Patent Publication Number: US-2010111002-A1

Title: Method for Establishing a Packet Switched Call at a Dual Mode Access Terminal

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to dual mode access terminals that can process both packet switched and circuit switched calls. In particular, the invention relates to establishing a packet switched call at an access terminal that is processing an established circuit switched call. 
     BACKGROUND OF THE INVENTION 
     An Internet Protocol (IP) Multimedia Subsystem (IMS) is a core network that combines IP multimedia and telephony. IMS standards have been provided by the Third Generation Partnership Project (3GPP), Third Generation Partnership Project 2 (3GPP2), and Internet Engineering Task Force (IETF) organizations and define a generic architecture for Voice over IP (VoIP) and multimedia services. In conjunction with cellular/Wi-Fi dual-mode wireless communication devices, users are able to employ IMS to obtain seamless mobility and Voice Call Continuity (VCC) between conventional circuit switched (CS) networks and packet switched (PS) networks. 
     Evolution of cellular communications has resulted in a proliferation of networks that use different technologies and corresponding different air interfaces. An example of a circuit radio access network (RAN) is a code division multiple access (cdma) cdma2000 1X RAN providing only circuit voice or circuit data service. Some examples of packet data network technologies employed in packet RANs include cdma2000 high rate packet data (HRPD), also known as 1XEV-DO (1X Evolution Data Only), cdma2000 1XRTT, cdma2000 1X-EV-DV (1X Evolution Data/Voice), IEEE 802.11a/b/g, and IEEE 802.16. The associated packet RANs can provide various multimedia services, such as video telephony (VT) services. 
     As a result, during the course of an established call in one RAN, it is often desirable to provide service notification information concerning another incoming call from another RAN. If a user accepts the new incoming service, it is then also desirable to establish a service connection in the other RAN. However, IMS standards and technologies have not provided efficient means for notifying a dual-mode device, which is currently conducting a circuit-switched service in a circuit RAN, that a packet-switched service has been requested in a packet RAN. Also, current standards and technologies have not provided efficient means for establishing a new service in a second RAN after a user decides to accept the new service. Yet such notifications and service establishment can be useful to enable device users to obtain the full benefits of IMS networks. 
     SUMMARY OF THE INVENTION 
     According to one aspect, the present invention is a method for establishing a packet switched call at an access terminal that is processing an established circuit switched call. The method comprises adding a call type indication to a call setup message in a serving network, where the call type indication indicates that the packet switched call is requested to be setup in a packet data radio access network. In response to the call setup message, a message indicating that the packet switched call has been requested is then transmitted from a mobile switching center in the serving network to a circuit switched base station in the serving network. A message indicating that the packet switched call has been requested is then transmitted from the circuit switched base station to the access terminal. A message from the access terminal indicating that the packet switched call has been accepted is then processed at the circuit switched base station. Next, a message indicating that the packet switched call has been accepted is transmitted from the circuit switched base station to the mobile switching center. Finally, the packet switched call is connected. 
     Advantages of embodiments of the present invention thus include enabling an access terminal that has dual mode capabilities, such as a dual mode cellular telephone or other wireless communication device, that is operating in a circuit switched mode and processing an established circuit switched call, to receive a message indicating that a packet switched call, such as a video telephony call, to the access terminal has been requested. Using a call waiting feature of the access terminal, a user is able to either accept or reject the requested packet switched call. If the packet switched call is accepted, the circuit switched call is released and the packet switched call is established. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention may be readily understood and put into practical effect, reference now will be made to exemplary embodiments as illustrated with reference to the accompanying figures, wherein like reference numbers refer to identical or functionally similar elements throughout the separate views. The figures together with a detailed description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the present invention, where: 
         FIG. 1  is a block diagram illustrating a wireless communication system in accordance with some embodiments of the present invention. 
         FIG. 2  is a block diagram illustrating a wireless communication system in accordance with some other embodiments of the present invention. 
         FIG. 3  is a block diagram illustrating an architecture of an access terminal (AT), in accordance with some embodiments of the present invention. 
         FIG. 4  is a block diagram illustrating an architecture of a base station (BS), in accordance with some embodiments of the present invention. 
         FIG. 5  is a block diagram illustrating an architecture of a mobile switching center (MSC), in accordance with some embodiments of the present invention. 
         FIG. 6  is a block diagram illustrating an architecture of a media gateway control function (MGCF), in accordance with some embodiments of the present invention. 
         FIG. 7  is a block diagram illustrating an architecture of a voice call continuity application server (VCC AS), in accordance with some embodiments of the present invention. 
         FIGS. 8A and 8B  are message sequence charts illustrating a method for establishing a video telephony (VT) call at an AT that is processing an established circuit switched (CS) call through a circuit services network, according to some embodiments of the present invention; 
         FIGS. 9A and 9  B are message sequence charts illustrating a method for establishing a video telephony (VT) call at an AT that is processing an established circuit switched (CS) call through a circuit services network, according to some other embodiments of the present invention; 
         FIGS. 10A and 10B  are message sequence charts illustrating a method for establishing a video telephony (VT) call at an AT that is processing an established circuit switched (CS) call through a circuit services network, according to still other embodiments of the present invention; and 
         FIG. 11  is a general flow diagram illustrating a method for establishing a packet switched call at an access terminal that is processing an established circuit switched call, according to some embodiments of the present invention. 
     
    
    
     Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. 
