Abstract:
A method and system for interworking between cellular networks and wireless local area networks (WLANs) are disclosed. At least one cellular network, at least one WLAN, and an IP network may be deployed. A wireless transmit/receive unit (WTRU) may establish a connection to a WLAN and a tunnel between an access point (AP) and a packet data gateway (PDG) may be established. The PDG may establish a tunnel to an IP network. The WTRU may invoke a service which is delivered through the WLAN. As signal quality from the AP degrades below a predetermined threshold, a handover from the WLAN to the cellular network may be performed. A new connection to the cellular network may be established either before or after breaking the current connection to the WLAN or the two connections may be maintained simultaneously.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/285,684, filed Nov. 22, 2005, which claims the benefit of U.S. Provisional Application Ser. No. 60/634,679 filed Dec. 9, 2004. The contents all of the above-referenced applications are hereby incorporated by reference herein. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention is related to wireless communication systems. More specifically, the present invention is a method and system for interworking between cellular networks and wireless local area networks (WLANs). 
       BACKGROUND 
       [0003]    Different types of wireless communication networks are presently deployed, such as WLANs and cellular networks. A multi-mode wireless transmit/receive unit (WTRU) supports wireless communication in more than one wireless communication network. As a user of the multi-mode WTRU roams between different networks, it is necessary to perform handover from one network to the other while receiving services continuously. For example, a wireless subscriber may roam between a WLAN and a third generation (3G) network while maintaining continuity in the wireless service provided to the user. Therefore, there is a need for coordination between the WTRU and the networks such that the service continuity is maintained as the user roams between different wireless networks. 
       SUMMARY 
       [0004]    The present invention is related to a method and system for interworking between cellular networks and WLANs. At least one cellular network, at least one WLAN and an IP network are deployed. The WLAN includes an access point (AP). The cellular network includes a radio access network and a core network. The radio access network includes a Node-B and a radio network controller, and the core network includes a packet data gateway (PDG), a serving GPRS support node (SGSN) and a gateway GPRS support node (GGSN). 
         [0005]    A WTRU first establishes a connection to a WLAN and a tunnel between an AP and a PDG is established. The PDG further establishes a tunnel to an IP network. The WTRU then invokes a service which is delivered through the WLAN. As signal quality from the AP degrades below a predetermined threshold, a handover from the WLAN to the cellular network is performed. A new connection to the cellular network may be established either before or after breaking the current connection to the WLAN or the two connections may be maintained simultaneously. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    A more detailed understanding of the invention may be had from the following description of preferred embodiments, given by way of example and to be understood in conjunction with the accompanying drawings, wherein: 
           [0007]      FIG. 1  is a block diagram of a UMTS-WLAN architecture; 
           [0008]      FIG. 2  is a signaling diagram of a process for access to 3G based services through a WLAN; 
           [0009]      FIG. 3  is a signaling diagram of a process for interworking in accordance with a first embodiment of the present invention; 
           [0010]      FIG. 4  is a signaling diagram of an alternative process for interworking in accordance with an alternative to the first embodiment of the present invention; 
           [0011]      FIG. 5  is a signaling diagram of a process for interworking in accordance with a second embodiment of the present invention; 
           [0012]      FIG. 6  is a signaling diagram of an alternative process for interworking in accordance with an alternative to the second embodiment of the present invention; and 
           [0013]      FIG. 7  is a signaling diagram of a process for interworking in accordance with a third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    The present invention will be described with reference to the drawing figures wherein like numerals represent like elements throughout. 
         [0015]    When referred to hereinafter, the terminology “WTRU” includes but is not limited to a user equipment, a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. When referred to hereinafter, the terminology “Node-B” and “AP” includes but is not limited to a base station, a site controller or any other type of interfacing device in a wireless environment. 
         [0016]    The present invention provides methods for maintaining service continuity and seamless handover between a WLAN and a cellular network by defining steps for establishing the cellular network connectivity, steps for performing a handover, and steps for breaking the connectivity between the user and the WLAN. It should be noted that the cellular network can be any type of cellular network including, but not limited to, a universal mobile telecommunication system (UMTS), cdma2000 and a global system for mobile communication (GSM), and the WLAN can be any type of WLAN including, but not limited to, an IEEE 802.x network. 
         [0017]      FIG. 1  is a block diagram of a UMTS-WLAN interworking network  100 . WLANs  130   a ,  130   b , (e.g., WLAN hot spots), are deployed in the coverage area of the UMTS  110 . Each WLAN  130   a ,  130   b  includes at least one AP  132   a ,  132   b  for radio access. The AP  132   a ,  132   b  is connected to an access router (AR)  134  for access to external networks, such as an IP network  140 , (e.g., Internet), or a cellular core network  120  for 3G-based services through the WLAN hotspot. 
