commands and so on. Furthermore, a UE is provided with a processor entity and a memory means. An example UE is described in more detail with respect to Figure 6.

The core network (CN) entities typically include various switching and other control entities and gateways for enabling the communication via a number of radio access networks and also for interfacing a single communication system with one or more communication system such as with other cellular systems and/or fixed line communication systems. In some systems the radio access network controller is typically connected to an appropriate core network entity or entities such as, but not limited to, a serving general packet radio service support node (SGSN), see controllers 133 and 139 of Figure 1 . The radio access network is in communication with the serving GPRS support node via an appropriate interface, for example on an lu interface. Although not shown, the SGSNs 133, 139 typically have access to designated subscriber database configured for storing information associated with the subscription of the respective user equipment. The serving GPRS support node, in turn, typically communicates with a gateway GPRS support node via a GPRS backbone network 132. This interface is commonly a switched packet data interface.

A packet data session may be established to carry traffic flows over the network. Such a packet data session is often referred as a packet data protocol (PDP) context. A PDP context may include a radio bearer provided between the user equipment and the radio network controller, a radio access bearer provided between the user equipment, the radio network controller and the SGSN, and switched packet data channels provided between the serving GPRS service node and the gateway GPRS service node. Each PDP context usually provides a communication pathway between a particular user equipment and the gateway GPRS support node and, once established, can typically carry multiple flows. Each flow normally represents, for example, a particular service and/or a media component of a particular service. The PDP context therefore often represents a logical communication pathway for one or more flow across the network. To implement the PDP context between user equipment and the serving GPRS support node, radio access bearers (RAB) need to be established which commonly allow for data transfer for the user equipment. The implementation of these logical and physical channels is known to those skilled in the art and is therefore not discussed further herein.

The communication systems may be such that services are provided for the user equipment by means of various functions of the data network that are handled by controller entities, such as servers. For example, several different servers providing various control functions are used for the service provision control. These may include functions such as the call state control functions (CSCFs). The call state control functions may be divided into various categories such as a proxy call state control function (P-CSCF), interrogating call state control function (l-CSCF), and serving call state control function (S-CSCF). It shall be appreciated that similar functions may be referred to in different systems with different names. For example, in certain applications the CSCFs may be referenced to as the call session control functions.

A user who wishes to use services provided by the IMS system may need first to register with a serving controller, such as the serving call session control function (S-CSCF) 136. As shown in Figure 1 , communication between the S-CSCF 136 and the user equipment may be routed via at least one proxy call session control function (P-CSCF) 135. The proxy CSCF 135 is thus for proxying messages from the GGSN 134 to a serving call session control function 136. The serving controller, i.e. CSCF 136 in Figure 1 , in turn, provides the control entity the user equipment 130 and/or node 140 needs to be registered with. The registration is required to enable the user equipment or node to request for a service from the communication system.

A policy control entity 137 may also be provided. The policy control entity 137 may be configured to store required policy information in a database 138 thereof. The network entities may query the policy control entity for various purposes, such as for obtaining information regarding the media policy of a particular user. The policy control entity may be provided by means of a policy decision function (PDF), which is connected to P-CSCF and GGSN.

Figure 1 also shows an application server 150. The user equipment 130 and/or node 140 may connect, via the GPRS network, to the application server that is connected to one or more data networks such as, but not limited to, the exemplifying Internet Protocol (IP) network. It shall be appreciated that a great number of application servers may be connected to each data network. The application server 150 comprises a memory 152, which may comprise a database.

Some embodiments may be used in the Serving Call State Control Function (S- CSCF) selecting an Application Server (AS) node from the given FQDN which resolves in multiple IPs. The SCSF 136 would query the DNS 153.

Though network nodes in the IMS Core Network should have a high or very high availability, some maintenance downtime and occasional failures are unavoidable. Communication links although designed with robust protocols between the networks elements are also subject to failures.

Therefore a recovery mechanism may be required to reduce the service outage time in case a network node, such as a P-CSCF, is out of service or unreachable.

The current standard defines HSS (home subscriber server) based P-CSCF restoration, and PCRF (policy and charging rules function) based P-CSCF restoration (see 3GPP TS 24.229, 3GPP TS 23.380) to handle P-CSCF outage use cases. However, P-CSCF restoration is applicable only for 3GPP and non 3GPP access type UEs and it does not work for a fixed line subscriber. As S-CSCF is not able to distinguish between the access type of UEs (e.g. distinguish between mobile and fixed-line subscribers -from an incoming request message such as an INVITE, the S- CSCF triggers P-CSCF restoration (HSS or PCRF based on the configuration) for fixed line subscribers as well. A problem with existing methods is that the methods are not able to communicate with the fixed-line (e.g. fixed phone line) in order to trigger registration again following P-CSCF failure. This is explained in more detail with respect to Figure 2.

Figure 2 is a signalling diagram showing communication between entities in an IMS. An originating side node is shown schematically at 240. The user 240 could be fixed-line or a mobile user (e.g. VOLTE (voice over LTE).An HSS is schematically shown at 252, and an S-CSCF is shown schematically at 236. A“failed” P-CSCF is shown at 235. The failed P-CSCF 235 may for example be undergoing maintenance or system downtime, as schematically represented by cross 254. A“new” or backup P-CSCF is shown at 250. The new P-CSCF 250 may operate to replace the failed P- CSCF 235. An MME/SGSN is schematically shown at 239 and a UE 230 is schematically shown at 230. The UE 230 is communicating using a fixed line. For example the UE 230 may comprise a fixed-line phone, or a phone or other type of UE connected to a fixed-line (e.g. IP-PBX).

At S1 a session initiation protocol (SIP) message is sent from originating side node 240 to S-CSCF 236. The SIP message is forwarded from S-CSCF 236 to failed- P-CSCF 235. The S-CSCF 236 has no knowledge that P-CSCF 235 has failed, and therefore S-CSCF 236 forwards the SIP message to the failed P-CSCF as shown at S2. Owing to the fact that P-CSCF 235 is in a failed state, no response is received at S-CSCF 236 from P-CSCF 235. This is shown at S3. At S4, a server assignment request (SAR) is sent from S-CSCF 236 to HSS 252. In this example the SAR includes a P-CSCF restoration indication. The restoration indication informs the HSS 252 of the P-CSCF 235 failure. At S5 a server assignment response (SAA) is sent from HSS 252 to S-CSCF 236. In this example the SAA comprises an error response as the request for restoration is for a fixed line subscriber. In this example the S-CSCF 236 informs the HSS 252 through a P-CSCF Restoration indication flag in Cx:SAR message to de- register/unregister the subscriber i.e. UE 230. When this P-CSCF Restoration indication is received by HSS 252, the HSS 252 verifies the supported feature flag for this subscriber in its S6a/S6d connection for the connected MME/SGSN. If the supported feature has the P-CSCF restoration flag, which is received from the MME/SGSN during the attach, then HSS 252 sends the S6a/S6d:IDR message by setting this P-CSCF Restoration flag in Feature-List ID AVP (Feature-List ID2) to MME/SGSN. The MME/SGSN is expected to inform IMS PDN to release the corresponding UE from the faulty P-CSCF. However for the fixed line subscriber (e.g. UE 230) there won’t be any information at HSS that supports restoration. Hence an error response will be sent.

Figure 3 is a signalling diagram showing communication between entities in an IMS. More particularly Figure 3 shows PCRF (policy and charging rules function) based P-CSCF restoration. The system of Figure 3 comprises a PCRF 358. S1 to S3 of Figure 3 are the same as S1 to S3 of Figure 2. In the example of Figure 3, the S- CSCF is aware of the new P-CSCF 350. Accordingly, at S4 the S-CSCF 336 sends the SIP message to new P-CSCF 350. At S5, the new P-CSCF 350 sends an authorization authentication request (AAR) to PCRF 358. In response, at S6 the PCRF 358 sends an authorization authentication answer (AAA) to new P-CSCF 350. At S7, the New P-CSCF sends a SIP negative response to S-CSCF. This informs the S- CSCF that the session cannot be initiated, because the PCRF does not have the fixed line subscriber information.

According to some examples, there is provided a method which assists the S- CSCF in identifying when an incoming request (e.g. a SIP request) is for (or terminates towards) a fixed line subscriber or fixed line node. The request may originate from a fixed-line or mobile user. When it is determined that a P-CSCF serving that fixed line subscriber is in a failed state, then using this information that the request is for a fixed- line subscriber then the S-CSCF can trigger P-CSCF restoration. Therefore in some examples the method comprises a step of determining if the P-CSCF is“alive” (e.g. active), or has failed (e.g. inactive). The restoration may comprise restoring the failed P-CSCF or introducing a new P-CSCF to replace the failed P-CSCF. When it is determined that the P-CSCF has failed, then in response a check or determination is made as to whether the incoming call or request is for a fixed line subscriber. This is described in more detail below.

As per 3GPP TS 24.229 and 3GPP TS 29.228, a reference location is configured in a user profile of the fixed line subscriber. The S-CSCF can obtain the reference location from HSS during registration of the fixed line subscriber (e.g. DSL, XDSL, IP-PBX (private branch exchange). The S-CSCF may store this reference location in registration context or field. When the P-CSCF is out of service and any incoming request is made to the fixed line subscriber, the S-CSCF checks the registration context before invoking any P-CSCF restoration method. If the reference location information is present in the registration context then S-CSCF will know that the incoming request is for a fixed line subscriber. The S-CSCF will then trigger a message, for example a NOTIFY message, to the back-up P-CSCF. The NOTIFY message may comprise information, for example an event attribute of that <contact>. Any sub-element(s) may be set to "shortened" as defined in TS 24.229 section 5.1.1.5A. Fixed line node (e.g. IP-PBX) may initiate registration again as mentioned in TS 24.229 section 5.1.1.5A and 5.1.1.4. According to some examples the S-CSCF maintains a list of P-CSCF(s) which may be used for sending the NOTIFY message to the fixed line subscriber.

This is explained in more detail with respect to Figure 4, which is a signalling diagram of communication between entities in an IMS according to an example.

The system shown in Figure 4 comprises a fixed-line 460. In some examples the fixed line 460 may be considered a fixed line node. The fixed-line 460 may comprise, for example, an IP-PBX or a fixed line phone. The fixed line 460 may alternatively comprise a DSL or XDSL node. A UE-A 430 is associated with fixed line 460. For example UE-A 430 may be a UE which connects to the IMS via the fixed line 460. Failed P-CSCF is shown at 435 (the failure schematically represented by cross 454), and back-up P-CSCF is shown at 450. In examples there may be more than one back-up P-CSCF. S-CSCF is shown at 436. A second UE, UE-B is shown at 440. UE- B 440 may be a mobile UE or a fixed line UE.

In the example of Figure 4, UE-B 440 wants to establish a communication session with UE-A 430. In order to do this the UE-B 440 will need to communicate with the UE-A 430 via fixed line 460.

At S1 the UE-B 440 sends a message to S-CSCF 436. The message at S1 may comprise an INVITE message. For example the INVITE message may contain information that UE-B 440 wants to establish a communication session with UE-A 430. In other words, S1 may comprise the S-CSCF 436 receiving a request for communication between a first node and a second node. At S2, the S-CSCF 436 forwards the INVITE message to the failed P-CSCF 435 (at this point the S-CSCF 436 is unaware that P-CSCF 435 has failed).

The S-CSCF 436 receives no response from the P-CSCF 435, as shown at S3. In other words, the S-CSCF 436 determines failure of the P-CSCF 435. In some examples the S-CSCF 436 operates a timer, and if a response is not received from the P-CSCF within a certain time then that P-CSCF may be considered to have failed.

As shown at S4 the S-CSCF determines whether the request is for a fixed line subscriber. In some examples, identification of P-CSCF failure triggers or causes the S-CSCF to check registration information of the P-CSCF. The registration information may be a registration context. In some examples the registration context is comprised in the received request for communication between the first node and second node (e.g. in the INVITE message). If there is location information comprised in the registration information, then the S-CSCF may determine that the request (e.g. the INVITE message) was for a fixed-line subscriber, since only fixed line subscribers would have this reference location information. In other words, the determining whether the UE 430 is communicating using a fixed line comprises determining location information. The location information may comprise PANI (P-Access- Network-lnformation).

In response to determining that the request is for a fixed line subscriber, the S- CSCF 436 will send a message to a back-up P-CSCF. For example this may be carried out using a NOTIFY message. This is shown at S5. In some examples the NOTIFY message is triggered by the S-CSCF for certain subscribers only. For example the NOTIFY message may only be triggered for subscribers which do not use IPSec (internet protocol security) or TLS (transport layer security). In some examples existing procedures may be used in the S-CSCF to identify IPSec and TLS users during registration.

In some examples the P-CSCF location information is stored in the S-CSCF as part of a user profile. In some examples, the user profile is sent in server assignment response (SAA) by HSS to S-CSCF during registration. Below is an example of such reference location information:

"<Extension>

<AccessType>XDSL</AccessType> <Accesslnfo>dsl-location</Accesslnfo>

<AccessValue>Bangalore</AccessValue>

</Referencel_ocationlnformation>

</Extension>"

For example this location information may comprise location information of the fixed line 460 (e.g. IP-PBX) configured by an operator.

In some examples the S-CSCF maintains a table which contains a list of P- CSCFs. Using this information the S-CSCF can identify a back-up P-CSCF to use in case of a P-CSCF failure. For example the S-CSCF can use the information in the table to identify a backup P-CSCF to send a NOTIFY message to in case of P-CSCF failure. In some examples the list of proxy/P-CSCF addresses is configured manually for fixed line subscribers. In some examples the list of proxy/P-CSCF addresses is configured automatically for fixed line subscribers. Therefore in some examples a same set of P-CSCF addresses will be configured in the table which can be maintained by the S-CSCF.

Returning to Figure 4, at S5 the S-CSCF 436 sends the NOTIFY message to the back-up P-CSCF 450. The back-up P-CSCF 450 can be identified by the S-CSCF 436 as explained above.

At S6 the back-up P-CSCF 450 sends the NOTIFY message to fixed-line node

460.

At S7 the fixed-line node 460 sends a confirmation/acknowledgement message to the backup P-CSCF 450. The confirmation message may comprise a 200OK message.

At S8 the backup P-CSCF 450 sends a confirmation message to S-CSCF 436. Again, the confirmation message may comprise a 200OK message.

At S9 the fixed-line node 460 sends a message to S-CSCF 436 to initiate registration of fixed line node 460 to backup P-CSCF 450. The registration may be carried out according to existing procedures.

Once registered, communications can begin between UE-B 440 and UE-A 430, via fixed-line node 460. All subsequent calls (both originating and terminating) involving fixed-line node 460 can then be completed successfully.

Figure 5 is a flow chart of a method according to an example. At S1 a request is received for communication between a first node and a second node.

At S2 a determination is made as to whether the second node is communicating using a fixed-line. In some examples this determination is in response to determining a failure of a proxy call-state control function serving the second node.

At S3, when it is determined that the second node is communicating using a fixed-line, proxy call-state control function restoration initiation is initiated.

Reference has been made to communication devices or user equipment. Figure 6 shows a schematic, partially sectioned view of a communication device 600. Such a communication device is often referred to as user equipment (UE) or terminal. An appropriate mobile communication device may be provided by any device capable of sending and receiving radio signals. Non-limiting examples comprise a mobile station (MS) or mobile device such as a mobile phone or what is known as a’smart phone’, a computer provided with a wireless interface card or other wireless interface facility (e.g., USB dongle), personal data assistant (PDA) or a tablet provided with wireless communication capabilities, or any combinations of these or the like. A mobile communication device may provide, for example, communication of data for carrying communications such as voice, electronic mail (email), text message, multimedia and so on. Users may thus be offered and provided numerous services via their communication devices. Non-limiting examples of these services comprise two-way or multi-way calls, data communication or multimedia services or simply an access to a data communications network system, such as the Internet. Users may also be provided broadcast or multicast data. Non-limiting examples of the content comprise downloads, television and radio programs, videos, advertisements, various alerts and other information.

A wireless communication device may be for example a mobile device, that is, a device not fixed to a particular location, or it may be a stationary device. The wireless device may need human interaction for communication, or may not need human interaction for communication. In the present teachings the terms UE or“user” are used to refer to any type of wireless communication device.

The wireless device 600 may receive signals over an air or radio interface 607 via appropriate apparatus for receiving and may transmit signals via appropriate apparatus for transmitting radio signals. In Figure 6 transceiver apparatus is designated schematically by block 606. The transceiver apparatus 606 may be provided for example by means of a radio part and associated antenna arrangement. The antenna arrangement may be arranged internally or externally to the wireless device.

A wireless device is typically provided with at least one data processing entity 601 , at least one memory 602 and other possible components 603 for use in software and hardware aided execution of tasks it is designed to perform, including control of access to and communications with access systems and other communication devices. The data processing, storage and other relevant control apparatus can be provided on an appropriate circuit board and/or in chipsets. This feature is denoted by reference 604. The user may control the operation of the wireless device by means of a suitable user interface such as key pad 605, voice commands, touch sensitive screen or pad, combinations thereof or the like. A display 608, a speaker and a microphone can be also provided. Furthermore, a wireless communication device may comprise appropriate connectors (either wired or wireless) to other devices and/or for connecting external accessories, for example hands-free equipment, thereto. The communication devices 602, 604, 605 may access the communication system based on various access techniques.

Figure 7 shows an example of a control apparatus for a communication system, for example to be coupled to and/or for controlling a station of an access system. For example an S-CSCF, P-CSCF, SGSN, GGSN may comprise such a control apparatus. The control apparatus 700 can be arranged to provide control on communications in the service area of the system. The control apparatus 700 comprises at least one memory 701 , at least one data processing unit 702, 703 and an input/output interface 704. Via the interface the control apparatus can be coupled to a receiver and a transmitter. The receiver and/or the transmitter may be implemented as a radio front end or a remote radio head. For example the control apparatus 700 or processor 702, 703 can be configured to execute an appropriate software code to provide the control functions.

In general, the various embodiments may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. Some aspects of the invention may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the invention is not limited thereto. While various aspects of the invention may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

As used in this application, the term“circuitry” may refer to one or more or all of the following: (a) hardware-only circuit implementations (such as implementations in only analog and/or digital circuitry) and (b) combinations of hardware circuits and software, such as (as applicable) :(i) a combination of analog and/or digital hardware circuit(s) with software/firmware and (ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and (c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation. This definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and/or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device.

The embodiments of this invention may be implemented by computer software executable by a data processor of the mobile device, such as in the processor entity, or by hardware, or by a combination of software and hardware. Computer software or program, also called program product, including software routines, applets and/or macros, may be stored in any apparatus-readable data storage medium and they comprise program instructions to perform particular tasks. A computer program product may comprise one or more computer-executable components which, when the program is run, are configured to carry out embodiments. The one or more computer-executable components may be at least one software code or portions of it.

Further in this regard it should be noted that any blocks of the logic flow as in the Figures may represent program steps, or interconnected logic circuits, blocks and functions, or a combination of program steps and logic circuits, blocks and functions. The software may be stored on such physical media as memory chips, or memory blocks implemented within the processor, magnetic media such as hard disk or floppy disks, and optical media such as for example DVD and the data variants thereof, CD. The physical media is a non-transitory media.

The memory may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The data processors may be of any type suitable to the local technical environment, and may comprise one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASIC), FPGA, gate level circuits and processors based on multi core processor architecture, as non-limiting examples.

Embodiments of the inventions may be practiced in various components such as integrated circuit modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate.

The foregoing description has provided by way of non-limiting examples a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention as defined in the appended claims. Indeed there is a further embodiment comprising a combination of one or more embodiments with any of the other embodiments previously discussed.

Claims

1. An apparatus comprising at least one processor, and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: receiving a request for communication between a first node and a second node; and in response to determining a failure of a proxy call-state control function serving the second node, determining whether the second node is communicating using a fixed-line; and when it is determined that the second node is communicating using a fixed-line, initiating proxy call-state control function restoration.

2. An apparatus according to claim 1 , wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform determining the failure of the proxy call-state control function by determining failure of the proxy call-state control function to respond to a connection request message from the apparatus.

3. An apparatus according to claim 1 or claim 2, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform sending a message to a back-up proxy call- state control function when initiating the proxy call-state control function restoration.

4. An apparatus according to claim 3, the message to the back-up proxy call-state control function configured to be forwarded to the second node, for enabling the second node to perform registration with the back-up proxy call-state control function.

5. An apparatus according to claim 3 or claim 4, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform registering the apparatus with the second node in response to receiving a confirmation message at the apparatus, the confirmation message received from the back-up proxy call-state control function.

6. An apparatus according to any of claims 3 to 5, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform storing identities of a plurality of proxy call- state control functions, the back-up proxy call state control function comprising one of the plurality of proxy call-state control functions.

7. An apparatus according to any of claims 1 to 6, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform determining whether the second node is communicating using a fixed line by determining location information.

8. An apparatus according to claim 7, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform obtaining the location information from a registration message from the first node, the registration message comprised in the request for communication between the first node and the second node.

9. An apparatus according to any of claims 3 to 6, wherein the message to the back-up proxy call-state control function comprises a NOTIFY message, the NOTIFY message comprising a shortened contact element or sub-element.

10. An apparatus according to any of claims 1 to 8, the second node serving a user equipment, the request for communication comprising a request for communication between the first node and the user equipment, the communication being via the second node.

11. An apparatus according to any of claims 1 to 10, the second node comprising a private branch exchange node.

12. A method comprising: receiving at an apparatus a request for communication between a first node and a second node; and in response to determining a failure of a proxy call-state control function serving the second node, determining whether the second node is communicating using a fixed-line; and when it is determined that the second node is communicating using a fixed- line, initiating proxy call-state control function restoration.

13. A method according to claim 12, comprising determining the failure of the proxy call-state control function by determining failure of the proxy call-state control function to respond to a connection request message from the apparatus.

14. A method according to claim 12 or claim 13, comprising sending a message to a back-up proxy call-state control function when initiating the proxy call-state control function restoration.

15. A method according to claim 14, comprising configuring the message to the back-up proxy call-state control function for forwarding to the second node, for enabling the second node to perform registration with the back-up proxy call-state control function.

16. A method according to claim 14 or claim 15, comprising registering the apparatus with the second node in response to receiving a confirmation message at the apparatus, the confirmation message received from the back-up proxy call-state control function.

17. A method according to any of claims 14 to 16, comprising storing identities of a plurality of proxy call-state control functions, the back-up proxy call state control function comprising one of the plurality of proxy call-state control functions.

18. A method according to any of claims 12 to 17, comprising determining whether the second node is communicating using a fixed line by determining location information.

19. A method according to claim 18, comprising obtaining the location information from a registration message from the first node, the registration message comprised in the request for communication between the first node and the second node.

20. A method according to any of claims 14 to 17, wherein the message to the back-up proxy call-state control function comprises a NOTIFY message, the NOTIFY message comprising a shortened contact element or sub-element.

21. A method according to any of claims 12 to 20, the second node serving a user equipment, the request for communication comprising a request for communication between the first node and the user equipment, the communication being via the second node.

22. A method according to any of claims 12 to 21 , the second node comprising a private branch exchange node.

23. A non-transitory computer readable medium comprising program instructions for causing an apparatus to perform at least the following: receiving at an apparatus a request for communication between a first node and a second node; and in response to determining a failure of a proxy call-state control function serving the second node, determining whether the second node is communicating using a fixed-line; and when it is determined that the second node is communicating using a fixed- line, initiating proxy call-state control function restoration.

24. A computer program comprising instructions stored thereon for performing at least the method of claims 12 to 22.