Abstract:
A method and device may be used for selecting a network for initiating an emergency call. The network selection may be based on stored information and/or a network connection status. The stored information may include a network priority for placing the emergency call.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/124,719, filed May 9, 2005, which issued on Mar. 27, 2012 as U.S. Pat. No. 8,145,182, which claims the benefit of U.S. Provisional Application No. 60/569,014, filed May 7, 2004, which is hereby incorporated by reference. 
    
    
     FIELD OF INVENTION 
     The present invention generally relates to wireless local area networks (WLANs), and more particularly, to supporting emergency calls in a WLAN. 
     BACKGROUND 
     Existing 802 technology (802.11 WLANs, 802.15 wireless personal area networks (WPANs), etc.) traditionally does not have to support emergency calls like cellular does. For cellular, support of emergency calls often resulted from regulatory requirements imposed on the technology and is therefore widely implemented in most of today&#39;s deployed wireless cellular networks and handsets. Support for emergency calls involves many aspects across all communication layers, especially signaling support and mandated procedures, which are non-existent for 802.11 and 802.15 technologies. With the advent of voice over Internet Protocol (VoIP) in WLANs and increased everyday usage of WLANs, support for emergency calls in WLANs will become necessary. 
     Even “fixed” VoIP phone service offerings for the residential market have limited emergency call support. Number location information cannot always be tracked by a dispatcher in a public safety answering point (PSAP), call back is not always possible, and address registration may be required upon purchase of the equipment. When the VoIP phone is moved to a new location, the emergency call will still be sent based on the registered address location. The registered address can be changed in principle, but delays are at least on the order of days or weeks in updating the information at the PSAP. In addition, some users might not update their registration information in a timely manner, if at all. 
     This situation worsens with more mobility as enabled by VoIP phones using WLANs. WLAN based VoIP phones can work from any location and the user can be expected to roam seamlessly between locations, such as from an office to a home to public hotspots, etc. 
     Certain 802.11-specific issues exist, including radio access, access point (AP) location, caller location, and emergency call admission. In regard to radio access, no priorities for emergency calls currently exist in the 802.11 standards, and there is no means to distinguish an emergency call from a regular call for the WLAN access network. The location of an AP or a STA is currently unknown to the network in a non-proprietary manner, even if for example, the AP&#39;s identification can easily be determined. It is also not currently possible to map the caller&#39;s location in a non-proprietary manner. 
     In regard to admission, a tightly managed WLAN may prevent emergency callers from establishing an emergency call if the caller is not authorized to enter the network. The normal connection procedure between a STA and an AP requires the STA to send an Association request, followed by negotiation with the AP prior to associating the STA to the AP. If the STA is unable to indicate that it is making an emergency call, it would have to go through the entire association procedure to determine if it could be admitted. As an example of this type of difficulty, if a STA does not have the proper password or authentication credentials to access the system (if the AP is configured to require passwords or require authentication credentials, as might exist for example with a private hotspot or enterprise/office WLAN), the AP will bluntly refuse the STA&#39;s association request. But even if the STA has the proper password or authentication credentials, the AP could still refuse admittance to the network based on its configured maximum capacity for voice users. In this case, the right decision for the AP would be to admit this new emergency call (at the highest priority) and to discontinue another existing voice call. Because the AP currently lacks means to make this distinction in the first place, such a feature cannot be implemented with existing state-of-the-art WLAN technology. Contrast this with the operation of a cellular system, in which any device can make an emergency call, even a device without a SIM card. 
     SUMMARY 
     A method and apparatus may use various system operation aspects for enabling emergency call handling support with 802.11 and 802.15 technology. Some of the embodiments may pertain to new L2 signaling messages or information elements to indicate emergency calls to APs. New procedures and control mechanisms may be used for emergency situations. In addition, procedures for dual-mode (WLAN and second generation (2G) or third generation (3G) cellular) implementations are addressed. Because emergency call requirements are often coupled to regulatory requirements on location reporting of the position of the emergency caller, one or more devices and signaling procedures may be used to allow requesting and reporting of geographic positions in a WLAN network. The position information may be coupled to emergency calls or may be implemented separately. 
     A benefit of a STA being able to identify an emergency call is that simple logic may be installed at the AP that allows the AP to distinguish between a STA that it should treat normally (i.e., should follow regular association procedures) and a STA that should be admitted under all circumstances, regardless of how the network is configured (i.e., bypass any security requirements to admit an emergency call). 
     A method and device may be used for selecting a network for initiating an emergency call. The network selection may be based on stored information and/or a network connection status. The stored information may include a network priority for placing the emergency call. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more detailed understanding of the invention may be had from the following description of a preferred embodiment, given by way of example, and to be understood in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a diagram of a standard medium access control (MAC) frame; 
         FIG. 2A  is a diagram of a MAC frame with a bit flag indicating an emergency call; 
         FIG. 2B  is a diagram of a MAC frame with an information element (IE) indicating an emergency call; 
         FIG. 3  is a diagram of a standard ready to send (RTS) frame; 
         FIG. 4A  is a diagram of a RTS frame with a bit flag indicating an emergency call; 
         FIG. 4B  is a diagram of a RTS frame with an IE indicating an emergency call; 
         FIG. 5  is a flowchart of a method for using a RTS frame as shown in  FIG. 4A  or  4 B; 
         FIG. 6  is a flowchart of a method for switching radio technologies to complete an emergency call; 
         FIG. 7  is a diagram of a SOS beacon frame indicating an emergency call; 
         FIG. 8  is a flowchart of a method for transmitting and using the SOS frame shown in  FIG. 7 ; and 
         FIG. 9  is a flowchart of a method to determine whether to apply a proxy function. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereafter, the term “station” (STA) includes, but is not limited to, a wireless transmit/receive unit, a user equipment, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, the term “access point” (AP) includes, but is not limited to, a base station, a Node B, a site controller, or any other type of interfacing device in a wireless environment. 
     The present invention is applicable to all WLANs, personal area networks (PANs), and metropolitan area networks (MANs), but in particular to 802.11-based WLANs, 802.15-based wireless PANs, 802.16/20 based wireless MANs, and equivalents thereto. In one realization, the present invention is applicable to WTRUs implementing a combination of these access technologies including WLAN, PAN, MAN, and cellular multi-mode WTRUs. 
     The present invention for handling emergency support will be described hereinafter as being grouped into three major areas. This is, however, for convenience of explanation and should not be taken as a limitation of the invention. 
     I. Air Interface Related Signaling/Support and Procedures 
     A. Indication of Emergency Calls in MAC Frames and MAC Signaling Messages 
     A standard MAC frame  100  is shown in  FIG. 1 . The MAC frame  100  includes a frame control field  102 , a duration/ID field  104 , one or more address fields  106   a - 106   d , a sequence control field  108 , a quality of service (QoS) control field  110 , a frame body  112 , and a frame check sequence (FCS) field  114 . The QoS control field  110  is divided into a plurality of subfields, as shown. 
     A priority for emergency calls can be indicated in MAC frames by a bit flag, by an emergency message type IE, by an emergency message field part on an existing or new IE, or by an emergency call code implemented using a reserved (currently unused) value in any existing IE or field of a MAC frame. The indicator allows an AP to know that it needs to admit the emergency call. For similar purposes, QoS priorities or requirements are indicated by means of QoS classes (for example DiffServ). Any existing MAC frame type (control, management, or data) can be modified to include the emergency call indicator. The emergency call indicator can be added to any location in the MAC frame, in the header or the body using any of the mechanisms described. 
     As shown in  FIG. 2A , a MAC frame  200  includes fields  202 - 214 , which are the same as fields  102 - 114  described above in connection with  FIG. 1 . In one embodiment, a simple bit flag  220  is used to indicate to the receiver that this is an emergency call. As shown in  FIG. 2A , one possible location for the bit flag  220  is in the reserved bit (bit  7 ) of the QoS control field  210 . A person of ordinary skill in the art would note that it is possible to place the bit flag  220  in any currently reserved location in any of the existing header or frame body fields in the MAC frame. 
     As shown in  FIG. 2B , a MAC frame  250  includes a frame control field  252 , a length field  254 , and an emergency call IE  256  to indicate an emergency call. The emergency call IE  256  can include, but is not limited to, an emergency call flag  260 , a reason code field  262 , a capability information field  264 , a location information field  266 , a voice codec application field  268 , and additional fields  270 . The emergency call IE  256  can be added to any MAC frame. In addition, the information contained in the emergency call IE  256  can be added to an existing IE type. 
     The emergency call flag  260  can be a simple indicator (e.g., a bit flag) to identify that the call is an emergency call. The reason code field  262  indicates the reason for the emergency call (e.g., fire, medical emergency, etc.). The capability information field  264  includes the capabilities of the STA placing the emergency call and is used to assist in completing the emergency call as quickly as possible. The location information field  266  contains the location of the STA placing the emergency call. The voice codec application field  268  identifies the voice codec used by the STA, and is used in case there is any incompatibility between the STA and the AP attempting to handle the emergency call. Additional information that can be included into the emergency call IE (as fields  270 ) are timestamps and WTRU and/or operator service capability information. 
     Existing MAC frames under 802.11e have call priorities. The Transmission Specification (TSPEC) IE includes a three bit priority subfield in a transmission specification information field. The principles of the present invention can also be implemented in the TSPEC IE by defining a value for an emergency call. In cellular systems, a similar mechanism (a signaling frame) is used to send the call parameters to the network and includes a reserved field to identify emergency calls. 
     B. Location Information 
     Location information can also be attached to these new MAC frames  200 ,  250  (e.g., in the location information field  266 ), in addition to conveying the emergency call establishment reason. For example, the AP or STA can use the basic service set (BSS) ID, the AP or STA MAC addresses, static or dynamically assigned IP addresses, or global positioning system (GPS) information from an AP or STA implementing this functionality, and forward this information to the emergency call center. It is noted that the location information can also be conveyed separately from the emergency call information. 
     Other means of locating the emergency STA include, but are not limited to, identifying the STA placing the emergency call by caller ID, utilizing a callback number, and using known addresses to help locate the STA by the emergency call center (e.g., using the MAC addresses of the current point of attachment for the STA, such as the AP or the network ID, or AP geographical coordinates). 
     For example, a MAC signaling mechanism for a WLAN can be used where the AP can request the position from a STA. The STA would report back to the AP with its position. One possible implementation includes the use of assisted GPS (A-GPS) coordinates, that are currently in widespread use in cellular handsets. Multiple positioning methods can be supported for different access networks, including, but not limited to, uplink time difference of arrival (U-TDOA), enhanced observed time difference (E-OTD), idle period downlink observed time difference of arrival (IPDL-OTDOA), A-GPS, Universal Geographic Coordinates (for example, as defined in IEEE standard 802.11k or IETF RFC 3825) and methods using WLAN AP locations, cell site, or sector information and timing advance or roundtrip time measurements. While the preceding examples of conveying location information have been particularly mentioned, one skilled in the art would note that any format to convey geographic coordinates can be used. 
     The emergency call functions can be performed independently from (yet complementary with) the location reporting functions. To illustrate, it is possible to: (1) attach location information to the emergency call signaling frames when the STA actually issues the emergency call, and (2) signal location updates as a stand-alone functionality without an emergency call. An example of the latter would be to keep the AP informed and updated about the latest STA position either periodically (e.g., every few seconds), polled as part of the AP background operation, or by unsolicited regular location reporting by the STA to the AP. Maintaining location information at the AP may be preferable because when the STA issues the emergency call, the AP already has a reasonably recent estimate of the STA&#39;s position, such that the STA is not required to explicitly piggyback its location onto the emergency call request. 
     For example, such stand-alone STA location information reporting can be used to allow implementation of location-dependent services in a WLAN network, in parallel to addressing regulatory requirements. 
     As such, the position information can also be provided to location services (LCS) applications existing within an interworking WLAN (I-WLAN), public land mobile network (PLMN), or in the STA. In addition, the originating party&#39;s serving cell identity or serving AP identity can be provided to the LCS client. 
     C. Extend the Existing RTS/CTS Frame Exchange Mechanism and Procedure 
     A standard RTS frame  300  is shown in  FIG. 3 . The RTS frame  300  includes a frame control field  302 , a duration field  304 , a receiver address (RA) field  306 , a transmitter address (TA) field  308 , and a FCS field  310 . 
     A STA that wants to transmit an emergency call transmits an extended RTS frame  400  containing a special signaling flag as shown in  FIG. 4A  or an extended RTS frame  450  containing a new IE as shown in  FIG. 4B . 
       FIG. 4A  shows an RTS frame  400 . Fields  402 - 410  of the RTS frame  400  are the same as fields  302 - 310  of the RTS frame  300  described above in connection with  FIG. 3 . The frame control field  402  has several subfields, including a protocol version subfield  412 , a type subfield  414 , a subtype subfield  416 , a to distribution system (DS) subfield  418 , a from DS subfield  420 , a more fragments subfield  422 , a retry subfield  424 , a power management subfield  426 , a more data subfield  428 , a Wired Equivalent Privacy (WEP) subfield  430 , and an order subfield  432 . 
     The signaling flag can be added to any reserved bit in the RTS frame  400 . Potential locations for the reserved bit include the protocol version subfield  412 , the type subfield  414 , and the subtype field  416 . It is noted that one skilled in the art could place the signaling flag in any reserved bit in the RTS frame  400 . 
       FIG. 4B  shows an extended RTS frame  450 , including a frame control field  452 , a duration field  454 , a RA field  456 , a TA field  458 , a purpose IE  460 , and a FCS field  462 . The purpose IE  460  can be similar in content to the emergency call IE  256 , described above. All STAs receiving the extended RTS frame  450  are then required to stop any transmission attempt for a pre-determined amount of time to idle the wireless medium and to give the STA in emergency an opportunity to transmit. 
     In one embodiment, upon receipt of an extended RTS frame, the receiving STAs enter a modified backoff process in order to give the STA placing the emergency call a higher probability to succeed in obtaining access to the medium. Two implementations of modifying the backoff process are possible: (1) shortening the backoff time for the STA placing the emergency call relative to other STAs, or (2) lengthening the backoff time for non-emergency STAs. In either implementation, the end result is that the emergency STA has a shorter backoff time than non-emergency STAs. 
     A method  500  for using the RTS frame  400  or  450  is shown in  FIG. 5 . The purpose of the method  500  is to idle the transmission medium in order to permit a STA to transmit an emergency call. The method begins with a STA placing an emergency call by sending a RTS frame  400  or  450  (step  502 ). An AP receives the RTS frame (step  504 ) and responds to the STA with a standard CTS frame (step  506 ). The backoff type to be used by the AP is determined (step  508 ). There are two possible backoff types, both of which would permit the STA placing the emergency call to access the medium prior to all other STAs waiting to transmit. 
     If the backoff type is that the STA in emergency (i.e., the STA placing the emergency call) has a shorter backoff time, then the STA in emergency waits for the shortened backoff time (step  510 ) and then transmits the emergency call (step  512 ). All other STAs that are attempting to access the medium wait for the standard backoff time (step  514 ) and are then able to transmit (step  516 ). The method then terminates (step  518 ). 
     If the backoff type is that all other STAs have a longer backoff time (step  508 ), then the STA in emergency waits for the standard backoff time (step  520 ) and transmits the emergency call (step  522 ). All of the other STAs wait for a longer backoff time (step  524 ) and are then able to transmit (step  516 ). The method then terminates (step  518 ). 
     In general, when a STA enters a backoff procedure, the STA attempts to transmit randomly in one out of a series of N timeslots. If there is a transmission collision, the STA will backoff again and increase the value of N, up to a predetermined maximum value for N. Before a STA can attempt to transmit, the STA must wait for M timeslots. This basic procedure provides any STA an equal chance of winning access to the medium. In 802.11e, to implement QoS, there are two ways to ensure that a particular station has a greater chance of winning access to the medium. The first is to reduce the value of M, thereby giving the STA a shorter wait time. The second is to use a smaller value for N, which increases the chances that a STA will be able to transmit in a particular timeslot. 
     In the method  500 , there are several possible means for a STA to know which backoff value to use. A first means is to use hard-coded values for M and N in connection with an emergency call, such that these hard-coded values for M and N will be used by a STA in emergency. A second means is to explicitly signal values for M and N from the AP to the STA in emergency. The AP would typically send those parameters to the STA by either using broadcast or dedicated management frames during normal system operation. STAs read the emergency call-related configuration parameters to be used in case they need to set up an emergency call. One example is that the AP, as part of the Beacon or Probe Response management frames, sends other BSS configuration values to all STAs in its BSS. Adding the emergency call-related M and N parameters is a natural extension to these. For example, 802.11e QoS-related configuration parameters per access category (backoff values, windows, etc.) to be used by all STAs in the BSS are signaled today by the AP using a similar mechanism. 
     A third means is a combination of the first and second means, by which a STA has hard-coded default values for M and N that it uses normally, and if the STA is in emergency, the AP will signal new values for M and N to override the hard-coded default values. One skilled in the art could envision additional means for communicating the appropriate backoff times to a STA in emergency and all other STAs seeking access to the medium. 
     D. Mandated Switch-Over to Another Radio Technology for Dual-Mode WLAN STAs (for Example 3G and WLAN) 
     In case of an emergency, a dual-mode WLAN STA will attempt any emergency call first on the cellular network, instead of on the WLAN. This is in principle a “hard-coded” procedure in the STA alone. A method  600  for implementing this procedure is shown in  FIG. 6 . 
     The method  600  begins by the user making an emergency call at the STA (step  602 ). A determination is made whether the STA is capable of operating on a cellular network or a WLAN (step  604 ). If the STA is operating on a cellular network (i.e., is currently connected to the cellular network), then the STA remains on the cellular network for the emergency call (step  606 ). If the STA is capable of operating on the cellular network, but is not presently connected to the cellular network, then the STA establishes a connection to the cellular network (step  608 ) and makes the emergency call on the cellular network (step  606 ). If the STA is operating on a WLAN, then the STA switches to the cellular network to make the emergency call (step  610 ). 
     After the emergency call has been placed, a determination is made whether the emergency call went through on the cellular network (step  612 ). If so, then the method  600  terminates (step  614 ). If the emergency call did not go through on the cellular network, then the STA switches to the WLAN to make the call (step  616 ) and the method terminates (step  614 ). 
     In case an emergency call needs to be issued by a dual-mode WLAN-cellular handset, the preferred procedure is to have the handset fallback onto the cellular modem (i.e., establish the emergency call on a cellular radio link), because emergency call support may not be available or may be less reliable over the WLAN. 
     Alternatives for the method  600  include: (1) establishing a preferred, mandated, or recommended order of radio technologies (e.g., WLAN or cellular) to switch to when attempting to send an emergency call; (2) the system operator configures emergency call behavior on a SIM card or similar device for dual mode handsets; (3) maintain a VoIP call on the cellular network or move the call onto a traditional circuit switched voice channel in case of an emergency; (4) the system operator signals a preferred local order of radio technologies over the wireless interface; or (5) the user manually configures the policy setting. 
     E. Bypassing Authentication and Security when Attempting an Emergency Call 
     A procedure is mandated that any 802.xx STA seeking to establish an emergency call in a WLAN must be allowed by the AP. This includes bypassing authentication like 802.1x and other security measures on the network side. This procedure could be triggered by using the extended RTS/CTS method  500  (as shown in  FIG. 5 ) or by a bit flag, IE, header, reserved information field, or bit/sequence value in the MAC frame (as shown in  FIGS. 2A and 2B ). 
     II. WTRU Behavior/Procedure in Case of Emergency 
     A. WLAN Sends SOS Beacon Signal to Facilitate in Finding the Caller 
     A procedure is mandated in the STA or configured by the network that once the emergency call is over (or even during), the STA and/or the involved AP start transmitting SOS type signaling frames  700  as shown in  FIG. 7  on regular intervals. 
     An SOS signaling frame  700  is a modified version of a Probe Request frame. The SOS signaling frame  700  includes a frame control field  702 , a duration field  704 , a destination address (DA) field  706 , a source address (SA) field  708 , a BSSID field  710 , a sequence control field  712 , a SSID IE  714 , a supported rates IE  716 , and an emergency call IE  718 . The emergency call IE  718  can be the same as the emergency call IE  256  described above in connection with  FIG. 2B . It is noted that the supported rates IE  716  is optional, and can be eliminated from the SOS signaling frame  700  without affecting its functionality. 
     In one embodiment, the SOS signal frame can be defined as a Probe Request frame sent with short interframe space (SIFS) priority or priority interframe space (PIFS) priority, to ensure access to the medium. The SOS signal frame contains new emergency call related elements in emergency call IE, such as 911 ID (e.g., caller ID), equipment details (such as international mobile equipment identification (IMEI)), network affiliations, user name, and emergency reason code. The reason code may be obtained by the device prompting the user to identify the reason for the emergency call (e.g., “press 1 if fire emergency”, etc.). A reason code would provide some ability to handle the emergency in case there is no way to terminate the in-progress call. 
     The SOS signal frame can be scheduled for transmission every 100 mSec or so to facilitate location logging and tracking. An AP would be required to log any SOS signal frame reception with a timestamp and details of the signal. The signal strength details include signal strength, signal quality, antenna azimuth and gain, and caller details such as IMEI, user name (if available), and other 802.11 device information useful for identity and capability purposes. An AP receiving an SOS signal frame would also be required to report the event to an emergency network node responsible for emergency response, radio resource coordination, location, and tracking of the calling device. 
     This is an active probing mechanism where the SOS signaling frames are sent and can be received by emergency workers as they approach the caller. One analogy is the emergency beacon in the black box of an airplane. A new MAC frame can be introduced for this purpose or an existing MAC frame, for example a probe request frame, can be extended by new IEs (such as the emergency call IE  256 ) to fulfill this purpose. 
     A method  800  for utilizing a SOS signaling frame is shown in  FIG. 8 . The user makes an emergency call from a STA (step  802 ). The STA begins to transmit SOS frames (step  804 ). Based on the desired implementation, the SOS frames can either be sent as probes or be used to establish a direct connection with an emergency worker (step  806 ). 
     If the SOS frames are to be sent as probes, a transmission period is set and a determination is made whether the end of the transmission period has been reached (step  810 ). If the transmission period has not ended, then the STA continues transmitting the SOS frames (step  812 ) and the method returns to step  810 . If the end of the transmission period has been reached (step  810 ), then the STA stops transmitting the SOS frames (step  814 ) and the method terminates (step  816 ). 
     If the SOS frames are to be used to establish a direct connection with an emergency worker (step  806 ), then a determination is made whether the emergency worker in within range of the STA (step  820 ). If the emergency worker is not within range of the STA, then the STA continues transmitting SOS frames (step  822 ) and the method continues with step  820 . If the emergency worker is within range of the STA (step  820 ), then the STA stops transmitting SOS frames and establishes a direction connection between the caller and the emergency worker (step  824 ) and the method terminates (step  816 ). 
     In a first alternative (steps  810 - 814 ), the SOS frames transmitted by the STA could be triggered by signaling from the AP or higher layer protocols like session initiation protocol (SIP) once the emergency call is over. The duration/frequency of the SOS frames are contained in this trigger signal. Sending the SOS frames after the emergency call is over prevents transmission of unnecessary SOS frames in case the emergency call was a mistake or if there was no need for an emergency worker to come in response to the call. 
     In a second alternative (steps  820 - 824 ), a direct VoIP connection between the emergency worker and the caller is established when they are within range of each other. Other STAs that hear the SOS frames may treat the SOS frames like the extended RTS frame described above in connection with  FIGS. 4A ,  4 B, and  5  (i.e., the other STAs will not try to access the medium, so that the emergency caller has better access to the bandwidth). 
     B. Network (for Example, the AP) Implements a Call Back Functionality to Handle Emergency Calls 
     Once an emergency call is established, the WLAN maintains an active connection to the user that initiated the emergency call for a certain period after the emergency call is over in case of call back. This functionality may be transparent to the user. 
     III. Functionality in the Infrastructure 
     A. Proxy Function 
     A method  900  for determining whether an AP needs to act as a proxy for the STA is shown in  FIG. 9 . The STA makes an emergency call (step  902 ) and the AP receives the emergency call (step  904 ). A determination is made whether the STA has the capabilities to complete the emergency call based on the network used to carry the call (step  906 ). The AP checks if the STA has all required functionality (e.g., SIP/H.323 protocol termination, vocoder, etc.) to support the call. This information may be indicated as part of the MAC frame (e.g., MAC frames  200 ,  250 ) or it may be part of the subscriber information in the network that the AP can access. 
     If the STA has all of the necessary capabilities, then the STA proceeds with the call as normal (step  908 ). The AP can add location information to the call as necessary, including the location of the STA and/or the location of the AP (such as the network ID, the AP&#39;s MAC address, etc.) (step  910 ). The method then terminates (step  912 ). 
     If the STA does not have all of the necessary capabilities to complete the call (step  906 ), then the AP acts as a proxy for the STA, providing any necessary functionality (step  914 ). The AP adds location information to the call as needed (step  910 ) and the method terminates (step  912 ). 
     If the AP determines that the STA does not have all the required functionality to fully complete the emergency call in the current environment, then the AP will act as a proxy for the STA (step  914 ). For example, if the STA does not have SIP protocol support, then the AP can act as a SIP proxy for the STA. As another example, if the STA has SIP support but the network only supports H.323, then the AP can interwork the SIP messages from the STA to H.323 messages to the rest of the network. In the extreme case that the STA does not even have a vocoder, the AP can download a thin vocoder client to the STA and interwork to more standard vocoders elsewhere in the network. 
     Another method is that the AP spoofs (i.e., reads the contents and/or type information even if it is officially not supposed to) on the contents of the IP packets that are used for signaling or normal traffic by the STA and its correspondent in the network. For example, SIP signaling protocol messages over IP are typically used today for call handling. Such SIP signaling contains useful information, such as capability information and destination addresses, for the AP to fulfill its role as proxy. In addition to the previously described methods, if the AP extracts such information from spoofing on the STA-remote destination higher layer (i.e., above L2 MAC) message contents, it can fulfill its role more efficiently. A person skilled in the art would recognize that SIP is one example of a management protocol for IP-based calls, and that other equivalent protocols exist and are widely used in the industry. Therefore, this method is not limited to SIP alone. 
     B. Linking an AP to an Emergency Call Center 
     Once an AP learns of a STA making an emergency call, the AP needs to establish a link to an emergency call center in order to properly route the call from the STA. There are several possible transfer mechanisms to get the emergency call from the AP to the call center. For example, the AP can communicate with a gateway, linking it to the call center. 
     The emergency network node concept can be extended to include an emergency response operations center with man-in-the-loop capability. The emergency network node would be an extended service set (ESS) or network tailored to the infrastructure application. For example, on a university campus, the designated emergency network node would be the campus police department. As another example, in a manufacturing plant, the emergency network node would be the security office. The emergency network node would include an operator who would receive the VoIP call, log call information, screen calls, and then place an emergency call on a public switched telephone network (PSTN) to alert the appropriate authorities. 
     The emergency network node concept can be further extended to include an automated node with a direct line to a PSTN. The automated node would act as voice circuit bridge to dial and connect the wireless caller to the PSTN emergency center. 
     The method for connection to the emergency network node could be extended to include the ability to route and handle a call without authentication, authorization, or security features. This would permit a direct unencrypted connection or a tunneled connection between the wireless caller and the emergency network node. 
     The function of the emergency network node could be extended to include call handing, call handoff, and roaming coordination. This functionality would preauthorize resources in neighbor APs (APs adjacent to the AP serving the wireless call), so that the caller may roam without losing the wireless connection and without the need to reestablish a new emergency call when moving across AP boundaries, thus eliminating duplicate calls on same emergency. 
     The concepts of the present invention can be extended beyond the specific examples illustrated above. For example, the present invention can be extended to mesh networks and ad-hoc networks. As an alternative, instead of a human user, the present invention can be extended to machine-to-machine usage scenarios for emergency handling with WLANs. One possibility would be using 802.11 in home security systems, i.e., a WLAN is used in place of hard wired telephone lines (that can be cut). In this example, instead of a human user generating a WLAN emergency call, the home security system auto-generates an emergency call to a security call center when someone breaks in. Alternatively, the home security system could start sending emergency SOS frames, as described above. 
     Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone (without the other features and elements of the preferred embodiments) or in various combinations with or without other features and elements of the present invention.