HK1111039B - Method and system for interworking of cellular networks and wireless local area networks - Google Patents

Description

Method and system for interconnecting cellular network and wireless local area network

Technical Field

The present invention relates to wireless communication systems. More particularly, the present invention relates to a method and system for interworking between a cellular network and a Wireless Local Area Network (WLAN).

Background

Currently, different types of wireless communication networks are deployed in the market, such as: wireless local area networks and cellular networks. A multi-mode wireless transmit/receive unit (WTRU) supports wireless communications with a single or multiple wireless communication networks. When a user of a multi-mode wtru roams between different networks, the user of the multi-mode wtru needs to perform a handover (handoff) from one network to another while continuing to receive service. For example, a wireless subscriber may roam between a wireless local area network and a third generation (3G) network while maintaining continuity of wireless service provided to the wireless subscriber. In view of the foregoing and other objects, there is a need for a method for coordinating wtrus with networks, whereby service continuity is maintained when a wireless subscriber roams between different wireless networks.

Disclosure of Invention

The present invention relates to a method and system for interworking between cellular networks and wireless local area networks. At least one cellular network, at least one wireless local area network, and an Internet Protocol (IP) network are deployed. The wireless area network includes an Access Point (AP). The cellular network includes a node B and a Radio Network Controller (RNC), and the core network includes a Packet Data Gateway (PDG), a serving General Packet Radio Service (GPRS) support node (SGSN), and a gateway general packet radio service support node (GGSN).

First, the wtru establishes a connection with the wlan, and a tunnel (tunnel) is established between the ap and the pdn gateway. The packet data gateway further establishes a tunnel with the internet protocol network. The wtru then triggers a service delivered via the wlan. When the signal quality from the ap degrades below a predetermined threshold, a handover from the wlan to the cellular network is performed. The new connection to the cellular network may be established before or after the current connection to the wlan is broken, or both connections may continue to be maintained at the same time.

According to the present invention, there is provided a method for interworking between a cellular network and a WLAN in a wireless communication system having the cellular network, the WLAN and an IP multimedia subsystem IMS, the method being adapted for a user service provided by the IMS, the method comprising the steps of: a wireless transmit/receive unit (WTRU) establishing a connection with a wireless local area network; the WTRU obtaining a packet data gateway, PDG, address through the WLAN using a domain name service, DNS; the WTRU establishes a channel between the WTRU and the PDG;

the WTRU registering to a third generation network IP multimedia subsystem IMS through the WLAN using session initiation protocol SIP; the WTRU triggering an IP session for a user service from the IMS; the WTRU receiving the user service from the IMS through a channel between the WLAN and the WTRU and the PDG; the WTRU establishing a connection to a cellular network; the WTRU initiating a handoff of the subscriber service from the WLAN to the cellular network; and the WTRU performing the handover while continuously receiving the user service from the IMS.

Drawings

A further detailed understanding of the present invention may be obtained from the following detailed description of the preferred embodiments, which are to be construed in an illustrative, as well as a detailed description of the preferred embodiments, which is provided by way of example only, and which is to be read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a block diagram illustrating a Universal Mobile Telecommunications System-Wireless local area network (UMTS-WLAN) architecture;

FIG. 2 is a signaling diagram illustrating a procedure for accessing third generation network services via a wireless local area network;

FIG. 3 is a signaling diagram illustrating an interworking procedure according to a first preferred embodiment of the present invention;

figure 4 is a signaling diagram illustrating an alternative interconnection procedure according to a first alternative preferred embodiment of the present invention;

fig. 5 is a signaling diagram illustrating an interworking procedure according to a second preferred embodiment of the present invention;

figure 6 is a signaling diagram illustrating an alternative interconnection procedure according to a second alternative preferred embodiment of the present invention; and

fig. 7 is a signaling diagram illustrating an interconnection procedure according to a third preferred embodiment of the present invention.

Detailed Description

The present invention will be described in detail below with reference to the attached drawings, and in the detailed description of the preferred embodiments, like reference numerals denote like elements.

In the detailed description of the preferred embodiments, the term "wtru" includes, but is not limited to, User Equipment (UE), mobile stations, fixed or mobile subscriber units, pagers, and any other type of device capable of operating in a wireless environment. In addition, in the detailed description of the preferred embodiments, the terms "node B" and "access point" are any other type of interfacing device including, but not limited to, a Base Station (BS), a location controller, or a wireless environment.

The present invention provides methods for maintaining service continuity and seamless (seamless) handover between wlan and cellular networks, comprising the steps of: establishing cellular network connectivity; performing a handover; and interrupting the connectivity between the user and the wireless local area network. It should be noted that the cellular network may be any type of cellular network including, but not limited to, Universal Mobile Telecommunications System (UMTS), code division multiple access 2000(CDMA2000), and global system for mobile communications (GSM), and the wireless area network may be any type of wireless area network including, but not limited to, IEEE 802.XX networks.

Fig. 1 is a block diagram illustrating a universal mobile telecommunications system-wireless local area network (UMTS-WLAN) internetwork 100. The wlan 130A and 130B (e.g., wlan hotspots) are deployed in the coverage area of the umts system 110. Each wlan 130A and 130B has at least one ap 132A and 132B for wireless access. Access points 132A and 132B connect to an Access Router (AR)134 to access external networks, such as: the ip network 140 (e.g., the internet) or the mobile core network 120 is further served by the third generation network through the wlan hotspot.

The base stations 112 are deployed in the coverage area of the umts to access the umts network. The base stations 112 are connected to a radio network controller, and a Radio Network Controller (RNC)114 is connected to a mobile core network 120.

The mobile core network 120 includes a circuit switched core network (not shown) and a packet switched core network (shown in fig. 1). The packet-switched core network 120 includes: a serving general packet radio service support node 122, an Authentication Authorization Accounting (AAA) server 124, a Home Location Register (HLR)/Home Subscriber Server (HSS)126, a gateway general packet radio service support node 128, a packet data gateway 129, and a wireless area network access gateway (WAG) 121.

Please refer to fig. 1 and 2, which illustrate a procedure 200 for accessing third generation network services through a wlan. The wtru 102 is currently located in the service area of the wlan hotspot 130A. The wtru 102 obtains system information of the wlan hotspot 130A through an active or passive scanning process (step 202). In the active scanning procedure, the wtru 102 transmits a probe (probe) request to the ap 132A, and the ap 132A transmits a probe response in response to the probe request (steps 202A and 202B). The wtru 102 may receive beacons (beacons) via more than a single access point. In this case, the wtru 102 generally selects the ap with the strongest signal. In the passive scanning procedure, the wtru 102 listens for a beacon transmitted periodically via the ap 132A (step 202C).

After acquiring the system information of the wlan hotspot 130A, the wlan association procedure and the authentication procedure are performed. The wtru 102 sends an association request signal to the selected ap 132A (step 204) and the ap 132A associates a response signal to the wtru 102 (step 206). At this point, an association is established and a wlan authentication procedure is performed (step 208).

The wtru 102 registers with the umts network that receives the umts network service via the wlan 130A (step 210), and then starts a subscription and service authentication procedure. The wireless area network 130A recognizes a network access identification number (ID) (NAI) provided by the wtru 102. The access router 134 utilizes a network access identification number (ID) (NAI) to route the authentication authorization accounting message to the associated authentication authorization accounting server 124 of the umts core network 120. The access router 134 triggers Extensible Authentication Protocol (EAP), -Authentication Key Agreement (AKA) authentication and relays the message to the umts authentication authorization accounting server 124. Whenever the wtru 102 receives the authentication success message, the wtru 102 receives an internet protocol address using Dynamic Host Configuration Protocol (DHCP) and then initiates a tunnel establishment with the pdn gateway 129 via a Wlan Access Gateway (WAG) 121. The wtru 102 establishes a Fully Qualified Domain Name (FQDN) and a domain name service query of the packet data gateway 129 via the Domain Name Service (DNS) server 142 (step 212). The wtru 102 selects a pdn gateway via the received list of domain name service query responses and establishes a point-to-point tunnel between the selected pdn gateway 129 and the wtru 102 (step 214).

Fig. 3 is a signaling diagram illustrating an interworking procedure 300 according to a first preferred embodiment of the present invention. According to the first preferred embodiment of the present invention, the new connection to the umts network is established before the current connection to the wlan hotspot is broken (i.e., "pre-break connection"). When the channel is established using step 214 of fig. 2, the wtru 102 indicates a specific application, such as: a voice over internet protocol (VoIP) service and a channel is established in response to this particular application. The tunnel is established by the wtru 102 sending a request to the ap 132A (step 302A) and forwarding the request to the pdn gateway 129 using the ap 132A (step 302B). After the tunnel is established between the wtru 102 and the pdn gateway 129, the wtru 102 triggers the indicated specific service (step 302).

After indicating a particular application, two options exist. First, a request is sent to the packet data gateway 129 to establish a connection with the internet protocol multimedia subsystem (IMS)150 and to set up a proxy call state control function (P-CSCF) or a Session Initiation Protocol (SIP) proxy of the wtru 102. Second, the request is sent to the pdn gateway 129 to establish a tunnel and wait for the wtru 102 to request a connection with the ims 150, and after the connection request, the setting of the sip proxy or proxy call state control function is performed. The first option is advantageous because it saves additional relays for setting up the call. However, the second option may also be implemented in certain situations. The step 304 between the pdn 129 and the ims 150 indicates the respective steps to establish connectivity between the pdn 129 and the ims 150, such as: session initiation protocol registration, setting of a proxy call state control function, and setting of a serving call state control function (S-CSCF). A Call State Control Function (CSCF) is a specific type of session initiation protocol server that is used to process session initiation protocol signaling packets of an internet protocol multimedia subsystem network. The proxy call state control function is to first contact the session initiation protocol proxy of the wtru. The serving call state control function is a central node of the signaling plane.

When the wtru 102 moves away from the current wlan hotspot 130A, as shown in fig. 1, a handover of the current wlan hotspot 130A to the umts network 110 is initiated. In accordance with the first preferred embodiment of the present invention, a new connection to the umts network 110 is established before the existing connection to the current wlan hotspot 130A is broken.

Referring again to fig. 3, the wtru 102 then proceeds to step 306 through step 310 to establish a connection with the gateway gprs support node 128, as indicated by the arrow 305. First, the wtru 102 establishes a Radio Access Bearer (RAB) with the node-B112 (step 306) and triggers a third generation network (3GPP) system attach (step 308). The wtru 102 then establishes a Packet Data Protocol (PDP) context to trigger third generation internet protocol connectivity (step 310). When the wtru 102 establishes a packet data communication protocol context, the wtru 102 selects an access point and an Access Point Name (APN) is determined. The access point name is for a domain name service query. Finally, this routine generates an ip address for the gateway gprs support node 128 to provide to the ap. The wtru 102 then triggers the third generation ip multimedia subsystem connectivity via the session initiation protocol registration in step 312, at which time connectivity between the gateway gprs support node 128 and the ip multimedia subsystem 150 is also established, as indicated by the arrow 312A.

Whenever connectivity to the umts network 110 is established, an interruption procedure for connectivity to the current wlan hotspot 130A is initiated. The wtru 102 sends a handover request to the ap 132A (step 314). The handover request identifies the channel end point, subscriber identification number (ID), radio resource, frequency channel, priority, or the like. The access point 132A then communicates a third generation (3GPP) network reconfiguration request to the packet data gateway 129 (step 316). For third generation (3GPP) network reconfiguration requirements, two options exist. The packet data gateway 129 may be removed via the call path after the connectivity to the wireless area network 130A is interrupted, or the packet data gateway 129 may continue to maintain the call path after the connectivity to the wireless area network 130A is interrupted. Fig. 3 is an illustration of a first option and fig. 4 is an illustration of a second option, which will be described in further detail below.

In the first preferred embodiment of the present invention shown in fig. 3, the packet data gateway 129 may be removed via the call path after the connectivity to the wlan 130A is interrupted. The packet data gateway 129 forwards the request to the gateway gprs support node 128 and the gateway gprs support node 128 forwards the request to the ims 150 (steps 318 and 320). The tunnel between the PDN gateway 129 and the GGSN 128 is only continued during the existence of connectivity with the WLAN 130A, and a new connection between the GGSN 128 and the IMS 150 is established, and traffic is forwarded directly to the GGSN 128, to which the WTRU 102 is currently connected, via the IMS 150.

The ims 150 transmits the reset response to the gateway gprs support node 128 and the gateway gprs support node 128 forwards the reset response to the packet data gateway 129 (steps 322 and 324). The ap 132A then releases resources after sending the handover complete signal to the wtru 102 (step 328). The gateway GPRS support node 128 also sends a handover complete message (i.e., HO complete) to obtain the resource configuration of the node B112 via the serving GPRS support node 122 (steps 330 and 332). The node-B112 then sends a handover complete message to the wtru 102 (step 334). Subsequently, the service from the ims 150 is provided through the umts network 110 (i.e., from the ims 150, through the ggsn 128, the sgsn 122, and the node B112 to the wtru 102 as indicated by arrows 336A to 336C) (steps 336 and 338).

Fig. 4 is a signaling diagram illustrating an alternative interconnection process 400 according to a first preferred embodiment of the present invention. The alternative interworking procedure 400 is similar to the interworking procedure 300 shown in fig. 3 except that the packet data gateway 129 is maintained in the call path after the connectivity with the wireless area network 130A is interrupted. After the handover, the packet data gateway 129 will be located in the center of the call path. The handover is performed by switching the signalling path of the proxy call state control function to the gateway general packet radio service support node 128 via the packet data gateway 129. Network traffic is directed to the gateway general packet radio service support node 128 via the packet data gateway 129.

Steps 402 to 416 are identical to steps 302 to 316 of the corresponding interconnect procedure 300, and therefore are not repeated below. After receiving the reset request through the access point 132A, the packet data gateway 129 transmits a channel setup request to the gateway general packet radio service support node 128, and the gateway general packet radio service support node 128 replies with a channel setup response in response (steps 418 and 420). In this case, a tunnel is established between the packet data gateway 129 and the gateway gprs support node 128. The gateway gprs support node 128 establishes session initiation protocol connectivity with the ims 150 via the packet data gateway 129 (steps 422 and 424). The packet data gateway 129 transmits a reset response to the access point 132A (step 426). The ap 132A then releases resources after sending the handover complete message to the wtru 102 (step 428). The gateway GPRS support node 128 also sends a handover complete message to provide the resource configuration of the node B112 through the serving GPRS support node 122 (steps 430 and 432). The node-B112 then sends a handover complete message to the wtru 102 (step 434). Subsequently, the service from the ims 150 is provided via the umts network 110 (i.e., from the ims 150 via the packet data gateway 129, the gateway gprs support node 128, the serving gprs support node 122, the node B112 to the wtru 102 as indicated by arrows 436A to 436C) (step 436).

Fig. 5 is a signaling diagram illustrating an interworking procedure 500 according to a second preferred embodiment of the present invention. In accordance with the second preferred embodiment of the present invention, the wtru 102 can maintain multiple sessions simultaneously and the existing connectivity to the wlan 130A does not need to be interrupted after the handover is completed. Both connections are maintained simultaneously and this particular application is the transfer via one network to another (that is, "simultaneously").

After the tunnel between the wtru 102 and the pdn gateway 129 is established, the wtru 102 triggers a service, such as a voice over internet protocol (VoIP) call service (step 502). The wtru 102 sends a request to the access point 132A (step 502A) and the access point 132A forwards the request to the packet data gateway 129 (step 502B). Step 504 between the pdn gateway 129 and the ims 150 indicates the respective steps for establishing a connection between the pdn gateway 129 and the ims 150, such as: session initiation protocol registration, setting of proxy call state control functions, and setting of serving call state control functions.

The wtru 102 simultaneously establishes additional connections with the umts network 110. The wtru 102 then proceeds to step 506 to step 510 to establish a connection with the gateway gprs support node 128 as indicated by arrow 505. First, the wtru 102 establishes a Radio Access Bearer (RAB) with the node-B112 (step 506) and triggers a third generation network system attach (step 508). The wtru 102 then establishes a pdp context to trigger third generation ip connectivity (step 510). When the wtru 102 establishes the packet data communication protocol context, the wtru 102 selects an access point and the access point name is determined. The access point name is for a domain name service query. Finally, this routine generates an ip address for the gateway gprs support node 128 to provide to the ap. The wtru 102 then triggers the third generation ims connectivity via the session initiation protocol registration in step 512, at which time connectivity between the gateway gprs support node 128 and the ims 150 is also established, as indicated by the arrow 512A.

When the wtru 102 moves away from the current wlan hotspot 130A, as shown in fig. 1, this particular application is transferred to the umts network 110 via the current wlan hotspot 130A without breaking the existing connection with the wlan hotspot 130A. The wtru 102 sends a handover request to the ap 132A (step 514). The handover request identifies the channel end point, subscriber identification number (ID), radio resource, frequency channel, priority, or the like. The access point 132A then communicates a third generation (3GPP) network reconfiguration request to the packet data gateway 129 (step 516). As described in the first preferred embodiment of the present invention and its alternatives, the pdn gateway 129 may be removed via the call path after the handover to the umts network, or the pdn gateway 129 may continue to maintain the call path after the handover to the umts network. Fig. 5 illustrates the first option and fig. 6 illustrates the second option, which will be described in further detail below.

The packet data gateway 129 forwards the request to the gateway gprs support node 128 and the gateway gprs support node 128 forwards the request to the ims 150 (steps 518 and 520). After the connection with the wlan 130A is handed off, the pdn gateway 129 is removed via the call path. The tunnel between the packet data gateway 129 and the gateway gprs support node 128 is only continued for a specific period of time, and a new connection between the gateway gprs support node 128 and the ims 150 is established and traffic is forwarded directly to the gateway gprs support node 128 to which the wtru 102 is connected via the ims 150.

The ims 150 transmits the reset response to the gateway gprs support node 128 and the gateway gprs support node 128 forwards the reset response to the packet data gateway 129 (steps 522 and 524). Access point 132A is transmitting a reset response to access point 132A (step 526). The ap 132A then releases resources after sending the handover complete signal to the wtru 102 (step 528). The gateway GPRS support node 128 also sends a handover complete message to obtain the resource configuration of the node B112 through the serving GPRS support node 122 (steps 530 and 532). The node-B112 then sends a handover complete message to the wtru 102 (step 534). Subsequently, the service from the ims 150 is provided through the umts network 110 (i.e., from the ims 150, through the gateway gprs support node 128, the serving gprs support node 122, and the node B112 to the wtru 102, as indicated by arrows 536A to 536C) (steps 536 and 538).

Fig. 6 is a signaling diagram illustrating an alternative interconnection procedure 600 in accordance with a second preferred embodiment of the present invention. The alternative interworking procedure 600 is similar to the interworking procedure 500 shown in fig. 5 except that the packet data gateway 129 remains in the call path after the connectivity handover with the wireless area network 130A. After the handover, the packet data gateway 129 will be located in the center of the call path.

Steps 602 through 616 are identical to steps 502 through 516 of the corresponding interconnect procedure 500, and thus are not repeated below. After receiving the reconfiguration request through the access point 132A, the packet data gateway 129 transmits a channel setup request to the gateway general packet radio service support node 128, and the gateway general packet radio service support node 128 replies with a channel setup response in response (steps 618 and 620). In this case, a tunnel is established between the packet data gateway 129 and the gateway gprs support node 128. The gateway gprs support node 128 establishes a session initiation protocol connectivity with the ims 150 via the packet data gateway 129 (steps 622 and 624). The packet data gateway 129 transmits a reset response to the access point 132A (step 626). The ap 132A then releases resources after sending the handover complete message to the wtru 102 (step 628). The gateway GPRS support node 128 also sends a handover complete message to provide the resource configuration of the node B112 via the serving GPRS support node 122 (steps 630 and 632). The node-B112 then sends a handover complete message to the wtru 102 (step 634). Subsequently, the service from the ims 150 is provided via the umts network 110 (i.e., from the ims 150, via the packet data gateway 129, the gateway gprs support node 128, the serving gprs support node 122, the node B112, and to the wtru 102, as indicated by arrows 636A to 636C) (step 636).

Fig. 7 is a signaling diagram illustrating an interworking procedure 700 according to a third preferred embodiment of the present invention. According to the third preferred embodiment of the present invention, the existing connectivity to the wlan 130A is interrupted (i.e., "disconnected before handover") before handover to the umts network 110 is performed. After the tunnel between the wtru 102 and the pdn gateway 129 is established, the wtru 102 triggers the indicated specific service (step 702). To trigger the indicated specific service, the wtru 102 sends a request to the ap 132A (step 702A), and the ap 132A forwards the request to the pdn gateway 129 (step 702B). Step 704 between the pdn 129 and the ims 150 indicates the respective steps for establishing a connection between the pdn 129 and the ims 150, such as: session initiation protocol registration, setting of proxy call state control functions, and setting of serving call state control functions.

When the wtru 102 moves away from the current wlan hotspot 130A, as shown in fig. 1, a handover is performed to the umts network 110 via the current wlan hotspot 130A. According to the third preferred embodiment of the present invention, after the existing connectivity to the current wlan hotspot 130A is interrupted (e.g., loss of signaling), a new connection to the umts network 110 is established.

When the signal from ap 132A is lost (step 706), the wtru 102 may initiate a handover to the umts network 110, or alternatively, the wlan may initiate a handover to the umts network 110. Since the wlan is connected to the pdn gateway 129, the wlan can initiate a handover to the target umts network 110. When a loss of signal is detected, ap 132A sends a signal (reset request) to the pdn gateway 129 (step 708). This session is then maintained for a certain interval (step 710).

The wtru 102 then proceeds from step 712 to step 716 to establish a connection with the gateway gprs support node 128 as indicated by arrow 711. The wtru 102 establishes a Radio Access Bearer (RAB) with the node-B112 (step 712) and triggers a third generation network system attach (step 714). The wtru 102 then establishes a pdp context to trigger third generation ip connectivity (step 716). When the wtru 102 establishes the packet data communication protocol context, the wtru 102 selects an access point and the access point name is determined. The access point name is used for domain name service queries. Finally, this routine generates an ip address for the gateway gprs support node 128 to provide to the ap. The wtru 102 then triggers the third generation ims connectivity via the session initiation protocol registration in step 718, at which time connectivity between the gateway gprs support node 128 and the ims 150 is also established, as indicated by the arrow 718A.

Subsequently, a handover session is initiated (step 720). The wtru 102 sends information regarding the existing session to the ims 150 (i.e., the sip server). Information relating to existing sessions includes session/service identification, original and terminating internet protocol addresses, a request to redirect network traffic to a Universal Mobile Telecommunications (UMTS) network using new contact information (i.e., current internet protocol address), and the like. The ims 150 then updates the new route for the call/session. The ims 150 establishes a proxy-paging state control function and a serving-paging state control function for the new session.

The ims 150 then sends a handover request notification to the pdn gateway 129, wherein the handover request notification has session related information and an indication that the call/session has been redirected and that the previously reserved data should be released (step 722). The pdg 129 then sends a reset response to the ap 132A along with the session related information and the wtru Identity (ID) (step 724). Subsequently, the access point 132A releases the setting resources of the wtru 102. The session is returned between the wtru 102 and the ims 150 (steps 726A-726D), and the user triggered service is provided from the ims 150 via the gprs support node 128, the sgsn 122, and the node B112 (step 728).

The pdn gateway 129 may initiate a handover to the ims 150. Alternatively, the wtru 102 may initiate a handover to the ims 150 and provide old connection information.

Although the various features and elements of the present invention are described in the preferred embodiments in particular combinations, it will be understood by those skilled in the art that the various features and elements of the preferred embodiments of the present invention can be used alone without the other features and elements of the preferred embodiments of the present invention or in various combinations without departing from the spirit and scope of the present invention.

Claims (6)

1. In a wireless communication system having at least one cellular network, at least one Wireless Local Area Network (WLAN) and an Internet Protocol (IP) multimedia subsystem (IMS), a method for interworking between the cellular network and the WLAN for a user service provided by the IMS, the method comprising:

a wireless transmit/receive unit (WTRU) establishing a connection with a wireless local area network;

the WTRU obtaining a packet data gateway, PDG, address through the WLAN using a domain name service, DNS;

the WTRU establishes a channel between the WTRU and the PDG;

the WTRU registering to a third generation network IP multimedia subsystem IMS through the WLAN using session initiation protocol SIP;

the WTRU triggering an IP session for a user service from the IMS;

the WTRU receiving the user service from the IMS through a tunnel between the WLAN and the WTRU and the PDG;

the WTRU establishing a connection to a cellular network;

the WTRU initiating a handoff of the subscriber service from the WLAN to the cellular network; and

the WTRU performs the handover while continuously receiving the user service from the IMS.

2. The method of claim 1, further comprising:

the WTRU disassociates from the PDG in response to completion of the handoff.

3. The method of claim 1 wherein the subscriber service is a voice call service.

4. The method of claim 1 wherein obtaining the address of the PDG using DNS comprises:

transmitting a DNS query for the address of the PDG; and

receiving a response to the DNS query, the response including the address of the PDG.

5. The method of claim 1, further comprising:

the WTRU aborts the connection with the WLAN in response to establishing the connection with the cellular network.

6. The method of claim 1, further comprising:

the WTRU maintains a connection with the WLAN and simultaneously maintains a connection with the cellular network.