Abstract:
A system and method in a radio telecommunications network for reconnecting an emergency call between a roaming mobile station with an emergency (emergency MS) and a public safety answering point (PSAP) when the call has been disconnected before the call was finished. The network includes a serving mobile switching center (MSC) serving the emergency MS, a border MSC bordering the serving MSC, and a home location register (HLR) that stores features and subscription data for the emergency MS. When the emergency call begins, a hotlining feature for the emergency MS is activated in the HLR which automatically reconnects the emergency MS to the PSAP when the MS accesses the network through a registration or by originating a subsequent call. Inter-exchange messages ensure that the emergency MS is automatically connected to the PSAP, even if the emergency MS accesses the network in the border MSC.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     This invention relates to telecommunication systems and, more particularly, to a system and method of handling emergency calls from roaming mobile stations in a radio telecommunications network. 
     2. Description of Related Art 
     When a mobile subscriber initiates an emergency call such as a 911 call, the call is connected through a serving mobile switching center (MSC) to an end office (EO) known as a Public Safety Answering Point (PSAP). There may be situations in which the call is then prematurely disconnected. This disconnection may be due to the subscriber hanging up, loss of radio signal, border rescan problems, loss of the subscriber&#39;s mobile station (MS) during an attack, or other reasons. Current and previous versions of TR45.2 (PN 3581) state that under these circumstances, the serving MSC attempts to reestablish the call. In other words, the serving MSC pages for the MS, and if a page response is received, an alert signal is sent to the responding MS. If the alert signal is answered, the call is reconnected to the PSAP. 
     However, in existing networks, it is currently not possible to reconnect the emergency MS under certain circumstances. For example, the emergency MS may immediately originate another call to a different destination before the emergency call can be reconnected to the PSAP. Also, the MS may have been operating near the border of the serving MSC when the emergency call was dropped, and by the time the serving MSC attempts to reestablish the call the MS may have travelled into a bordering MSC. In this case, the emergency call cannot be reestablished. 
     There are no known prior art teachings of a solution to the aforementioned deficiency and shortcoming. In order to overcome the disadvantage of existing solutions, it would be advantageous to have a system and method of handling emergency calls in a radio telecommunications network in which an emergency MS is reconnected to the PSAP following a premature disconnection. The invention would reconnect the call to the PSAP even if the emergency MS attempts to originate a different call, or if the emergency MS has travelled into the service area of a bordering MSC. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention is a method in a radio telecommunications network of reconnecting an emergency call between a mobile station with an emergency (emergency MS) and a public safety answering point (PSAP) when the call has been disconnected before the call was finished. The network includes a serving mobile switching center (MSC) serving the emergency MS, and a home location register (HLR) that stores features and subscription data for the emergency MS. The method comprises the steps of activating a hotlining feature in the HLR when the emergency call is begun, determining that the emergency call was disconnected before the call was finished, determining that the emergency MS attempts to access the network after the emergency call was disconnected, and automatically connecting the emergency MS to the PSAP. 
     The network may include a plurality of MSCs, and the emergency MS may register in a second MSC other than the serving MSC after the hotlining feature has been activated. The method then includes the steps of notifying the HLR that the emergency MS has registered in the second MSC, determining in the HLR that the emergency MS has an unfinished emergency call, and instructing the second MSC to connect the emergency MS to the PSAP. 
     If the emergency MS registers within the service area of a second MSC other than the serving MSC before the hotlining feature has been activated, the method includes the steps of sending a registration notification (REGNOT) message from the second MSC to the HLR, sending a registration cancellation (REGCANC) message from the HLR to the previous serving MSC identifying the emergency MS, and sending a return REGCANC message from the previous serving MSC to the HLR identifying that the emergency MS has an unfinished emergency call and providing a PSAP number to the HLR. This is followed by sending a qualification directive (QUALDIR) message from the HLR to the second MSC with the PSAP number and instructions to connect the emergency MS to the PSAP. Thereafter, when a registration or a call origination is received from the emergency MS in the second MSC, the call is automatically connected to the appropriate PSAP. 
