Patent Publication Number: US-2006002355-A1

Title: Method and system for providing handoff between mobile communication network and wireless local area network, and switching device therefor

Description:
PRIORITY  
      This application claims the benefit under 35 U.S.C. §119(a) of an application entitled “Method and System for Providing Handoff between Mobile Communication Network and Wireless Local Area Network, and Switching Device Therefor” filed in the Korean Intellectual Property Office on Jul. 5, 2004 and assigned Serial No. 2004-52125, the entire contents of which are hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to a method and system for providing inter-network handoffs. In particular, the present invention relates to a method and system for providing a handoff between a mobile communication network supporting a circuit-switched voice service and a wireless local area network (WLAN) supporting a voice over Internet protocol (VoIP)-based voice service, and a switching device therefor.  
      2. Description of the Related Art  
      Generally, mobile communication networks supporting circuit-switched voice service are classified according to their communication methods. The mobile communication networks are classified into one of either a Frequency Division Multiple Access (FDMA) mobile communication network in which a full frequency band is divided into a plurality of frequency channels and then uniquely allocated to a plurality of subscribers, a Time Division Multiple Access (CDMA) mobile communication network in which one frequency channel is time-shared by a plurality of subscribers, or a Code Division Multiple Access (TDMA) mobile communication network in which codes are uniquely allocated to the subscribers that use the same frequency band in the same time band.  
      With the rapid progress of communication technology, the latest mobile communication networks provide not only the existing voice service but also high-speed data service in which a user with a mobile terminal can enjoy multimedia services including E-mail, still image and moving image services. Also, it is well known that a 3 rd  generation (3G) mobile communication system supporting both the voice service and the packet service includes a synchronous CDMA 2000 1x system, a 1x Evolution Data Only (EV-DO) system capable of high-speed packet transmission, an Evolution of Data and Voice (EV-DV) system, and an asynchronous Universal Mobile Telecommunication Systems (UMTS) system.  
      In the conventional data transmission method, the voice service is provided through a circuit-switched network such as a public switched telephone network (PSTN) and the packet service is provided through an Internet/Public Serving Data Network (PSDN), which is an Internet protocol (IP) network. However, the well-known VoIP technology has been proposed to provide the voice service even through the IP network. With the development of the IP network, the VoIP can enable a high-quality voice call by overcoming the 56-Kbps voice bandwidth limit of the circuit-switched network and also enable inexpensive international calls for only the associated Internet service provider fee. In addition, the VoIP can provide various application solutions and additional services. Due to the advantages, the number of VoIP users is increasing rapidly.  
      While the conventional VoIP service was provided through a personal computer (PC) or a wire network, the VoIP service can be provided even through a wireless local area network (WLAN). Therefore, subscribers can receive the VoIP service even through a mobile terminal such as a cellular phone.  
       FIG. 1  is a block diagram illustrating a simple configuration of a mobile communication network supporting the conventional VoIP service. In  FIG. 1 , a mobile terminal (MT)  110  is a dual-mode terminal that can access both a circuit-switched network and a packet network, and can also access a WLAN via an access point (AP)  120 .  
      The MT  110  performs Pulse Code Modulation (PCM) on an analog voice signal of its user, compresses the PCM-modulated signal, converts the compressed signal into a voice packet appropriate for a WLAN standard, and transmits the voice packet to the AP  120 . The AP  120  delivers the received voice packet to a VoIP gateway  130  via an IP network  1 . The VoIP gateway  130 , intervening between the IP network  1  and a PSTN  2 , transmits and receives various control signals for transmission of voice signals, and performs data conversion. In other words, the VoIP gateway  130  converts the voice packet received from the AP  120  into a voice signal appropriate for the PSTN  2 , and transmits the voice signal to the PSTN  2 . Further, the VoIP gateway  130  inversely converts a voice signal of the other party, received from the PSTN  2 , into a voice packet, and delivers the voice packet appropriate for a WLAN standard to the MT  110  via the AP  120 .  
      With the use of the dual-mode MT  110 , the user can receive the voice service through both the general cellular mobile communication network and the WLAN, which is a packet network. As for the mobility of the MT  110 , a handoff in the mobile communication network and the WLAN is possible, but inter-network handoff between the mobile communication network and the WLAN has not been taken into consideration. It is expected that the need for the inter-network handoff between the mobile communication network and the WLAN will dramatically increase as a WLAN environment will become more commonly provided in homes and offices. In particular, there is a demand for seamless handoff taking into account a time delay occurring during handoff of the voice service to the WLAN environment.  
     SUMMARY OF THE INVENTION  
      It is, therefore, an object of the present invention to provide a method and system for providing a handoff between a mobile communication network supporting a circuit-switched voice service and a wireless local area network (WLAN) supporting a voice over Internet protocol (VoIP)-based voice service.  
      It is another object of the present invention to provide a handoff method and system for reducing power consumption of a mobile terminal by simplifying a handoff process from a mobile communication network to a WLAN.  
      It is further another object of the present invention to provide a switching device for providing a handoff between a mobile communication network supporting a circuit-switched voice service and a WLAN supporting a VoIP-based voice service.  
      According to one aspect of the present invention, there is provided a method for providing handoff between a mobile communication network comprising a mobile switching center (MSC) for controlling call processing such that a circuit-switched voice service is provided to a mobile terminal (MT), and a home location register (HLR) in which subscriber information is registered, and a wireless local area network (WLAN) comprising an access point (AP) and a voice over Internet protocol (VoIP) gateway to provide a VoIP service. The method comprises the steps of registering, by the AP, subscriber information for the MT comprising an Internet protocol (IP) address of the AP, in the HLR; transmitting, by the MT, a handoff request to the MSC if the MT connected to the mobile communication network has moved to a region of the WLAN; transmitting, by the HLR, the subscriber information to the VoIP gateway; allocating an IP address and a WLAN channel to the MT; and setting a session for a VoIP service of the MT.  
      According to another aspect of the present invention, there is provided a system for providing a handoff between a mobile communication network supporting a circuit-switched voice service to a mobile terminal (MT) and a wireless local area network (WLAN) supporting a voice over Internet protocol (VoIP)-based voice service. The system comprises an access point (AP) for allocating an Internet protocol (IP) address and a wireless channel used by the MT if the MT desires to access the WLAN; a home location register (HLR) for registering therein predetermined subscriber information for the MT including an IP address of the AP; a mobile switching center (MSC) for forwarding a handoff request message to the WLAN upon receiving the handoff request message from the MT; and a switching device for, upon receiving the handoff request message from the MT, receiving the subscriber information from the HLR, performing a predetermined authentication process using the subscriber information, and performing session connection for WLAN access by the MT.  
      According to another further aspect of the present invention, there is provided a switching device included in a wireless location area network (WLAN) that provides a voice over Internet protocol (VoIP) service to at least one mobile terminal (MT). The switching device comprises a wireless soft switch (WSS) for registering subscriber information for the MT including Internet protocol (IP) information of an access point (AP) for WLAN access by the MT in a home location register (HLR) of a mobile communication network, and upon receiving a handoff request of the MT from a mobile switching center (MSC) of the mobile communication network, performing session connection for the MT using the subscriber information; and a media gateway (MGW) connected to the WSS through a gateway control protocol, for performing data conversion between the mobile communication network and the WLAN. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
       FIG. 1  is a block diagram illustrating a brief configuration of a mobile communication network supporting the conventional VoIP service;  
       FIG. 2  is a block diagram illustrating a network configuration of a handoff system between a mobile communication network and a wireless local area network (WLAN) according to an embodiment of the present invention;  
       FIGS. 3A and 3B  are flowcharts for a description of a handoff method between a mobile communication network and a WLAN according to an embodiment of the present invention; and  
       FIG. 4  is a block diagram illustrating a structure of a switching device for handoff between a mobile communication network and a WLAN according to an embodiment of the present invention. 
    
    
      Throughout the drawings it should be understood that like reference numerals refer to like features, structures and elements.  
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
      Several exemplary embodiments of the present invention will now be described in detail with reference to the annexed drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for the sake of conciseness.  
       FIG. 2  is a block diagram illustrating a network configuration of a handoff system between a mobile communication network and a wireless local area network (WLAN) according to an embodiment of the present invention. The handoff system comprises a dual-mode mobile terminal (MT)  210  supporting access to both the mobile communication network, which is a cellular network, and the WLAN, the mobile communication network, connected to the MT  210  via a wireless network, for providing a circuit-switched voice service. The WLAN is connected to the MT  210  via the wireless network and located in a cell of the mobile communication network, for providing not only IP network access service but also the VoIP service.  
      The embodiments of the present invention will be described on the following assumption. The MT  210  is a dual-mode terminal, capable of communicating in an indoor region, where a WLAN service is provided, and in an outdoor region where a cellular voice service is provided via a mobile communication network. In a home location register (HLR)  250  of the mobile communication network, a vocoder type used in the indoor region upon service initiation of the MT  210  should be previously defined. In an access gateway (AGW)  270 , an IP address of a wireless soft switch (WSS)  280  that supports a mobile identification number (MIN) of the MT  210  and serves as a signaling gateway for a VoIP service of the MT  210  should be previously specified.  
      A detailed description will now be made of the network elements provided to perform seamless handoff between a mobile communication network and a WLAN in the system of  FIG. 2 .  
      The MT  210 , when it accesses the mobile communication network, operates according to a CDMA 2000 1x standard or a UMTS scheme. The MT  210  operates according to an IEEE 802.1x scheme, when it accesses the WLAN. The MT  210  accesses a mobile switching center (MSC)  240  via a base transceiver system (BTS)  220  and a base station controller (BSC)  230 , which constitute a wireless access network of the mobile communication network.  
      The MSC  240 , connected to a public switched telephone network (PSTN)  2 , downloads, from the HLR  250 , information on a subscriber that receives not only a circuit-switched voice service but also a VoIP service during handoff from the mobile communication network to the WLAN, and registers the downloaded information in an undepicted visitor location register (VLR). The MSC  240  performs call processing (call termination or origination) and call setup for the subscriber depending on the current location information of the MT  210 , and thereafter, relays a voice traffic transmission. In the MSC  240 , preferably, a Signaling System 7 (SS7) point should be previously specified.  
      In order to provide a VoIP service during handoff of the MT  210 , the information on a subscriber registered in the HLR  250  preferably includes an IP address of an access point (AP)  260  to which the MT  210  is connected in a WLAN region and information on a vocoder type used by the MT  210  in the corresponding WLAN region. The HLR  250  performs extensible authentication protocol (EAP) authentication with the AP  260  such that subscriber authentication through an undepicted authentication, authorization and accounting (AAA) server (or authentication server) is achieved when the MT  210  performs handoff to the WLAN.  
      Herein, during the initial booting of the AP  260 , the HLR  250  previously registers and stores an IP address of the AP  260 , which was previously delivered from the AP  260  via the WSS.  
      The MT  210 , when performing handoff to the WLAN, accesses an IP network  1  via the AP  260  and performs packet communication such as the VoIP service. It is assumed herein that the AP  260  includes an undepicted access point controller (APC) for controlling packet communication.  
      The MT  210  includes an IP multimedia subsystem (IMS) session initiation protocol (SIP) client that receives an SIP Invite message (or call setup request message) delivered from the WSS, which performs a session connection for the VoIP service, and transmits a 200 OK message (or response message) to the WSS in response to the SIP Invite message.  
      The SIP refers to a session initiation protocol used in an IMS that accesses a core network of another communication network such as the mobile communication network or the PSTN  2  via a gateway and provides an IP-based communication service to subscribers.  
      Herein, the AGW  270  for delivering an IP address of the AP  260  to an WSS, the WSS for receiving the subscriber information from the HLR  250  of the mobile communication network during handoff of the MT  210  and transmitting predetermined pre-authentication information for previously performing subscriber authentication on the MT  210  to the AP  260 , and a media gateway (MGW) for performing data conversion between the mobile communication network or the PSTN  2  and the IP network  1 , constitute an IMS domain.  
      The pre-authentication information includes a medium access control (MAC) address, an international mobile subscriber identity (IMSI), a MIN and a subscriber identity module (SIM) of the MT  210  in the case of an UMTS terminal, and includes a MAC address, an IMSI and a MIN of the MT  210 , and a network access identifier (NAI) representing an address of the MT  210  in the form of an Internet domain in the case of a CDMA terminal.  
      A WSS/MGW  280  includes the WSS serving as a signaling gateway for a VoIP service and the MGW serving as a media gateway for data conversion between a voice signal of the mobile communication network and a voice packet of the WLAN. The AGW  270  and the WSS/MGW  280  form a VoIP gateway supporting a handoff service between the mobile communication network and the WLAN.  
      In this configuration, the MT  210  moving from the mobile communication network to the WLAN sends a handoff request to the MSC  240 , while maintaining the connection to the mobile communication network, and the MSC  240  forwards the handoff request from the MT  210  to the WSS/MGW  280  belonging to the IMS domain. The WSS/MGW  280 , receiving the handoff request from the MT  210 , receives subscriber information including an IP address of the AP  260  accessed by the handoff requesting MT  210  and vocoder type information, from the HLR  250  of the mobile communication network.  
      The WSS/MGW  280  sets up an IP tunnel to the AP  260  and delivers pre-authentication information for the MT  210  to the AP  260  through the IP tunnel, and the AP  260  performs EAP authentication with the HLR  250  through an undepicted AAA server using the pre-authentication information. Thereafter, the AP  260  allocates an IP address for an access to the WLAN by the MT  210 , and delivers channel information available for the MT  210  to the WSS/MGW  280 .  
      In addition, the WSS/MGW  280  delivers the received channel information to the MSC  240 , and the MSC  240  delivers channel information available in the WLAN to the MT  210  connected to the mobile communication network. The MT  210 , after receiving the channel information, sets up a session connection to the WSS/MGW  280  and performs a seamless handoff from the mobile communication network to the WLAN.  
      That is, at the handoff request of the MT  210 , according to embodiments of the novel method the steps of previously transmitting the pre-authentication information to the AP  260  to reduce any authentication delay occurring during the IP allocation and the channel allocation for the MT  210 , transmits the channel information available in the WLAN to the MT  210  connected to the mobile communication network via the MSC  240 , performing an IP allocation to the MT  210 , and thereafter, performing an association (or connection setup) between the MT  210  and the AP  260 , thereby reducing the time required for handoff are performed. In this manner, embodiments of the novel method reduce the power consumption of the MT that must maintain the connection of both a channel to the mobile communication network and a channel to the WLAN until an end of the handoff process, and also provide seamless service to the user during the handoff from the mobile communication network to the WLAN.  
      Embodiments of the present invention can implement a selected one of an operation of decreasing an authentication delay by transmitting the pre-authentication information and an operation of decreasing an access time to the WLAN by previously transmitting channel information.  
      A detailed description will now be made of the WSS/MGW  280 , which is a switching device serving as a signaling gateway for a VoIP service of the MT  210  that performs handoff from the mobile communication network to the WLAN.  
       FIG. 4  is a block diagram illustrating a structure of a switching device for performing a handoff between a mobile communication network and a WLAN according to an embodiment of the present invention.  
      In  FIG. 4 , a WSS  280   a  registers subscriber information for access to the WLAN by the MT  210 , such as IP information of the AP  260  in the HLR  250 , and sends a request for a session connection to the MT  210  to the AP  260  using an SIP message. The WSS  280   a  is designed such that it delivers pre-authentication information of the MT  210  handing off to the AP  260  and previously delivers channel information of the WLAN to which the MT  210  is connected to the MSC  240 . The MSC  240  reduces a WLAN access time of the MT  210  by transmitting a handoff command message including channel information to the MT  210 .  
      An MGW  280   b  is preferably connected to the WSS  280   a  via a media gateway control protocol (MGCP)/MEGACO, which is a gateway control protocol, and performs data conversion between the mobile communication network and the WLAN. In particular, the MGW  280   b  is designed such that it supports transcoding between various codecs (G.711, G.729, G.723, and the like) for voice traffic, and various transmission layers (IP, ATM, and TDM).  
      The WSS  280   a  comprises an HLR interworking block  281  for registering subscriber information of the MT  210  in the HLR  250 , an MGW controller  283  for handling connection and or release of a bearer by controlling the MGW  280   b , and an SIP message processor  285  for processing an SIP message for call connection and or release of the MT  210 . Further, the WSS  280   a  comprises an MSC/PSTN interworking block  287 , a visitor location register (VLR)  288 , and a call control function (CCF)/service switching function (SSF)  289  for processing a call state of a subscriber and an additional service.  
      The HLR interworking block  281  receives, from the point to point protocol (PPP)-connected AP  260 , not only an IP address of the corresponding AP  260  but also an IP address registration message including IMSIs and/or MINs of the MTs  210  served by the AP  260  in a WLAN region, and delivers the received IP addresses to the HLR  250 . The HLR interworking block  281  can use an interim standard (IS)-41 MAP as a signaling protocol for interworking with the HLR  250 . If a call connection and or release signal is received from the SIP message processor  285  or the MSC/PSTN interworking block  287 , the MGW controller  283  exchanges a control signal for a bearer connection and or release with the MGW  280   b.    
      The SIP message processor  285  processes call connection and or releases for the VoIP service or processes SIP messages used for providing various additional services. Herein, in the process of performing a session connection for the MT  210 , the SIP message processor  285  transmits an SIP Invite message to the MT  210  via the AP  260  and receives a 200 OK message from the MT  210  in response to the SIP Invite message.  
      The MSC/PSTN interworking block  287  receives a handoff request message of the MT  210  from the MSC  240 , and delivers available channel information for the MT  210  received from the AP  260  to the MSC  240 . The MSC/PSTN interworking block  287  controls a call connection service to the MSC  240  or an undepicted PSTN according to a location of a called terminal, and the VLR  288  stores a subscriber profile delivered from the HLR  250 , if a location of the MT  210  is registered therein. The CCF/SSF  289  delivers pre-authentication information for the MT  210  to the AP  260 , manages the call states of subscribers and controls various additional services, and interworks with the MSC/PSTN interworking block  287  and the SIP message processor  285 .  
       FIGS. 3A and 3B  are flowcharts for a handoff method between a mobile communication network and a WLAN according to an embodiment of the present invention. A detailed description of the proposed handoff method will be made herein below with reference to  FIGS. 2, 3A  and  3 B.  
      If an xDSL modem (not shown) used by an AP  260  is booted up, the AP  260  and an AGW  270  set a point-to-point protocol (PPP) therebetween. After the PPP setup, the AP  260  transmits its IP address registration message to the AGW  270  in step  301 . Herein, the AP  260  previously stores IMSIs and MINs of its MTs, and transmits the IMSIs and MINs of the MTs using the IP address registration message.  
      In step  303 , an AGW  270 , upon receiving the IP address registration message from the AP  260 , forwards the IP address registration message of the corresponding AP  260  to a WSS/MGW  280  that manages the IMSIs and MINs of a plurality of MTs, included in the registration message. In step  305 , the WSS/MGW  280  forwards the received IP address registration message to an HLR  250 . Although the IP address of the AP  260  is allocated herein through PPP setup between the AP  260  and the AGW  270 , the PPP setup process can be omitted by allocating a fixed IP to the AP  260 .  
      In step  307 , the HLR  250  receives the IP address registration message of the AP  260 , transmitted from the WSS/MGW  280 , and registers and stores an IP address of the corresponding AP  260  as subscriber information along with vocoder type information upon service initiation of the MT  210 . In step  309 , if there is an MT  210  that has entered a WLAN region from a mobile communication network region, the MT  210  transmits a handoff request message including its MAC address, MIN, SIM and NAI to the MSC  240 .  
      In step  311 , the MSC  240  transmits a handoff message including a MAC address, a MIN and an IMSI of the handoff requesting MT  210  to the WSS/MGW  280 . For example, a facilities directive (FACDIR) message used during hard handoff of the MSC  240  can be used as the handoff message transmitted in step  311  to the WSS/MGW  280 . Upon receiving the handoff message, the WSS/MGW  280  performs a MAP operation using the MIN to acquire subscriber information from the HLR  250 .  
      In step  313 , the WSS/MGW  280  transmits to the HLR  250  a request for transmission of subscriber information of the handoff requesting MT  210 . In step  315 , the HLR  250  transmits subscriber information including an IP address of the corresponding AP  260  and a vocoder type of the MT  210  to the WSS/MGW  280 , and the WSS/MGW  280  stores the received subscriber information and then performs service control on a VoIP call. In step  317 , the WSS/MGW  280  receiving the subscriber information sets an IP tunnel to the AP  260  to which the MT  210  belongs, to previously perform WLAN access authentication for the MT  210 . In step  319 , the WSS/MGW  280  transmits pre-authentication information for WLAN access authentication for the MT  210  to the corresponding AP  260 .  
      The pre-authentication information preferably comprises a MAC address, an IMSI, a MIN and a SIM of the MT  210  in the case of an UMTS terminal, and comprises a MAC address, an IMSI, a MIN, and NAI of the MT  210  in the case of a CDMA terminal. In this manner, embodiments of the present invention can reduce the authentication time required in the process of accessing the WLAN by the MT  210  by transmitting pre-authentication information to the AP  250  before the IP allocation and channel allocation for the MT  210 .  
      In steps  321  and  323 , the AP  260  performs EAP authentication for WLAN access by the HLR  250  and the MT  210  via an AAA server. After the successful EAP authentication, the AP  260  allocates an IP address to the corresponding MT  210  in step  325 . In step  327 , the AP  260 , after performing the EAP authentication, delivers a predetermined authentication response message preferably including an IMSI, a MIN, an IP address of the MT  210 , and the channel information available for the MT  210  to the WSS/MW  280 .  
      In step  329 , the WSS/MGW  280  extracts channel information included in the authentication response message and delivers the extracted channel information to the MSC  240 . In step  331 , the WSS/MGW  280  transmits an SIP Invite message to the AP  260  to request session connection for WLAN access by the MT  210 . In step  333 , the MSC  240 , which has received the channel information for the handing off MT  210  in step  329 , transmits a handoff command message including the received channel information to the MT  210 .  
      In step  335 , the MT  210  performs an association (or connection setup) with the AP  260  so that the MT  210  accesses the WLAN. In step  337 , the AP  260  transmits an SIP Invite message to the MT  210  using SIP session information that is being maintained after the SIP Invite message was received from the WSS/MGW  280 , thereby requesting call setup to the WLAN. In step  339 , the MT  210  transmits a 200 OK message to the WSS/MGW  280  in response to the SIP Invite message received in step  331 . In step  341 , the WSS/MGW  280  transmits an acknowledgement (ACK) message to the MT  210 .  
      In steps  343  and  345 , upon receiving the ACK message, the MT  210  releases the connection to the mobile communication network and performs a VoIP voice call with the MSC  240  via the WSS/MGW  280 . Herein, the voice traffic transmitted in step  343  is, for example, a voice packet transmitted via the IP network  1 , while the voice traffic transmitted in step  345  is, for example, a PCM voice signal. The WSS/MGW  280  converts voice signals from the mobile communication network into voice packets and transmits the voice packets to the WLAN. Further, the WSS/MGW  280  converts voice packets from the WLAN into PCM voice signals and transmits the PCM voice signals to the mobile communication network.  
      As can be understood from the foregoing description, embodiments of the present invention can provide a seamless handoff service from a mobile communication network supporting a circuit-switched voice service to a WLAN supporting a VoIP service. In addition, embodiments of the present invention can dramatically reduce the WLAN access time of an MT by providing pre-authentication information for the handoff MT to an AP of the WLAN.  
      Moreover, embodiments of the present invention can provide an SIP-based VoIP service that can be handed off from the mobile communication network, to a subscriber of an office or home network where a WLAN service is provided. Further, embodiments of the present invention can reduce power consumption of the MT by simplifying a handoff process from the mobile communication network to the WLAN.  
      While the invention has been shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.