Abstract:
A method for delivering SMS to ATs in a first communication network providing DO and 1x interfaces and for offloading the delivery of the SMS from a second communication network including an MSC, comprising monitoring a DO control channel for pages by an AT and delivering the SMS in SIP to the AT over the DO interface. The method may further comprise tuning the AT to the DO interface and determining whether the AT is SIP registered for using the DO interface. An application server determines whether the AT is SIP registered for using the DO interface. When the DO interface is not available, the SMS may be delivered over the 1x interface and the method further comprises the AT sending a special SMS to an SMS gateway, which causes the application server to remember that the AT is now monitoring the first communication network including a circuit-switched network.

Description:
CLAIM OF PRIORITY UNDER 35 U.S.C. §119 
     The present Application for Patent claims priority to Provisional Application No. 60/750,234, entitled “System and Method for Delivering Short Messages on DO and 1x Networks,” filed Dec. 13, 2005, and assigned to the assignee hereof and expressly incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field 
     The present invention generally relates to communication and, in particular, to techniques for providing the Short Message Service (SMS) to be delivered on DO and 1x networks or interfaces and for decreasing the load on the Mobile Switching Center (MSC) interface. 
     2. Background 
     Wireless communication networks are widely deployed to provide various types of services such as voice, packet data, broadcast, and so on. These wireless networks include Code Division Multiple Access (CDMA) networks, Global System for Mobile Communications (GSM) networks, Universal Mobile Telecommunications System (UMTS) networks, and so on. A network typically refers to a deployment of a system, although these two terms are also used interchangeably. 
     Each wireless network utilizes a particular air interface to support over-the-air communication and typically further implements a particular mobile networking protocol that supports roaming and advanced services. For example, a CDMA network utilizes a CDMA air interface and an ANSI-41 networking protocol. The CDMA network may implement one or more CDMA standards such as IS-2000 (1xEV), IS-856 (1x-EVDO), IS-95, and so on. The CDMA network may provide a broadcast service that broadcasts messages to users within the network. The broadcast messages can carry various types of information such as news, traffic reports, weather information, and so on. 
     The broadcast services in the CDMA networks are implemented with Short Message Service (SMS), which is a service that supports the exchange of short messages between a wireless network and wireless devices (e.g., cellular phones). SMS is network technology dependent, and different SMS implementations have been defined for ANSI-41. Each SMS implementation has different capabilities and utilizes different message types and formats for sending SMS messages. The SMS implementation for an ANSI-41 network is described in a document TIA/EIA-637-B, entitled “Short Message Service for Wideband Spread Spectrum Systems,” which is publicly available and incorporated herein by reference. As the amount of broadcast messages increases in the CDMA networks, there is a need to decrease the load on the Mobile Switching Centers (MSCs) by delivering the SMS on the DO interface when it is available. 
     SUMMARY 
     The invention relates to systems and methods for decreasing the load on the MSCs by delivering the SMS on the DO interface when it is available. In one aspect, a method is described for delivering SMS to access terminals in a first communication network providing the DO interface and the 1x interface and for offloading the delivery of the SMS from a second communication network. The method comprises monitoring a DO control channel for pages by an Access Terminal (AT) and delivering the SMS encapsulated in Session Initiation Protocol (SIP) to the AT over the DO interface when it is available. In this aspect, the second communication network includes the MSC. The method may further comprise tuning the AT to the DO interface when the DO interface is available, and determining whether the AT is SIP registered for using the DO interface. In this aspect, an application server determines whether the AT is SIP registered for using the DO interface. 
     In another aspect, when only the 1x interface is available, the method may further comprise notifying the second communication network that the AT did not detect the DO interface, deregistering the AT from the SIP network for SMS service, and delivering the SMS over the 1x interface. With this aspect, the method further comprises the AT sending a special SMS to an SMS gateway, wherein the SMS gateway causes an application server to remember that the AT is now monitoring the first communication network including a circuit-switched network. The SMS gateway further deregisters the AT from the SIP network for SMS service. When the DO interface is available again, the method further comprises performing a SIP registration for the AT to allow the SMS to be delivered over the DO interface. 
     In another aspect, the AT and the first and second communication networks may engage in a SIP notification where the AT indicates its ability to receive or send SMS on the DO interface and the networks may indicate either a positive or a negative response. With this aspect, the AT originates SMS using SIP on the DO interface if a SIP registration succeeds and if the SIP notification indicates SMS capability. 
     In another aspect, the OTASP may be used to provision the AT with the capability of SMS over the DO interface. With this aspect, if the AT gets provisioned with SMS over the DO interface capability, then the AT will send or receive SMS over SIP on the DO interface when the DO interface is available. 
     In another aspect, the method may further comprise a CCCF/NeDS for engaging the first and second communication networks in a SIP notification to determine whether the SMS or voice call is to be delivered on the DO interface or the 1x interface. With this aspect, if the AT is not SIP registered, then the method may further comprise delivering the SMS or voice call on the 1x interface. If the AT is SIP registered, wherein if the AT is indicated as being capable of receiving voice using SIP notification, and wherein if a record for indicating whether the AT is monitoring the DO interface or the 1x interface is set to the DO interface, then the voice call is delivered on the DO interface. If the AT is SIP registered, wherein if the AT is indicated as being capable of receiving SMS using SIP notification, and wherein if a record for indicating whether the AT is monitoring the DO interface or the 1x interface is set to the DO interface, then the SMS is delivered on the DO interface. 
     In another aspect, wherein if all of the active set members support Voice over IP (VoIP), then the AT sends a SIP notification indicating that a voice call is to be delivered on the DO interface. With this aspect, wherein if at least one of the active set members does not support VoIP, then the AT sends a SIP notification indicating that the voice call is to be delivered on the 1x interface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a network supporting signaling flows for SMS origination and termination; 
         FIG. 2  illustrates a signaling flow for a scenario where a terminal is SIP registered and tuned to HRPD, and the terminal receives an SMS message; 
         FIG. 3  illustrates a signaling flow for a scenario where a terminal is not IMS registered but is CS registered and tuned to 1x, and the terminal receives an SMS message; 
         FIG. 4  illustrates a signaling flow for a scenario where a terminal is not IMS registered but is CS registered and tuned to 1x, and the SMS-GW queries the application server to determine whether the terminal is IMS registered; 
         FIG. 5  illustrates a signaling flow for a scenario where a terminal is CS registered and receives an SMS message; 
         FIG. 6  illustrates a signaling flow for a scenario where a terminal that is SIP registered and tuned to HRPD to originate an SMS message; 
         FIG. 7  illustrates a call flow of a UE initiated notification after 1x CS registration; 
         FIG. 8  illustrates a signaling flow of how the invention addresses the issue denial-of-service between multiple access terminals; and 
         FIG. 9  illustrates a signaling flow of how the invention avoids the race condition. 
     
    
    
     DETAILED DESCRIPTION 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. 
     The present invention defines the interactions and signaling flows between, among other things, the Short Message Service-Gateway (SMS-GW) and the
         Home Subscriber Server (HSS) and   Serving-Call/Session Control Function (S-CSCF).       

     In particular, the invention allows the core network to know as closely as possible the current accessibility of the UE and to deliver services efficiently across the appropriate Access Network (AN) while minimizing the impact on the legacy systems. 
     The following are acronyms and definitions used to describe the invention: 
     ACRONYMS 
     ANSI American National Standards Institute 
     AS Application Server 
     AT Access Terminal 
     AUTHR Authentication Response 
     BGCF Border Gateway Control Function 
     BSC Base Station Controller 
     CIC Circuit Identification Code 
     CS Circuit-Switched 
     CSCF Call/Session Control Function 
     DBM Data Burst Message 
     ESN Electronic Serial Number 
     ESS Extended Service Set 
     GW Gateway 
     HDR High Data Rate 
     HLR Home Location Register 
     HO Handoff 
     HRPD High Rate Packet Data 
     HSS Home Subscriber Server 
     IMS IP Multimedia Subsystem 
     IMSI International Mobile Subscriber Identity 
     IPv4 Internet Protocol version 4 
     IPv6 Internet Protocol version 6 
     I-CSCF Interrogating-CSCF 
     ISUP ISDN User Part 
     MDN Mobile Directory Number 
     Megaco Media Gateway Control 
     MGCF Media Gateway Control Function 
     MOW Media Gateway 
     MIN Mobile Identification Number 
     M-L Mobile-to-Land 
     M-M Mobile-to-Mobile 
     MMD Multimedia Domain 
     MS Mobile Station 
     MSC Mobile Switching Center 
     OTASP Over the Air Service Provisioning 
     PCM Pulse Code Modulation 
     P-CSCF Proxy-CSCF 
     PDIF Packet Data Interworking Function 
     PDSN Packet Data Serving Node 
     PSTN Public Switched Telephone Network 
     RTP Real-time Transport Protocol 
     SIP Session Initiation Protocol 
     SMS Short Message Service 
     S-CSCF Serving-CSCF 
     SDP Session Description Protocol 
     STA Station 
     TDM Time Division Multiplexing 
     TLDN Temporary Local Directory Number 
     UDP User Datagram Protocol 
     UE User Equipment 
     VCC AS Voice Call Continuity Application Server. 
     VoIP Voice over IP 
     VLR Visitor Location Register 
     WLAN Wireless Local Area Network 
     DEFINITIONS 
     
         
         VCC AS An entity that:
       (1) assists in routing calls received from either the IMS network or PSTN to a terminal that is either 1x CS registered or IMS registered, or both and   (2) is involved in voice call setup signaling to facilitate HRPD/WLAN VoIP-to-1x circuit-switched voice handoffs and 1x circuit-switched voice to HRPD/WLAN VoIP handoffs.   
     
         SMS-GW An entity that stores and forwards SMS messages to and from a terminal that is either IMS registered or 1x CS registered. 
       
    
     An HDR subscriber station, referred to herein as an access terminal, may be mobile or stationary, and may communicate with one or more HDR base stations, referred to herein as Modem Pool Transceivers (MPTs). An access terminal transmits and receives data packets through one or more modem pool transceivers to an HDR Base Station Controller (BSC) or a Modem Pool Controller (MPC). Modem pool transceivers and modem pool controllers are parts of a network called an access network. An access network transports data packets between multiple access terminals. The access network may be further connected to additional networks outside the access network, such as a corporate intranet or the Internet, and may transport data packets between each access terminal and such outside networks. An access terminal that has established an active traffic channel connection with one or more modem pool transceivers is called an active access terminal, and is said to be in a traffic state. An access terminal that is in the process of establishing an active traffic channel connection with one or more modem pool transceivers is said to be in a connection setup state. An access terminal may be any data device that communicates through a wireless channel or through a wired channel. An access terminal may further be any of a number of types of devices including but not limited to PC card, compact flash, external or internal modem, or wireless or wireline phone. The communication link through which the access terminal sends signals to the modem pool transceiver is called a reverse link. The communication link through which a modem pool transceiver sends signals to an access terminal is called a forward link. 
     Signaling Flows for SMS 
     Referring to  FIG. 1 , there is shown an architecture  100  supporting signaling flows for SMS origination and termination. For example, if a mobile is registered in both a home network  102  and a visited network  104 , an SMS-GW  106  may decide which channel to use for initial SMS delivery attempt to the mobile, i.e., either IMS or over the circuit-switched network. More specifically, the SMS-GW  106  may select the delivery channel based on either the SMS-GW local configuration information (i.e., operator&#39;s preference), or the mobile user provisioning information (i.e., the subscriber&#39;s preference), or both. If both the local configuration and the user provisioning information are supported by the operator, then the SMS-GW  106  may base the channel selection on the user provisioning information (i.e., the subscriber&#39;s preference). In case the initial preferred delivery attempt fails, then the SMS-GW  106  may try to deliver via the alternate channel. 
     Initial IMS Delivery Attempt: Success Scenario 
     Referring to  FIG. 2 , there is shown a signaling flow for a scenario where a terminal is SIP registered and tuned to HRPD, and receives an SMS message. With this aspect,
         1. The SMS-GW  106  for UE  200  receives an ANSI-41 SMDPP message  202  for UE  200  from an originating SMS-GW. The originating SMS-GW is not shown for brevity.   2. The SMS-GW  106  responds by sending an ANSI-41 smdpp message  204  back to the originating SMS-GW.   3. The SMS-GW  106  is provisioned to use IMS, and hence the SMS-GW  106  sends a Diameter User-Data-Request message  206  to the HSS  108  to determine whether or not UE  200  is IMS registered. The SMS-GW  106  queries the HSS  108  using the MDN of the UE  200  received in step  1 .   4. The HSS  108  responds by sending a Diameter User-Data-Answer message  208  to the SMS-GW  106  indicating that UE  200  is IMS registered. If the UE  200  is IMS registered, then the HSS  108  also returns UE  200 &#39;s S-CSCF  114  address.   5. The SMS-GW  106  sends a SIP MESSAGE  210  to UE  200 &#39;s S-CSCF  114  containing the SMS message received in Step  1 . The Content-Type value associated with the SIP MESSAGE may be “application/vnd.3gpp2.sms”. The payload of the SIP MESSAGE may all contain a binary encoded SMS transport layer message.   6. The S-CSCF  114  forwards the SIP MESSAGE  212  to UE  200  via UE  200 &#39;s P-CSCF. The UE  200 &#39;s P-CSCF is not shown for brevity.   7. The UE  200  responds by sending a SIP  200  OK message  214  back to the SMS-GW  106  via UE  200 &#39;s P-CSCF and S-CSCF  114 . The UE  200 &#39;s P-CSCF is not shown for brevity.   8. The UE  200 &#39;s S-CSCF  114  forwards the SIP  200  OK message  216  to the SMS-GW  216 .   9. If required in the original ANSI-41 SMDPP message, then the SMS-GW  106  generates ANSI-41 SMDPP message  218  to the originating SMS-GW to inform it about the delivery status.   10. The originating SMS-GW responds by sending an ANSI-41 smdpp message  220  back to SMS-GW  106 .
 
Initial IMS Delivery Attempt, Not IMS Registered: CS Delivery Success
       

     In another aspect,  FIG. 3  illustrates a signaling flow for a scenario where a terminal is not IMS registered but is CS registered and tuned to 1x, and receives an SMS message. With this aspect,
         1. The SMS-GW  106  for the UE receives a MAP SMDPP message  302  for the UE from an originating SMS-GW. The originating SMS-GW and the UE are not shown for brevity.   2. The SMS-GW  106  responds by sending a MAP smdpp message  304  back to the sender of the MAP SMDPP message.   3. The SMS-GW  106  is provisioned to use IMS, and hence the SMS-GW  106  sends a Diameter User-Data-Request message  306  to the HSS  108  to determine whether or not the UE is IMS registered. The SMS-GW  106  queries the HSS  108  using the MDN of the UE received in step  1 .   4. The HSS  108  responds by sending a Diameter User-Data-Answer message  308  to the SMS-GW  106  indicating that the UE is not IMS registered.   5. The SMS-GW  106  sends a MAP SMSREQ  310  to the HLR  116  containing the UE&#39;s MDN in order to determine the UE&#39;s current routing information and retrieve the UE&#39;s MIN info.   6. The HLR  116  sends a MAP SMSREQ message  312  back to the SMS-GW  106  containing the UE&#39;s MIN and SMS address (i.e., MSC/VLR  118  address).   7. The SMS-GW  106  sends a MAP SMDPP message  314  containing the UE&#39;s MIN to a MSC  300  identified by the SMS address (i.e., MSC/VLR  118  address) in the Visited Network for the UE.   8. The SMS message is delivered to the UE on 1x and a Layer 2 ACK is received. The UE is not shown for brevity.   9. The MSC  300  in the Visited Network for the UE sends a MAP smdpp message  318  back to the SMS-GW  106 .   10. If required in the original ANSI-41 SMDPP message, then the SMS-GW  106  generates an ANSI-41 SMDPP message  320  to the originating SMS-GW to inform it about the delivery status.   11. The originating SMS-GW responds by sending an ANSI-41 smdpp message  322  back to the SMS-GW  106 .
 
Initial IMS Delivery Attempt: Querying Application Server Whether Terminal is IMS Registered
       

     In another aspect,  FIG. 4  illustrates a signaling flow for a scenario where a terminal is not IMS registered but is CS registered and tuned to 1x, and the SMS-GW queries the application server to determine whether the terminal is IMS registered. With this aspect,
         1. The SMS-GW  106  for the UE receives a MAP SMDPP message  402  for the UE from an originating SMS-GW. The originating SMS-GW and the UE are not shown for brevity.   2. The SMS-GW  106  responds by sending a MAP smdpp message  404  back to the sender of the MAP SMDPP message.   3. The SMS-GW  106  is provisioned to use IMS, and hence the SMS-GW  106  sends a query  406  to the VCC AS  107  to determine whether or not the UE is IMS registered. The SMS-GW  106  queries the VCC AS  107  using the MDN of the UE received in step  1 .   4. The VCC AS  107  replies by sending a response  408  to the SMS-GW  106  indicating that the UE is not IMS registered.   5. The SMS-GW  106  sends a MAP SMSREQ  410  to the HLR  116  containing the UE&#39;s MDN in order to determine the UE&#39;s current routing information and retrieve the UE&#39;s MIN info.   6. The HLR  116  sends a MAP SMSREQ message  412  back to the SMS-GW  106  containing the UE&#39;s MIN and SMS address (i.e., MSC/VLR  118  address).   7. The SMS-GW  106  sends a MAP SMDPP message  414  containing the UE&#39;s MIN to a MSC  300  identified by the SMS address (i.e., MSC/VLR  118  address) in the Visited Network for the UE.   8. The SMS message is delivered to the UE on 1x and a Layer 2 ACK is received.   9. The MSC  400  in the Visited Network for the UE sends a MAP smdpp message  418  back to the SMS-GW  106 .   10. If required in the original ANSI-41 SMDPP message, then the SMS-GW  106  generates an ANSI-41 SMDPP message  420  to the originating SMS-GW to inform it about the delivery status.   11. The originating SMS-GW responds by sending an ANSI-41 smdpp message  422  back to the SMS-GW  106 .
 
Initial CS Delivery Attempt: Success Scenario
       

     In another aspect,  FIG. 5  illustrates a signaling flow for a scenario where a terminal is CS registered and receives an SMS message. With this aspect,
         1. The SMS-GW  106  for the UE receives a MAP SMDPP message  502  for the UE from an originating SMS-GW. The originating SMS-GW is not shown for brevity. Also, the UE is not shown for brevity.   2. The SMS-GW  106  responds by sending a MAP smdpp response message  504  back to the sender of the MAP SMDPP message.   3. The SMS-GW  106  is provisioned to use CS, and hence the SMS-GW  106  sends a MAP SMSREQ  506  to the HLR  116  containing the UE&#39;s MDN in order to determine the UE&#39;s current routing information and retrieve the UE&#39;s MIN info.   4. The HLR  116  sends a MAP smsreq response message  508  back to the SMS-GW  106  containing the UE&#39;s MIN and SMS address (i.e., MSC/VLR  118  address).   5. The SMS-GW  106  sends a MAP SMDPP message  510  containing the UE&#39;s MIN to the MSC  500  identified by the SMS address (i.e., MSC/VLR  118  address) in the Visited Network for the UE.   6. The SMS message is delivered to the UE on 1x and a Layer 2 ACK is received. The UE is not shown for brevity.   7. The MSC  500  in the Visited Network for the UE sends a MAP smdpp success response message  512  back to the SMS-GW  106 .   8. If required in the original MAP SMDPP message, then the SMS-GW  106  generates a new MAP SMDPP message  514  to the originating SMS-GW to inform it about the delivery status.   9. The originating SMS-GW responds by sending a MAP smdpp response message  516  back to the SMS-GW  106 .
 
SMS Origination by UE that is IMS Registered
       

     In another aspect,  FIG. 6  illustrates a signaling flow for a scenario where a terminal that is SIP registered and tuned to HRPD originates an SMS message. With this aspect,
         1. AT/MS sends a SIP MESSAGE  602  to another SMS user via UE  600 &#39;s P-CSCF and S-CSCF  114 . The UE  600 &#39;s P-CSCF is not shown for brevity. The Content-Type value associated with the SIP MESSAGE may be “application/vnd.3gpp2.sms”. The payload of the SIP MESSAGE may contain a binary encoded SMS transport layer message.   2. Based upon a filter Service Point Trigger, the S-CSCF  114  forwards the SIP MESSAGE  604  to the SMS-GW  106  for AT/MS  600 .   3. The SMS-GW  106  responds by sending a SIP  202  Accepted message  606  to the UE  600  via AT/MS  600 &#39;s S-CSCF and P-CSCF.   4. The S-CSCF  114  forwards the SIP MESSAGE  608  to the UE  600  via the UE  600 &#39;s P-CSCF. The UE  600 &#39;s P-CSCF is not shown for brevity.   5. The SMS-GW  106  sends an ANSI-41 SMDPP message  610  to the terminating SMS-GW. The terminating SMS-GW is not shown for brevity.   6. The terminating SMS-GW responds by sending an ANSI-41 smdpp message  612  to the SMS-GW  106  for the UE  600 .
 
Registration Notification
       

     In another aspect, in order to remove any ambiguity in the network for call delivery, the Voice Call Continuity Application Server (VCC AS) needs to know where the MS is reachable. That is, when the UE does not detect the DO air-interface (i.e., only 1x is available), the UE sends an SMS addressed to a predetermined number, e.g., E.164, associated with the VCC AS PSI. Referring to  FIG. 7 , the following describes a call flow of the UE initiated notification after 1x CS registration.
         1. On detecting EV-DO loss of coverage, the UE  700  registers on 1x if necessary. The UE  700  encapsulates the notification update  712  in an SMS message addressed to the VCC AS  107  (i.e., addressed to a predetermined number, e.g., E.164, associated with the VCC AS PSI, which is either provisioned at the UE or received during IMS registration procedures).   2. An ADDS_transfer message  714  is sent from the 1x BSC  702  to the Visited MSC  704 .   3. The Visited MSC  704  performs a VLR lookup for the SMSC address to deliver the SMS message  716 .   4. The Visited MSC  704  forwards the SMS message  718  to the SMSC  708 .   5. On receipt of a SMS message, the SMSC  708  performs HLR  116  lookup  720  to locate the target address to deliver the message.   6. The SMSC  708  then delivers the message  722  to the VCC AS  107  the E.164 number resolved to.   7. The VCC AS  107  updates the state of the VCC UE in order to deliver all incoming voice calls to the UE  700  on 1x.   8. The VCC AS  107  responds to the delivered SMS message with a positive acknowledgement  724 .   9-11. The delivery report message is forwarded through the SMSC/MSC to the originating UE.   12. When the DO becomes available again, the UE  700  performs an IMS re-registration and the VCC AS  107  is updated via a third party registration so that future calls are delivered using the DO air-interface.       

     Thus, the invention discloses techniques for providing the SMS to be delivered on DO and 1x networks or interfaces and for decreasing the load on the MSC interface. In particular, the invention offloads the MSC by delivering the SMS encapsulated in SIP on DO when DO is available. That is, if the AT monitors the DO Control Channel for pages, then the SIP-encapsulated SMS is delivered to it after reception of the DO page on a DO connection. Other aspects of the invention as described above include:
         When the AT does not detect the DO air-interface (i.e., only 1x is available), the AT sends an SMS to a pre-provisioned number which is intercepted and consumed by the SMS-GW associated with the AT. The SMS-GW de-registers the mobile (from the SIP network) upon reception of this special SMS. Alternatively, the SMS-GW attempts to de-register the AT for the SMS service.   When the DO becomes available again, the AT does a SIP register so that SMS may be delivered using the DO air-interface.   This off-loads the MSC with respect to the SMS processing.   The AT and AN engage in a SIP EVENT NOTIFICATION package where the AT indicates its capability with respect to SMS delivery on DO and the network indicates either positive or negative response.   The AT originates SMS using SIP on EV-DO if SIP registration succeeds and if the SIP EVENT NOTIFICATION indicates SMS capability.   Alternatively, OTASP may be used to provision the AT with the capability of SMS over DO. If the AT gets provisioned with “SMS over DO capability”, then the AT will send SMS over SIP on DO when DO is available.       

     Referring to  FIG. 8 , there is shown another aspect of the invention where if a first AT (with a valid subscription) sends a special SMS to the SMS-GW requesting SMS delivery for a second AT to be performed on 1x (instead of DO), this will not lead to a denial-of-service problem as the MSC authenticates the originator as the ADDS Transfer message containing IMSI and AUTHR. That is, the SMS-GW compares the identity of the mobile for which the “special SMS” is sent with the SMS_OriginalOriginatingAddress. The SMS-GW then de-registers the mobile whose address is indicated by the SMS_OriginalOriginatingAddress from the SIP network. In the example illustrated in  FIG. 8 , the SMS_OriginalOriginatingAddress would be the address of the second AT and, therefore, the second AT may not cause de-registration of the first AT from the SIP network. 
     In another aspect as illustrated in  FIG. 9 , there is shown a scenario where after the AT sends the special SMS to the SMS-GW, it is possible that it tunes to DO and sends a SIP register message. A concern here would be what if the de-registration by the SMS-GW occurs after the SIP registration. That is, would that lead to the AT being de-registered from IMS while on DO and, therefore, SMS messages would not be delivered to the AT. The invention avoids the race condition described in the above scenario by providing the SMS-GW to send an SMS to the AT confirming that the de-registration has been performed. More specifically, the AT should not tune to DO before it receives this confirmation SMS. 
     The invention addresses the problem of delivering SMS on DO (and hence off-loading the 1x MSC) while the voice service is still delivered on 1x. It should be noted that the invention further addresses the scenario in which the VoIP is offered on DO and the voice service is to be delivered on DO (as well as SMS). More specifically, when the AT enters the DO coverage, it performs SIP registration and when it exits the DO coverage and enters the 1x-only coverage, it sends the special SMS to the SMS-GW that leads to de-registering the AT from SIP. When a voice call arrives to a Call Continuity Control Function/Network Domain Selection (CCCF/NeDS), it would query the HLR if the AT is not SIP registered; otherwise, it will deliver the voice call on the IMS/DO. 
     The invention has been described such that those of skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
     Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention. 
     The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in Random Access Memory (RAM), flash memory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM), registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal. 
     The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.