Abstract:
An apparatus and method is provided for facilitating the handoff of a mobile terminal from a first access device in a first network to a second access device in a second network. Prior to performing a handoff operation, the first access device inquires as to whether the mobile terminal is authorized to be handed off to the second access device and, in response thereto, completes the handoff. If the mobile terminal is not authorized to be handed off to the second access device, the handoff operation is inhibited, preferably without using any radio frequency resources of the second access device.

Description:
FIELD OF THE INVENTION 
   The invention relates generally to telecommunications networks. More particularly, the invention provides a method and apparatus for pre-authorizing handovers of mobile terminals among access routers in communication networks, such as wireless networks. 
   BACKGROUND OF THE INVENTION 
   Mobile devices can provide both voice-based connections and packet-based data connections using different base stations and infrastructures. For example, a Web-enabled cell phone might maintain a voice connection using a first transmission channel and maintain a mobile IP connection using a second (and independent) transmission channel, such that handoffs occur independently for the two channels. Alternatively, voice services can be combined with packet services, such that a single connection is maintained for both services. Voice connections can also be provided over IP in a combined service. 
     FIG. 1  shows a network with mobility features that covers three service areas SA 1 , SA 2 , and SA 3 . As shown in  FIG. 1 , a mobile terminal MT is within service area SA 1  served by base station BS 1  (also called an access point or AP). A service area generally refers to the radio coverage associated with a radio tower/base station. 
   Base station BS 1  is connected to an access router AR 1 , which provides access to the Internet. Other base stations such as BS 3  may also be connected to access router AR 1 , such that a common IP address is used for mobile terminals even though the terminals may pass through different service areas. In other words, although there may be a hand off of radio frequency channels when the mobile terminal moves between service area SA 1  and service area SA 3 , it may not be necessary to change the IP address used to communicate with the mobile terminal because the Internet connection is still served by the same access router AR 1 . 
   A second service area SA 2  is served by a separate base station BS 2 , which is in turn connected to a different access router AR 2 . Due to the network topology, access routers AR 1  and AR 2  use different blocks of IP addresses for communicating with mobile terminals roaming within their associated service areas. If mobile terminal MT moves from service area SA 1  to service area SA 2 , some mechanism is needed to hand off the Internet connection from access router AR 1  to access router AR 2 . Similarly, if service areas SA 1  and SA 2  are separated by a large logical distance (e.g., AR 1  and AR 2  are connected to different ISPs), some coordination mechanism is needed to permit data transmitted to a terminal previously operating in service area SA 1  to be forwarded to service area SA 2  if that terminal moves into area SA 2 . 
   One conventional scheme for handing off IP connections is depicted in  FIG. 2 . Service area SA 1  is served by access router AR 1 , which is designated the “home agent” for communicating with a particular mobile terminal MT. While mobile terminal MT moves within service area SA 1 , correspondence nodes communicate with the mobile terminal using an IP address that is assigned by the access router AR 1  to the mobile node. IP packets (e.g., e-mail, Web pages, and the like) are transmitted over the Internet to the home network and are forwarded to the mobile terminal through the home agent. 
   If the mobile terminal MT moves to a different service area SA 2 , served by a different access router AR 2 , packets that were previously transmitted to AR 1  will no longer reach the mobile terminal. 
   One conventional approach for handing off mobile nodes is to advertise (e.g., broadcast) the existence of access router AR 2  in service area SA 2 , such that when mobile terminal MT moves into service area SA 2 , it is notified of the existence of access router AR 2 , and it receives a new IP address for communicating within service area SA 2 . The mobile terminal MT then sends a binding update to home agent AR 1  (e.g., through a land line LL or over the Internet), so that home agent AR 1  knows the IP address that will allow packets to reach the mobile terminal in service area SA 2 . The home agent treats this address as a “care of” address, and all further packets to the mobile terminal&#39;s home address are forwarded to the new IP address. In essence, two separate IP addresses are used to communicate with the mobile terminal: a home agent address and a care of address that changes when the MT moves into a new service area. 
   When a mobile node moves from one access router to another, the packet forwarding path of sessions to and from the mobile node changes. In order to minimize the impact of a change in access routers, relevant context is transferred from the originating access router to the new access router. As described in H. Syed et al., “General Requirements for a Context Transfer Framework,” draft Internet Engineering Task Force Seamoby requirements work in progress (May 2001), the context transfer protocol entities may, in the process of establishing and supporting context transfer, acquire information that would be useful to the handover process in determining the new forwarding path; for example, the outcome of an admission control decision at a receiving access router. 
   A mobile terminal may move into an area that is served by two or more access routers. As with cellular telephone roaming, however, the mobile terminal may not be authorized to be handed off to certain access routers. Conventional handovers of mobile terminals from an originating access router to a target access router occur before determining whether the target access router is in fact authorized to service the mobile terminal. After the target access router accepts a handoff of a mobile terminal, it may perform a check to determine whether the mobile terminal is authorized to be serviced. If it is not, the service connection is dropped. 
   In other words, the handover decision from one AR to another AR is conventionally handled independently of whether the mobile terminal is authorized to roam into the network of the new AR. Typically, the mobile terminal is handed over to the new AR, then an authorization process ensues to determine whether the mobile terminal is authorized to roam into the new network. However, dropping the service connection with an unauthorized mobile terminal unnecessarily wastes resources, such as maintaining a connection with the mobile terminal for a period of time before the unauthorized service is discovered. It also wastes radio frequency spectrum, since radio resources are allocated to the mobile node prior to authorization. If the mobile node is determined not to be authorized, then radio resources must be revoked upon such determination of lack of authorization. 
   If there are several candidate access routers to which handover could result, for example those providing different access technologies (e.g., IEEE 802.11 WLANS or Bluetooth), a mobile node may not be authorized to roam into the network of certain service providers. Consequently, the conventional scheme for performing handoffs to access routers wastes resources and can delay handover processing. 
   What is needed is a system and method for addressing some or all of the aforementioned problems. 
   SUMMARY OF THE INVENTION 
   The invention provides a system and method to facilitate handoffs among access routers in networks such as wireless networks. According to one aspect of the invention, an originating access router inquires as to whether a target access router is authorized to accept a handoff of a mobile terminal and, if such authorization exists, initiates the handoff to the target access router. According to another variation of the invention, the target access router queries a home network to determine whether the mobile terminal is authorized to be handed off to the target access router and does not initiate the handoff operation until such authorization has been obtained. Authorization may be provided on the basis of static information, such as administrative approval, or on the basis of dynamic information, such as loading conditions. 
   In both embodiments, authorization of a mobile node&#39;s handover from one access router to another takes place prior to the actual handover. The inventive mechanism also allows for authorization for mobile nodes before a handover takes place even between heterogeneous networks. If there are several candidate access routers to which handover could result, then obtaining authorization information could help in determining the most favorable access router to which the handover should be made. Where more than one service provider permits roaming for a given mobile node, knowledge of relevant authorization information from the different service providers can allow for an optimal handover decision. Radio resources associated with the target access router are not used until a decision to hand over the mobile terminal has been made. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a conventional network covering three service areas SA 1 , SA 2 , and SA 3 . 
       FIG. 2  shows a conventional scheme for handing off a mobile terminal between access routers, wherein the mobile terminal registers with a home agent AR 1  but also communicates using a second IP address through a “care of” agent AR 2 . 
       FIG. 3  shows a system according to one aspect of the invention, wherein the handoff of a mobile terminal from an originating access router to a target access router does not occur until after a determination is made as to whether the target access router is authorized to service the mobile terminal. 
       FIG. 4  shows a second embodiment of a system according to the invention using session initiation protocol (SIP) to traverse one network boundary and AAA protocol to traverse another network boundary. 
       FIG. 5  shows one possible configuration for an authorization database  501 , authorization checker  503 , and loading detection module  502 . 
       FIG. 6  shows a flow chart illustrating steps of a method for handing off a mobile terminal to a target access router only after verifying that the target access router is authorized to accept a hand-off of the mobile terminal. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  shows a system employing various principles of the invention. As shown in  FIG. 3 , a first access router AR 1  serves a first service area SA 1  in which a mobile terminal MT may be located. Although not explicitly shown in  FIG. 3 , it is assumed that each access router transmits and receives data packets through one or more base stations that cover corresponding geographic areas. It is also assumed that each access router provides Internet-compatible services (e.g., IP protocol compatibility) such that data packets received at each router can be forwarded to one or more mobile terminals within the corresponding service area, although the invention is not limited in this respect. 
   Suppose that mobile terminal MT moves from area SA 1 , which is served by access router AR 1 , to area SA 2 , which is served by access router AR 2 . It is assumed that access router AR 1  and AR 2  communicate either directly or indirectly (e.g., through the Internet, land lines, other devices, or wireless means) as depicted by path  301  such that AR 1  can effect a handoff of mobile terminal MT to AR 2 . AR 2  further communicates with an AAA server AS 1  as depicted by path  302 . AAA refers to Authentication, Authorization and Accounting, which generally defines protocols and services relating to accounting and authorization for network services, see, e.g., IETF RFC 2924, September 2000 and “Diameter Mobile Ipv4 Application,” Internet Draft, July 2001. Server AS 1  communicates with a home server HS located in a home network SA 3 , as depicted by path  303 . Home server HS contains authorization information AUTH as explained in more detail below. 
   As shown in  FIG. 3 , communication path  301  between AR 1  and AR 2  may be implemented using the Session Initiation Protocol (SIP), whereas communication path  302  between AR 2  and AS 1  may be implemented using an AAA protocol such as DIAMETER. Communication path  303  between AS 1  and home server HS may also be implemented according to the DIAMETER protocol. In one variation, AR 1  transmits a SIP message with an OPTION method that contains details regarding the mobile terminal and the target access router AR 2 . This message is then translated into a suitable DIAMETER message for transport over the interfaces where DIAMETER is used. The Session Initiation Protocol (SIP) is described in the Internet Engineering Task Force (IETF) Request for Comment number 2543. 
   According to one aspect of the invention, prior to performing the handoff of mobile terminal MT from AR 1  to AR 2 , access router AR 1  contacts AR 2  to inquire about authorization information for mobile terminal MT. AR 2  in turn contacts server AS 1  for such information, which in turn contacts home server HS via path  303  to determine whether the mobile terminal is authorized to be handed off to access router AR 2 . If the mobile terminal is authorized, the handoff proceeds; otherwise, the handoff is aborted. Arranging a handoff may include procedures of context transfer (see, e.g., R. Koodli and C. Perkins, “A Context Transfer Framework for Seamless Mobility,” Work in Progress, Internet Draft, February 2001), or fast handover (see, e.g., G. Tsirtsis et al., “Fast Handovers for Mobile IPv6,” Work in Progress, Internet Draft, April 2001). 
   According to one aspect of the invention, radio resources are not used during the authorization process in order to determine whether the handoff should proceed. For example, AR 2  need not allocate a radio channel to communicate with the mobile terminal until after it has been determined that the mobile terminal is authorized to be handed off to AR 2 . 
   The linking of an access router such as AR 2  to an AAA server such as AS 1  via DIAMETER, and the further linking of an AAA server such as AS 1  to a home network server such as HS via the DIAMETER protocol, is conventional and described in the 3 rd  Generation Partnership Project (3GPP2) specification TS 23.228 version 5.0.0 (April 2001). However, the use of the architecture in the manner described above to perform pre-handover authorization between two access routers serving different networks is not conventional. 
     FIG. 4  shows an alternate embodiment according to the invention. According to this embodiment, AR 1  communicates with AR 2  using the SIP protocol as indicated by path  401 . AR 2  communicates with a SIP server SS 1  also according to the SIP protocol as indicated by path  402 . SIP server SS 1  communicates with a SIP server SS 2  in the home network as indicated by path  403 . Finally, SIP server SS 2  communicates with AAA server HS using the DIAMETER protocol, as indicated by path  404 . All of these communication paths are intended to be illustrative only; other protocols and communication methods can be used to inquire about authorization information in accordance with the invention. For example, authorization information can be pre-stored or cached in a particular access router, avoiding the need to access a home network. 
     FIG. 5  shows one possible embodiment of an authorization database such as database AUTH shown in  FIG. 3  and  FIG. 4 . Authorization information may comprise static information (e.g., an administratively created list of mobile terminals and the access routers to which they are authorized to be handed off), or dynamically changing information (e.g., authorization based on dynamic loading conditions or other criteria). Additionally, authorization information may be limited by time of day, or a subscription plan to which a mobile subscriber belongs (e.g., a “gold” plan allowing access to all routers; a “silver plan” allowing access to certain routers; and a “basic” plan allowing access to a limited number of access routers). 
   In one embodiment, subscribers using mobile terminals are identified according to an identifier such an International Mobile Subscriber Identity (IMSI), which is typically recorded in the nonvolatile memory of a mobile terminal such as a mobile telephone. The location of mobile terminals can be identified according to a hierarchical identification scheme, such as a concatenation of Public Line Mobile Network (PLMM) identifier, local area identifier, and base station identifier. Other schemes for identifying subscribers of mobile terminals as well as their location, and storing authorization information for such mobile terminals in a database, are possible. 
   As shown in  FIG. 5 , an authorization checker function  503 , which may be implemented in software within home server HS, checks for a given mobile ID whether the mobile ID is authorized to be handed off to a given candidate access router. In one variation, the authorization comprises a simple table look-up based on a list of allowed access routers for a given mobile terminal. In alternate embodiments, more sophisticated authorization may be stored. For example, certain mobile terminals may be restricted from accessing certain access routers except during a particular time of day. Mobile terminal subscribers may be assigned to a subscription plan that determines the level of access (e.g., how many access routers and under what conditions they can be accessed for handoff operations). Moreover, dynamic authorization information can be used to authorize mobile terminals on the basis of dynamic conditions such as loading of a particular access router. In this regard, a loading detection module  502  can be implemented to operate in conjunction with a loading parameter to modify the allowed list of access routers based on how heavily the access routers are loaded, such that subscribers who pay extra money get preference during peak loading conditions. Current loading conditions can be provided from access routers to home network servers in order to share information concerning loading conditions. Other variations are of course possible. 
     FIG. 6  shows a flow chart illustrating various steps that can be carried out in accordance with the invention. In step  601 , a mobile terminal wishes to move from an area serviced by a first access router AR 1  to an area serviced by a second access router AR 2 . This can be determined by the mobile terminal receiving an advertisement from the second access router including a router ID. In some cases, the current AR may detect the mobile terminal roaming into another service area and wish to instruct the mobile terminal to go to a particular router and connect to a particular access point. 
   In step  602 , access router AR 1  sends an inquiry to AR 2  inquiring about authorization for the mobile terminal to be handed off to AR 2 . In an alternate embodiment, shown at steps  608  and  609 , AR 1  sends the inquiry to the home server for an authorization check, bypassing steps  602  through  604 . As discussed above, access router AR 2  may have pre-stored information regarding authorizations for particular mobile terminals to be handed off, avoiding the need for steps  602  through  604  and  608  altogether. In other words, the database query could occur locally within AR 2  rather than requiring transmissions to another computer. 
   In step  603 , AR 2  forwards the inquiry to an AAA server, which in step  604  forwards the request to a home server corresponding to the mobile terminal. In step  605 , a determination is made as to whether the mobile terminal is authorized to be handed off to AR 2  using the principles outlined above. If the mobile terminal is not authorized to be handed off to AR 2 , then in step  606  the handoff is rejected, preferably without using any radio frequency resources between AR 2  and the mobile terminal. On the other hand, if the mobile terminal is authorized, then in step  607  the handoff to AR 2  is initiated. In the embodiment shown in  FIG. 4 , the steps are modified slightly to account for the existence of SIP servers SS 1  and SS 2 . 
   In an alternate embodiment, access router AR 1  can query home server HS through another path (e.g., directly or over the Internet), rather than going through access router AR 2 . In this embodiment, access router AR 1  may learn of the existence of AR 2  through other means (e.g., from the mobile terminal; through an administrative table; or through a learning function that detects the existence of AR 2  through queries and responses). 
   It should be appreciated that the principles of the invention can be applied not only to mobile IP networks, but to networks of other types. For example, the inventive principles can be applied to perform handovers between a wireless LAN and a GPRS network. 
   It should also be appreciated that access routers may cache authorization information, avoiding the need to query the home network for authorization information. In the configuration shown in  FIG. 3 , for example, access router AR 2  may be provided with authorization information for a plurality of mobile terminals from home network server HS. Thereafter, AR 2  can query its locally cached version of authorization information in order to respond to an authorization inquiry from AR 1 . Moreover, access router AR 1  may query authorization information from home server HS through another path (e.g., over the Internet), without going through access router AR 2 . Other combinations and paths are of course possible. 
   Any or all of the functions depicted in the figures can be implemented using computer software executing on a general-purpose or special-purpose digital computer. The authorization information can be stored in a computer memory, relational database, or other data structure. Conventional access routers can be modified to incorporate the functions illustrated in  FIG. 3 . 
   As used herein, the term “mobile terminal” should be understood to include IP-enabled cellular telephones and wireless telephones equipped to communicate using other protocols; wirelessly accessible Personal Digital Assistants (PDAs) such as those manufactured by PALM Inc.; notebook computers that can communicate wirelessly; and other mobile devices that can communicate using packetized digital communications over various transmission technologies (including CDMA, GSM, TDMA, and others) or media (radio, infrared, laser, and the like). 
   The term “access router” should be understood to include computer-implemented devices that route packets, such as IP packets, to addresses in a network based on routing information. However, it should be understood that access routers are generally distinct from base stations/access points, which may rely on different transmission schemes to transmit information (e.g., GSM or CDMA). One or more base stations could be associated with a single access router, as shown in  FIG. 1 . Alternatively, more than one access router could be associated with a single base station. 
   The term “mobile IP network” should be understood to include a network or networks (even if incompatible in transmission technology or operated by different carriers) that communicate wirelessly with mobile terminals using Internet Protocol. 
   While the invention has been described with respect to specific examples including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques that fall within the spirit and scope of the invention as set forth in the appended claims. Any of the method steps described herein can be implemented in computer software and stored on computer-readable medium for execution in a general-purpose or special-purpose computer.