Patent Publication Number: US-7590092-B2

Title: Method and system for inter-technology handoff of a hybrid access terminal

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from provisional application Ser. No. 60/809,270, entitled “METHOD AND SYSTEM FOR INTER-TECHNOLOGY HANDOFF OF AN ACCESS TERMINAL,” filed May 30, 2006, which is commonly owned and incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to wireless communication systems, and more specifically to handover of an access terminal between cellular communication networks implementing different air interface technologies. 
     BACKGROUND OF THE INVENTION 
     The evolution of cellular communications has resulted in a proliferation of networks of different technologies and corresponding different air interfaces. As a result, during the course of a single call, a wireless mobile station may roam among multiple radio access networks (RANs), wherein each such RAN implements a different technology than the other RANs of the multiple RANs. Some examples of the different packet data network technologies 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 802.11b/g, and IEEE 802.16. An example of a circuit RAN is a cdma2000 1X RAN providing only circuit voice or circuit data service. 
     As the mobile station roams among a circuit services RAN, such as a cdma2000 1X RAN, and a packet data RAN providing packet data services, it may be beneficial to system performance to handoff the mobile station from the circuit services RAN to the packet data RAN. For example, the channel conditions associated the latter RAN may be more favorable than the channel conditions associated with the former RAN due to such factors as fading, adjacent and co-channel interference, and available power at a serving base station (BS) or radio access network (RAN). By way of another example, an operator of both a legacy circuit network and an HRPD network may desire to move the mobile station from one such network to the other network for purposes of system loading. 
     Currently, the only defined method for executing a handoff from a legacy cdma2000 1X circuit services RAN to an HRPD packet data RAN is an execution of a dormant hard handoff as defined by the 3GPP2 (Third Generation Partnership Project 2) A.S0008-B v0.2 and A.S0009-B v0.2 HRPD Inter Operability Specification (IOS) standards (V&amp;V versions), wherein a mobile station must go dormant and drop a radio resource of a network of a first cdma2000 technology and then acquire a radio resource of a network of a second cdma2000 technology. A result is a brief period of time during which the mobile station is not actively engaged in a communication session with either network. Further, when executing a dormant hard handoff there is no linkage between the two networks as the mobile station must drop the first network and acquire the second network without any assistance from the BS or AN of either network. As a result, voice/data traffic may be lost during the handoff, resulting in poor system performance and efficiency and disgruntled end users. 
     Therefore, a need exists for a method and apparatus for implementing an active hard handoff of a communication session from a circuit services RAN providing circuit voice and data services support to a packet data RAN providing packet data services support that minimizes an amount of time that a mobile station is not actively engaged in a communication session with either network during the handoff. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a wireless communication system in accordance with various embodiments of the present invention. 
         FIG. 2  is a block diagram of a Hybrid Access Terminal in accordance with an embodiment of the present invention. 
         FIG. 3  is a block diagram of a base station in accordance with an embodiment of the present invention. 
         FIG. 4  is a block diagram of a packet data node in accordance with an embodiment of the present invention. 
         FIG. 5A  is a signal flow diagram of a method executed by the communication system of  FIG. 1  in handing off of a communication session from a circuit services network of  FIG. 1  to a packet data network of  FIG. 1  in accordance with various embodiments of the present invention. 
         FIG. 5B  is a continuation of the signal flow diagram of  FIG. 5A  depicting a method executed by the communication system of  FIG. 1  in handing off of a communication session from a circuit services network of  FIG. 1  to a packet data network of  FIG. 1  in accordance with various embodiments of the present invention. 
         FIG. 5C  is a continuation of the signal flow diagrams of  FIGS. 5A and 5B  depicting a method executed by the communication system of  FIG. 1  in handing off of a communication session from a circuit services network of  FIG. 1  to a packet data network of  FIG. 1  in accordance with various embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     To address the need that exists for a method and apparatus that implements a handoff of a communication session from a circuit services network to a packet data network, a communication system is provided that utilizes an interface terminating at a base station (BS) of the circuit services network and a packet data node in the packet data network and DOS over DBM signaling between the BS and an access terminal (AT), thereby permitting the communication system to tunnel SIP messaging between the AT and the packet data node via the BS and to tunnel HRPD traffic channel information and an HRPD identifier from the packet data network to the AT via the BS. Thus, a handoff of a voice call involving the AT may be initiated from circuit services network to the packet data network prior to the AT establishing a call dialogue with the packet data network. By facilitating an exchange of handoff information among packet data network, and in particular the wireless packet data node, and the AT via circuit services network, and in particular the BS, handoff latency delays are reduced. 
     Generally, an embodiment of the present invention encompasses a method for inter-network handoff of a circuit voice call from a circuit services network to a Voice over Internet Protocol (VoIP) call on a packet data network. The method includes conveying forward link voice traffic to, and receiving reverse link voice traffic from, an AT via the circuit services network and determining to hand off the call. The method further includes, in response to determining to hand off the call and while continuing to support the call, conveying, by a BS of the circuit services network to a wireless packet data node of the packet data network, a request to hand off the call to the packet data network as a VoIP call and, in response to conveying the request, receiving a confirmation from the packet data network to accept the hand-in as a VoIP call. The method further includes conveying, by the BS to the AT, information associated with the packet data network and a new active set for facilitating a handoff to the packet data network and acquiring the AT by the packet data network and providing the AT with packet data services. 
