Abstract:
Methods and apparatuses for routing emergency calls originated from mobile wireless devices in a Voice-over-IP (VoIP) system are described herein. In one or more embodiments, emergency calls are routed in a VoIP system by receiving incoming emergency calls originated from dual-mode mobile devices connected to the VoIP system through wireless access points (WAPs), determining locations associated with the incoming emergency calls, and redirecting callers to a cellular network. In other embodiments, emergency calls are routed in a VoIP system by mapping WAP identifiers to emergency answering points (EAPs), receiving an incoming emergency call from an originating WAP, identifying an EAP relating to the originating WAP, and directing the emergency call to the identified EAP.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/755,926, filed Jan. 3, 2006, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to converged cellular and wireless broadband networks, and particularly relates to routing emergency calls in converged networks. 
         [0003]    The convergence of cellular and wireless broadband networks allows subscribers to move between the networks with seamless voice and data session continuity, just as subscribers move between cells within a cellular network. Wireless network convergence effectively creates a dual radio access network. When it is efficient to route information such as data or voice over a cellular network, a mobile device utilizes the cellular network for communication. Conversely, when it is more efficient to route information over a wireless broadband network, the mobile device utilizes the wireless broadband network for communication. 
         [0004]    One issue relating to the convergence of cellular and wireless broadband networks is the routing of emergency calls to the appropriate local emergency personnel. Various governments require communication service providers to support emergency calls made from cellular handsets, e.g., the E-911 mandate issued by the Federal Communications Commission (FCC) in the United States. Additionally, the FCC will require Voice-over-IP (VoIP) service providers to comply with the E-911 mandate in the near future. For example, VoIP providers will be required to deliver all 911 calls to the customer&#39;s local emergency operator and provide emergency operators with the call back number and location information of their customers. 
         [0005]    Location-based services are widely used in cellular networks for identifying caller location when handling emergency calls placed by cellular handsets. For example, device-centric technologies such as the Global Positioning System (GPS) can pinpoint the location of a mobile device to an accuracy of ten meters or less. Network-assisted technologies such as assisted-GPS (AGPS) for Code Division Multiple Access (CDMA) cellular networks and Enhanced Observed Time Difference (EOTD) for Global System for Mobile communications (GSM) networks can pinpoint the location of a mobile device to an accuracy of one hundred meters or less. 
         [0006]    However, location identification technology for mobile devices that access wireless broadband networks is less mature. Further, the nature of broadband communication, e.g., the use of Internet Protocol (IP) bearers for communicating between remote devices, removes all information associated with the location of a caller. As such, the convergence of cellular and wireless broadband networks presents a new challenge for identifying the location of mobile wireless devices when the devices communicate over a wireless broadband network. For example, as a mobile wireless device seamlessly transitions from a cellular network to a wireless broadband network, the device may no longer be capable of determining and/or communicating its position when connected to the wireless broadband network. VoIP service providers face a particularly daunting task if mandated to support E-911 for mobile devices placing VoIP calls using wireless broadband access technology. 
       SUMMARY OF THE INVENTION 
       [0007]    The methods and apparatuses taught herein provide a method of routing emergency calls originated from mobile wireless devices in a Voice-over-IP (VoIP) system. In one example, the method comprises receiving incoming emergency calls originated from dual-mode mobile devices connected to the VoIP system through wireless access points (WAPs), determining locations associated with the incoming emergency calls, and redirecting callers to a cellular network. Corresponding to the above emergency call routing method, a complementary VoIP system comprises a call processing server configured to receive incoming emergency calls originated from dual-mode mobile devices connected to the VoIP system through WAPs. The call processing server is further configured to determine locations associated with the incoming emergency calls, and redirect callers to a cellular network. 
         [0008]    Several embodiments described herein enable VoIP systems to acquire location information associated with mobile wireless devices accessing VoIP systems and to use the acquired location information to route emergency calls to appropriate emergency answering points (EAPs). In one example, WAP identifiers are mapped to EAPs. As such, when an incoming emergency call is received from an originating WAP, an EAP relating to the originating WAP is identified and the emergency call is directed to the identified EAP. 
         [0009]    In another example, an incoming emergency call originated from a mobile wireless device connected to a VoIP system through a WAP is received by the VoIP system. Location information associated with the mobile wireless device is acquired from the mobile wireless device and the emergency call is directed to an EAP that services a geographic area corresponding to the location information acquired from the mobile wireless device. 
         [0010]    In yet another example, an incoming emergency call originated from a mobile wireless device connected to a VoIP system through a WAP is received by the VoIP system. Location information derived by a device in-range of the mobile wireless device is acquired. The emergency call is directed to an EAP that services a geographic area corresponding to the location information acquired from the in-range device. 
         [0011]    Of course, the present invention is not limited to the above features and advantages. Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a block diagram of an embodiment of a Voice-over-IP (VoIP) system. 
           [0013]      FIG. 2  is a logic flow diagram of an embodiment of processing logic for identifying wireless access points to a VoIP system. 
           [0014]      FIG. 3  is a logic flow diagram of an embodiment of processing logic for relating wireless access points to emergency answering points. 
           [0015]      FIG. 4  is a block diagram of an embodiment of a database included in or associated with the VoIP system of  FIG. 1 . 
           [0016]      FIG. 5  is a block diagram of an embodiment of a VoIP system that acquires location information from a device in-range of a mobile wireless device. 
           [0017]      FIG. 6  is a logic flow diagram of an embodiment of processing logic for providing a wireless access point identifier to a VoIP system during an emergency call. 
           [0018]      FIG. 7  is a logic flow diagram of one embodiment of processing logic for routing emergency calls in a VoIP system. 
           [0019]      FIG. 8  is a logic flow diagram of an embodiment of processing logic for providing mobile wireless device location information to a VoIP system during an emergency call. 
           [0020]      FIG. 9  is a logic flow diagram of an embodiment of processing logic for providing location information associated with a mobile wireless device to a VoIP system during an emergency call. 
           [0021]      FIG. 10  is a logic flow diagram of another embodiment of processing logic for routing emergency calls in a VoIP system. 
           [0022]      FIG. 11  is a logic flow diagram of yet another embodiment of processing logic for routing emergency calls in a VoIP system. 
           [0023]      FIG. 12  is a logic flow diagram of an embodiment of processing logic for redirecting emergency calls received by a VoIP system over a cellular network. 
           [0024]      FIG. 13  is a logic flow diagram of an embodiment of processing logic for redirecting an emergency call by a dual-mode mobile device over a cellular network. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]      FIG. 1  illustrates an embodiment of a Voice-over-IP (VoIP) system  10  that provides packet-based voice and data services to mobile wireless devices such as a dual-mode mobile communication device  12 . The dual-mode mobile device  12  gains access to the VoIP system  10  via a Wireless Access Point (WAP)  14 , e.g., an IEEE 802.11 (WiFi), IEEE 802.16 (WiMax), or IEEE 802.20 (Mobile Broadband Wireless Access) compatible WAP. The dual-mode mobile device  12  is directly or indirectly coupled to the VoIP system  10 , e.g., through a Packet-Switched Data Network (PSDN)  16  such as the Internet. The VoIP system  10  comprises a call processing server  18  for managing VoIP connections traversing the VoIP system  10 , including emergency calls. 
         [0026]    The dual-mode mobile device  12  and the VoIP system  10  communicate both control information and packet-based communication data. To establish and control packet-based calls, the dual-mode mobile device  12  and the VoIP system  10  use a signaling protocol, e.g., Session Initiation Protocol (SIP) or H.323. For example, the call processing server  18  of the VoIP system  10  and a communication processor  20  of the dual-mode mobile device  12  use SIP in conjunction with client code such as Java to control handling of emergency calls initiated by the device  12 . 
         [0027]    The communication processor  20  manages network communication for the dual-mode mobile device  12 , including establishing and maintaining communication channels, initiating and managing calls, and acquiring the location of the dual-mode mobile device  12 . The communication processor  20  may comprise one or more general or special purpose microprocessors, digital signal processors, application specific integrated circuits, field programmable gate arrays, and/or other types of digital processing circuits, configured according to computer program instructions implemented in software (or firmware). 
         [0028]    Likewise, the call processing server  18  manages packet-based communication for the VoIP system  10 . The call processing server  18  comprises hardware and/or software and can be deployed as a single server, cluster of servers, or a server farm having distributed functionality. The call processing server  18  manages device communication, maintains various mappings and translations, and opens and closes communication channels between devices. For example, the call processing server  18  includes a call agent  22  for providing VoIP call signaling and control functions. The call agent  22  manages signaling and control flows associated with devices that access the VoIP system  10 , e.g., by originating, terminating or forwarding calls. In a non-limiting example, the call agent  22  may include a SIP server (not shown) for providing SIP call signaling and control functions, e.g., by routing and forwarding SIP requests. 
         [0029]    Further, the call processing server  18  includes an application server  24  for executing one or more applications or services not managed by the call agent  22 , e.g. voice mail, conference calling, and emergency call handling. The call processing server  18  interfaces with a media gateway controller/media gateway (MGC/MG)  26 . The MGC/MG  26  contains call control logic and hardware for interfacing with the Public-Switched Telephone Network (PSTN)  28 . As such, the call processing server  18  gains access to the PSTN  28  via the MGC/MG  26 . 
         [0030]    As part of managing packet-based connections in the VoIP system  10 , the call processing server  18  processes emergency calls received from various devices connected to the system  10 , including mobile wireless devices such as the dual-mode mobile device  12 . Emergency calls received by the VoIP system  10  may include proprietary emergency voice calls,  911  emergency voice calls, emergency text messages, emergency instant messages or the like. The call processing server  18  routes received emergency calls to Emergency Answering Point (EAPs)  30 , i.e., designated statewide default answering points such as Public Service Answering Points (PSAPs), appropriate local emergency authorities or other emergency answering points or proprietary emergency answering points such as Onstar. To route an emergency call to an appropriate EAP, the call processing server  18  acquires information associated the location of the packet-based call, e.g., geospacial or civic location information such as latitude, longitude, altitude, street address, phone number, building name, etc. The call processing server  18  uses such location information to identify an appropriate EAP for receiving a particular emergency call. 
         [0031]    The VoIP system  10  routes emergency calls to the EAPs  30  via either the PSTN  28  or an emergency services network  32  such as the wireline E911 network or a proprietary emergency call handling network capable of routing emergency calls and related information to the EAPs  30 . To route an emergency call via the PSTN  28 , the call processing server  18  uses location information associated with the call to identify an address of an appropriate EAP and then forwards the call to the EAP address over the PSTN  28  via the MGC/MG  26 . When routing calls via the emergency services network  32 , the call processing server  18  forwards the emergency call along with acquired location information to the emergency services network  32  directly via a gateway (not shown) or indirectly via the PSDN  16  or the PSTN  28 . The emergency services network  32  uses the location information to identify an address of an appropriate EAP for responding to the emergency call. 
         [0032]    Several embodiments are described herein that enable the VoIP system  10  to acquire location information associated with mobile wireless devices accessing the system  10  and to use the acquired location information to route emergency calls to an appropriate EAP. In one embodiment, the call processing server  18  populates and manages a database  34  that relates WAPs to the EAPs  30  using location information associated with mobile wireless devices. Particularly, the call processing server  18  uses location information associated with mobile wireless devices as an approximation of WAP location and relates one or more of the EAPs  30  to particular WAPs using the location information. Thus, when the VoIP system  10  receives an emergency call from a known WAP, i.e., a WAP having an entry in the database, the call processing server  18  identifies an EAP associated with the WAP and routes the emergency call to the identified EAP. 
         [0033]      FIG. 2  illustrates an embodiment of processing logic for identifying WAPs and providing location information associated with identified WAPs to the VoIP system  10 . Prior to connecting to the VoIP system  10 , a mobile wireless device gains wireless broadband access, e.g. to a wireless Local Area Network (WLAN) (Step  100 ). For example, the dual-mode mobile device  12  gains wireless broadband access via the WAP  14  using a WLAN radio  36  included in the device  12 . The WAP  14  implements a network access authentication procedure for determining whether the dual-mode mobile device  12  is an authorized device. 
         [0034]    After gaining access to a wireless broadband network, the mobile wireless device logs into or is otherwise authenticated by the VoIP system  10  (Step  102 ). After authentication is completed, or alternatively, as part of the authentication process, the mobile wireless device sends to the VoIP system  10  an identifier associated with the originating WAP, i.e., the WAP through which the device gains access to the VoIP system  10  (Step  104 ). For example, the dual-mode mobile device  12  provides an identifier associated with the originating WAP  14 . Each identifier uniquely identifies a particular WAP to the VoIP system  10 , e.g., a media access control (MAC) address, a service set identifier (SSID), or an internet protocol (IP) address. Upon request from the VoIP system  10  or automatically, the mobile wireless device sends location information associated with the mobile device to the VoIP system  10  (Step  106 ). 
         [0035]      FIG. 3  illustrates an embodiment of processing logic for populating the database  34  with WAP information provided by mobile wireless devices. When a mobile wireless device accesses the VoIP system  10  via a wireless broadband connection, e.g., during non-emergency calls, the device logs into or otherwise authenticates itself to the VoIP system  10  (Step  108 ). As part of the login process, the mobile wireless device sends to the VoIP system  10  an identifier associated with a WAP through which the device communicates with the VoIP system  10 . For example, the dual-mode mobile device  12  provides an identifier associated with the originating WAP  14  to the VoIP system  10 . 
         [0036]    The call processing server  18  verifies whether the originating WAP  14  is known to the VoIP system  10  (Step  110 ). If the originating WAP  14  is known, the call processing server  18  processes the incoming call (Step  112 ). Conversely, if the originating WAP  14  is unknown, the VoIP system  10  acquires location information from the dual-mode mobile device  12  (Step  114 ). The acquired location information serves as an approximation of the location of the originating WAP  14 . The database  34  is then updated with the acquired location information (Step  116 ). Particularly, the database  34  maps the new WAP identifier with one or more of the EAPs  30  that service a geographic area corresponding to location information associated with the newly identified WAP, as illustrated by  FIG. 4 . Further, the VoIP system  10  may acquire location information from multiple mobile wireless devices that access the system  10  through the same WAP. The call processing server  18  may use the plurality of acquired location information to refine or pinpoint the location of a particular WAP. 
         [0037]    The dual-mode mobile device  12  can acquire its location in various ways. For example, the dual-mode mobile device  12  may include a GPS device (not shown) for determining its location. Alternatively, the dual-mode mobile device  12  may communicate with a cellular network  38  to acquire its location. For example, a cellular radio  40  included in the dual-mode mobile device  12  can establish a radio connection to the cellular network  38 . Once connected, the dual-mode mobile device  12  acquires its location by cellular network-derived techniques such as Enhanced Observed Time Difference (EOTD), assisted GPS, or Time Difference of Observed Arrival (TDOA). In yet another example, a user of the dual-mode mobile device  12  inputs location information into the device, e.g., by inputting alphanumeric characters into a keypad of the device  12  or by voice command. 
         [0038]      FIG. 5  illustrates an embodiment where a mobile wireless device such as the dual-mode mobile device  12  or the VoIP system  10  acquires location information from an in-range device  42 , i.e., a device in sufficient proximity with the mobile wireless device such that a wireless connection can be established between the devices. The location information acquired from the in-range device  42  can be used to approximate the location of the dual-mode mobile device  12  when the device  12  is unable to ascertain its own location. The dual-mode mobile device  12  either obtains location information from the in-range device  42  and provides the location information to the VoIP system  10  or initiates a connection between the VoIP system  10  and the in-range device  42 . 
         [0039]    In one example, the dual-mode mobile device  12  acquires location information from the in-range device  42  and provides it to the VoIP system  10 . As such, the in-range device  42  is unknown to the VoIP system  10 . During an emergency call, a SIP signaling connection is established between the communication processor  20  of the dual-mode mobile device  12  and the call processing server  18  of the VoIP system  10 . In addition, a media connection is also established between the VoIP system  10  and the dual-mode mobile device  12  for exchanging information between the communication processor  20  and the call processing server  18 . Upon determining that the location of the dual-mode mobile device  12  is not known or cannot be approximated, the dual-mode mobile device  12  establishes a SIP connection with a communication processor  44  of the in-range device  42 . As part of the SIP connection with the in-range device  42 , a media connection is also established. The dual-mode mobile device  12  then requests location information from the in-range device  42 . The dual-mode mobile device  12  acquires the location information from the in-range device  42  via the media connection between the two devices. The dual-mode mobile device  12  then provides the location information to the VoIP system  10  via the media connection between the dual-mode device  12  and the VoIP system  10 . 
         [0040]    In another non-limiting example, the call processing server  18  establishes new SIP and media connections with the communication processor  44  of the in-range device  42 . Using the preexisting media connection with the in-range device  42 , the dual-mode mobile device  12  may acquire a device identifier from the in-range device  42 , e.g., a MAC address, SSID, IP address, or phone number. The dual-mode mobile device  12  then forwards the device identifier acquired from the in-range device  42  to the VoIP system  10  via the preexisting media connection between the system  10  and the dual-mode device  12 . The call processing server  18  uses the device identifier to establish new SIP and media connections between the VoIP system  10  and the in-range device  42 . As such, the call processing server  18  can then acquire location information from the in-range device  42  over the newly established media channel. Those skilled in the art will appreciate that the call processing server  18  can contact one or more in-range devices while maintaining an emergency call connection with the dual-mode mobile device  12 . 
         [0041]    In yet another non-limiting example, the call processing server  18  communicates with the in-range device  42  through the dual-mode device  12 . Particularly, the dual-mode device  12  routes or passes information between the VoIP system  10  and the in-range device  42  using the SIP and media connections established between the dual-mode device  12  and the VoIP system  10  and between the dual-mode device  12  and the in-range device  42 . That is, the dual-mode mobile device  12  can function as a relay to establish communication between the in-range device  42  and the VoIP system  10 . As such, the dual-mode mobile device  12  functions as a router or pass-through device, enabling the call processing server  18  to use the preexisting connections with the dual-mode mobile device  12  to acquire location information from the in-range device  42 . 
         [0042]      FIG. 6  illustrates an embodiment of processing logic for placing an emergency call to the VoIP system  10  by a mobile wireless device via a WAP. The mobile wireless initiates an emergency call with the VoIP system  10  via a wireless broadband connection (Step  200 ). For example, the dual-mode mobile device  12  initiates an emergency call via a wireless broadband connection established by the WAP  14 . The mobile wireless device sends to the VoIP system  10  an identifier associated with a WAP through which the device communicates with the VoIP system  10  (Step  202 ). For example, the dual-mode mobile device  12  provides an identifier associated with the originating WAP  14 . 
         [0043]      FIG. 7  illustrates an embodiment of processing logic for routing an emergency call received by the VoIP system  10  to an appropriate EAP using the WAP/EAP relationships provided by the database  34 . For example, after the dual-mode mobile device  12  is authenticated by the originating WAP  14 , the device  12  initiates an emergency call via the wireless broadband connection established by the WAP  14  (Step  204 ). The VoIP system  10  receives from the dual-mode mobile device  12  an identifier associated with the originating WAP  14  (Step  206 ). The call processing server  18  then queries or mines the database  34  using the WAP identifier received from the dual-mode mobile device  12  to identify an EAP associated with the originating WAP  14  (Step  208 ). The call processing server  18  directs the emergency call to the identified EAP (Step  210 ), e.g., via the PSTN  28  or the emergency services network  32 . 
         [0044]      FIG. 8  illustrates an embodiment of processing logic for placing an emergency call to the VoIP system  10  by a mobile wireless device that provides its location to the system  10  as part of the emergency call. The mobile wireless initiates an emergency call with the VoIP system  10  via a wireless broadband connection (Step  300 ). For example, the dual-mode mobile device  12  initiates an emergency call via a wireless broadband connection established by the WAP  14 . The mobile wireless device provides to the VoIP system  10  location information associated with the mobile wireless device (Step  302 ). For example, the dual-mode mobile device  12  provides to the VoIP system  10  GPS-derived, cellular network-derived, or user-derived location information each as previously described. 
         [0045]    Alternatively,  FIG. 9  illustrates an embodiment of processing logic for placing an emergency call to the VoIP system  10  by a mobile wireless device that provides the location of an in-range device to the system  10  as an approximation of the mobile wireless device&#39;s location. The mobile wireless initiates an emergency call with the VoIP system  10  via a wireless broadband connection (Step  304 ). If the mobile wireless device cannot identify its own location, the mobile wireless device establishes a connection with an in-range device (Step  306 ). For example, the communication processor  20  of the dual-mode mobile device  12  establishes SIP and media connections with the communication processor  44  of the in-range device  42 . The mobile wireless device then acquires location information from the in-range device via the connection between the two devices (Step  308 ). The mobile wireless device provides the acquired in-range device location information to the VoIP system  10  via the connection established between the system  10  and the mobile wireless device resulting from the emergency call (Step  310 ). 
         [0046]      FIG. 10  illustrates an embodiment of processing logic for routing an emergency call received by the VoIP system  10  to an appropriate EAP using location information received from a mobile wireless device placing the emergency call. For example, after the dual-mode mobile device  12  is authenticated by the originating WAP  14 , the device  12  places an emergency call via the wireless broadband connection established by the WAP  14  (Step  312 ). 
         [0047]    In addition to receiving the emergency call, the VoIP system  10  also receives from the dual-mode mobile device  12  solicited or unsolicited location information acquired by the device  12  (Step  314 ). In one example, the device  12  acquires the location information after a user initiates an emergency call via the device  12 , but before the device  12  places the call to the VoIP system  10 . In another example, the device  12  provides location information previously acquired and stored by the device  12 . Regardless of when the device  12  acquires its location, the location information may be automatically provided to the VoIP system  10  as part of the emergency call or may be provided by the device  12  upon request by the VoIP system  10 . The call processing server  18  then directs the emergency call to an EAP that services the geographic area corresponding to the unsolicited location information (Step  316 ), e.g., via the PSTN  28  or the emergency services network  32 . 
         [0048]      FIG. 11  illustrates an embodiment of processing logic for routing an emergency call received by the VoIP system  10  to an appropriate EAP using location information received from a device in-range of a mobile wireless device placing the emergency call. According to this particular embodiment, a mobile wireless device is unable to acquire its location, but is in-range of a device that has or can obtain location information. During an emergency call, the call processing server  18  uses location information acquired from an in-range device as an approximation of the location of the mobile wireless device that placed the emergency call. For example, the processing logic “begins” with the dual-mode mobile device  12  placing an emergency call to the VoIP system  10  via a wireless broadband connection established by the originating WAP  14  (Step  400 ). In addition to receiving the emergency call, the VoIP system  10  also receives from the dual-mode mobile device  12  address information associated with the in-range device  42  and uses the address information to establish a connection with the in-range device  42  (Step  402 ). The VoIP system  10  then acquires location information from the in-range device  42  via the newly established connection between the system  10  and the in-range device  42  (Step  404 ). The call processing server  18  directs the emergency call to an EAP that services the geographic area corresponding to the in-range device location information (Step  406 ), e.g., via the PSTN  28  or the emergency services network  32 . 
         [0049]      FIG. 12  illustrates an embodiment of processing logic for re-directing an incoming emergency call received by the VoIP system  10  when the system  10  is unable to acquire location information associated with the emergency call. The processing logic “begins” with the VoIP system  10  receiving an emergency call placed by a mobile device capable of both cellular and wireless communication such as the dual-mode mobile device  12  (Step  500 ). Upon receiving the emergency call, the call processing server  18  determines whether a location associated with the emergency call is identifiable (Step  502 ), e.g., by one or more of the embodiments described herein. If a location is identifiable, the call processing server  18  routes the emergency call to an appropriate EAP (Step  504 ). 
         [0050]    If the location is unidentifiable, i.e., the call processing server  18  is not able to determine the location or an approximate location of the dual-mode mobile device  12 , the emergency call is re-directed to an alternate carrier such as a cellular carrier associated with the cellular network  38  (Step  506 ). In one example, the call processing server  18  provides a call redirection instruction to the dual-mode mobile device  12  after the server  18  determines that the location of the device  12  is unidentifiable, thus instructing the dual-mode device  12  to re-direct the emergency call. In another example, the dual-mode mobile device  12  recognizes that it cannot acquire its location, and in doing so, re-directs the call to the cellular network  38  without instruction from the call processing server  18 . 
         [0051]    The communication processor  20  manages emergency call redirection in the dual-mode mobile device  12 . When the location of the dual-mode mobile device  12  is unidentifiable, the communication processor  20  establishes a cellular communication channel with the cellular network  38 , as illustrated by Step  508  of  FIG. 13 . In one example, the call processing server  18  of the VoIP system  10  provides a call redirection instruction to the dual-mode mobile device  12 , causing the communication processor  20  to “re-direct” the emergency call by placing a subsequent emergency call over the cellular network  38 . In another example, the communication processor  20  recognizes that it cannot acquire the location of the dual-mode mobile device  12 , and in doing so, generates an internal call redirection instruction causing the dual-mode device  12  to “re-direct” the call without instruction from the call processing server  18 . Regardless of how a call redirection instruction is generated, the communication processor  20  “re-directs” the emergency call by placing a subsequent emergency call over the cellular network  38  in response to a call redirection instruction, as illustrated by Step  510  of  FIG. 13 . As such, the emergency call is serviced by a cellular-based system (not shown) when the location of the dual-mode mobile device  12  is unidentifiable. Those skilled in the art will appreciate that the communication processor  20  can establish a cellular communication channel while maintaining a call connection with the VoIP system  10  if the WLAN and cellular radios  36 ,  40  do not substantially interfere with each other. 
         [0052]    With the above embodiments in mind, it should be understood that emergency call routing in VoIP systems as taught herein provides for a VoIP system, e.g., the system  10  that is configured to route an emergency call placed by a mobile wireless device to an EAP that services a geographic area corresponding to an approximate location of the mobile wireless device. The VoIP system is also configured to re-direct emergency calls received from dual-mode mobile devices over a cellular network when the calls lack location information sufficient for the VoIP system to route the calls to appropriate EAPs. 
         [0053]    Thus, while the invention has been described in terms of specific embodiments, it should be understood that the present invention is not limited by the foregoing description, nor is it limited by the accompanying drawings. Instead, the present invention is limited only by the following claims and their legal equivalents.