     In another aspect, the present invention is a system in a radio telecommunications network for reconnecting an emergency call between an emergency MS and a PSAP when the call has been disconnected before the call was finished. The system comprises means within the HLR for activating a hotlining feature for the emergency MS when the emergency call is begun, means within the serving MSC for determining that the emergency call was disconnected before the call was finished, means within the serving MSC for determining that the emergency MS attempts to access the network after the emergency call was disconnected, and means within the serving MSC for automatically connecting the emergency MS to the PSAP. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood and its numerous objects and advantages will become more apparent to those skilled in the art by reference to the following drawings, in conjunction with the accompanying specification, in which: 
     FIG. 1 (Prior Art) is a simplified block diagram of an existing radio telecommunications network in which an emergency call is prevented from being reestablished with an emergency MS due to the MS moving out of the service area of the serving mobile switching center (MSC) that first established the emergency call; 
     FIG. 2 is a simplified block diagram of a radio telecommunications network in which the system of the present invention has been implemented; 
     FIG. 3 is a message flow diagram of the messages utilized to set up a hotline between the emergency MS and the PSAP when the emergency MS originates another call before the emergency call can be reconnected; 
     FIG. 4 is a message flow diagram of the messages utilized to set up a hotline between the emergency MS and the PSAP when the emergency MS roams into the service area of a bordering MSC before the emergency call can be reconnected; and 
     FIG. 5 is a message flow diagram of the messages utilized to set up a hotline between the emergency MS and the PSAP when the emergency MS roams into the service area of the bordering MSC and registers before the emergency call can be reconnected and before the serving MSC has notified the HLR of the unfinished emergency call. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     FIG. 1 is a simplified block diagram of an existing radio telecommunications network  10  in which an emergency call is prevented from being reestablished between an emergency MS  11  and a Public Safety Answering Point (PSAP)  12  due to the MS moving out of the service area  13  of the serving mobile switching center MSC)  14  that first established the emergency call, and out of range of base station  15  . If the call is prematurely disconnected while the MS is in the service area  13 , and then the MS  11  travels into the service area  16  of a bordering MSC  17 , then the emergency call cannot be reestablished by the serving MSC  14 . The emergency MS must dial 911 again to gain access to the network through the BS  18  and the bordering MSC  17 . In this case, the call will probably be connected to a different PSAP or PSAP operator who is not familiar with the emergency, and any information that was passed before the call was disconnected must be repeated. 
     FIG. 2 is a simplified block diagram of a radio telecommunications network  20  in which the system of the present invention has been implemented. An emergency MS  21  is shown to be operating in the service area  23  of a serving MSC  24  and BS  25 . The serving MSC is connected to a PSAP  22 . The service area  23  is bordered by service area  26  of a bordering MSC  27  and BS  28 . The serving MSC includes a reconnection timer  29  and a signaling mechanism  30 , the functions of which are described below. Each of the MSCs is connected to a home location register (HLR)  31  which contains a subscriber profile  32  for the emergency MS  21 . A new subscriber category (Hot Line)  33  is added to the subscriber profile. The function of the Hot Line category is described below. 
     FIG. 3 is a message flow diagram of the messages utilized to set up a hotline between the emergency MS  21  and the PSAP  22 . In this embodiment, the present invention solves the problem associated with the emergency MS immediately originating a call to another destination after the emergency call is disconnected, thereby preventing the original emergency call from being reestablished. In cellular telecommunication networks, when a MS ends a call, the radio connection is released so that the channel can be reallocated. The MS&#39;s serving MSC can call him back; however, nothing prevents the MS from immediately originating a call to another destination, thereby preventing the original emergency call from being reestablished. 
     The method illustrated in FIG. 3 is a “front-end method” in which the hotlining feature is set up at the front end of the emergency call. The emergency MS  21  initiates an emergency call at  41 , and the serving MSC  24  delivers the call at step  42  to the PSAP  22 . The signaling mechanism  30  implemented in the serving MSC  24  notifies the emergency MS&#39;s HLR  31  of the emergency call origination. This may be accomplished by causing the serving MSC to send a Registration Notification (REGNOT) Invoke message  43  to the HLR whenever an emergency call is originated. The REGNOT Invoke message includes a new parameter such as “started 911 call, PSAP ID/PSAP number”. Alternatively, the serving MSC may send the parameter in a modified Qualification Request (QUALREQ) Invoke message. The HLR sets the Hot Line category  33  for the emergency MS to ON at  44 . The HLR may set up the hotlining feature by allocating the Origination Indicator OI- 8  subscriber category (or equivalent overriding category) in the subscriber profile, along with the indication of the unfinished emergency call and the PSAP&#39;s directory number. This ensures that upon the next call origination from the emergency MS, the call is automatically re-routed to the proper PSAP. The HLR then returns a REGNOT Return Result message  45  to the serving MSC instructing the MSC to hotline the emergency MS to the PSAP. 
     Thereafter, if the emergency call is prematurely disconnected for any reason at  46 , and the serving MSC cannot reestablish the call at  47 , the serving MSC sends a MSInactive Invoke message  48  to the HLR and includes a Deregistration Type parameter set to a new value as “unfinished 911 call”. The MSInactive Invoke message may also include the ID number or directory number of the PSAP which was involved in the call. At step  49 , the HLR marks the subscription as inactive, and sends a MSInactive Return Result message  51  to the serving MSC. Thereafter, at step  52 , the emergency MS may attempt to access the network by re-registering or originating another call. If the emergency MS re-registers, it is paged, and then reconnected to the PSAP at step  53 . If the emergency MS originates another call at  52 , the serving MSC delivers the call at step  53  to the PSAP. 
     At step  54 , the emergency call is properly completed and disconnected. The serving MSC  24  then sends a REGNOT Invoke message  55  to the HLR reporting the finished 911 call. At step  56 , the HLR resets the Hot Line category to OFF, and sends a REGNOT Return Result message  57  to the serving MSC. 
     The HLR may also be implemented with discrimination logic which enables or disables the hot lining feature if additional registrations are received from cloned MSs before the emergency call is properly completed. 
     FIG. 4 is a message flow diagram of the messages utilized to set up a hotline between the emergency MS  21  and the PSAP  22  when the emergency MS roams into the service area  26  of the bordering MSC  27  before the emergency call can be reconnected. As noted above, when an MS ends a call the radio connection is released so that the channel can be reallocated. The MS&#39;s serving MSC can call him back; however, nothing prevents the MS from roaming outside the service area of the serving MSC, thereby preventing the call from being reestablished. 
     The method illustrated in FIG. 4 is also a “front-end method” in which the hotlining feature is set up at the front end of the emergency call. The emergency MS initiates an emergency call at  61 , and the serving MSC  24  delivers the call at step  62  to the PSAP  22 . The serving MSC  24  notifies the emergency MS&#39;s HLR  31  of the emergency call origination by, for example, sending a REGNOT Invoke message  63  from the serving MSC to the HLR whenever an emergency call is originated. The REGNOT invoke message includes a new parameter such as “started 911 call, PSAP ID/PSAP number”. The HLR sets the Hot Line category  33  for the emergency MS to ON at  64  by, for example, allocating the Origination Indicator OI-8 subscriber category. This ensures that upon the next call origination from the emergency MS, the call is automatically re-routed to the proper PSAP. The HLR then returns a REGNOT Return Result message  65  to the serving MSC instructing the MSC to hotline the emergency MS to the PSAP. 
     Thereafter, if the emergency call is prematurely disconnected for any reason at  66 , and the serving MSC cannot reestablish the call at  67 , the serving MSC sends a MSInactive Invoke message  68  to the HLR and includes a Deregistration Type parameter set to a new value as “unfinished 911 call”. The MSInactive Invoke message may also include the ID number or directory number of the PSAP which was involved in the call. The HLR sends a MSInactive Return Result message  69  to the serving MSC and marks the subscription as inactive at step  70 . 
     At  71 , the emergency MS then makes a system access (for example, a periodic registration) with the border MSC  27 . The border MSC sends a REGNOT (or QUALREQ) Invoke message  72  to the HLR to obtain subscriber data for the registering MS. The HLR checks the subscription data at  73 , and it is found that the MS has been marked as having an unfinished 911 call. Therefore, the HLR sends a REGNOT (or QUALREQ) Return Result message  74  to the border MSC and includes an indication that the MS must be re-connected to the PSAP. The HLR also includes the PSAP directory number provided to the HLR by the serving MSC in either the REGNOT message  63  or the MSInactive message  68 . Thereafter, at step  75 , the emergency MS may re-register or may originate another call in the border MSC. If the emergency MS re-registers, it is paged, and then reconnected to the PSAP at step  76 . If the emergency MS originates another call at  75 , the border MSC automatically delivers the call at step  76  to the PSAP. 
     At step  77 , the emergency call is properly completed and disconnected. The border MSC  27  then sends a REGNOT Invoke message  78  to the HLR reporting the finished 911 call. At step  79 , the HLR resets the Hot Line category to OFF, and sends a REGNOT Return Result message  80  to the border MSC. 
     FIG. 5 is a message flow diagram of the messages utilized to set up a hotline between the emergency MS  21  and the PSAP  22  when the emergency MS roams into the service area  26  of the bordering MSC  27  and registers before the emergency call can be reconnected and before the serving MSC has notified the HLR of the unfinished emergency call. The method illustrated in FIG. 5 is a “back-end method” in which the hotlining feature is set up after the emergency call is disconnected because there was no opportunity to set up the feature at the front end of the emergency call. The emergency MS initiates an emergency call at  81 , and the serving MSC  24  delivers the call at step  82  to the PSAP  22 . At step  83 , the emergency call is prematurely disconnected. The serving MSC attempts to reconnect the call at  84 , but the attempt fails. Thereafter, the emergency MS registers at  85  in the border MSC  27 . When this occurs, the border MSC accepts the registration and sends a REGNOT Invoke message  86  to the HLR to retrieve subscription data for the emergency MS. The HLR returns a REGNOT Return Result message  87  to the border MSC, and sends a Registration Cancellation (REGCANC) Invoke message  88  for the emergency MS to the old serving MSC  24 . The old serving MSC then sends a REGCANC Return Result message  89  to the HLR and includes a parameter indicating that the MS is currently involved in an unfinished 911 call along with the PSAP directory number. The HLR sets the Hot Line category to ON at  91 , and instructs the new serving (border) MSC  27  to connect the emergency MS to the PSAP in a QUALDIR Invoke message  92 . The border MSC responds with a QUALDIR Return Result message  93 . Thereafter, at step  94 , the emergency MS may re-register or may originate another call in the border MSC. If the emergency MS re-registers, it is paged, and then reconnected to the PSAP at step  95 . If the emergency MS originates another call at  94 , the border MSC automatically delivers the call at step  95  to the PSAP. Thus, the emergency MS is automatically routed to the PSAP whenever it registers or originates a call in the bordering MSC  27 . 
     At step  96 , the emergency call is properly completed and disconnected. The border MSC  27  then sends a REGNOT Invoke message  97  to the HLR reporting the finished 911 call. At step  98 , the HLR resets the Hot Line category to OFF, and sends a REGNOT Return Result message  99  to the border MSC. 
     It is thus believed that the operation and construction of the present invention will be apparent from the foregoing description. While the system and method shown and described has been characterized as being preferred, it will be readily apparent that various changes and modifications could be made therein without departing from the scope of the invention as defined in the following claims.