     Another embodiment of the present invention encompasses a communication system that provides for inter-network handoff of a voice call from a circuit services network to a packet data network. The system includes a BS that supports a circuit switched voice call and that transmits forward link voice traffic to an AT via a forward link and receives reverse link voice traffic from the AT via a reverse link as part of the circuit switched voice call. The BS further determines to hand off the call to the packet data network, in response to determining to hand off the call and while continuing to support the call, conveys to a packet data node of the packet data network a request to hand off the call to the packet data network as a VoIP call, in response to conveying the request to handoff, receives a confirmation of the packet data network&#39;s acceptance of the hand-in as a VoIP call, and conveys to the AT information associated with the packet data network and a new active set for facilitating a handoff to the packet data network. The system further includes a packet data node of the packet data network that is in communication with the BS and that receives from the BS the request to hand off the call, in response to receiving the request, conveys a confirmation of an acceptance of the hand-in as a VoIP call to the base station, and acquires the AT and provides the AT with packet data services. 
     Yet another embodiment of the present invention encompasses a BS that supports a circuit switched voice call and that is configured to transmit forward link voice traffic to an AT via a forward link and receive reverse link voice traffic from the AT via a reverse link as part of the circuit switched voice call, determine to hand off the call to a packet data network, in response to determining to hand off the call and while continuing to support the call, convey, to a wireless packet data node of the packet data network, a request to hand off the call to the packet data network as a VoIP call, in response to conveying the request to handoff, receive from the packet data node a confirmation of an acceptance by the packet data network of a hand-in as a VoIP call, and convey to the AT information associated with the packet data network and a new active set for facilitating a handoff of the AT to the packet data network. 
     Still another embodiment of the present invention encompasses a packet data node that supports a packet switched voice call and that is configured to receive, from a BS in a circuit services network, a request to hand off a voice call associated with an AT to a packet data network comprising the wireless packet data node as a VoIP call, in response to receiving the request, convey a confirmation of acceptance of a hand-in of the call as a VoIP call, and acquire the AT and provide the AT with packet data services. 
     Turning now to the drawings, the present invention may be more fully described with reference to  FIGS. 1-5C .  FIG. 1  is a block diagram of a wireless communication system  100  in accordance with various 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  via the 3GPP2 C.S0024 protocol and with BS  112  via the 3GPP2 C.S0001-C.S0005 protocols. 
     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 . 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 a call control and mobility management functionality (not shown), such as a Visited Location Register (VLR), and a 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  118  with respect to messages received from Access Terminals (ATs) serviced by the BSC and further provides transcoding functionality  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 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 a remote end point  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 a remote end point. 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 Inetrworking 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. For example, the circuit service network may transport an HRPD air interface message in an HRPD Data Over Signaling (DOS) message over a circuit services network Data Burst Message (DBM) (DOS over DBM). 
     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 when the packet data node 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 when the packet data node includes a PCF and/or an AN, to circuit services network  110 . 
     If circuit services network  110  includes support for packet data services and a packet data session has been established for an AT such as AT  102 , a Short Data Burst (SDB) feature of network  110  permits the transfer of packet data frames between the network and AT  102  over a traffic channel of network  110  when the AT is engaged in a circuit voice call or active packet data call, and over a common channel of network  110  if the AT&#39;s packet data session is dormant and the AT is not engaged in a circuit voice call. Packet data network  130  provides a Data Over Signaling (DOS) feature for a transfer of ‘higher layer’ data between the packet data network and an AT, such as AT  102 , over either a common channel or a traffic channel of the packet data network when its packet data session has been established and is anchored in the packet data network. For example, the SDB and DOS features may be used to transfer instant messaging text, email, or a web page to an AT without requiring a reactivation of the AT&#39;s packet data session, that is, while the AT&#39;s packet data session is dormant. 
     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 (C-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 Network Domain Selection (NeDS)  158  functionality and a Call Continuity Control Function (CCCF)  160 , hereinafter collectively referred to as a CCCF/NeDS  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). Although  FIG. 1  depicts NeDS  158  and CCCF  160  as being implemented in a single network element, such as a single server, those who are of ordinary skill in the art realize that NeDS  158  and CCCF  160  may be implemented in separate network elements without departing from the spirit and scope of the present invention. Similarly, Although  FIG. 1  depicts I-CSCF and C-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 C-CSCF may be implemented in separate network elements without departing from the spirit and scope of the present invention. CCCF/NeDS  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 a path for an exchange of voice information. 
     Referring now to  FIG. 2 , an architecture of AT  102  is provided in accordance with an embodiment of the present invention. AT  102  may include at least one transceiver  202  that allows the AT to transmit or receive in each of the two networks. Transceiver  202  is coupled to a vocoder  206  and a processor  208 , which processor is further coupled to an at least one memory device  210 . AT  102  may maintain a priori information in at least one memory device  210  that facilitates the switching between networks  110  and  130 . Processor  208  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 processor is configured to execute the functions described herein as being executed by AT  102 . The at least one memory device  210  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  210  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 now to  FIGS. 3 and 4 , an architecture of each of BS  112  and packet data node  134  is provided in accordance with an embodiment of the present invention. Each of BS  112  and packet data node  134  and includes a respective processor  302 ,  402 , 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 is configured to execute the functions described herein as respectively being executed by the BS and packet data node. Each of BS  112  and packet data node  134  further includes a respective at least one memory device  304 ,  404  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 and that allow the BS and packet data node to perform all functions necessary to operate in communication system  100 . 
     The functionality described herein as being performed by AT  102 , BS  112 , and packet data node  134  is implemented with or in software programs and instructions stored in the respective at least one memory device  210 ,  304 , and  404  of the AT, BS, and packet data node and executed by an associated processor  208 ,  302 , and  402 , of the AT, BS, and packet data node. When packet data node  134  comprises an AN and a PCF, the functions described herein as being performed by the packet data node may be performed by a processor included in the AN or a processor included in the PCF or may be distributed among the processors of the AN and the PCF based on data and programs respectively stored in a corresponding at least one memory device of the AN and the PCF. When BS  112  comprises BTS  114  and a BSC  116 , the functions described herein as being performed by the BS 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. Preferably, circuit services network  110  is a cdma2000 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. Preferably, packet data network  130  is a cdma2000 (Code Division Multiple Access) communication system that provides HRPD communication services to subscribers serviced by the network 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. And preferably the IP Multimedia Core Network Subsystem (IMS) of Home Network  150  operates in accordance with the 3GPP2 X.S0013 standards, which describes the operation, elements, and interfaces of an IMS. 
     Further, circuit services network  110  and AT  102  preferably 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  preferably 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 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. 
     It may be assumed, for purposes of the present invention, that AT  102  is engaged in a circuit voice call with circuit services network  110 . In order to access circuit services network  110 , AT  102  tunes to an operating frequency assigned to the circuit services network, 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 packet data network  152  and requests circuit services network  110  to page AT  102  to request the AT move to the packet data network so that the packet data can be delivered to the AT. 
     When AT  102  is not engaged in a voice call with, or monitoring a paging channel in, circuit services network  110 , the AT may initiate a packet data call and register with packet data network  130 , and more particularly with PDSN  150 . AT  102  may then establish a data link with PDSN  138  in accordance with a Layer 2 protocol such as the 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 preferably comprising 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. 
     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 is assigned a traffic channel in air interface  132  and packet data is transferred to the AT 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 may roam through the system. 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 and 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 another example, the costs associated with operating AT  102  on circuit services network  110  may be different from the costs associated with operating AT  102  on packet data network  130 . In turn, an operator (or operators) of networks  110  and  130  may charge a different fee for use of each network. As a result, a user of AT  102  may program into the subscriber a directive to operate on the lower cost network whenever the AT is able to obtain a traffic channel in the lower cost network. When AT  102  is engaged in a voice call in a higher cost network and is able to obtain a traffic channel in the lower cost network, the subscriber unit, or the user of the subscriber unit if the user is informed of the availability of a traffic channel in the lower cost network, may initiate a handoff to the lower cost network. By way of yet another example, it may be desirable to move a 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 service rather than a voice call, and packet data network  130  supports video telephony but circuit services network  110  does not. By way of still another example, for load leveling purposes, for network cost consideration purposes, or due to a need to clear traffic channels in a coverage area in order to facilitate emergency communications, an operator of a communication system such as communication system  100  may find it desirable to move an AT, such as AT  102 , that is actively engaged in a voice call in circuit services network  110  to the other network. 
     In order to facilitate a handoff of an AT, such as AT  102 , communication system  100  provides a method and apparatus for an active handoff of the AT from circuit services network  110  to packet data network  130  when the AT is actively engaged in a voice call in the circuit services network. By providing for an active handoff of a voice call from circuit services network  110  to packet data network  130 , communication system  100  assures that the AT is actively engaged in a communication session with at least one of networks  110  and  130  at nearly all times, thereby minimizing the likelihood that voice or data traffic may be lost during the handoff. 
     Referring now to  FIGS. 5A ,  5 B, and  5 C, a signal flow diagram  500  is provided that illustrates a method executed by communication system  100  in handing off AT  102  from circuit services network  110  to packet data network  130  in accordance with various embodiments of the present invention. Signal flow diagram  500  begins when AT  102  is engaged in a voice call with a remote end point  172  via circuit services network  110 . In order to participate in the call, AT  102  must already be registered with circuit services network  110 . As is known in the art, when AT  102  is pre-registered with the IMS, in order to set up the call MGCF  124  sets up  502  a Session Initiation Protocol (SIP) dialogue with CCCF/NeDS  156  via I/S-CSCF  154  and, in turn, CCCF/NeDS  156  sets up  504  a SIP dialogue remote with end point  172  via I/S-CSCF  154  and far end network  170 . A path is then established for a transfer of voice information between AT  102  and remote end point  172 . 
     In exchanging voice information, AT  102  exchanges  506  vocoded speech with BS  112  via air interface  104 . BS  112  receives  506  vocoded speech from AT  102  via a reverse link traffic channel of air interface  104 , transcodes the vocoded speech to 64 kbps Pulse Code Modulation (PCM) (G.711), and conveys  508  the transcoded speech to MSC  120 . Similarly, BS  112  transcodes communications received from MSC  120  to a vocoder format compatible with AT  102  and transmits  506  the transcoded information to AT  102  via a forward link traffic channel of air interface  104 . MSC  120  forwards  510  the PCM signals received from BS  112  to MGW  122  and forwards  510  PCM signals received from the MGW to the BS. MGW  122  transcodes PCM signals received from MSC  102  to vocoded speech, includes the vocoded speech in data packets, preferably formatted in an RTP/UDP/IP format, and routes  512  the data packets to remote end point  172  via far end network  170 . MGW  122  further converts vocoded speech included in data packets received from remote end point  172  via far end network  170  to 64 kbps PCM (G.711) and routes  512  the PCM signals to MSC  120 . 
     While AT  102  is in an active voice call in circuit services network  110 , the AT monitors qualities, in particular a signal strength or alternatively any of a variety of other signal qualities such as a signal-to-noise ratio (SNR), a carrier-to-interference ratio (C/I), pilot power-to-total power (Ec/Io) ratio, a bit error rate (BER), or a frame error rate (FER), of pilots associated with each of BS  112  of circuit services network  110  and packet data node  134  of packet data network  130 . AT  102  may monitor the pilots of each network  110 ,  130  concurrently or may switch between networks in monitoring the pilots. AT  102  may self-determine when or whether to monitor the pilots associated with packet data node  134  of packet data network  130  or may monitor the pilots in response to receiving an instruction to do so from circuit services network  110 , and in particular one of BS  112  and MSC  120 . 
     AT  102  reports  514  the monitored pilot(s) in accordance with well known reporting procedures. For example, when a monitored pilot exceeds a 1X intra-system soft handoff add threshold, the MS reports this pilot, and the measured pilot channel strength, to BS  112 , and in particular to BSC  116 , in a Pilot Strength Measurement Message (PSMM) conveyed to the BS via the reverse link of air interface  104 . Similarly, when a monitored pilot falls below a 1X intra-system soft handoff drop threshold, the MS reports this pilot, and the measured pilot signal strength, to BS  112 , and in particular to BSC  116 , in a PSMM conveyed to the BS via the reverse link of air interface  104 . BSC  116  then stores the reported network pilot measurements. A report of a pilot that exceeds a 1X intra-system soft handoff add threshold is an indicator to add a BS associated with that pilot to soft handoff with the MS and a report of a pilot that falls below a 1X intra-system soft handoff drop threshold is an indicator to drop a BS associated with that pilot from soft handoff with the MS. 
     When a quality of a pilot of packet data network  130 , and more particularly of air interface  132 , is measured by AT  102  when operating in circuit services network  110  and exceeds an inter-system hard handoff add threshold, or a quality of a previously reported pilot of packet data network  130 , and more particularly of air interface  132 , is measured by AT  102  when operating in circuit services network  110  and falls below an inter-system hard handoff drop threshold, the AT reports  514  the monitored HRPD pilot(s) back to BS  112 , and more particularly BSC  116 . BSC  116  then stores the reported HRPD pilot measurements. 
     AT  102  may report HRPD pilot strengths by sending a circuit services network  110  message delivery mechanism, such as a 1X Data Burst Message (DBM), to BS  112  on a reverse link dedicated signaling channel (r-dsch) of air interface  104  or a reverse link traffic channel of air interface  104  assigned to the call. The reverse link signaling message comprising the DBM may include a request for confirmation of delivery for the signaling message. Preferably, the DBM includes a BURST_TYPE field, or another identifier, that identifies the message as reporting HRPD pilot signal strengths. In response to receiving the DBM, BS  112  parses the message and recognizes the message as comprising an HRPD pilot signal strength measurement. 
     Based on the pilot measurements associated with BS  112  and the HRPD pilot measurements associated with packet data node  134  and reported by AT  102 , circuit services network  110 , and in particular BS  112  or MSC  120 , may then determine to handoff AT  102  to packet data network  130  and packet data node  134 . For example, when a pilot of BS  112  compares unfavorably to (is below, in the case of a signal strength threshold) the 1X intra-system soft handoff drop threshold and/or a pilot of packet data node  134  compares favorably to (exceeds, in the case of a signal strength threshold) the HRPD inter-system hard handoff add threshold, this may indicate a desirability of a handoff. By way of another example, costs associated with operating AT  102  on network  110  may be different from the costs associated with operating AT  102  on network  130 . In turn, an operator (or operators) of networks  110  and  130  may charge a different fee for use of each network. If packet data network  130  is the lower cost network, a user of AT  102  may program into the MS a directive to operate on the second network  130  whenever a measurement of a pilot associated with the second network compares favorably to the HRPD inter-system hard handoff add threshold. By way of still another example, for load leveling purposes, for network cost consideration purposes, or due to a need to clear traffic channels in a coverage area in order to facilitate emergency communications, an operator of communication system  100  may find it desirable to move a AT, such as AT  102 , that is actively engaged in a voice call in circuit network  110  to packet data network  130  whenever a measurement of a pilot associated with the second network compares favorably to the HRPD inter-system hard handoff add threshold. 
     In response to determining to handoff AT  102  from circuit network  110  and BS  112  to packet data network  130  and packet data node  134 , BS  112  assembles a request to handoff the voice call as a Voice over IP (VoIP) call on the packet data network. Preferably, the request requests resources to support the call and comprises an A21-Resource Allocation Request message that includes a request that the packet data network allocate resources for AT  102 . BS  112  then conveys  516  the request to packet data node  134 , preferably via the inter-RAN interface such as an A21 interface. In response to receiving the resource allocation request, packet data node  134  determines whether AT  102  has an HRPD packet data session anchored in packet data network  130  and has an SSIR (session state information record) with an associated HRPD Radio Link Protocol (RLP) flow and an access terminal identifier, such as a Unicast Access Terminal Identifier (UATI)) assigned to the AT. For example, a dormant packet data session associated with the AT may be maintained by packet data network  130 . As noted above, an AT&#39;s packet data session is considered to be in the dormant state when a PPP/Layer connection has been established between the AT and PDSN, A10 bearer resources have been allocated to the AT, but traffic channel and A8 bearer resources (between an AN and PCF of the packet data node) are not allocated to the AT. 
     If AT  102  does not have an HRPD packet data session anchored in packet data network  130 , a pre-negotiated and pre-established ‘dummy’ session from a reserved pool of resources (for incoming ATs that don&#39;t have an assigned session) with a pre-configured session state information record (including an SSIR and at least an HRPD IP flow with appropriate QoS to support VoIP service) and a Point-to-Point Protocol (PPP) connection (Layer 2 protocol) may be assigned to the AT by packet data network  130 , and preferably by packet data node  134 , along with an access terminal identifier, such as a UATI, that is uniquely associated with the AT for the session. The pre-configured session may include pre-setup A10 connections and IP flows to support the VoIP call. AT  102  is aware of the packet data session configuration (SSIR) and PPP session when a hand-in to packet data network  130  is required either by operator configuration of the AT or by provision of this information at the time of the handoff, that is, by provision of this information to AT  102  by packet data node  134  via BS  112 . The information then is maintained in the at least one memory device  210  of the AT and also is maintained in the at least one memory device  404  of the packet data node or otherwise is maintained in a location in packet data network  130  that is accessible by the packet data node. In either case, the SSIR and PPP session information may be preconfigured with default values such that PPP or HRPD session negotiation is not required at the time of the handoff, thereby allowing AT  102  to be handed off to packet data network  130  without requiring that a new packet data session be set up for the AT at the time of the handoff and without first requiring a conveyance of an SSIR to the AT over the air or negotiating SSIR with the AT, thereby reducing handoff latency delays. In response to receiving the request to allocate resources, packet data node  134  allocates resources to AT  102 , if resources are available either from a pre-existing (for example, dormant) HRPD packet data session or from a preconfigured/reserved HRPD packet data session that is assigned to the AT, and assembles and conveys  518  to BS  112 , preferably via the inter-RAN interface, a packet data network air interface signaling response, more particularly an HRPD DOS message, that comprises a confirmation by packet data network  130  to accept a hand-in of the call as a VoIP call. 
     When no resource is available at packet data node  134  for assignment to AT  102 , packet switched node  134  may set a ‘Cause’ field in the response conveyed to BS  112 , preferably an A21-Resource Reservation Response, to ‘no resource.’ The handoff may then end. 
     When a resource is available at packet data node  134  for assignment to AT  102 , either from a pre-existing packet data session or from a preconfigured/reserved HRPD session to be assigned to AT, then the response conveyed by packet data node  134  to BS  112  may comprise an A21-Resource Allocation Response message. The message may include one or more of traffic channel assignment information for AT  102  and associated with a traffic channel in air interface  132 , the pre-configured session state information, and the UATI assigned to the AT for the session. If the AT changed HRPD subnets during the circuit switched voice call, then packet data node  134  may use an A13 procedure to retrieve the AT&#39;s information from a source packet data node before packet data node  134  allocates resources to the AT. In response to receiving the response from packet data node  134 , BS  112  conveys  520  a message to AT  102  comprising information for facilitating a handoff to packet data network  130 , and preferably to packet data node  134 , including one or more of the traffic channel information associated with traffic channel assigned in air interface  132 , the pre-configured session state information, the UATI assigned to the AT for the session, and a new active set. 
     In one embodiment of the present invention, BS  112  may assemble a DBM that indicates that a resource has been reserved for the AT, preferably by setting a ‘Reason’ field of the DBM to ‘resource reserved’ to indicate to AT  102  that the AT is to start bearer path modification procedures. The DBM further may include a ‘Data Burst Type’ field that identifies the data as packet data and may or may not include at least a portion of the response received from packet data node  134 , that is, the A21-Resource Allocation Response, in a payload portion of the DBM. BS  112  then conveys the DBM to AT  102  via either a forward link common signaling channel of air interface  104 , a dedicated signaling channel of the air interface, or the traffic channel of the air interface assigned to AT  102 . 
     If, when assigning resources at packet data node  134  to AT  102 , a dormant packet data session does not exist and the packet data node assigns resources to the AT from a reserved pool of resources, then in response to receiving the traffic channel assignment information from the packet data node, AT  102  performs  522  a discovery of P-CSCF  140  in accordance with well-known techniques, except that the messages used in the P-CSCF discovery are tunneled between BS  112  and packet data node  134  via the A21 interface and between the BS and AT  102  using DOS over DBM. Further, if packet data node  134  assigns resources to AT  102  from a reserved pool of resources, the AT also initiates  524  a registration with the IP Core Network Multimedia Subsystem (IMS) of Home Network  150 , and more particularly with CCCF/NeDS  156 . 
     Messages sent and received between BS  112  and packet data node  134  may be passed directly over the A21 interface when the IWS  126  is collocated in circuit services network  110 , such as within BS  112 . When a direct A21 interface between circuit services network  110  and packet data network  130  is not available, the messages may be passed indirectly, for example, to an MSC which is connected to packet data node  134  via an Al interface or to an MSC which is connected via an A1 interface to the IWS which is connected to the packet data node via an A21 interface. Preferably the AT initiates the IMS registration by conveying a SIP Register message to BS  112 , preferably using DOS over DBM signaling. For example, AT  102  may include the SIP Register message in a DBM that may further include data fields comprising an HRPD service option (SO), such as SO 59, that identifies the Data Burst as destined for packet data network  130 , and RLP information associated with the packet data, that is, an identification of the HRPD RLP flow to which the packet data is to be sent (HRPD RLPFlowID). BS  112  then sends the SIP Register message to packet data node  134 , such as to an AN (A.S0008-B v0.2 architecture) or a PCF (A.S0009-B v0.2 architecture) of the packet data node. BS  112  may include the SIP Register message in an Application Data Delivery Service (ADDS) message that includes the message in an Application Data Message field in an ADDS User Part element of the ADDS message or in a DOS Deliver message and may send the message to the packet data node via the A21 interface or via any other signaling connection that the BS may have to the packet data node. Packet data node  134  then parses the message received from BS  112  to extract the SIP Register message and sends the SIP Register message to a PDSN supporting AT  102 &#39;s packet data session in the packet data network, that is, PDSN  138 , as normal packet data via the A10 interface, mapping the data flow to the RLP flow specified by AT  102 . In this way, AT  102  may convey SIP messages to packet data node  134  and further to packet data network  130  without having to leave circuit services network  110 . PDSN  138  then forwards the received packet data to I/S-CSCF  154  and the I/S-CSCF forwards the packet data to CCCF/NeDS  156 . 
     When, at signal flow  520 , AT  102  receives the DBM from BS  112  indicating that a resource has been reserved for the AT, the AT initiates the bearer path modification by initiating an IMS call to CCCF/NeDS  156 . More particularly, AT  102  conveys  526  a session invitation, more particularly a SIP Invite, to BS  112  that includes a routing identifier, such as an E. 164  number or a SIP URI, associated with the recipient of the session invitation, that is, CCCF/NeDS  156 , and a Session Description Protocol (SDP) proposal associated with an establishment of a bearer path via air interface  132  and packet data network  130 . Again, AT  102  may send the SIP Invite to BS  112  using DOS over DBM signaling, wherein the DBM may further include data fields comprising an HRPD service option (SO), such as SO 59, that identifies the Data Burst as destined for packet data network  130 , and identification of the HRPD RLP flow to which the packet data is to be sent. As is known in the art, the SDP proposal may include one or more of a type of media, such as video, audio, and so on, a transport protocol, such as RTP/UDP/IP, H.320, and so on, a format of the media, such as H.261 video, MPEG video, and so on, a remote address for media, and a transport port for contact address. SDP is well-known and is described in detail in 3GPP2 Request for Comments (RFC) 2327. 
     BS  112  then sends the SIP Invite message to packet data node  134 , such as to an AN (A.S0008-B v0.2 architecture) or a PCF (A.S0009-B v0.2 architecture) of the packet data node, via the A21 interface. Again, BS  112  may include the SIP Invite message in an ADDS message that includes the message in an Application Data Message field in the ADDS User Part element of the ADDS message or in a DOS Deliver message. Packet data node  134  then parses the message received from BS  112  to extract the SIP Invite message and sends the SIP Invite message to PDSN  138  as normal packet data via the A10 interface, mapping the data flow to the RLP flow specified by AT  102 . PDSN  138  then forwards  532  the SIP Invite to I/S-CSCF  154  and the I/S-CSCF forwards  534  the SIP Invite to CCCF/NeDS  156 . 
     In response to receiving the SIP Invite, CCCF/NeDS  156  assembles and conveys  536  an updated session invitation comprising the SDP proposal and a routing identifier associated with the recipient of the SDP proposal, that is, remote end point  172 , to the remote end point. More particularly, CCCF/NeDS  156  assembles a SIP Re-invite with the updated SDP proposal and conveys  536  the SIP Re-invite to I/S-CSCF  154 . I/S-CSCF  154  then forwards  538  the SIP Re-invite to remote end point  172  via far end network  170 . 
     Based on the SDP proposal included in the session invitation, that is, the SIP Re-invite, remote end point  172  determines whether to accept the SDP proposal. When the remote end point  172  accepts the proposal or suggests a counter-proposal (as opposed to an unconditional rejection of the proposal), remote end point  172  acknowledges receipt of the updated session invitation and informs  540  I/S-CSCF  154  via far end network  170  of the acceptance of the SDP proposal included in the SIP Re-invite message or of the counter-proposal. Preferably, remote end point  172  acknowledges the proposal and conveys the acceptance of the proposal or the counter-proposal by assembling and conveying a SIP response, preferably a SIP 200 OK message, comprising the accepted SDP proposal or the counter-proposal. I/S-CSCF  154  forwards  542  the acceptance of the SDP proposal or the counter-proposal, that is, the SIP 200 OK message, to CCCF/NeDS  156 . CCCF/NeDS  156  then routes the acceptance of the SDP proposal or the counter-proposal, that is, the SIP 200 OK message, to AT  102  via circuit services network  110 . That is, CCCF/NeDS  156  forwards  544 ,  546  the SIP 200 OK message to P-CSCF  140  via I/S-CSCF  154 . P-CSCF  140  forwards  548  the SIP 200 OK message to packet data node  134 , and the packet data node tunnels  550  the SIP 200 OK message to BS  112  via the A21 interface. BS  112  then assembles a DBM that includes the acceptance of the SDP proposal or the counter-proposal, that is, the SIP 200 OK message, and forwards  552  the DBM to AT  102  via a forward link signaling channel or the assigned forward link traffic channel of air interface  102 . 
     In response to receiving the acceptance of the SDP proposal or the counter-proposal, that is, the SIP 200 OK message, AT  102  acknowledges to remote end point  172  receipt of remote end point  172 &#39;s acceptance of the AT&#39;s SDP proposal or the AT&#39;s acceptance of the remote end point&#39;s counter-proposal. Preferably, AT  102  acknowledges receipt of remote end point  172 &#39;s acceptance of the AT&#39;s SDP proposal or the AT&#39;s acceptance of the remote end point&#39;s counter-proposal by assembling and conveying  554  a SIP ACK to BS  112  via a reverse link signaling channel of air interface  104 . Again, AT  102  may send the SIP ACK to BS  112  using DOS over DBM signaling, wherein the DBM that may further include data fields comprising an HRPD service option (SO), such as SO 59, that identifies the Data Burst as destined for packet data network  130 , and identification of the HRPD RLP flow to which the packet data is to be sent. 
     BS  112  then sends the SIP ACK message to packet data node  134  via the A21 interface. Again, BS  112  may include the SIP ACK message in an ADDS message that includes the message in an Application Data Message field in the ADDS User Part element of the ADDS message or in a DOS Deliver message. Packet data node  134  then parses the message received from BS  112  to extract the acknowledgment of the acceptance of the AT&#39;s SDP proposal or the AT&#39;s acceptance of the remote end point&#39;s counter-proposal, that is, the SIP ACK, from the message, and forwards  558  the SIP ACK as normal packet data to P-CSCF  140 . AT  102  may discover the address of the P-CSCF  140  using DHCP (Dynamic Hierarchical Control Protocol) or the address of the P-CSCF may be configured in the AT. P-CSCF  140  forwards  560  the SIP ACK to I/S-CSCF  154  and the I/S-CSCF forwards  562  the SIP ACK to CCCF/NeDS  156 . CCCF/NeDS  156  then routes  564 ,  566  the SIP ACK to remote end point  172  via I/S-CSCF  154  and far end network  170 . 
     In addition, in response to receiving the SIP 200 OK message, AT  102  requests to be handed off to packet data network  130 , and more particularly to packet data node  134 , by assembling and conveying  568 , to BS  112 , a DBM comprising a Handoff Status message. In response to receiving the Handoff Status message, BS  112  conveys  570  a DBM to AT  102  instructing the AT to handoff to the packet data network and further comprising the traffic channel assignment information associated with a traffic channel in air interface  132  and the UATI assigned to AT  102  that was included in the A21-Resource Reservation Response message received by the BS from packet data node  134 . In response to receiving the instruction to handoff, traffic channel assignment information associated with a traffic channel in air interface  132 , and UATI, AT  102  tunes to packet data network  130 , and more particularly one or more of a forward link signaling channel and the assigned traffic channel in air interface  132 , and establishes  572  a PPP (Point-to-Point Protocol) communication session with PDSN  138  via air interface  132  and packet data node  134  in accordance with well known techniques. In turn, PDSN  138  sets up a bearer path with remote end point  172  via far end network  170  via which vocoded speech is conveyed in data packets, preferably formatted in an RTP/UDP/IP format, to the far end network. In this manner, packet data network  130 , and in particular packet data node  134 , acquires AT  102  on a packet data VoIP call. 
     Further, in response to receiving the SIP ACK message, CCCF/NeDS  156  initiates a termination of the leg of the voice call that circuit services network  110  has continued to support and maintain until such termination. More particularly, in response to receiving the SIP ACK message, CCCF/NeDS  156  conveys  574  a SIP BYE to I/S-CSCF  154  and the I/S-CSCF forwards the SIP BYE message to MGCF  124 . In response to receiving the SIP BYE message, MGCF  124  requests that MGW  122  ‘subtract’ the MGW&#39;s ephemeral terminations, preferably by using a Megaco Subtract to ‘subtract,’ that is, remove, the termination from the bearer path, thus removing the MGW from the bearer path of the voice call. Further, in response to receiving the SIP BYE message, MGCF  124  requests  578  that MSC  120  release the resources allocated by circuit services network  110  to AT  102 , preferably by conveying an ISUP: REL message to the MSC. 
     In response to receiving the request to release resources from MGCF  124 , MSC  120  conveys  580  a request to BS  112  to release resources allocated at the BS to AT  102  for the voice call, preferably by conveying a CLEAR COMMAND message to the BS. In response to receiving the resource release request from MSC  120 , BS  112  releases the resources and informs  582  MSC  120  that the resources allocated to AT  102  have been released, preferably by conveying a CLEAR COMPLETE message to the MSC. In response to being informed by BS  112  that the resources allocated to AT  102  for the voice call have been released, MSC  120  releases the TDM trunk resources allocated to AT  102  for the voice call and informs  584  MGCF  124  that the resources allocated to AT  102  for the voice call have been released, preferably by conveying an ISUP: RLC message to the MGCF. In turn, in response to being informed by MSC  120  that the resources allocated to AT  102  for the voice call have been released, MGCF  124  informs  586  I/S-CSCF  154  that the leg of the voice call maintained by circuit services network  110  has been terminated by conveying, to the I/S-CSCF, an acknowledgment, preferably a SIP 200 OK message, of the SIP BYE message received from I/S-CSCF. I/S-CSCF  154  then informs  588  CCCF/NeDS  156  that the leg of the voice call maintained by circuit services network  110  has been terminated by forwarding the acknowledgment, that is, the SIP 200 OK message, of the SIP BYE message that the CCCF/NeDS sent to the I/S-CSCF. In response to receiving the SIP 200 OK message, CCCF/NeDS  156  sets up  590 ,  592  a Session Initiation Protocol (SIP) dialogue with each of AT  102  and remote end point  172  as is known in the art and a bearer path is then established  594  for a transfer of voice information between AT  102  and remote end point  172 . Signal flow diagram  500  then ends. 
     By utilizing the A21 interface between BS  112  and wireless packet data node  134  and DOS over DBM signaling between BS  112  and AT  102 , communication system  100  is able to tunnel SIP messaging between the AT and the wireless packet data node via circuit services network  110  and to tunnel HRPD traffic channel information and an HRPD identifier from the packet data network to the circuit services network  110 , and via the circuit services network to the AT. Thus a handoff of a voice call involving AT  102  may be initiated from circuit services network  110  to packet data network  130  prior to the AT establishing a call dialogue with the packet data network. By facilitating an exchange of handoff information among packet data network  130 , and in particular wireless packet data node  134 , and AT  102  via circuit services network  110 , handoff latency delays may be reduced. 
     More particularly, BS  112 , while supporting a circuit switched voice call, determines to hand off the call and conveys, to wireless packet data node  134  via the A21 interface, a request to hand off the call to packet data network  130 . In response to receiving the request, wireless packet data node  134  determines an access terminal identifier associated with AT  102  and conveys the access terminal identifier and traffic channel assignment information to BS  112  via the A21 interface. In determining the access terminal identifier, packet data node  134  determines whether AT  102  has a packet data session anchored in the packet data network. When AT  102  has a packet data session in packet data network  130 , packet data node  134  may determine an access terminal identifier by determining an access terminal identifier that is associated with the MS/AT&#39;s packet data session. When AT  102  does not have a packet data session anchored in packet data network  130 , the packet data node determines an access terminal identifier by assigning, to the AT, a previously reserved packet data session and an access terminal identifier. BS  112  may then convey the access terminal identifier, packet data traffic channel information, and a new active set to the AT prior to the AT establishing a call/SIP dialogue with packet data network  130 . Packet data node  134  then acquires the access terminal and BS  112  may terminate the circuit switched leg of the call. 
     While the present invention has been particularly shown and described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various changes may be made and equivalents substituted for elements thereof without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather then a restrictive sense, and all such changes and substitutions are intended to be included within the scope of the present invention. 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms “comprises,” “comprising,” or any 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. The terms ‘including’ and/or ‘having’, as used herein, are defined as comprising. Furthermore, unless otherwise indicated herein, the use of relational terms, if any, such as first and second, top and bottom, and the like are 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. An element preceded by “. . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that the element.