     DETAILED DESCRIPTION 
     Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of method steps and apparatus components related to establishing a packet switched call at an access terminal that is processing an established circuit switched call. Accordingly, the apparatus components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention, so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
     In this document, relational terms such as left and right, first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element preceded by “comprises a . . . ” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. 
     Turning now to the drawings, the present invention may be more fully described with reference to  FIGS. 1-11 . Referring to  FIG. 1 , a block diagram illustrates a wireless communication system  100 , in accordance with some embodiments of the present invention. Communication system  100  includes a wireless access terminal (AT)  102 , for example but not limited to a cellular telephone, a radiotelephone, or a Personal Digital Assistant (PDA), personal computer (PC), or laptop computer equipped for wireless voice communications. In various communications systems, AT  102  may also be referred to as a subscriber unit (SU), a mobile station (MS), a hybrid terminal, or a user&#39;s equipment (UE). AT  102  is capable of engaging in a packet data call with packet data network  130  and is further capable of engaging in a circuit voice or data call with circuit services network  110 , and more particularly is capable of communicating with packet data node  134 . 
     As depicted in  FIG. 1 , AT  102  is associated with a first, home network  150  but resides in a second, visited network  142 . Visited network  142  includes both a wireless circuit services cellular communication network  110 , such as a cdma2000 (Code Division Multiple Access 2000) 1X network, and a wireless packet data communication network  130  that provides VoIP services, such as a cdma2000 HRPD (High Rate Packet Data) packet data communication network  130 . 1X is a spectrally efficient technology for circuit-switched voice communications, which enables applications such as multimedia messaging and GPS-based location services. Circuit services network  110  includes a Base Station (BS)  112  that comprises a Base Transceiver Station (BTS)  114  operably coupled to a Base Station Controller (BSC)  116 . BS  112  is coupled to a Mobile Switching Center (MSC)  120  via both a signaling (A1) and a bearer (A2) interface. MSC  120  includes call control and mobility management functionality (not shown), such as a Visited Location Register (VLR), and switching functionality (not shown) and is coupled to a Media Gateway (MGW)  122  via a bearer interface, preferably a Pulse Code Modulation over Time Division Multiplexing (PCM over TDM) interface. Circuit services network  110  further includes a Media Gateway Control Function (MGCF)  124  that is coupled to each of MGW  122  and MSC  120  via a signaling interface, preferably to MGW  122  via a Media Gateway Control protocol (Megaco) interface and to MSC  120  via an ISDN User Part (ISUP) interface. 
     BSC  116  provides selection and distribution unit functionality with respect to messages received from Access Terminals (ATs) serviced by the BSC and further provides transcoding functionality in functional block  118  with respect to transcoding between the vocoder formats provided by the ATs and the vocoder formats provided by networks coupled to network  110 , such as a 64 kbps PCM format (ITU-T G.711). However, in other embodiments of the present invention, the transcoding functionality may reside in MSC  120  instead of BSC  116 . MGW  122  provides a gateway for circuit services network  110  to far end network  170 , for example, an external data network such as an Internet Protocol (IP) network such as the Internet. When AT  102  is engaged in a voice call with an other end point (OEP)  172  via circuit services network  110  and far end network  170 , MGW  122  converts Pulse Code Modulation (PCM) signals received from MSC  120  to data packets, for example, based on a Real Time Protocol/User Datagram Protocol/Internet Protocol (RTP/UDP/IP) protocol suite, for routing to external data network  170  and converts voice data received from data network  170  to a PCM over TDM (Time Division Multiplex) format for routing to MSC  120 . 
     Packet data network  130  comprises a packet data node  134  coupled to a Packet Data Serving Node (PDSN)  138 , or when packet data network  130  is a WLAN network to a Packet Data Interworking Function (PDIF), via a bearer (A10) interface and a signaling (A11) interface. PDSN  138  further has a signaling control path connection with a Proxy-Call Session Control Function (P-CSCF)  140  and is connected to far end network  170  via an interface supporting the RTP/UDP/IP protocol suite for an exchange of packet data when engaged in a packet data session with the OEP  172 . Packet data node  134  provides wireless packet data communication services to ATs located in a coverage area of the packet data node. Packet data node  134  comprises a wireless Access Network (AN) (not shown), such as a BTS coupled to a BSC, an Access Point (AP), or a Node B coupled to a Radio Network Controller (RNC). Packet data node  134  may further comprise a Packet Control Function (PCF) (not shown) that may be coupled to the AN via one or more of a bearer connection and a signaling connection, such as an A8 and an A9 interface. When packet data node  134  comprises an AN and a PCF, the functionality described herein as being performed by packet data node  134  may be performed by either the AN or the PCF or may be distributed among the AN and the PCF. 
     Each of BS  112  and packet data node  134  provides wireless communication services to Access Terminals (ATs) located in a coverage area of the BS or packet data node via a respective 1X air interface  104  and HRPD air interface  132 . Each air interface  104 ,  132  includes a forward link that includes a pilot channel, at least one forward link traffic channel, and forward link common and dedicated signaling channels. Each air interface  104 ,  132  further includes a reverse link that includes at least one reverse link traffic channel, reverse link common and dedicated signaling channels, and an access channel. 