         [0018]    Base stations  112  are deployed in the UMTS coverage area for access to UMTS networks. The base station  112  is connected to a radio network controller (RNC)  114  which is connected to the cellular core network  120 . 
         [0019]    The cellular core network  120  comprises a circuit switched core network (not shown) and a packet switched core network (shown in  FIG. 1 ). The packet switched core network  120  comprises an SGSN  122 , an authentication, authorization and accounting (AAA) server  124 , a home location register (HLR)/home subscriber server (HSS)  126 , a GGSN  128 , a PDG  129  and a WLAN access gateway (WAG)  121 . 
         [0020]    Referring to  FIGS. 1 and 2 , a process  200  for access to 3G-based services through WLAN is explained hereinafter. A WTRU  102  is currently in a service area of the WLAN hotspot  130   a . The WTRU  102  acquires system information of the WLAN hotspot  130   a  through active or passive scanning (step  202 ). In active scanning, the WTRU  102  sends a probe request to the AP  132   a  and the AP  132   a  sends a probe response in response to the probe request (steps  202   a ,  202   b ). The WTRU  102  may receive beacons from more than one AP. In such case the WTRU typically selects the AP having the strongest signal. In passive scanning, the WTRU  102  listens to the beacon transmitted from the AP  132   a  periodically (step  202   c ). 
         [0021]    After acquiring the system information, WLAN association and authentication procedures are performed. The WTRU  102  sends an association request message to the selected AP  132   a  (step  204 ) and the AP  132   a  sends an association response message to the WTRU  102  (step  206 ). At such point, an association is established and WLAN authentication procedure is performed (step  208 ). 
         [0022]    The WTRU  102  then initiates subscription and service authentication procedures by registering with the UMTS network for receiving UMTS-based services through the WLAN  130   a  (step  210 ). The WLAN  130   a  resolves the Network Access Id (NAI) provided by the WTRU  102 . The AR  134  uses the NAI to route AAA messages to the relevant AAA server  124  in the UMTS core network  120 . The AR  134  triggers extensible authentication protocol (EAP)-authentication key agreement (AKA) authentication and relay messages to a UMTS AAA server  124 . Once the WTRU  102  receives an authentication success message, the WTRU  102  uses dynamic host configuration protocol (DHCP) to receive an IP address and then initiates a tunnel establishment with the PDG  129  through the WAG  121 . The WTRU  102  constructs a fully qualified domain name (FQDN) and performs a domain name service (DNS) query for the PDG  129  from a DNS  142  (step  212 ). The WTRU  102  selects a PDG from the received list in the DNS query response and establishes an end-to-end tunnel between the selected PDG  129  and the WTRU  102  (step  214 ). 
         [0023]      FIG. 3  is a signaling diagram of a process  300  for interworking in accordance with the first embodiment of the present invention. In accordance with the first embodiment, a new connection to the UMTS network is established before breaking the current connection to the WLAN hotspot, (i.e., “make before break”). When establishing the tunnel at step  214  in  FIG. 2 , the WTRU indicates an application, such as voice over IP (VoIP) services, and the tunnel is set up for this certain application. The tunnel is established by the WTRU  102  sending a request to the AP  132   a  (step  302   a ) and the AP  132   a  forwarding the request to the PDG  129  (step  302   b ). After the tunnel between the WTRU  102  and the PDG  129  is established, the WTRU  102  invokes the indicated service (step  302 ). 
         [0024]    There are two options that may follow the indication of the application. One is that a request is sent to the PDG  129  to establish the connection to the IP Multimedia Subsystem (IMS)  150  and allocate the Proxy Call State Control Function (P-CSCF) or the Session Initiation Protocol (SIP) proxy for the WTRU  102 . The other option is that a request is sent to the PDG  129  to establish the tunnel and wait for the WTRU  102  to request a connection to the IMS  150  and the allocation of the SIP proxy or the P-CSCF is performed after the request for connection. The first option is preferred since it will save additional delay in setting up the call. However, the second option may be the implementation in certain situations. The step  304  between the PDG  129  and the IMS  150  indicates the steps taking place to establish the connection between the PDG  129  and the IMS  150 , such as SIP registration, allocation of P-CSCF and the allocation of Serving CSCF (S-CSCF). A CSCF is a specific type of SIP server, which is used to process SIP signaling packets in an IMS network. A P-CSCF is an SIP proxy that is the first point of contact for the WTRU. An S-CSCF is a central node of the signaling plane. 
         [0025]    As the WTRU  102  moves away from the current WLAN hotspot  130   a , as shown in  FIG. 1 , a handover from the current WLAN hotspot  130   a  to the UMTS network  110  is initiated. In accordance with this embodiment, a new connectivity to the UMTS network  110  is established before breaking the existing connectivity to the current WLAN hotspot  130   a.    
         [0026]    Referring again to  FIG. 3 , the WTRU  102  establishes a connection to the GGSN  128  as indicated by arrow  305  by the following steps  306 - 310 . The WTRU  102  first establishes a radio access bearer (RAB) to a Node-B  112  (step  306 ) and invokes a 3GPP system attachment (step  308 ). The WTRU  102  then invokes 3GPP IP connectivity by establishing a packet data protocol (PDP) context (step  310 ). When the WTRU  102  sets up a PDP context, the WTRU  102  selects an access point and an access point name (APN) is determined. The APN is used in a DNS query. This process finally gives an IP address of the GGSN  128  which serves the access point. The WTRU  102  then invokes 3GPP IMS connectivity through SIP registration at step  312  at such point the connection between the GGSN  128  and the IMS  150  is also established as indicated by arrow  312   a.    
         [0027]    Once the connectivity to the UMTS network  110  is established, a process for breaking the connectivity to the current WLAN hotspot  130   a  is initiated. The WTRU  102  sends a handover request to the AP  132   a  (step  314 ). The handover request identifies the tunnel end points, the user ID, radio resources, frequency channels, priority, or the like. The AP  132   a  then sends a 3GPP relocation request to the PDG  129  (step  316 ). There are two options with respect to the 3GPP relocation request. The PDG  129  may be removed from the call path after the connectivity to the WLAN  130   a  is terminated or the PDG  129  may remain on the call path after the connectivity to the WLAN  130   a  is terminated.  FIG. 3  illustrates the first option and the second option will be explained with reference to  FIG. 4  hereinafter. 
         [0028]    In the first embodiment shown in  FIG. 3 , the PDG  129  is removed from the call path after the connectivity to the WLAN  130   a  is terminated. The PDG  129  forwards the request to the GGSN  128 , and the GGSN  128  forwards the request to the IMS  150  (steps  318 ,  320 ). The tunnel between the PDG  129  and the GGSN  128  lasts only for the duration the connectivity to the WLAN  130   a  exists, and then a new connection between the GGSN  128  and the IMS  150  is established and traffic is forwarded directly from the IMS  150  to the GGSN  128  where the WTRU  102  is now connected. 
         [0029]    The IMS  150  sends a relocation response to the GGSN  128 , which forwards the response to the PDG  129  (steps  322 ,  324 ). The PDG  129  sends a relocation response to the AP  132   a  (step  326 ). The AP  132   a  then releases the resources after sending a handover complete message to the WTRU  102  (step  328 ). The GGSN  128  also sends the handover complete message, (i.e., HO complete), for resource allocation to the Node-B  112  via the SGSN  122  (steps  330 ,  332 ). The Node-B  112  then sends the handover complete message to the WTRU  102  (step  334 ). The services from the IMS  150  are then provided through the UMTS network  110 , (i.e., from the IMS  150  via the GGSN  128 , the SGSN  122  and the Node-B  112  to the WTRU  102  as indicated by arrows  336   a - 336   c ) (steps  336 ,  338 ). 
         [0030]      FIG. 4  is a signaling diagram of an alternative process  400  to the first embodiment. Process  400  is similar to process  300  except the PDG  129  remains on the call path after the connectivity to the WLAN  140   a  is terminated. The PDG  129  will be in the middle of the call path after the handover. The handover is performed by switching the signaling path in the P-CSCF toward the GGSN  128  from the PDG  129 . The traffic is directed from the PDG  129  to the GGSN  128 . 
         [0031]    Steps  402 - 416  are the same as corresponding steps  302 - 316  and will not be repeated herein. After receiving the relocation request from the AP  132   a , the PDG  129  sends a tunnel establishment request to the GGSN  128  and the GGSN  128  responds with a tunnel establishment response. At such point a tunnel is established between the PDG  129  and the GGSN  128 . The GGSN  128  establishes the SIP connectivity to the IMS  150  through the PDG  129  (steps  422 ,  424 ). The PDG  129  sends a relocation response to the AP  132   a  (step  426 ). The AP  132   a  then releases the resources after sending a handover complete message to the WTRU  102  (step  428 ). The GGSN  128  also sends the handover complete message for resource allocation to the Node-B  112  via the SGSN  122  (steps  430 ,  432 ). The Node-B  112  then sends the handover complete message to the WTRU  102  (step  434 ). The services from the IMS  150  are then provided through the UMTS network  110 , (i.e., from the IMS  150  via the PDG  129 , the GGSN  128 , the SGSN  122  and the Node-B  112  to the WTRU  102  as indicated by arrows  436   a - 436   c ) (step  436 ). 
         [0032]      FIG. 5  is a signaling diagram of a process  500  for interworking in accordance with a second embodiment of the present invention. In accordance with the second embodiment, the WTRU  102  may maintain multiple sessions simultaneously and the existing connectivity to the WLAN  130   a  is not torn down after the handover is complete. Two connections are maintained simultaneously and the application is transferred from one network to the other, (i.e., “simultaneous”). 
         [0033]    After the tunnel between the WTRU  102  and the PDG  129  is established, the WTRU  102  invokes a service, such as VoIP call services (step  502 ). The WTRU  102  sends a request to the AP  132   a  (step  502   a ) and the AP  132   a  forwards the request to the PDG  129  (step  502   b ). The step  504  between the PDG  129  and the IMS  150  indicates the steps taken place to establish the connection between the PDG  129  and the IMS  150 , such as SIP registration, allocation of P-CSCF and the allocation of S-CS CF. 
         [0034]    The WTRU  102  establishes an additional connection to the UMTS network  110  concurrently. The WTRU  102  establishes a connection to the GGSN  128  as indicated by arrow  505  by the following steps  506 - 510 . The WTRU  102  establishes an RAB to a Node-B  112  (step  506 ) and invokes a 3GPP system attachment (step  508 ). The WTRU  102  then invokes 3GPP IP connectivity by establishing a PDP context (step  510 ). When the WTRU  102  sets up a PDP context, the WTRU  102  selects an access point and an APN is determined. The APN is used in a DNS query. This process finally gives an IP address of the GGSN  128  which serves the access point. The WTRU  102  then invokes 3GPP IMS connectivity through SIP registration at step  512  at such point the connection between the GGSN  128  and the IMS  150  is also established as indicated by arrow  512   a.    
         [0035]    As the WTRU  102  moves away from the current WLAN hotspot  130   a , as shown in  FIG. 1 , the application is transferred from the WLAN  130   a  to the UMTS network  110  without breaking the existing connection to the WLAN  130   a . The WTRU  102  sends a handover request to the AP  132   a  (step  514 ). The handover request identifies the tunnel end points, the user ID, radio resources, frequency channels, priority, or the like. The AP  132   a  then sends a 3GPP relocation request to the PDG  129  (step  516 ). As stated hereinbefore with respect to the first embodiment and its alternative, the PDG  129  may be removed from the call path after the connection is switched to the UMTS or may remain on the call path.  FIG. 5  illustrates the first option and the second option will be explained with reference to  FIG. 6  hereinafter. 
         [0036]    The PDG  129  forwards the request to the GGSN  128 , and the GGSN  128  forwards the request to the IMS  150  (steps  518 ,  520 ). The PDG  129  is removed from the call path after the connectivity to the WLAN  130   a  is switched. The tunnel between the PDG  129  and the GGSN  128  lasts only for a certain interval, and a new connection between the GGSN  128  and the IMS  150  is established and traffic is forwarded directly from the IMS  150  to the GGSN  128  where the WTRU  102  is connected. 
         [0037]    The IMS  150  sends a relocation response to the GGSN  128 , which forwards the response to the PDG  129  (steps  522 ,  524 ). The PDG  129  sends a relocation response to the AP  132   a  (step  526 ). The AP  132   a  then releases the resources after sending a handover complete message to the WTRU  102  (step  528 ). The GGSN  128  also sends the handover complete message for resource allocation to the Node-B  112  via the SGSN  122  (steps  530 ,  532 ). The Node-B  112  then sends the handover complete message to the WTRU  102  (step  534 ). The services from the IMS  150  are then provided through the UMTS network  110 , (i.e., from the IMS  150  via the GGSN  128 , the SGSN  122  and the Node-B  112  to the WTRU  102  as indicated by arrows  536   a - 536   c ) (steps  536 ,  538 ). 
         [0038]      FIG. 6  is a signaling diagram of a process  600  which is an alternative to the second embodiment of the present invention. Process  600  is similar to process  500  except the PDG  129  remains on the call path after the connectivity to the WLAN  130   a  is switched. The PDG  129  will be in the middle of the call path after the handover. 
         [0039]    Steps  602 - 616  are the same as corresponding steps  502 - 516  of process  500  and will not be repeated herein. After receiving the relocation request from the AP  132   a , the PDG  129  sends a tunnel establishment request to the GGSN  128  and the GGSN  128  responds with a tunnel establishment response (steps  618 ,  620 ). At such point a tunnel is established between the PDG  129  and the GGSN  128 . The GGSN  128  establishes the SIP connectivity to the IMS  150  through the PDG  129  (steps  622 ,  624 ). The PDG  129  sends a relocation response to the AP  132   a  (step  626 ). The AP  132   a  then releases the resources at step  629  after sending a handover complete message to the WTRU  102  (step  628 ). The GGSN  128  also sends the handover complete message for resource allocation to the Node-B  112  via the SGSN  122  (steps  630 ,  632 ). The Node-B  112  then sends the handover complete message to the WTRU  102  (step  634 ). The services from the IMS  150  are then provided through the UMTS network, (i.e., from the IMS  150  via the PDG  129 , the GGSN  128 , the SGSN  122  and the Node-B  112  to the WTRU  102  as indicated by arrows  636   a - 636   c ) (step  636 ). 
         [0040]      FIG. 7  is a signaling diagram of a process  700  for interworking in accordance with a third embodiment of the present invention. In accordance with the third embodiment, the existing connectivity to the WLAN  130   a  is torn down before handover to the UMTS network  110 , (i.e., “break before make”). After the tunnel between the WTRU  102  and the PDG  129  is established, the WTRU  102  invokes the indicated service (step  702 ). To invoke the indicated service, the WTRU  102  sends a request to the AP  132   a  (step  702   a ) and the AP  132   a  forwards the request to the PDG  129  (step  702   b ). The step  704  between the PDG  129  and the IMS  150  indicates the steps taken place to establish the connection between the PDG and the IMS, such as SIP registration, allocation of P-CSCF and the allocation of S-CSCF. 
         [0041]    As the WTRU  102  moves away from the current WLAN hotspot  130   a , as shown in  FIG. 1 , handover from the current WLAN hotspot  130   a  to the UMTS network  110  is performed. In accordance with this embodiment, a new connectivity to the UMTS network  110  is established after breaking the existing connectivity to the current WLAN hotspot  130   a , (e.g., loss of signal). 
         [0042]    When the signal from the AP  132   a  is lost (step  706 ), the WTRU may initiate the handover to the UMTS system or alternatively the WLAN may initiate the handover. Since the WLAN is connected to the PDG  129 , the WLAN may initiate the handover to the target UMTS system. When the signal loss is detected, the AP  132   a  sends a message, (a relocation request), to the PDG  129  (step  708 ). The session is then maintained for a certain interval (step  710 ). 
         [0043]    The WTRU  102  then establishes a connection to the GGSN  128  as indicated by arrow  711  by the following steps  712 - 716 . The WTRU  102  establishes an RAB to a Node-B  112  (step  712 ) and invokes a 3GPP system attachment (step  714 ). The WTRU  102  then invokes 3GPP IP connectivity by establishing a PDP context (step  716 ). When the WTRU  102  sets up a PDP context, the WTRU  102  selects an access point and an APN is determined. The APN is used in a DNS query. This process finally gives an IP address of the GGSN  128  which serves the access point. The WTRU  102  then invokes 3GPP IMS connectivity through SIP registration at step  718 , at such point the connection between the GGSN  128  and the IMS  150  is also established as indicated by arrow  718   a.    
         [0044]    A handover bending session is then initiated (step  720 ). The WTRU  102  sends the information related to the existing session to the IMS  150 , (i.e., SIP server). The information includes the session/service identification, originating and terminating IP addresses, a request to redirect the traffic to the UMTS system with the new contact information, (i.e., current IP address), or the like. The IMS  150  then updates the new routing of the call/session. The IMS  150  establishes a new P-CSCF and S-CSCF for the new session. 
         [0045]    The IMS  150  then sends a handover request notification to the PDG  129  with information regarding the session and indications that the call/session has been redirected and resources previously reserved should be released (step  722 ). The PDG  129  then sends a relocation response to the AP  132   a  along with the session information and WTRU identity (step  724 ). The AP  132   a  then releases resources allocated for the WTRU  102 . The session is resumed between the WTRU  102  and the IMS  150  (steps  726   a - 726   d ) and user invoked services are provided from the IMS  150  via the GGSN  128 , the SGSN  122  and the Node-B  112  to the WTRU  102  (step  728 ). 
         [0046]    The PDG  129  may indicate a handover to the IMS  150 . Alternatively, the WTRU  102  may indicate the handover to the IMS  150  and provide the old connection information. 
         [0047]    Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention.