Patent Publication Number: US-8537781-B2

Title: Low latency handover between wireless communication networks using different radio access technologies

Description:
RELATED APPLICATIONS 
     This application is a Continuation of U.S. application Ser. No. 11/984,462, filed on Nov. 19, 2007 now U.S. Pat. No. 8,027,309, the disclosure of which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to wireless communication networks, and more particularly, to systems and methods for supporting handover performed when a user&#39;s mobile station (MS) moves from a wireless communication network that uses one radio access technology to a wireless communication network that uses another radio access technology. 
     BACKGROUND 
     A modern wireless communication network, such as a 3rd or 4th generation wireless network, combines a radio access network (RAN) with an Internet Protocol (IP) network. The RAN supports wireless connectivity over the airlink with a user&#39;s mobile station (MS), whereas the IP network provides the MS with access to IP services. For example,  FIG. 1  illustrates a wireless communication network  100  based on the Evolved 3rd Generation Partnership Program (3GPP) Packet Switching Domain. 
     The wireless network  100  involves a radio access technology that enables an MS  10  to access an IP network  102  over an Evolved Universal Terrestrial Radio Access Network (E-UTRAN)  12 . A base station called an Evolved Node B (eNB)  14  may serve as a hub for radio communications over the E-UTRAN  12 . One or more eNBs  14  may be provided in the E-UTRAN  12  for supporting all sub-layers of an airlink protocol carried out for transmitting and/or receiving data packets to and/or from the MS  12 . 
     A service access entity gateway (SAE GW)  16  terminates the MS interface towards the E-UTRAN  12 . The SAE GW  16  performs packet routing and forwarding, provides lawful interception of the MS traffic, serves as a local mobility anchoring point for supporting handover between the eNBs  14 , and relays traffic between the E-UTRAN  12  and the IP network  102 . The network  100  also includes a Mobility Management Entity (MME)  18  for performing mobility management functions such as MS authentication, keeping track of its current location, paging and roaming. 
     A Packet Data Network Gateway (PDN GW)  20  is provided between the IP network  102  and the SAE GW  16 . The PDN GW  20  supports an interface to the IP network  200 . In particular, the PDN GW  20  allocates an IP address to the MS  10 , provides IP access policy enforcement, performs lawful interception of the IP traffic, supports billing and charging for IP services, and provides per-user based packet filtering. 
     Based on the coverage provided by the E-UTRAN  12 , the network  100  may support MS communications in a limited area. When the MS  10  initiates an active call in the network  100 , and then moves to an area which is not covered by the network  100 , the initiated call has to be transferred to another network that may use another radio access technology. When the call is transferred to another network, the context relating to the active call, such as quality of service (QoS) context, security associations, compression context, link layer context (e.g. Point-to-Point Protocol), billing and charging information, and multicast session related information, is required to be transferred. 
     In addition, if the MS  10  is not capable of keeping both radio access technologies active at the same time, it needs to setup new radio access resources and link layer related information using the new radio access mechanism. 
     In existing IP-based wireless networks, when transitioning between network technologies, the call handover latency is unacceptably high for real-time traffic applications such as Voice over IP (VoIP) or streaming video. Due to the high call handover latency, a user may experience a large break in a video or a voice conversation, or a call may be dropped. 
     Therefore, there is a need for a handover mechanism that would minimize the handover latency to enable the MS to move seamlessly between networks with different radio access technologies. 
     SUMMARY OF THE DISCLOSURE 
     A data communication system and method are disclosed herein, which incorporate concepts to address the above noted problems with switching between different radio access technologies during an active wireless communication session. 
     In accordance with one aspect of the disclosure, a communication system for providing a user&#39;s mobile station (MS) with an Internet Protocol (IP) connectivity, has an IP network gateway for allocating an IP address to the MS to enable it to access an IP network. The system may include first and second base stations respectively configured to support communications of the MS over first and second radio access networks using different radio access technologies. 
     A first access gateway provides an interface between the IP network gateway and the first radio access network, whereas a second access gateway provides an interface between the IP network gateway and the second radio access network. For example, the first access gateway may operate in accordance with the 3rd Generation Partnership Project (3GPP) standard, and the second access gateway may operate in accordance with the Code Division Multiple Access 2000 (CDMA2000) standard. 
     A handover interface is provided between the first access gateway and the second access gateway for enabling the MS to switch between the first radio access network and the second radio access network with minimum latency. 
     An IP address allocated by the IP network gateway to the MS connected to the first radio access network may be maintained when the MS is connected to the second radio access network. The first and second access gateways may communicate with the IP network gateway using Proxy Mobile IP protocol. 
     The handover interface may transfer between the first access gateway and the second access gateway session-related context associated with an active wireless communication session originated before the user&#39;s mobile station performs switching between the first radio access network and the second radio access network. 
     The session-related context may provide context required for the MS to continue an IP session performed over the first radio access network when the MS switches from the first radio access network to the second radio access network. 
     In particular, the session-related context may include context for setting up a link layer to provide radio access of the user&#39;s mobile station required to continue the IP session. 
     Further, the session-related context may include context for establishing a network layer to provide IP connectivity of the user&#39;s mobile station required to continue the IP session. 
     Also, the session-related context may include policy and security context required to continue the IP session. 
     Moreover, the session-related context may include compression context defining a compression mechanism used for the IP session. 
     Further, the session-related context may include quality of service context defining quality of service information for the IP session. 
     Also, the session-related context may include billing and charging context defining billing and charging information for the IP session. 
     Further, the handover interface may provide a bearer path between the first access gateway and the second access gateway. This bearer path may enable the MS to continue a call performed over the first radio access network when the MS switches from the first radio access network to the second radio access network. 
     In accordance with the embodiment of the disclosure, when the MS initiates switching to the second radio access network, the first base station may provide the first access gateway with a first handover request. The first access gateway may respond by producing a second handover request transmitted to the second access gateway. 
     In response to the second handover request, the second access gateway may produce a handover response transmitted to the first access gateway to set up a link layer tunnel for providing communications between the first access gateway and the second access gateway. In particular, context-related information may be transferred over this gateway. 
     When the MS performs handover from the first radio access network to the second radio access network, the IP network gateway may use this tunnel to communicate with the MS. 
     In accordance with another aspect of the disclosure, a method of enabling an MS communicating over a first radio access network to move to a second radio access network involves initiating connection of the MS to the second radio access network based on a radio access condition in the first radio access network, and establishing a handover interface between the first access gateway and the second access gateway for transferring session-related context required to continue over the second radio access network an IP session performed by the MS over the first radio access network. 
     Additional advantages and novel features will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and the accompanying drawings or may be learned by production or operation of the examples. The advantages of the present teachings may be realized and attained by practice or use of various aspects of the methodologies, instrumentalities and combinations set forth in the detailed examples discussed below. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The following detailed description of the embodiments of the present disclosure can best be understood when read in conjunction with the following drawing figures that depict concepts by way of example, not by way of limitations. In the figures, like reference numerals refer to the same or similar elements. 
         FIG. 1  depicts an exemplary communication network for providing a user&#39;s mobile station with Internet Protocol (IP) connectivity. 
         FIG. 2  illustrates an example of a fast handover architecture of the present disclosure. 
         FIG. 3  illustrates a handover bearer path configured when a user&#39;s mobile station (MS) switches between different radio access technologies. 
         FIG. 4  illustrates setting up Layer 2 when the MS switches between different radio access technologies. 
         FIG. 5  shows an exemplary handover call flow for switching the MS between different radio access technologies. 
         FIG. 6  shows active data flows at various stages of MS switching between different radio access technologies. 
         FIG. 7  shows another example of a handover call flow for switching the MS between different radio access technologies. 
     
    
    
     DETAILED DISCLOSURE OF THE EMBODIMENTS 
     In the following detailed description, numerous specific details are set forth by way of examples in order to provide a thorough understanding of the relevant teachings. However, it should be apparent to those skilled in the art that the present teachings may be practiced without such details. In other instances, well known methods, procedures, components, and circuitry have been described at a relatively high-level, without detail, in order to avoid unnecessarily obscuring aspects of the present teachings. 
     The present disclosure will be made using the example of a handover mechanism applied when a user&#39;s mobile station (MS) moves from a wireless network using a 4th generation (4G) radio access technology such as that defined in the 3rd Generation Partnership Program (3GPP) TS 23.401 technical specification, to a wireless network involving a 3rd generation (3G) radio access technology such as that defined in the Code Division Multiple Access (CDMA) 2000 standard. However, one skilled in the art will realize that the disclosed handover mechanism is applicable for MS switching between any wireless networks that use different radio access technologies. 
       FIG. 2  illustrates exemplary context transfer operations performed when an MS  202  moves from a wireless network using a 4G radio access technology defined in the 3GPP TS 23.401 technical specification to a wireless network using a 3G radio access technology defined in the CDMA2000 standard. In particular, the drawing shows such elements of the 4G wireless network as an Evolved Node B(eNB)  204  that serves as a base station, a service access entity gateway (SAE GW)  206 , a mobile management entity (MME)  208  and a packet data network gateway (PDN GW)  210 . 
     As discussed in connection with  FIG. 1 , the 4G wireless network involves a radio access technology that enables an MS  202  to access an IP network  211  over a 4G radio access network (RAN)  203  such as an Evolved Universal Terrestrial Radio Access Network (E-UTRAN). The eNB  204  that serves as a hub for radio communications over the E-UTRAN is responsible for supporting all sub-layers of an airlink protocol for transmitting and/or receiving data packets to and/or from the MS  202  over a radio access link. 
     The SAE GW  206  performs packet routing and forwarding, provides lawful interception of the MS traffic, serves as a local mobility anchoring point for supporting handover between eNBs  204 , and relays traffic between the E-UTRAN and the IP network  211 . The MME  208  performs mobility management functions such as MS authentication, keeping track of its current location, paging and roaming. 
     The PDN GW  210  provides an interface to the IP network  211 . In particular, the PDN GW  210  allocates an IP address to the MS  202 , provides IP access policy enforcement, performs lawful interception of the IP traffic, supports billing and charging for IP services, and provides per-user based packet filtering. 
     The 3G wireless network is represented by a 3G base station/radio network controller (3G BS/RNC)  212 . In the 3G network, the base station (BS) may be arranged on the same platform with the radio network controller (RNC) or on different platforms. The BS supports packet transmission and/or reception to and/or from a user&#39;s MS over a 3G radio access network (RAN)  213  such as a CDMA2000 radio access network. The RNC provides control functionalities for the BS. 
     Another component of the 3G wireless network defined in the CDMA2000 standard is a packet data service node (PDSN)  214  that acts as the connection point between the radio access network and the IP network  211 . The PDSN is responsible for managing Point-to-Point Protocol (PPP) sessions between the IP network  211  and the MS. It provides mobility management functions and packet routing functionality. 
     The PDN GW  210  provides an interface to the IP network  211  for both the 4G and 3G wireless networks represented in  FIG. 2 . As discussed in more detail later, to avoid interruption of an IP-based session originated in the 4G network when the MS  202  moves to the 3G network, the IP address allocated by the PDN GW  210  to the MS  202  in the 4G network is maintained in the 3G network. In accordance with the present disclosure, this feature may be supported, for example, by establishing a network based mobility protocol called Proxy Mobile IP (PMIP) between the PDN GW  210  and the PDSN  214 , as well as between the PDN GW  210  and the SAE GW  206 . The PMIP is a protocol defined in Internet Draft “Proxy Mobile IPv6 draft-ietf-netlmm-proxymip6-06.txt” submitted on Sep. 23, 2007. The PMIP is based on an external node acting as a proxy Mobile Node that registers the location of an IP-based communication device and is accessible while the device is on the network. When the MS  202  is active on the 4G network, PMIP between the PDN GW  210  and the SAE GW  206  is active. When the MS  202  moves from the 4G network to the 3G network, PMIP becomes active between the PDN GW  210  and the PDSN  214 . In both cases, the PDN GW  210  acts as an IP anchor point and provides an IP address to the MS  202 . Although the present disclosure is made with the example of the PMIP protocol, one skilled in the art will realize that the disclosed handover mechanism may be implemented using any network based mobility protocol such as Generic Transport Protocol (GTP). 
     As illustrated in  FIG. 2 , when the MS  202  uses the 4G wireless network for a wireless communication session, such as a Voice over IP (VoIP) call, signaling and bearer paths are established from the MS  202  to the PDN GW  210  via the eNB  204  and the SAE GW  206  (line  220 ). During the session, the MS  202  performs a periodic measurement of radio signals from the eNB  204 . At some point of time, the MS  202  can move out of the range of the eNB  204 . If the MS  202  is in the range covered by the 3G BS/RNC  212  of the 3G wireless network, the MS  202  needs to perform handover to the 3G wireless network. 
     To carry out the handover, the MS  202  indicates to the SAE GW  206  that handover from the eNB  204  to the 3G BS/RNC  212  is initiated (line  222 ). Thereafter, the handover of the MS  202  from the eNB  204  to the 3G BS/RNC  212  is performed over a data link layer, i.e. Layer 2 of the Open Systems Interconnection (OSI) Basic Reference Model. The Layer 2 handover from one base station to another base station is well known to one skilled in the art of wireless communications. 
     When the SAE GW  206  receives the handover indication from the MS  202 , the SAE GW  206  sends a Handover Request to the PDSN  214  to initiate a Fast Handover Interface  224  to the PDSN  214 , and begins a context transfer procedure to transfer to the PDSN  214  session-related context, i.e. the context of an active wireless communication session, such as an IP-based telephone call or any other IP-based transaction, involving the MS  202  and originated before the MS  202  initiated switching between the 4G network and the 3G network. A call flow that illustrates operations performed in connection with the Fast Handover Interface  224  is described in more detail later. Although the handover operations are explained using the example of switching from the 4G network to the 3G network, one skilled in the art would realize that moving from the 3G network to the 4G network involves similar handover operations. 
     The context transfer procedure includes establishing a Layer 2 tunnel between the SAE GW  206  and the PDSN  214  over the Fast Handover Interface  224 . The Layer 2 tunnel may be used to carry transit bearer traffic between the SAE GW  206  and the PDSN  214 . For example, the Layer 2 tunnel may utilize Generic Routing Encapsulation (GRE), which is a tunneling protocol developed by Cisco Systems to encapsulate network layer packets inside IP tunneling packets. The original packet is the payload for the final packet. Alternatively, the Layer 2 tunnel may use IP tunneling, which is a known process of embedding one IP packet inside of another, for the purpose of simulating a physical connection between two remote networks across an intermediate network. 
     The session-related context transferred from the SAE GW  206  to the PDSN  214  over the Fast Handover Interface  224  may include Layer 2 context prepared using 4G radio access technology for application in the 3G network. The Layer 2 context may represent PPP connection or an equivalent data link layer. When the MS  202  is being connected to the 3G wireless network, PPP connection is required. To minimize interruption time during the handover from the 4G network to the 3G network, the PPP connection is established between the MS  202  and the PDSN  214 , while the MS  202  is still connected to the 4G wireless network. When the MS  202  indicates that it needs a handover, the SAE GW  206  initiates establishing the Fast Handover Interface  224  to the PDSN  214 . The SAE GW  206  along with the eNB  204  provides a transport connection between the MS  202  and the PDSN  214  to establish the PPP connection. The setting up of a Layer 2 context is explained in more detail later. 
     Further the context transfer procedure may include transferring policy and security context. In particular, when the MS  202  connects to the 4G wireless network, subscriber authentication is performed. For example, the authentication may be carried out using security keys such as IPsec keys. If the MS  202  moves from the 4G network to the 3G network, authentication may be again required. This will add up to the handover interruption time. To minimize handover interruption time, the context transfer procedure of the present disclosure involves transferring authentication information from the SAE GW  206  to the PDSN  214  when the MS  202  moves from the 4G network to the 3G network. 
     Also, the context transfer procedure may include transferring compression context. When an MS operates in a wireless network, a compression mechanism is used between the MS and a serving gateway to reduce airlink overhead. For example, in the 4G network, the compression context is set up using negotiations between an MS and an SAE GW. The compression content may involve an IP/UDP/RTP (Internet Protocol/User Datagram Protocol/Real-Time Transport Protocol) header compression information. In a conventional system, when an MS moves from one wireless access technology to another wireless access technology, it has to begin operations in a new network in an uncompressed mode. Then the MS has to re-negotiate compression context before a compression mechanism can be utilized. To enable the MS  202  to use the compression mechanism without re-negotiating the compression context when it moves from the 4G network to the 3G network, the context transfer procedure of the present disclosure may include transferring the compression context over the Fast Handover Interface  224  from the SAE GW  206  to the PDSN  214 . 
     Further, the context transfer procedure may include transferring Quality of Service (QoS) context. As 4G wireless networks are based on IP protocols, all real-time applications carried out in 4G networks, such as a VoIP application, depend on IP protocols. QoS is an important aspect of delivering quality signals in IP networks. For example, the MS  202  may perform a voice call in the 4G network. A voice call requires a specific QoS treatment to meet latency and jitter requirements. The SAE GW  206  is responsible for QoS operations with IP packets in the 4G network. In particular, the SAE GW  206  may perform QoS bit marking to form an 8-bit Differentiated Services Code Point (DSCP) byte or an 8-bit Type of Service (ToS) byte in an IP packet. The QoS bit marking is different for different types of service. When the MS  202  moves from the 4G network to the 3G network, the PDSN  214  has no knowledge of what type of QoS treatment needs to be provided to a specific IP packet flow to or from the MS  202 . To provide this information, the context transfer procedure of the present disclosure may include transferring the QoS context, such as a type of QoS bit marking, over the Fast Handover Interface  224  from the SAE GW  206  to the PDSN  214  when the MS  202  moves from the 4G network to the 3G network. 
     Also, the context transfer procedure may include transferring Layer 2 Handoff context that relates to the Layer 2 context required for setting a Layer 3 (network layer in the OSI model). In particular, when the MS  202  moves between the 4G network and the 3G network, Layer 3 handover should be performed because an IP-related context of an active VoIP call changes due to a change in network nodes providing access to the IP network  211 . In some cases, the IP address assigned to the MS  202  may change. However, to perform active VoIP communication without interruptions, IP address assigned to the MS  202  in the 4G network should be maintained in the 3G network. 
     To maintain the IP address assigned to the MS  202 , the context transfer procedure of the present disclosure may include transfer of the Layer 2 context required to perform Layer 3 handover. This context may include information identifying radio channels, signals, neighboring cell lists required to transfer the MS  202  from a source base station to a destination base station. The context transfer procedure of the present disclosure may include transferring this information over the Fast Handover Interface from the SAE GW  206  to the PDSN  214  when the MS  202  moves from the 4G network to the 3G network. 
     Moreover, when the MS  202  is involved in an active call, billing and charging information is collected by a serving gateway. In particular, when the MS  202  is involved in an active call in the 4G network, billing and charging information is collected by the SAE GW  206 . When the MS  202  moves from the 4G network to the 3G network, billing and charging information has to be collected by the PDSN  214 . In a conventional system, the PDSN  214  will have no information on billing and charging that relates to a call performed over the 4G network. To enable the 3G network to obtain this information, the context transfer procedure of the present disclosure may include transferring billing and charging context over the Fast Handover Interface  224  when the MS  202  moves from the 4G network to the 3G network. 
     Hence, the Fast Handover Interface  224  between the SAE GW  206  and the PDSN  214  may be used for transferring context relevant to an active call involving the MS  202  and performed over the 4G network before the MS  202  moves to the 3G network. After transferring the relevant context, the Layer 3 handover is performed over the Fast Handover Interface  224  to set a Layer 3 connection defining IP-related parameters of the active call in the 3G network. As discussed above, as the Layer 2 context required for setting the Layer 3 connection in the 3G network was already transferred during the context transfer procedure, the PDN GW  210  may maintain in the 3G network the same IP address of the MS  202  as the IP address allocated in the 4G network. Therefore, an active VoIP call originated in the 4G network may continue in the 3G network without interruption. After establishing the Layer 3 connection, the MS  202  is enabled to communicate in the 3G network using the 3G BS/RNC  212 , the PDSN  214  and the PDN GW  210 . 
       FIG. 3  illustrates establishing in the 3G network a bearer path, such as a VoIP path, for a call originated in the 4G network when the MS  202  moves from the 4G network to the 3G network. When the MS  202  operates in the 4G network, a bearer path for a call involving the MS  202  is established between the MS  202  and the PDN GW  210  over the eNB  204  and the SAE GW  206  (line  226 ). Line  228  shows a bearer path when the MS  202  is in process of performing the Layer 3 handover between the 4G network and the 3G network. In this case, the bearer path still goes from the PDN GW  210  to the SAE GW  206  because the connection between the PDN GW  210  and the PDSN  214  is not ready yet. However, as the MS  202  has already switched from a radio access connection with the eNB  204  to a radio access connection with the 3G BS/RNC  212 , the SAE GW  206  directs data traffic over the Fast Handover Interface  224  to the PDSN  214  that delivers the traffic to the MS  202  over the 3G BS/RNC  212 . Similarly, traffic originated by the MS  202  goes to the PDN GW  210  via the 3G BS/RNC  212 , the PDSN  214 , the Fast Handover Interface  224  and the SAE GW  206 . 
     When the Layer 3 handover is completed and the connection between the PDN GW  210  and the PDSN  214  is established, a bearer path between the MS  202  and the PDN GW  210  goes over the 3G BS/RNC  212  and the PDSN  214  (line  230 ). Hence, the Fast Handover Interface  224  carries the data traffic flowing between the MS  202  and the PDN GW  210  when the MS  202  is in process of switching between the 4G network and the 3G network. 
       FIG. 4  illustrates preparing a Layer 2 link, such as a PPP link or an equivalent connection, to be used over the 3G network when the MS  202  switches from the 4G network to the 3G network. In particular, in response to a handover request from the MS  202 , the SAE GW  206  creates a transfer context link between the SAE GW  206  and the PDSN  214  over the Fast Handover Interface  224  (line  232 ). The transfer context link may be used to carry control signaling and as a bearer path. Over the transfer context link, the MS  202  sends a Layer 2 setup request to the PDSN  214 . Complete Layer 2 setup process is performed using the 4G radio access. The MS  202  continues to use the same IP address as the IP address assigned in the 4G network. 
     In response to the Layer 2 setup request, the PDSN  214  prepares the 3G BS/RNC  212  for establishing the Layer 2 connection with the MS  202  to be used as soon as the MS  202  jumps to the 3G network (line  234 ). The new radio resource allocation is communicated to the MS  202  from the PDSN  214  through a path established over the SAE GW  206  and the eNB  204 . 
     Once the MS  202  switches to the 3G radio access, it immediately uses a path to the PDN GW  210  established over the 3G BS/RNC  212  and the PDSN  214  (line  236 ). Hence, the Fast Handover Interface  224  is used to prepare the Layer 2 link setup for the 3G access, while the MS  202  is still on the 4G network. As a result, when the MS  202  switches from the 4G network to the 3G network, the handover interruption time is minimized. 
       FIG. 5  illustrates an exemplary handover call flow for a scenario when the MS  202  goes from the 4G radio access to the 3G radio access. When a call is set up in the 4G network, a data path  301  is established between the MS  202  and the SAE GW  206 . The data path  301  may carry any IP data such as Real Time Protocol (RIP) data for a VoIP call. The SAE GW  206  acts as a proxy mobile agent (PMA) for proxy mobile IP communication between the SAE GW  206  and the PDN GW  210  which acts as a local mobility agent (PMA). The proxy mobile IP communication between the SAE GW  206  and the PDN GW  210  is performed over a tunneled data path  302 . 
     Based on radio access conditions, the MS  202  may decide to switch from the 4G radio access provided over the 4G network to the 3G radio access offered by the 3G network, i.e. to initiate handover (HO). In this case, the eNB  204  sends a base station handover (BS HO) request  303  to the SAE GW  206 . The BS HO request  303  indicates that the MS  202  needs to perform handover to the 3G radio access network. 
     In response to the BS HO request  303 , the SAE GW  206  sends a handover request  304  to the PDSN  214  to create a horizontal context transfer link. A VoIP/RTP mechanism may be used to carry signaling traffic and transit bearer traffic over this link. 
     The PDSN  214  may respond with a handover response message  305  indicating that the PDSN  214  has resources available for handover from the 4G network to the 3G network and is ready to perform the handover. After receiving the PDSN  214  response, the SAE GW  206  initiates a Layer 2 (L2) tunnel setup  306  between the SAE GW  206  and the PDSN  214 . The Layer 2 tunnel may use the General Packet Radio Service tunneling protocol (GTP), Generic Routing Encapsulation (GRE) or any other similar tunneling protocol for carrying traffic required to perform one or more handover context transfers between the SAE GW  206  and the PDSN  214 . 
     After the PDSN  214  is able to communicate with the MS  202  through the SAE GW  206  and the eNB  204 , the PDSN  214  may perform Layer 2 negotiations and setup Layer 2 link (line  307 ). The MS  202  is assigned with the same IP address as the address used by the MS  202  in the 4G network. Further, the PDSN  214  may communicate with the MS  202  about 3G radio resource requirements. 
     Thereafter, a Layer 2 tunnel  308  may be set up between the 3G BS  212  and the PDSN  214 . The Layer 2 tunnel  308  may use GTP, GRE or any other similar tunneling protocol for carrying traffic. In particular, the PDSN  214  may inform the 3G BS  212  about radio resource requirements of the MS  202  and may provide information related to Layer 2 handover. In response, the 3G BS  212  may reserve required radio resources and may prepare for the Layer 2 handover. 
     Then, the PDSN  214  sends a message  308   a  to the MS  202  over the 4G network to inform the MS  202  about agreed upon 3G radio resources. After sending the message  308   a , the PDSN  214  is ready for switching the MS  202  to the 3G network. Therefore, the PDSN  214  sends a context information request  309  to the SAE GW  206  to request the context information related to the MS  202 . The context information is described above in connection with  FIG. 2 . The SAE GW  206  responds with a context information response  310  containing the requested context information. After receiving the context information, the PDSN  214  is ready to handle traffic relating to the MS  202 . 
     Layer 2 handover (L2 HO)  311  occurs between the eNB  204  and the 3G BS  212  to enable transfer of the MS  202  from the 4G radio access to the 3G radio access. An Airlink Start signal  313  is transmitted from the MS  202  to the 3G BS  212  to initiate 3G radio access. In response, the 3G BS  212  provides the PDSN  214  with an Ailink Start-3G radio ON signal  314  to indicate that the MS  202  is on the 3G network. 
     This signal triggers the PDSN  214  to set up a link to the PDN GW  210 . If a Layer 2 tunnel does not exist between the PDSN  214  and the PDN GW  210 , a Layer 2 tunnel  315  is established. This tunnel may use GTP, GRE or any other similar tunneling protocol to assist local mobility. 
     The PDSN  214  may make a Proxy Mobile IP (PMIP) registration request  316  to the PDN GW  210 . In response, the PDN GW  210  may generate a PMIP registration response  317 . The PMIP registration exchange is used for switching a tunnel linked to the PDN GW  210  and used for carrying bearer traffic for the MS  202 . As discussed above, a network based mobility protocol called Proxy Mobile IP (PMIP) may be established between the PDN GW  210  and the PDSN  214 , as well as between the PDN GW  210  and the SAE GW  206 . The PMIP is defined in the Request for Comments (RFC) 3344 entitled “IP Mobility Support for IPv4,” dated August 2002. The PMIP is based on an external node acting as a proxy Mobile Node that registers the location of an IP-based communication device and is accessible while the device is on the network. When the MS  202  is active on the 4G network, PMIP between the PDN GW  210  and the SAE GW  206  is active. When the MS  202  moves from the 4G network to the 3G network, PMIP becomes active between the PDN GW  210  and the PDSN  214 . Hence, when the MS  202  is moved to the 3G network, the PDN GW  210  switches the tunnel and begins using the tunnel to the PDSN  214  instead of the tunnel to the SAE GW  206 . In both cases, the PDN GW  210  acts as an IP anchor point and provides the same IP address to the MS  202 . 
     After switching the PDN GW tunnel, an active data flow  318  is carried over the 3G network between the MS  202  and the PDSN  214  through the 3G BS  212 , and a tunneled active data flow  319  is carried through the tunnel between the PDSN  214  and the PDN GW  210 . Thereafter, the 4G network releases resources dedicated to the MS  202 . 
       FIG. 6  illustrates data flows at various stages of MS switching. In particular, when the MS  202  uses the 4G network before handover to the 3G network is initiated, an active data flow  330  is carried between the MS  202  and the SAE GW  206 , and a tunneled active data flow  331  is carried through the tunnel between the PDN GW  210  and the SAE GW  206 . 
     When the MS  202  starts using the 3G network but still performs the handover procedure, an active data flow  332  is carried between the MS  202  and the PDSN  214 , a tunneled active data flow  333  is carried between the SAE GW  206  and the PDSN  214 , and a tunneled active data flow  334  is carried between the SAE GW  206  and the PDN GW  334 . 
     When the MS  202  uses the 3G network after completing the handover, an active data flow  335  is carried between the MS  202  and the PDSN  214 , and a tunneled active data flow  336  is carried between the PDSN  214  and the PDN GW  210 . 
       FIG. 7  illustrates another example of a call flow for switching the MS  202  from the 4G radio access technology to the 3G radio access technology. This call flow begins in the manner similar to the call flow discussed in connection with  FIG. 5 . In particular, when a call is setup in the 4G network, a data path  401  is established between the MS  202  and the SAE GW  206 . The data path  401  may carry any IP data such as Real Time Protocol (RTP) data for a VoIP call. The proxy mobile IP communication between the SAE GW  206  and the PDN GW  210  is performed over a tunneled data path  402 . 
     Based on radio access conditions, the MS  202  may decide to switch from the 4G network to the 3G network, i.e. to initiate handover. In this case, the eNB  204  sends a base station handover (BS HO) request  403  to the SAE GW  206 . The BS HO request  403  indicates that the MS  202  needs to perform handover to the 3G radio access network. 
     In response to the BS HO request  403 , the SAE GW  206  sends a handover request  404  to the PDSN  214  to create a horizontal context transfer link. The PDSN  214  may respond with a handover response message  405  indicating that the PDSN  214  has resources available for handover from the 4G network to the 3G network and is ready to perform the handover. After receiving the PDSN  214  response, the SAE GW  206  initiates a Layer 2 (L2) tunnel setup  406  between the SAE GW  206  and the PDSN  214 . 
     After this step, the call flow in  FIG. 7  differs from the call flow in  FIG. 5 . In particular, after establishing the Layer 2 tunnel between the SAE GW  206  and the PDSN  214 , the PDSN  214  may produce a context information request  406   a  to request the context information related to the MS  202 . The context information is described above in connection with  FIG. 2 . The SAE GW  206  may respond with a context information response  406   b  containing the requested context information. 
     After the context is transferred to the PDSN  214 , it may perform Layer 2 negotiations and setup Layer 2 link (line  407 ). The MS  202  is assigned with the same IP address as the address used by the MS  202  in the 4G network. Further, the PDSN  214  may communicate with the MS  202  about 3G radio resource requirements. 
     Thereafter, a Layer 2 tunnel  408  may be set up between the 3G BS  212  and the PDSN  214 . In particular, the PDSN  214  may inform the 3G BS  212  about radio resource requirements of the MS  202  and may provide information related to the Layer 2 handover. In response, the 3G BS  212  may reserve required radio resources and may prepare for the Layer 2 handover. Then, the PDSN  214  may send a message  408   a  to the MS  202  over the 4G network to inform the MS  202  about agreed upon 3G radio resources. 
     Layer 2 handover (L2 HO)  411  occurs between the eNB  204  and the 3G BS  212  to enable transfer of the MS  202  from the 4G radio access to the 3G radio access. An Airlink Start signal  413  is transmitted from the MS  202  to the 3G BS  212  to initiate 3G radio access. In response, the 3G BS  212  provides the PDSN  214  with an Ailink Start-3G radio ON signal  414  to indicate that the MS  202  is on the 3G network. 
     This signal triggers the PDSN  214  to set up a link to the PDN GW  210 . If a Layer 2 tunnel does not exist between the PDSN  214  and the PDN GW  210 , a Layer 2 tunnel  415  is established. The PDSN  214  may make a Proxy Mobile IP (PMIP) registration request  416  to the PDN GW  210 . In response, the PDN GW  210  may generate a PMIP registration response  417 . The PMIP registration exchange is used for switching a tunnel linked to the PDN GW  210  and used for carrying bearer traffic for the MS  202 . 
     After switching the PDN GW tunnel, an active data flow  418  is carried over the 3G network between the MS  202  and the PDSN  214  through the 3G BS  212 , and a tunneled active data flow  419  is carried through the tunnel between the PDSN  214  and the PDN GW  210 . Thereafter, the 4G network releases resources dedicated to the MS  202 . 
     Hence, the handover mechanism of the present disclosure minimizes interruption of an IP session involving an MS connected to a first wireless network using one radio access technology when the MS switches to a second wireless network that uses another radio access technology. 
     While the foregoing has described what are considered to be the best mode and/or other preferred examples, it is understood that various modifications may be made therein and that the invention or inventions disclosed herein may be implemented in various forms and examples, and that they may be applied in numerous applications, only some of which have been described herein. 
     It is intended by the following claims to claim any and all applications, modifications and variations that fall within the true scope of the present teachings.