     Circuit services network  110  and packet data network  130 , and more particularly BS  112  and packet data node  134 , communicate with each other via an Interworking Solution function (IWS)  126 . IWS  126  provides an interworking function between packet data network  130  and circuit services network  110  via an A21 inter-RAN interface and supports A21 signaling with the circuit services network. An inter-RAN interface is described in detail in U.S. patent application Ser. No. 11/141,926, attorney docket number CE13247R, which patent application is commonly owned and incorporated herein by reference in its entirety. Further, an A21 inter-RAN interface and an IWS are described in the 3GPP2 A.S0008-B v0.2 and A.S0009-B v0.2 standards. IWS  126  interfaces to packet data network  130  and supports packet data, and in particular HRPD, signaling. IWS  126  provides an interworking function allowing packet data network  130  to convey HRPD air interface signaling to an AT in the circuit services network, thereby permitting an HRPD message to be transported over circuit services network  110  to the AT  102 . 
     In one embodiment of the present invention, IWS  126  may be collocated at BS  112 , and further may be located in either BTS  114  or BSC  116 , and may be connected to packet data node  134  via an inter-RAN interface, that is, an interface terminating at BS  112  in circuit services network  110  and at packet data node  134  in the packet data network  130 , preferably an A21 interface. In another embodiment of the present invention, IWS  126  may collocated at packet data node  134 , and further may be located in either the AN or the PCF when the packet data node comprises an AN and/or a PCF, and may be connected to MSC  120  via an A1/A1p interface, and via the MSC to BS  112 . When IWS  126  is collocated at packet data node  134 , the A21 interface is internal to the packet data node. In yet another embodiment of the present invention, IWS  126  may be a standalone IWS that may be accessed by packet data node  134 , for example, via an A21 interface, and by MSC  120 , for example, via an A1/A1p interface. The A21 interface is used to transparently pass 1X air interface signaling messages between packet data node  134 , and in particular a PCF or an AN of the packet data node  134  when the packet data node  134  includes a PCF and/or an AN, and IWS  126  or, when the IWS is collocated at BS  112 , between packet data node  134  and the BS  112 . In communication system  100  and unlike in the prior art, the A21 interface is further used to pass HRPD air interface signaling from packet data node  134 , and in particular a PCF or an AN of the packet data node  134  when the packet data node  134  includes a PCF and/or an AN, to circuit services network  110 . 
     Each of circuit services network  110  and packet data network  130  communicates with an IP Multimedia Core Network Subsystem (IMS) of home network  150 . The IMS comprises an Interrogating Call Session Control Function (I-CSCF) and a Serving Call Session Control Function (S-CSCF), hereinafter collectively referred to as I/S-CSCF  154 , that are each coupled to a Home Subscriber Server (HSS)  152  via a signaling (Cx) interface. The IMS of home network  150  further comprises a Voice Call Continuity Application Server (VCC AS)  156 , that is coupled to HSS  152  via a signaling (Sh) interface and to I/S-CSCF  154  via a signaling interface capable of supporting Session Initiation Protocol (SIP). Similarly, Although  FIG. 1  depicts I-CSCF and S-CSCF as being implemented in a single network element, such as a single server, those who are of ordinary skill in the art realize that I-CSCF and S-CSCF may be implemented in separate network elements without departing from the spirit and scope of the present invention. VCC AS  156 , and MSC  120  as well, are each further coupled to a Home Location Register (HLR)  162  via a signaling interface that supports an inter-system protocol, such as Mobile Application Part (MAP). Although single interfaces have been described herein between many of the network elements of communication system  100 , each interconnection among elements may comprise multiple interconnections and/or interfaces, such as one or more of a signaling interface, for example, an interface for an exchange of SIP, ISUP, MAP, or Megaco messages, and a bearer interface or path, such as a path for an exchange of voice information. 
     Referring to  FIG. 2 , a block diagram illustrates a wireless communication system  200 , in accordance with some embodiments of the present invention. The wireless communication system  200  corresponds to the wireless communication system  100  with, however, the following changes. Components corresponding to the MGCF  124  and the MGW  122  in the visited network  142 , as shown in  FIG. 1 , are not present in the visited network  242  as shown in  FIG. 2 . Further, a VCC AS  256  in the wireless communication system  200  has a MAP interface that connects to a MSC  220 . The following elements in the wireless communication system  100  therefore correspond to the following elements in the wireless communication system  200 :  104  to  204 ,  110  to  210 ,  112  to  212 ,  114  to  214 ,  116  to  216 ,  118  to  218 ,  120  to  220 ,  126  to  226 ,  130  to  230 ,  132  to  232 ,  134  to  234 ,  138  to  238 ,  140  to  240 ,  142  to  242 ,  150  to  250 ,  152  to  252 ,  154  to  254 ,  156  to  256 , and  162  to  262 . 
     In  FIGS. 3-7  below, examples of architecture concerning the AT  102 , the BS  112 , the MSC  110 , the MGCF  124 , and the VCC AS  156 , respectively, are provided. For clarity and brevity, in the following description regarding  FIGS. 3-7 , references are made primarily to elements of only the wireless communication system  100 . However, as will be understood by those skilled in the art, the description below also applies generally to the corresponding elements of the wireless communication system  200 . 
     Referring to  FIG. 3 , a block diagram illustrates an architecture of the AT  102 , in accordance with some embodiments of the present invention. AT  102  may include at least one transceiver  302  that allows the AT  102  to transmit or receive in each of the two networks  110  and  130 . Transceiver  302  is coupled to a vocoder  306  and a processor  308 , which processor  308  is further coupled to an at least one memory device  310 . AT  102  may maintain apriori information in at least one memory device  310  that facilitates the switching between networks  110  and  130 . Processor  308  may comprise one or more microprocessors, microcontrollers, digital signal processors (DSPs), combinations thereof or such other devices known to those having ordinary skill in the art, which are configured to execute the functions described herein as being executed by AT  102 . The at least one memory device  310  may comprise random access memory (RAM), dynamic random access memory (DRAM), and/or read only memory (ROM) or equivalents thereof, that store data and programs that may be executed by the associated processor and that allow AT  102  to perform all functions necessary to operate in communication system  100 . When AT  102  has a dormant packet data session being maintained by packet data network  130 , the at least one memory device  310  may further maintain Radio Link Protocol (RLP) information associated with the packet data session, such as an identification of an HRPD RLP flow to which packet data is to be sent, for example, an ‘HRPD RLPFlowID.’ 
     Referring to  FIG. 4 , a block diagram illustrates an architecture of the BS  112 , in accordance with some embodiments of the present invention. BS  112  includes a respective processor  408 , such as one or more microprocessors, microcontrollers, digital signal processors (DSPs), combinations thereof or such other devices known to those having ordinary skill in the art, which processor  408  is configured to execute the functions described herein as respectively being executed by the BS  112 . BS  112  may include at least one transceiver  402  that allows the BS  112  to transmit or receive signals from the AT  102 . Transceiver  402  is coupled to a vocoder  406 , to the processor  408 , and to an at least one memory device  410 . The at least one memory device  410  may comprise random access memory (RAM), dynamic random access memory (DRAM), and/or read only memory (ROM) or equivalents thereof, that store data and programs that may be executed by the associated processor  408  and that allow the BS  112  to perform all functions necessary to operate in the communication system  100 . 
     Referring to  FIG. 5 , a block diagram illustrates an architecture of the MSC  110 , in accordance with some embodiments of the present invention. The MSC  110  includes a respective processor  502 , such as one or more microprocessors, microcontrollers, digital signal processors (DSPs), combinations thereof or such other devices known to those having ordinary skill in the art, which processor  502  is configured to execute the functions described herein as respectively being executed by the MSC  110 . The MSC  110  further includes a respective at least one memory device  504  that may comprise random access memory (RAM), dynamic random access memory (DRAM), and/or read only memory (ROM) or equivalents thereof, that store data and programs that may be executed by the associated processor  502  and that allow the MSC  110  to perform all functions necessary to operate in the communication system  100 . 
     Referring to  FIG. 6 , a block diagram illustrates an architecture of the MGCF  124 , in accordance with some embodiments of the present invention. The MGCF  124  includes a respective processor  602 , such as one or more microprocessors, microcontrollers, digital signal processors (DSPs), combinations thereof or such other devices known to those having ordinary skill in the art, which processor  602  is configured to execute the functions described herein as respectively being executed by the MGCF  124 . The MGCF  124  further includes a respective at least one memory device  604  that may comprise random access memory (RAM), dynamic random access memory (DRAM), and/or read only memory (ROM) or equivalents thereof, that store data and programs that may be executed by the associated processor  602  and that allow the MGCF  124  to perform all functions necessary to operate in the communication system  100 . 
     Referring now to  FIG. 7 , a block diagram illustrates an architecture of the VCC AS  156 , in accordance with some embodiments of the present invention. The VCC AS  156  includes a respective processor  702 , such as one or more microprocessors, microcontrollers, digital signal processors (DSPs), combinations thereof or such other devices known to those having ordinary skill in the art, which processor  702  is configured to execute the functions described herein as respectively being executed by the VCC AS  156 . The VCC AS  156  further includes a respective at least one memory device  704  that may comprise random access memory (RAM), dynamic random access memory (DRAM), and/or read only memory (ROM) or equivalents thereof, that store data and programs that may be executed by the associated processor  702  and that allow the VCC AS  156  to perform all functions necessary to operate in the communication system  100 . 
     The functionality described herein as being performed by AT  102 , BS  112 , MSC  110 , MGCF  124  and VCC AS  156  is implemented with or in software programs and instructions stored in the respective at least one memory device  310 ,  410 ,  504 ,  604  and  704  and executed by the associated processor  308 ,  408 ,  502 ,  602  and  702  of the AT  102 , BS  112 , MSC  110 , MGCF  124  and VCC AS  156 . When BS  112  comprises BTS  114  and a BSC  116 , the functions described herein as being performed by the BS  112  may be performed by a processor included in BTS  114  or a processor included in BSC  116  or may be distributed among the processors of BTS  114  and BSC  116  based on data and programs respectively stored in a corresponding at least one memory device of BTS  114  and BSC  116 . However, one of ordinary skill in the art realizes that the embodiments of the present invention alternatively may be implemented in hardware, for example, integrated circuits (ICs), application specific integrated circuits (ASICs), and the like, such as ASICs implemented in one or more of AT  104 , BS  112 , and packet data node  134 . Based on the present disclosure, one skilled in the art will be readily capable of producing and implementing such software and/or hardware without undo experimentation. 
     In order for AT  102  to engage in a circuit voice call or a packet data call respectively via circuit services network  110  or packet data network  130 , each of AT  102 , circuit services network  110 , and packet data network  130  operates in accordance with well-known wireless telecommunications protocols. For example, circuit services network  110  can be a cdma2000 (code division multiple access) communication system that provides circuit switched communication services to subscribers serviced by the network (it may also provide packet data services) and that operates in accordance with the 3GPP2 C.S0001 to C.S0005 standards, which provides an air interface compatibility standard for CDMA 1X systems. Packet data network  130  can be a cdma2000 communication system that provides HRPD communication services to subscribers serviced by the network  130  and that operates in accordance with the 3GPP2 (Third Generation Partnership Project 2) C.S0024-A standard, which provides an air interface compatibility standard for cdma2000 HRPD (High Rate Packet Data) systems and the 3GPP2 C.S0075 standard, which provides HRPD-1X inter-technology air interface support. The IP Multimedia Core Network Subsystem (IMS) of home network  150  operates in accordance with the 3GPP2 X.S0013 standards, which describe the operation, elements, and interfaces of an IMS. 
     Further, circuit services network  110  and AT  102  can operate in accordance with the 3GPP2 A.S0011-A.S0017 Inter Operability Specifications (IOS) standards, which provide a compatibility standard for cellular mobile telecommunications systems that operate as a cdma2000 1X system. In addition, packet data network  130  and again AT  102  can operate in accordance with one or more of the 3GPP2 A.S0008-B v0.2 or A.S0009-B v0.2 (v&amp;v versions) HRPD IOS standards, which provide compatibility standards for cellular mobile telecommunications systems that operate as a cdma2000 HRPD system. To ensure compatibility, radio system parameters and call processing procedures are specified by the standards, including call processing steps that are executed by an AT and a base station or other access network serving the AT and between the base station or other access network and associated infrastructure. However, those of ordinary skill in the art realize that packet data network  130  may operate in accordance with any one of a variety of wireless packet data communication systems that provide high rate packet data communication services, such as systems conforming to the IEEE (Institute of Electrical and Electronics Engineers) 802.xx standards, for example, the 802.11, 802.15, or 802.16 or 802.20 standards, and that circuit services network  110  may operate in accordance with any one of a variety of well-known conventional wireless telecommunication systems that provide circuit switched communication services. 
     In order to access circuit services network  110 , AT  102  tunes to an operating frequency assigned to the circuit services network  110 , acquires a pilot channel associated with a serving BS, such as BS  112 , and then registers with MSC  120  via BS  112  and a reverse link access channel of air interface  104 . Once AT  102  is registered, the AT may monitor a forward link paging channel of air interface  104 . The paging channel may then be used to notify AT  102  when a voice call arrives via circuit services network  110 . Alternatively, AT  102  may originate a circuit voice call after acquiring the pilot channel associated with BS  112  by requesting circuit voice service on a 3G1X reverse link access channel. The paging channel is further used when packet data network  130  has received packet data from home network  150  and requests circuit services network  110  to page AT  102  to request the AT  102  to move to the packet data network  130  so that the packet data can be delivered to the AT  102 . 
     When AT  102  is not engaged in a voice call with, or monitoring a paging channel in, circuit services network  110 , the AT  102  may initiate a packet data call and register with packet data network  130 , and more particularly with home network  150 . AT  102  may then establish a data link with PDSN  138  in accordance with a Layer 2 protocol such as a Point-to-Point Protocol (PPP). The Point-to-Point Protocol may then be used to assign an IP address to AT  102 . Once the IP address is assigned and a packet data session is established, AT  102  may communicate with packet data network  130  over a packet data network connection. The packet data network connection, comprising packet data node  134  and an AN and a PCF servicing AT  102  in network  130 , is communicated by the packet data network  130  to MSC  120  and is stored by the MSC  120 . 
     The C. S0024 standard provides for the packet data network packet data session to remain intact whether or not the connection is being used to support communications. That is, when AT  102  accesses packet data network  130  to establish a packet data session, the AT  102  is assigned a traffic channel in air interface  132  and packet data are transferred to the AT  102  via the traffic channel and the packet data network connection. During subsequent periods of inactivity in packet data network  130 , for example, when AT  102  is active in a voice call in circuit services network  110 , the traffic channel may be torn down but the packet data session remains intact. By maintaining the packet data session, AT  102  does not have to acquire a new IP address or establish a new PPP connection for a subsequent exchange of data. A packet data session that exists in the absence of a traffic channel is referred to as a “dormant” session. 
     In communication system  100 , when AT  102  is engaged in a circuit voice call in circuit services network  110 , the AT  102  may roam through the communication system  100 . As a result of the roaming, situations may arise where it is desirable to hand off AT  102  from circuit services network  110  to packet data network  130 . For example, as is known in the art, while roaming in communication system  100  and being serviced by BS  112 , AT  102  may receive a stronger signal from packet data node  134 . Typically signal strengths are determined by an AT, such as AT  102 , measuring a pilot channel associated with the packet data node or BS. When a pilot channel of a serving packet data node or BS is weaker than a threshold value and a pilot channel of another packet data node or BS, that typically indicates a desirability of a handoff. 
     By way of yet another example, it may be desirable to move an AT, such as AT  102 , that is actively engaged in a voice call in circuit services network  110  to packet data network  130  when the user of AT  102  prefers to use video telephony (VT) service rather than a voice call, and packet data network  130  supports video telephony but circuit services network  110  does not. 
     Referring to  FIGS. 8A and 8B , a message sequence charts illustrate a method for establishing a video telephony (VT) call at the AT  102  that is processing an established circuit switched (CS) call through the circuit services network  110  of the communication system  100 , according to some embodiments of the present invention. As described below, establishing the VT call includes notifying a user of the AT  102  of a request for the VT call. For purposes of  FIGS. 8A and 8B , consider that the AT  102  is registered in an internet protocol (IP) multimedia subsystem (IMS) of the visited network  142  and includes an activated call waiting feature. Further, consider that the IMS of the visited network  142  does support user selective call forwarding and call deflection. 
     At step  801 , the OEP  172  transmits a session initiation protocol (SIP) INVITE message, including a uniform resource identifier (URI) of the AT  102 , and session description protocol (SDP) information concerning a requested VT call from the OEP  172 , to the home network  150 , requesting that a VT call be established between the OEP  172  and the AT  102 . For example, the OEP  172  may be another cellular telephone or other type of communication device. At step  802 , the home network  150  then transmits an SIP INVITE message to the MGCF  124  in the visited network  142 . For example, the SIP INVITE message can include a temporary location directory number (TLDN) and SDP information that identifies the OEP  172 . The TLDN can be retrieved from a HLR in the home network. 
     At step  803 , the MGCF  124  then transmits an integrated systems digital network (ISDN) user part (ISUP) initial address message (IAM) to the MSC  120  in the visited network  142 . The ISUP IAM requests that the MGW  122  be configured with an ephemeral termination connected to the OEP  172 . The connection can be made for example with a physical pulse code modulation (PCM) trunk termination connected to the MSC  120 . The ISUP IAM includes a calling party number (CgPN) comprising: a) a service code that indicates that this is a VT call and, if the user accepts it, the VT call is requested to be setup in the HRPD radio access network (RAN) in the visited network  142 , and b) a mobile directory number (MDN) of the OEP  172 . For example, in the ISUP IAM a calling party number has a prefix before a number of the OEP  172 , where the prefix indicates that the new call is a VT call that needs to be setup in an HRPD system. 
     At step  804  the MSC  120  transmits an ISUP address complete message (ACM) back to the MGCF  124 . At step  805  the MGCF  124  then transmits an SIP  180  Ringing message to the home network  150 , and at step  806  the SIP  180  Ringing message is relayed to the OEP  172 . At step  807 , which can occur anytime after step  803 , the MSC  120  transmits a flash with information (FWI) message to the BS  112 . The FWI message comprises a call type indication that indicates that a new VT call is waiting. For example, the calling party number field in the FWI message contains a prefix that includes a service code that indicates that the new call is a HRPD VT call. Alternatively, an extended record type (ERT) can be used to indicate that the new call is a HRPD VT call. 
     At step  808 , the BS  112  transmits a FWI message to the AT  102  that indicates that a new VT call is waiting. As at step  807 , the calling party number field in the FWI message of step  808  contains a prefix that includes a service code that indicates that the new call is a HRPD VT call, or, alternatively, an extended record type (ERT) can be used to indicate that the new call is a HRPD VT call. Following step  808 , a user of the AT  102  can determine whether or not to accept the VT call from the OEP  172 . As will be understood by those skilled in the art, conventional call waiting features, such as audible tones, messages, or visual displays, can be used to inform a user of the AT  102  of the VT call. If it is determined to accept the VT call, then at step  809  the AT  102  transmits a FWI message to the BS  112  to indicate that the VT call from the OEP  172  should be forwarded to the AT  102 . The FWI message at step  809  can include, for example, a keypad facility information record that includes: a) a pre-programmed feature code field that indicates user selective call forwarding to a number stored in the AT  102  as the first digits in the field, and b) a forwarding to number that is set to a VCC application server (AS) E.164 number, which can immediately follow the pre-programmed feature code field. At step  810 , the FWI message is relayed from the BS  112  to the MSC  120 . 
     At step  811 , the MSC  120  then transmits to the MGCF  124  an ISUP call progress (CPG) message. At step  812 , the MGCF  124  transmits an SIP  181  call is being forwarded message to the home network  150 , and the SIP  181  message is then relayed at step  813  to the OEP  172 . At step  814 , which can occur anytime after step  811 , the MSC  120  transmits to the MGCF  124  an IAM by using the VCC AS E.164 number in a called party number field. At step  815 , the MGCF  124  determines the VCC AS SIP URI via, for example, an ENUM query and sends an SIP INVITE message to the home network  150 . 
     At step  816 , which can occur anytime after step  810 , the 1X established circuit switched call is released by the AT  102 . At step  817 , the AT  102  is tuned to the packet data network  130  and a packet data session is reactivated with the PDSN  138 . At step  818 , the VCC AS  156  in the home network  150  determines that the call needs to be delivered to the IMS, and sends an SIP INVITE message to a proxy call session control function (P-CSCF)  170 , to the PDSN  138 , and to the AT  102 . The SIP INVITE message includes a URI of the AT  102  and SDP information from the OEP  172 . At step  819 , an SIP  200  OK (INVITE) message is transmitted from the AT  102  to the PDSN  138 , to the P-CSCF  140 , and to the home network  150 . At step  820 , the SIP  200  OK (INVITE) message is then relayed from the home network  150  to the MGCF  124  and to the OEP  172 . At step  821  the MGCF  124  transmits to the MSC  120  an answer message (ANM). At step  822 , the OEP  172  transmits an SIP acknowledgement (ACK) message back to the home network  150 , and at step  823  the home network  150  relays the SIP ACK message to the P-CSCF  140 , to the PDSN  138 , and to the AT  102 . Finally, at step  824 , the VT call is established between the AT  102  and the OEP  172 , and video/audio data streams are transferred. As will be understood by those skilled in the art, other methods can be used to optimize the routing path. For purposes of clarity, the present description does not consider optimal routing paths. 
     Referring to  FIGS. 9A and 9B , message sequence charts illustrate a method for establishing a video telephony (VT) call through the circuit services network  110  of the communication system  100 , according to some other embodiments of the present invention. As described below, establishing the VT call includes notifying a user of the AT  102  of a request for the VT call. For purposes of  FIGS. 9A and 9B , consider that the AT  102  is registered in an internet protocol (IP) multimedia subsystem (IMS) of the visited network  142  and includes an activated call waiting feature. However, unlike in the message sequence chart described above with reference to  FIG. 8 , in  FIG. 9  consider that the IMS of the visited network  142  does not support user selective call forwarding and call deflection. At step  901 , the OEP  172  transmits a session initiation protocol (SIP) INVITE message, including a uniform resource identifier (URI) of the AT  102  and session description protocol (SDP) information concerning a requested VT call from the OPE  172 , to the home network  150 , requesting that a VT call be established between the OEP  172  and the AT  102 . At step  902 , the home network  150  then transmits an SIP INVITE message to the MGCF  124  in the visited network  142 . For example, the SIP INVITE message can include a temporary location directory number (TLDN) and SDP information that identifies the OEP  172 . 
     At step  903 , the MGCF  124  then transmits an integrated systems digital network (ISDN) user part (ISUP) initial address message (IAM) to the MSC  120  in the visited network  142 . The ISUP IAM requests that the MGW  122  be configured with an ephemeral termination connected to the OEP  172 . The connection can be made for example with a physical pulse code modulation (PCM) trunk termination connected to the MSC  120 . The ISUP IAM includes a calling party number (CgPN) comprising: a) a service code that indicates that this is a VT call and, if the user accepts it, the VT call is requested to be setup in the HRPD radio access network (RAN) in the visited network  142 , and b) a mobile directory number (MDN) of the OEP  172 . For example, in the ISUP IAM a calling party number has a prefix before a number of the OEP  172 , where the prefix indicates that the new call is a VT call that needs to be setup in an HRPD system. 
     At step  904  the MSC  120  transmits an ISUP address complete message (ACM) call waiting (CW) back to the MGCF  124 . At step  905  the MGCF  124  then transmits an SIP alerting ( 18   x ) message to the home network  150 , and at step  906  the SIP  18   x  message is relayed to the OEP  172 . At step  907 , which can occur anytime after step  903 , the MSC  120  transmits a flash with information (FWI) message to the BS  112 . The FWI message comprises a call type indication that indicates that a new VT call is waiting. For example, a calling party number field in the FWI message contains a prefix that includes a service code that indicates that the new call is a HRPD VT call. Alternatively, an extended record type (ERT) can be used to indicate that the new call is a HRPD VT call. 
     At step  908 , the BS  112  transmits a flash with information message to the AT  102  that indicates that a new VT call is waiting. As at step  907 , a calling party number field in the FWI message of step  908  contains a prefix that includes a service code that indicates that the new call is a HRPD VT call, or, alternatively, an extended record type (ERT) can be used to indicate that the new call is a HRPD VT call. Following step  908 , a user of the AT  102  can determine whether or not to accept the VT call from the OEP  172 . As will be understood by those skilled in the art, conventional call waiting features, such as audible tones, messages, or visual displays, can be used to inform a user of the AT  102  of the VT call. If it is determined to accept the VT call, then at step  909  the AT  102  transmits a FWI message to the BS  112  to indicate that the VT call from the OEP  172  should be forwarded to the IMS, and the BS  112  forwards the FWI message to the MSC  120 . However, if it is determined to reject the VT call, then the AT  102  continues the 1X established circuit switched call and does not reply to the FWI message received at step  908 . Considering that the VT call is accepted, at step  910  a call release message is transmitted from the AT  102  to the BS  112 , and is relayed by the BS  112  to the MSC  120 . The call release message indicates that the 1X established circuit switched call should be released due to the acceptance of the HRPD VT call. 
     According to some embodiments of the present invention, a release cause value is defined in a 1X air interface and clear command interoperability specification (IOS) to indicate that the 1X established circuit switched call is released due to a switch to an HRPD VT call. When the MSC  120  processes the release cause value, which can be included in the call release message at step  910 , the MSC  120  recognizes that an answer message (ANM) is not required to be sent to the MGCF  124 . 
     At step  911 , the AT  102  is tuned to the HRPD system and a packet data session is reactivated with the PDSN  138 . At step  912 , an SIP INVITE message is transmitted from the AT  102  to the PDSN  138 , to the P-CSCF  140  and to the home network  150 . The SIP INVITE message is addressed to the VCC AS  156  of the home network  150  and includes the AT  102 &#39;s SDP information. The combination of the known E.164 number and a P-Asserted-Identity value identify the SIP INVITE message and indicate that a handoff to the VT call has been initiated. At step  913 , the home network  150  transmits to the OEP  172  an SIP re-INVITE message that is addressed to the OEP  172  and includes SDP information identifying the AT  102 . At step  914 , the OEP  172  replies to the home network  150  by transmitting an SIP connect  200  OK message, which is then forwarded at step  915  by the home network  150  to the P-CSCF  140 , to the PDSN  138  and to the AT  102 . At step  916 , the AT  102  then replies with an SIP ACK message that is transmitted to the PDSN  138 , to the P-CSCF  140  and to the home network  150 . At step  917 , the home network  150  forwards the SIP ACK message to the OEP  172 . At step  918 , the home network  150  transmits an SIP release (BYE) message to the MGCF  124 . At step  919 , the MGCF  124  then transmits a release (REL) message to the MSC  120 , and at step  920  the MSC  120  responds to the MGCF  124  with a release complete (RLC) message. At step  921 , the MGCF  124  transmits an ACK message to the home network  150 . Finally, at step  922 , the VT call is established between the AT  102  and the OEP  172  and video/audio data streams are transferred. 
     Referring to  FIGS. 10A and 10B , message sequence charts illustrate a method for establishing a video telephony (VT) call through the circuit services network  210  of the communication system  200 , according to some other embodiments of the present invention. As described below, establishing the VT call includes notifying a user of the AT  102  of a request for the VT call. For purposes of  FIGS. 10A and 10B , consider that the AT  102  is registered in an internet protocol (IP) multimedia subsystem (IMS) of the visited network  242  and includes an activated call waiting feature. As in  FIG. 9 , consider in  FIG. 10  that the IMS of the visited network  242  does not support user selective call forwarding and call deflection. At step  1001 , the OEP  172  transmits a session initiation protocol (SIP) INVITE message, including a uniform resource identifier (URI) of the AT  102  and session description protocol (SDP) information concerning a requested VT call from the OPE  172 , to the home network  250 , requesting that a VT call be established between the OEP  172  and the AT  102 . At step  1002 , because the AT  102  is in a 1X call, the home network  150  transmits a data delivery redirection request (DDRREQ) message to the MSC  220 . The DDRREQ message contains a call type indication for the VT call. For example, the call type indication can comprise a prefix before the calling party number. At step  1003 , the MSC  220  transmits a response to the DDRREQ message to the home network  250 , and the home network  250  relays an SIP  18   x  message to the OEP  172 . 
     Steps  1005  through  1015  then proceed in a manner similar to the corresponding steps  907  through  917  as shown in  FIG. 9  concerning the communication system  100  and described in detail above. For brevity, the corresponding steps  1005  through  1015  concerning the communication system  200  are not described in detail as they will be understood by those skilled in the art in light of the description above. 
     Finally, at step  1016 , the VT call is established between the AT  102  and the OEP  172  and video/audio data streams are transferred. 
     Referring to  FIG. 11 , a general flow diagram illustrates a method  1100  for establishing a packet switched (PS) call, such as a video telephony (VT) call, at an access terminal (AT) that is processing an established circuit switched (CS) call, according to some embodiments of the present invention. A packet switched call refers to any of various types of voice or data services provided through a packet data network. At step  1105 , a call type indication is added to a call setup message, such as an initial address message or DDRREQ in a serving network, where the call type indication indicates that the PS call is requested to be setup in a high rate packet data (HRPD) radio access network (RAN) in the serving network. For example, at step  803  shown in  FIG. 8 , an ISUP IAM includes a calling party number (CgPN) comprising: a) a service code that indicates that a VT call is requested to be setup in the HRPD RAN in the visited network  142 , and b) a mobile directory number (MDN) of the OEP  172 . At step  1110 , a message in response to the IAM is transmitted from a mobile switching center (MSC) in the serving network to a circuit switched (CS) base station (BS) in the serving network. For example, at step  807  shown in  FIG. 8 , a flash with information (FWI) message is transmitted to the BS  112 . Next, at step  1115 , a message indicating that the PS call has been requested is transmitted from the BS to the AT. For example, at step  808  shown in  FIG. 8 , a FWI message is transmitted to the AT  102 . At step  1120 , a message from the AT indicating that the PS call has been accepted is processed at the BS. For example, at step  809  shown in  FIG. 8 , a FWI message is transmitted to the BS  112 . At step  1125 , a message indicating that the PS call has been accepted is transmitted from the BS to the MSC. For example, at step  810  shown in  FIG. 8 , a FWI message is transmitted to the MSC  120 . Finally, at step  1130 , the PS call is connected. For example, step  811  through step  824  shown in  FIG. 8  are completed. 
     Advantages of embodiments of the present invention thus include enabling an access terminal that has dual mode capabilities, such as a dual mode cellular telephone or other wireless communication device, that is operating in a circuit switched mode and processing an established circuit switched call, to receive a message indicating that a packet switched call, such as a video telephony call, to the access terminal has been requested. Using a call waiting feature of the access terminal, a user is able to either accept or reject the requested packet switched call. If the packet switched call is accepted, the circuit switched call is released and the packet switched call is established. 
     The above detailed description provides an exemplary embodiment only, and is not intended to limit the scope, applicability, or configuration of the present invention. Rather, the detailed description of the exemplary embodiment provides those skilled in the art with an enabling description for implementing the exemplary embodiment of the invention. It should be understood that various changes can be made in the function and arrangement of elements and steps without departing from the spirit and scope of the invention as set forth in the appended claims. It will be appreciated that embodiments of the invention described herein may be comprised of one or more conventional processors and unique stored program instructions that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of establishing a packet switched call at an access terminal that is processing an established circuit switched call as described herein. The non-processor circuits may include, but are not limited to, a radio receiver, a radio transmitter, signal drivers, clock circuits, power source circuits, and user input devices. As such, these functions may be interpreted as steps of a method for establishing a packet switched call at an access terminal that is processing an established circuit switched call. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. Thus, methods and means for these functions have been described herein. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation. 
     In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. The benefits, advantages, solutions to problems, and any elements that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as critical, required, or essential features or elements of any or all of the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims.