Patent Publication Number: US-8533348-B2

Title: Failover communication services

Description:
RELATED APPLICATION 
     U.S. patent application Ser. No. 12/948,984, filed Nov. 18, 2010, entitled “Communication Device for Configuring Failover Communication Services.” All sections of the aforementioned application are incorporated herein by reference. 
     FIELD OF THE DISCLOSURE 
     The subject disclosure relates generally to failover communication services. 
     BACKGROUND 
     In packet-switched communication systems, such as an Internet Protocol Multimedia Subsystem (IMS) network, it may be common for millions of communication devices to register with a server office of the IMS network. When a server office fails to provide services due to a catastrophic event such as an earthquake or other unexpected event, the communication devices may overload the IMS network with registration requests to other server offices that could adversely affect the performance of the IMS network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIGS. 1-2  depict illustrative embodiments of communication systems; 
         FIG. 3-6  depict illustrative embodiments of methods operating in portions of the systems described in  FIGS. 1-2 ; 
         FIGS. 7-8  depict illustrative embodiments of flow diagrams; and 
         FIG. 9  is a diagrammatic representation of a machine in the form of a computer system within which a set of instructions, when executed, may cause the machine to perform any one or more of the methods discussed herein. 
     
    
    
     DETAILED DESCRIPTION 
     The subject disclosure may describe, among other things, illustrative embodiments to avoid excessive re-registration requests initiated by communication devices due to a communication fault in an Internet Protocol Multimedia Subsystem (IMS) network that supplies communication services to a population of communication devices. An embodiment of the subject disclosure may avoid excessive re-registrations as a result of communication devices initiating a registration process with primary and secondary communication resources in an IMS network, and utilizing the secondary communication resource when initiating a call responsive to detecting the unavailability of the primary communication resource. 
     One embodiment of the subject disclosure may include a communication device having at least one memory coupled to at least one processor, wherein the at least one memory includes a set of instructions (e.g., computer-executable instructions, software, etc.). The set of instructions, when executed by at least one processor, may cause or configure the at least one processor or the communication device to perform one or more of the following: transmit a first Session Initiation Protocol (SIP) registration message to an Internet Protocol Multimedia Subsystem (IMS) network, receive a first acknowledgment from the IMS network indicating that the communication device has registered with a primary communication resource in the IMS network, transmit a second SIP registration message to the IMS network, wherein the second SIP registration message comprises an indicator for registering the communication device with a secondary communication resource in the IMS network, and/or receive a second acknowledgment from the IMS network indicating that the communication device has registered with the secondary communication resource. The set of instructions, when executed by at least one processor, may further cause or configure at least one processor or the communication device to perform one or more of the following: transmit a first SIP request message to the primary communication resource of the IMS network to establish a communication session with a second communication device, detect that the primary communication resource of the IMS network is unavailable to establish the communication session, and/or transmit a second SIP request message to the secondary communication resource to establish the communication session with the second communication device. 
     One embodiment of the subject disclosure may include an Interrogating Serving Call Session Control Function (I-CSCF) device having at least one memory coupled to at least one processor, where the at least one memory includes a set of instructions. The set of instructions, when executed by at least one processor, may cause or configure at least one processor or the I-CSCF device to perform one or more of the following: receive a first registration request initiated by a first communication device, transmit a request to a Home Subscriber Server (HSS) device responsive to receiving the first registration request, receive from the HSS a first message comprising a first identity of a primary communication resource, where the HSS device transmits the first message responsive to detecting that a first identity of the first communication device matches a second identity of a second communication device, and where the second communication device is registered with the primary communication resource, and register the first communication device with the primary communication resource. 
     One embodiment of the subject disclosure may describe an HSS having at least one memory coupled to at least one processor, where the at least one memory includes a set of instructions. The set of instructions, when executed by at least one processor, may cause or configure at least one processor or the HSS device to perform one or more of the following: store a first identity of a first communication device registered with a primary communication resource and a secondary communication resource, receive a request associated with a registration request initiated by a second communication device, detect that a second identity of the second communication device matches the first identity of the first communication device, and/or direct a registration of the second communication device with at least one of the primary communication resource or the secondary communication resource responsive to the detected match. 
     For illustration purposes only, the terms S-CSCF device, P-CSCF device, I-CSCF device, and so on, can be used without the word “device.” 
       FIG. 1  depicts an illustrative embodiment of a communication system  100  employing an IMS network architecture to facilitate the combined services of circuit-switched and packet-switched communication services. Communication system  100  can comprise an HSS  140 , a tElephone NUmber Mapping (ENUM) server  130 , and other common network elements of an IMS network  150 . The IMS network  150  can establish communications between or among IMS-compliant communication devices (CDs)  101 ,  102 , Public Switched Telephone Network (PSTN) CDs  103 ,  105 , and combinations thereof by way of a Media Gateway Control Function (MGCF)  120  coupled to a PSTN network  160 . The MGCF  120  may not be necessary when a communication session involves IMS CD to IMS CD communications. A communication session involving at least one PSTN CD, however, may utilize the MGCF  120 . It will be appreciated that IMS CDs  101 ,  102  can support packet-switched voice and packet-switched data communications and/or Long Term Evolution (LTE) communications protocol. Various IMS CDs  101 ,  102  may be embodied as cellular phones, smart phones, tablet computers, laptop computers, portable communication devices, appliances, and the like, whether wired or wireless. 
     IMS CDs  101 ,  102  can register with the IMS network  150  by establishing communications with a Proxy Call Session Control Function (P-CSCF) which communicates with an Interrogating CSCF (I-CSCF). The I-CSCF in turn can communicate with a Serving CSCF (S-CSCF) to register the CDs with the HSS  140 . To initiate a communication session between CDs, an originating IMS CD  101  can submit a Session Initiation Protocol (SIP INVITE) message to an originating P-CSCF  104  which can communicate with a corresponding originating S-CSCF  106 . The originating S-CSCF  106  can submit the SIP INVITE message to one or more application servers (AS)  117  that can provide a variety of services to IMS subscribers. 
     For example, one or more application servers  117  can be used to perform originating call feature treatment functions on the calling party number received by the originating S-CSCF  106  according to the SIP INVITE message. Originating treatment functions can include determining whether the calling party number has international calling services, call ID blocking, calling name blocking, 7-digit dialing, and/or is requesting special telephony features (e.g., *72 for forwarding calls, *73 for canceling call forwarding, *67 for caller ID blocking, and so on). Based on initial filter criteria (iFCs) in a subscriber profile associated with a CD, one or more application servers may be invoked to provide various call originating feature services. 
     The originating S-CSCF  106  can submit queries to the ENUM system  130  to translate an E.164 telephone number in the SIP INVITE message to a SIP Uniform Resource Identifier (URI) if the terminating CD is IMS-compliant. The SIP URI can be used by an I-CSCF  107  to submit a query to the HSS  140  to identify a terminating S-CSCF  114  associated with a terminating IMS CD such as CD  102 . Once identified, the I-CSCF  107  can submit the SIP INVITE message to the terminating S-CSCF  114 . The terminating S-CSCF  114  can then identify a terminating P-CSCF  116  associated with the terminating CD  102 . The P-CSCF  116  may then signal the CD  102  to establish Voice over Internet Protocol (VoIP) communication services, thereby enabling the calling and called parties to engage in voice and/or data communications. Based on the iFCs in a subscriber profile, one or more terminating application servers  115  may also be invoked to provide various call terminating feature services, such as call forwarding no answer, do not disturb, music tones, simultaneous ringing, sequential ringing, etc. 
     If the terminating communication device is instead a PSTN CD such as CD  103  or CD  105  (e.g., where the cellular phone  105  or other communication device only supports circuit switched voice communications via base station  121 ), the ENUM system  130  can respond with an unsuccessful address resolution which can cause the originating S-CSCF  106  to forward the call to the MGCF  120  via a Breakout Gateway Control Function (BGCF)  119 . The MGCF  120  can then initiate the call to the terminating PSTN CD over the PSTN network  160  to enable the calling and called parties to engage in voice and/or data communications. 
     It is further noted that cellular access base station  121  of  FIG. 1  can operate according to common wireless access protocols such as Global System for Mobile (GSM), Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), Universal Mobile Telecommunications (UMTS), World interoperability for Microwave (WiMAX), Software Defined Radio (SDR), Long Term Evolution (LTE), and so on. Other present and next generation wireless network technologies are contemplated by the subject disclosure. Multiple wireline and wireless communication technologies are also contemplated for the CDs of  FIG. 1 . 
     In the case where CD  105  and base station  121  support packet-switched voice and packet-switched data communications (e.g., LTE) or are otherwise IMS-compliant, the CD  105  may communicate via the cellular base station  121  and an Evolved Packet Core (EPC)  163  directly with the IMS network  150  as noted by the dotted lines interconnecting them. In this instance, CD  105  can operate as an IMS-compliant communication device. 
     When establishing communications between IMS CDs  101  and  102 , the communication process can be symmetric such that depending on which device is initiating the call, the terms “originating” and “terminating” can be interchangeable. In the case of asymmetric communications such as IMS CD  101  initiating communications with PSTN CD  103 , the communication request can follow the sequence of P-CSCF  104  to S-CSCF  106 , S-CSCF  106  to BGCF  119 , BGCF  119  to MGCF/MGW  120 , MGCF/MGW  120  to PSTN Network  160 , and PSTN Network  160  to PSTN CD  103 . If the PSTN CD  103  were to initiate communications with IMS CD  101 , the communication request can follow the sequence of PSTN Network  160  to MGCF/MGW  120 , MGCF/MGW  120  to I-CSCF  123 , I-CSCF  123  to S-CSCF  114 , S-CSCF  114  to I-CSCF  107 , I-CSCF  107  to S-CSCF  106 , S-CSCF  106  to P-CSCF  104 , and P-CSCF  104  to IMS CD  103 . 
     It is further noted that the communication system  100  can be adapted to support video conferencing. It is also contemplated that the CDs of  FIG. 1  can operate as wireline or wireless devices. The CDs of  FIG. 1  can further be communicatively coupled to the cellular base station  121 , a femtocell (not shown), a WiFi router, a DECT base unit, or another suitable wireless access unit to establish communications with the IMS network  150  of  FIG. 1 . 
     It is further understood that alternative forms of a CSCF can operate in a device, system, component, or other form of centralized or distributed hardware and/or software. Indeed, a respective CSCF may be embodied as a respective CSCF system having one or more computers or servers, either centralized or distributed, where each computer or server may be configured to perform or provide, in whole or in part, any method, step, or functionality described herein in accordance with a respective CSCF. Likewise, other functions, servers and computers described herein, including but not limited to, the HSS and ENUM server, the BGCF, and the MGCF, can be embodied in a respective system having one or more computers or servers, either centralized or distributed, where each computer or server may be configured to perform or provide, in whole or in part, any method, step, or functionality described herein in accordance with a respective function, server, or computer. 
     The IMS network  150  of  FIG. 1  also represents a single instance of network elements commonly utilized for end-to-end communications. In practice, however, there can be many instances of the IMS network elements shown in  FIG. 1 . Several instances of S-CSCF&#39;s and application servers can be clustered into a server office which provides communication services to potentially millions of subscribers. The P-CSCF&#39;s can be clustered in access offices with session border controllers (not shown in  FIG. 1 ). 
       FIG. 2  illustrates first and second server offices  202  and  204 . A communication device  205  can gain access to either of server offices  202  or  204  by way of first and second access offices  206  and  246 , each having a session border controller and P-CSCF pair. One or more instances of I-CSCF and HSS can be accessed by the network elements of either of server offices  202  and  204 , or the access offices  206  and  246  as shown by the dotted lines. 
     Server offices  202  and  204  can provide communication services to CD  205  by way of the S/BC  207  and P-CSCF  209  of access office  206 , and the S/BC  247  and P-CSCF  249  of access office  246 , respectively. As will be described by the methods of  FIGS. 3-6 , server office  202  can serve as a primary communication resource for serving the communication needs of CD  205 . Server office  204  may serve as a secondary communication resource for serving the communication needs of CD  205  when an unrecoverable or serious fault is detected, or any network element of server office  202  is otherwise unavailable as will be described by the methods of  FIGS. 3-6 . 
     Method  300  of  FIG. 3  will be described in conjunction with the flow diagrams of  FIGS. 7-8 . Method  300  can begin with step  302  in which a communication device such as the CD  205  of  FIG. 2  submits a SIP registration request to access office  206 . The S/BC  207  submits, at step  304 , the request to the P-CSCF  209  of access office  206 —see, e.g., step  702  of  FIG. 7 . At step  306 , the P-CSCF  209  submits the registration request to an I-CSCF  223  of the primary server office  202  (see step  704  of  FIG. 7 ). The I-CSCF  223  then transmits to the HSS  234  at step  308  a DIAMETER message in the form of a user authorization request (UAR), at step  706  of  FIG. 7 . The UAR message queries the HSS  234  for necessary information to register the CD  205 . 
     At step  310 , the HSS  234  can determine if the public user identity (PUID) of CD  205  matches a PUID of another communication device which has pre-registered with a primary and secondary communication resource of the IMS network  200 . It may be common for users to utilize more than one communication device (e.g., laptop, cellular phone, home landline phone, etc.) with the IMS network  200 . Under these circumstances, each communication device of the user can share a common PUID. In an example embodiment, a PUID can be a SIP URI (e.g., sip: user@operator) or a telephone (e.g., +1-234-456-6789). The communication devices may be distinguishable only by their Private User ID (PRID) or another unique identifier. If the HSS  234  detects that the PUID of the CD  205  does not match the PUID of other devices, then this condition may indicate to the HSS  234  that the CD  205  may be the first of a plurality of communication devices of the user which is registering with the IMS network  200  of  FIG. 2 . In this case, the HSS  234  responds to an I-CSCF at step  314  with a UAA message comprising the primary S-CSCF server name. It is I-CSCF&#39;s decision whether or not to send the registration to the same S-CSCF. 
     Upon detecting no other pre-registered communication devices with a common PUID, the HSS  234  proceeds to step  312  where it supplies the I-CSCF  223  with a server capability index transmitted in a DIAMETER message in the form of a user authorization answer (UAA), at step  708  of  FIG. 7 . The I-CSCF  223  can utilize the server capability index to choose a primary S-CSCF from one or more S-CSCFs operating in server office  202  according to the index. In the present illustration, the I-CSCF  223  selects in step  316  a primary S-CSCF shown illustratively as reference  222 . At  710  of  FIG. 7 , the I-CSCF  223  transmits a registration request to the primary S-CSCF  222  of server office  202 . 
     Upon receiving the registration request, the primary S-CSCF  222  can retrieve at step  318  from the HSS  234  authentication information associated with the CD  205 . This step can be accomplished by the primary S-CSCF  222  of server office  202  transmitting to the HSS  234  a multimedia authentication request (MAR) DIAMETER message, at step  712 , of  FIG. 7 . From this MAR message, the HSS  234  identifies the primary S-CSCF  222  selected by the I-CSCF  223 , which it records in its memory banks (or another accessible memory location, including a network memory location) and associates with CD  205 . The HSS  234  in turn can reply, at step  714  of  FIG. 7 , with a multimedia authentication answer (MAA) DIAMETER message that includes the authentication information requested by the primary S-CSCF  222 . 
     At  318 , the primary S-CSCF  222  can transmit an authentication challenge to the CD  205  as depicted by steps  716  through  720 . The authentication challenge can force the CD  205  to submit credentials in a new registration message as depicted by steps  722  through  730 . In this registration process, the CD  205  provides the primary S-CSCF  222  authentication data with the registration message. The primary S-CSCF  222  can compare this information to authentication data supplied by the HSS  234  to authenticate the CD  205 . 
     If the authentication succeeds, the primary S-CSCF  222  can proceed to  320  where it retrieves a subscription profile associated with the CD  205 . The primary S-CSCF  222  can accomplish this by transmitting to the HSS  234 , at step  732  of  FIG. 7 , a server assignment request (SAR) DIAMETER message. The HSS  234  can respond, at step  734 , with a server assignment (SAA) DIAMETER message which includes the subscription profile. The primary S-CSCF  222  can then notify the CD  205 , at step  322 , that it is registered with the IMS network  200  as depicted by steps  736  through  740  of  FIG. 7 . 
     The subscription profile can include initial filter criteria (iFCs) data which can be used to provision the application servers  224  of associated with the primary S-CSCF  222  (which will be referred to as primary application servers  224 ). iFCs can be used for providing call origination features such as caller ID blocking, 7-digit dialing, and so on as described earlier for  FIG. 1 . Each iFC can have two URIs, one that points to the primary application server  224  associated with the iFC, and another that points to a secondary application server  244  associated with the same iFC for backup communication services. At step  320 , the primary S-CSCF  222  can retrieve the iFCs from the subscription profile and register the CD  205  with the primary application servers  224  of server office  202 . Once this is accomplished, at step  322 , the primary S-CSCF  222  can return an acknowledgment to the CD  205  indicating that the registration was successful. 
     To establish a backup communication service, the CD  205  can be further adapted at step  324  to initiate a second SIP registration request which it forwards to the S/BC  247  of access office  246 . The second SIP registration request can include an indicator that specifies that the CD  205  desires backup communication services from the IMS network  200 . The flow diagram of  FIG. 8  describes how backup services may be established. The S/BC  247  submits the request to the P-CSCF  209  which in turn forwards the request to the I-CSCF  243  of server office  204 , which queries the HSS  234 . 
     Steps  324  through  326  are depicted by steps  802  and  806  of  FIG. 8 . The I-CSCF  243  detects the indicator in the second SIP registration request, and determines from this indicator that backup communication services are requested. The I-CSCF  243  can construct a UAR DIAMETER message that conforms to a diameter base protocol that is modified from current standards (such as defined by Request for Comments (RFC) 3588) to support a second registration indication in an attribute value pair (AVP) field of the UAR DIAMETER message. 
     The second registration indication included in the UAR DIAMETER message can inform the HSS  234  that the CD  205  is requesting backup services. At step  328  the HSS  234  can determine if another communication device sharing a common PUID has registered with the IMS network  200  thereby previously establishing primary and secondary S-CSCF resources. If a PUID match is not detected, the HSS  234  may assume that the CD  205  is the first to establish primary and secondary communication resources, and proceeds to step  330 , where it transmits to the I-CSCF  243  a UAA DIAMETER message (see step  808  of  FIG. 8 ) with a server capability index. Because of the second registration indication in the UAR DIAMETER message, the HSS  234  may know not to initiate a de-registration process that would deregister the CD  205  from the primary server office  202  as would commonly occur in prior art systems. Upon receiving the UAA DIAMETER message, I-CSCF  243  proceeds to step  332  where it selects from the server capability index a secondary S-CSCF shown illustratively as reference  242 . At step  810  of  FIG. 8 , the I-CSCF  243  transmits a registration request to the secondary S-CSCF  222  at server office  204 . 
     Upon receiving the SIP registration request from the I-CSCF  243 , the secondary S-CSCF  242  can retrieve, at step  336 , from the HSS  234  authentication information associated with the CD  205 . This step can be accomplished by the secondary S-CSCF  242  of server office  204  transmitting to the HSS  234  a multimedia authentication request MAR DIAMETER message, at step  812 , of  FIG. 8 . The MAR DIAMETER message includes the second registration indication in the AVP field of the MAR DIAMETER message. The HSS  234  may determine from the MAR DIAMETER message that the I-CSCF  243  has selected the secondary S-CSCF  242 , and may record this selection in its memory (or another accessible memory location, including a network memory location). The HSS  234  also detects from the second registration indication in the AVP field that the MAR DIAMETER message is associated with establishing backup services for the CD  205  and thereby does not initiate a deregistration of the CD  205  from the primary server office  202 . The HSS  234  can in turn reply, at step  814 , with a multimedia authentication answer MAA DIAMETER message that includes the authentication information requested by the secondary S-CSCF  242 . 
     At step  336 , the secondary S-CSCF  242  has the option to transmit another authentication challenge to the CD  205  as depicted by steps  816  through  820 . The authentication challenge can once again force the CD  205  to submit credentials in a new registration message as depicted at steps  822  through  830 . In this registration process, the CD  205  provides the secondary S-CSCF  242  authentication data with the registration message. The secondary S-CSCF  242  can compare this information to authentication data supplied by the HSS  234  to authenticate the CD  205 . 
     If the authentication succeeds, the secondary S-CSCF  242  can proceed to  338  where it retrieves a subscription profile associated with the CD  205 . The secondary S-CSCF  242  can accomplish this by transmitting to the HSS  234 , at step  832  of  FIG. 8 , a server assignment request SAR DIAMETER message. Once again the SAR DIAMETER message includes an AVP field with the second registration indication to prevent the HSS  234  from deregistering the CD  205  from the primary sever office  202 . The HSS  234  can respond, at step  834 , with a server assignment SAA DIAMETER command which includes the subscription profile. The secondary S-CSCF  242  can then notify the CD  205 , at step  340 , that it is registered with the IMS network  200  as depicted at steps  836  through  840  of  FIG. 8 . If some iFCs provide the information (e.g., server URI) for both primary and secondary application server instances, the S-CSCF will send a 3 rd -party registration request to the secondary application server. The 3 rd -party registration request will also flag the secondary indication. 
     The above embodiments describe a first registration of a single communication device of a user. Assume CD  205  has completed the primary and secondary registrations described above. Further assume that another communication device of the user sharing a common PUID attempts to register with the IMS network  200  of  FIG. 2 . The registration request can emerge from other access offices not shown in  FIG. 2 . During the registration process, the steps of  FIG. 3  would take place as described above. However, at step  310 , the HSS  234  (or another HSS in the IMS network  200 ) would detect that the PUID of CD  205  and the PUID of the second communication device of the user match. In this instance, an I-CSCF serving the second communication device of the user (which may differ from the I-CSCF  223  of server office  202 ) may be directed by the HSS  234  (or another HSS) to step  314 . In this step the I-CSCF would decide to register the second communication device of the user with the primary S-CSCF  222  selected for CD  205 . The primary S-CSCF  222  would continue the registration process as previously described at step  318  and subsequent steps thereafter. 
     Upon the second communication device initiating a second registration request for a secondary communication resource, the HSS  234  (or another HSS in the IMS network  200 ) would detect at step  328  that the PUID of CD  205  and the PUID of the second communication device of the user match. The I-CSCF serving the second communication device of the user (which may differ from the I-CSCF  243  of server office  204 ) may be directed by the HSS  234  to step  334 . In this step, the HSS responds with the secondary S-CSCF server name to which the CD had previously registered. The I-CSCF can register the second communication device of the user with the secondary S-CSCF  243  selected for CD  205  or another S-CSCF listed in the server capability index. Assuming the former, the secondary S-CSCF  242  would continue the registration process as previously described at step  336  and subsequent steps thereafter. 
     Based on the aforementioned embodiments, any number of communication devices of a user sharing a common PUID will share the same primary and secondary S-CSCF&#39;s  222  and  242  regardless of which access offices each device may be utilizing. 
     It is further noted that a determination of which S-CSCF from which server office  204  will provide backup services to the CD  205  can be preconfigured in the IMS network  200 , notably in all P-CSCFs, I-CSCFs and S-CSCFs. Each P-CSCF and I-CSCF can be preconfigured with a table of S-CSCFs and their companion secondary S-CSCFs, respectively. Additionally, each S-CSCF can be preconfigured with the information of its remote secondary S-CSCF. In one embodiment, each application server can also be preconfigured with information that identifies a remote secondary application server which provides backup services. In one embodiment, the primary and secondary application servers can be identified in the iFCs of the CD  205 , which can provide more flexibility to manage the IMS network  200  of  FIG. 2 . 
     It is also noted that the selection of server office  204  as a backup communication service can be based on a geographic disparity between server offices  202  and  204  or other criteria (such as load balancing, bandwidth utilization, etc.) suitable for satisfying the service provider&#39;s business or technical objectives. 
     Once registration process described above has been completed, the CD  205  (and other communication devices of the user), the P-CSCF  209  of access office  206 , and the I-CSCF  223  of the primary server office  202  can be configured to redirect communications from the primary server office  202  to the secondary server office  204  upon detecting a communication fault or other event that prevents utilization of the primary server office  202  for processing calls. Furthermore, in some embodiments, one or more registration processes may support registrations with more than two resources (e.g., primary and secondary communication resources). For example, there may be a registration for a tertiary communication resource, which enables access via a tertiary server office. Where more than one backup communication resource is available, the selection of a backup communication resources upon failure (e.g., of a primary communication resource) may be based upon current load, capacity, throughput and/or other measure of availability of a communication resource or based upon proximity of a CD to the respective backup communication resources. 
     Referring to  FIG. 4 , method  400  describes illustrative embodiments in which the CD  205  redirects a call to the secondary server office  204 . At step  402 , the CD  205  can initiate a call origination directed to a second communication device, which it forwards to access office  206 . In response, the P-CSCF  209  can transmit a SIP INVITE message as previously described for  FIG. 1  to the primary S-CSCF  222  for processing. If the primary S-CSCF  222  is operating properly, it will process the SIP INVITE message, at step  408 , and establish a call with a second communication device as previously described in  FIG. 1 . 
     If, however, the primary S-CSCF  222  or other network elements of the primary server office  202  such as, for example, the primary application servers  224  are malfunctioning or unavailable and the CD  205  fails to receive, at step  406 , a response within a timeout period established by the service provider, the CD  205  can attempt a retransmission of the SIP INVITE message. If a response timeout is encountered again, at step  406 , and the number of retransmissions exceeds a predetermined threshold set by the service provider, then the CD  205  can proceed to step  410  where it identifies a communication fault, and redirects the call to the secondary server office  204  by submitting a SIP INVITE message to access office  246 . 
     The CD  205  can decide that the primary server office  202  is not available for service when the primary server office  202  does not respond to SIP requests or the CD  205  receives, for example, a SIP  408  or  503  error message, at step  406 , which may indicate system level faults in any one of the network elements of access office  206  or the primary server office  202 . For example, an example error message could indicate a server or network unavailability, communication slowness or congestion, or type of failure. To detect communication faults early, the CD  205  can submit, at step  404 , SIP status check messages to the primary server office  202 . If no response is detected after the timeout period and retransmissions have exceeded an established threshold, then the CD  205  can proceed to steps  410  and  412  as previously described. 
     Referring to  FIG. 5 , method  500  describes illustrative embodiments in which the P-CSCF  209  of access office  206  may redirect calls to the secondary S-CSCF  242  of server office  204 . At step  502 , the P-CSCF  209  can detect a call origination initiated by the CD  205  at access office  206 . In response, the P-CSCF  209  can transmit a SIP INVITE message as previously described for  FIG. 1  to the primary S-CSCF  222  for processing. If the primary S-CSCF  222  is operating properly, it will process the SIP INVITE message, at step  508 , and establish a call between the CD  205  and a second communication device as previously described. 
     If, however, the primary S-CSCF  222  or other network elements such as, for example, the primary application servers  224  in the primary server office  202  are malfunctioning or unavailable and the P-CSCF  209  of access office  206  fails to detect, at step  506 , a response within a timeout period established by the service provider, the P-CSCF  209  can attempt a retransmission of the SIP INVITE message. If a response timeout is encountered again, at step  506 , and the number of retransmissions exceeds a predetermined threshold set by the service provider, then the P-CSCF  209  can proceed to step  510  where it identifies a communication fault, and returns to the CD  205  a SIP error message (e.g.,  503  or  408 ) at step  512  to indicate that a system level fault has been encountered, which directs the CD  205  to redirect the call origination at step  514  to the secondary server office. 
     A P-CSCF  209  can decide that a primary S-CSCF  222  is not available for service when the primary S-CSCF  222  does not respond to SIP requests from the P-CSCF or it receives a SIP  408  or  503  error message, at step  506 , to indicate system level faults in the primary server office  202 . To detect communication faults early, the P-CSCF  209  can submit, at step  504 , SIP status check messages to the primary S-CSCF  222 . If no response is detected after the timeout period and retransmissions have exceeded an established threshold, then the P-CSCF  209  can proceed to steps  510  and  512  as previously described. 
     Referring to  FIG. 6 , method  600  describes illustrative embodiments in which the I-CSCF  222  can redirect calls to the secondary server office  204  when an unrecoverable or serious communication fault is detected at the primary server office  202 . At step  602 , the I-CSCF  223  can receive a call origination SIP message as previously described for  FIG. 1  and query the HSS at step  604 . The I-CSCF  223  can receive from the HSS  234  at step  604  an identification of the primary S-CSCF  222  and secondary S-CSCF  242  server names. In one embodiment, the I-CSCF in step  608  forwards the SIP message to the primary S-CSCF  222  in step  608 . Step  612  represents that communications between the I-CSCF  223  and the primary S-CSCF  222  are successful in reaching the terminating communication device as was described for  FIG. 1 . 
     If, however, the I-CSCF  223  attempts to communicate with the primary S-CSCF  222  at step  608  and no response is detected after the timeout period, or an error message such as SIP  408  or  503  is detected at step  610  and retransmissions have exceeded an established threshold, then the I-CSCF  223  can proceed to step  614  where it determines an unrecoverable or serious communication fault has occurred. At step  616 , the I-CSCF  223  can establish communications with the secondary S-CSCF  242  and complete the call from the secondary server office  204 . At step  609 , the I-CSCF  223  can also be adapted to transmit status check messages to the primary S-CSCF  222 . If no response is received within a timeout period at step  610 , and retransmission attempts have exceeded a threshold set by the service provider, then the I-CSCF  223  can resort to utilizing the secondary S-CSCF  242  as described in steps  614  and  616 . 
       FIG. 9  depicts an exemplary diagrammatic representation of a machine in the form of a computer system  900  within which a set of instructions, when executed, may cause the machine to perform any one or more of the methods discussed above. One or more instances of the machine can operate, for example, as the CDs and the network elements of the IMS networks shown in  FIGS. 1-2  as described above. In some embodiments, the machine operates as a standalone device. In some embodiments, the machine may be connected (e.g., using a network) to other machines. In a networked deployment, the machine may operate in the capacity of a server or a client user machine in server-client user network environment, or as a peer machine in a peer-to-peer (or distributed) network environment. 
     The machine may comprise a server computer, a client user computer, a personal computer (PC), a tablet PC, a smart phone, a laptop computer, a desktop computer, a control system, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. It will be understood that a communication device of the subject disclosure includes broadly any electronic device that provides voice, video or data communication. Further, while a single machine is illustrated, the term “machine” shall also be taken to include any collection of machines that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methods discussed herein. 
     The computer system  900  may include a processor  902  (e.g., a central processing unit (CPU), a graphics processing unit (GPU, or both), a main memory  904  and a static memory  906 , which communicate with each other via a bus  908 . The computer system  900  may further include a video display unit  910  (e.g., a liquid crystal display (LCD), a flat panel, or a solid state display. The computer system  900  may include an input device  912  (e.g., a keyboard), a cursor control device  914  (e.g., a mouse), a disk drive unit  916 , a signal generation device  918  (e.g., a speaker or remote control) and a network interface device  920 . 
     The disk drive unit  916  may include a non-transitory machine-readable medium  922  on which is stored one or more sets of instructions (e.g., software  924 ) embodying any one or more of the methods or functions described herein, including those methods illustrated above. The instructions  924  may also reside, completely or at least partially, within the main memory  904 , the static memory  906 , and/or within the processor  902  during execution thereof by the computer system  900 . The main memory  904  and the processor  902  also may constitute machine-readable media. 
     Dedicated hardware implementations including, but not limited to, application specific integrated circuits, programmable logic arrays and other hardware devices can likewise be constructed to implement the methods described herein. Applications that may include the apparatus and systems of various embodiments broadly include a variety of electronic and computer systems. Some embodiments implement functions in two or more specific interconnected hardware modules or devices with related control and data signals communicated between and through the modules, or as portions of an application-specific integrated circuit. Thus, the example system is applicable to software, firmware, and hardware implementations. 
     In accordance with various embodiments of the subject disclosure, the methods described herein are intended for operation as software programs running on a computer processor. Furthermore, software implementations can include, but not limited to, distributed processing or component/object distributed processing, parallel processing, or virtual machine processing can also be constructed to implement the methods described herein. 
     The subject disclosure contemplates a machine readable medium containing instructions  924 , or that which receives and executes instructions  924  from a propagated signal so that a device connected to a network environment  926  can send or receive voice, video or data, and to communicate over a network  926  using the instructions  924 . The instructions  924  may further be transmitted or received over the network  926  via the network interface device  920 . 
     While the machine-readable medium  922  is shown in an example embodiment to be a single medium, the term “machine-readable medium” should be taken to include a single medium or multiple media (e.g., a centralized or distributed database, and/or associated caches and servers) that store the one or more sets of instructions. The term “machine-readable medium” shall also be taken to include any medium that is capable of storing, encoding or carrying a set of instructions for execution by the machine and that cause the machine to perform any one or more of the methods of the subject disclosure. 
     The term “machine-readable medium” shall accordingly be taken to include, but not be limited to: solid-state memories such as a memory card or other package that houses one or more read-only (non-volatile) memories, random access memories, or other re-writable (volatile) memories; and magneto-optical or optical medium such as a disk or tape. Accordingly, the disclosure is considered to include any one or more of a machine-readable medium or a distribution medium, as listed herein and including art-recognized equivalents and successor media, in which the software implementations herein are stored. 
     Although the present specification describes components and functions implemented in the embodiments with reference to particular standards and protocols, the disclosure is not limited to such standards and protocols. Each of the standards for Internet and other packet switched network transmission (e.g., TCP/IP, UDP/IP, HTML, HTTP) represent examples of the state of the art. Such standards are from time-to-time superseded by faster or more efficient equivalents having essentially the same functions. Wireless standards for device detection (e.g., RFID), short-range communications (e.g., Bluetooth, WiFi, Zigbee), and long-range communications (e.g., WiMAX, GSM, CDMA, LTE) are contemplated for use by computer system  900 . 
     The illustrations of embodiments described herein are intended to provide a general understanding of the structure of various embodiments, and they are not intended to serve as a complete description of all the elements and features of apparatus and systems that might make use of the structures described herein. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. Other embodiments may be utilized and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. Figures are also merely representational and may not be drawn to scale. Certain proportions thereof may be exaggerated, while others may be minimized. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense. 
     Although specific embodiments have been illustrated and described herein, it should be appreciated that any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations or variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the above description. 
     The Abstract of the Disclosure is provided with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter.