Patent Publication Number: US-6215782-B1

Title: Method for reconnecting calls in a wireless telecommunications system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to the application of Richard W. Buskens, Thomas F. LaPorta, Arun N. Netravali, and Krishnan K. Sabnani entitled “Method Of Using An Intelligent Mobile Terminal For Reconnecting Calls In A Wireless Telecommunications System”, Ser. application No. 08/771,739 filed on Dec. 20, 1996, which application is assigned to the assignee of the present invention. 
    
    
     TECHNICAL FIELD 
     This invention relates to wireless telecommunications, and more particularly, to efficiently reconnecting calls in wireless telecommunications systems. 
     BACKGROUND OF THE INVENTION 
     The hallmark of wireless telecommunications services is mobility gained by the elimination of wired connections. Indeed, the ability to use a mobile terminal (such as a cellular telephone) to originate and receive calls across a wide range of geographic locations is enabled by the use of an allocated portion of radio spectrum dedicated to transmitting voice, data, and control information. More particularly, mobile terminals send and receive voice, data, and control information from base stations over an air interface. The base station serves as a gateway point between the mobile terminal and a mobile switching center. The mobile switching center provides call processing services and resource allocation for establishing call connections in the wired network which is required for connecting mobile terminals to other parties. 
     To establish a call, a base station allocates air traffic channels to a mobile terminal for transmitting voice, data, and control information. In one implementation, a traffic channel operates at a carrier frequency comprising time slots during which the mobile terminal transmits data frames (including synchronization bits) to the base station and vice versa. One problem with mobile telecommunications is that synchronization needed to maintain a connection between a mobile terminal and base station is often lost. The most common cause for loss of synchronization is the inability of the terminal or base station to detect the carrier frequency of the traffic channels used in the call. Another cause of loss of synchronization is the inability to receive data frames from the mobile to the base station (or vice versa) in their anticipated time slots due to obstructions such as trees, buildings, tunnels or noise interference. In current implementations, both the serving base station and the mobile terminal wait for each others carrier frequencies (or data stream) to be reinstated for a specified period (a resynchronization “time-out” period) before the loss of synchronization precipitates call release procedures tearing down the existing call in the base station. From the base station perspective, releasing a call results in releasing the air traffic channels and other resources associated with the call. If the base station fails to resynchronize, it sends a call release message indicating disconnect to the mobile switching center so that call connections to the other party (or parties) are released. In the current art, from the mobile terminal perspective, releasing a call involves termination of data traffic on the air traffic channel allocated to the call, and resetting an internal state of the terminal so that new calls can proceed. 
     Abnormally released calls due to loss of synchronization result in subscriber frustration and a need for reestablishing the call. Reestablishing a call requires re-dialing and reestablishing connections interconnecting the mobile terminal to its parties. Hence, reconnecting disconnected calls not only requires subscriber action but also requires duplicated network action. Therefore, there is a need in the art for efficiently reconnecting mobile calls to decrease subscriber inconvenience and enhance network efficiency when disconnected calls are reconnected. 
     SUMMARY OF THE INVENTION 
     This need is addressed and a technological advance is achieved in the wireless telecommunications art by actively attempting call reconnection for a specified time period and performing call release procedures if the reconnection attempts fail. For purposes of the present invention, call disconnect is defined as the point at which loss of synchronization would ordinarily cause a call to be released. 
     In the preferred embodiment, a base station establishes a reconnection channel for interaction with mobile terminals equipped with a reconnection processor. Upon detection of disconnect, the base station attempts to establish new air traffic channels interconnecting the mobile terminal. The reconnection attempts are made by the base station for a specified time period. The reconnection channel serves as a broadcast means via which the mobile terminal and the base station interact to attempt reconnection. In the event that reconnection is unsuccessful, wired network call resources associated with the existing call are released. 
     In another preferred embodiment, a base station selectively reinitiates a reconnection process upon expiration of a resynchronization timer. The reconnection process requires the base station to hold wired call resources while it attempts to reestablish the call to the mobile terminal. If the call attempt is successful, the base station establishes new air traffic channels to bridge the wired network call resources with the new air traffic channels so that the call may continue. The reconnection attempt is made for a predetermined number of tries before the base station abandons call reconnect efforts and releases call resources. Upon release of call resources, a call release message indicating abnormal release is transmitted to a mobile switching center. 
     In still another preferred embodiment of the present invention, a mobile switching center includes a reconnection timer for establishing a predetermined time period during which the mobile switching center actively attempts to reconnect calls which have been requested to be disconnected by a base station. During reconnection efforts, the mobile switching center holds call resources unaffected by the loss of synchronization. If reconnection attempts are unsuccessful, all call resources are released. If reconnection attempts are successful, the unaffected call resources are bridged with new call resources established as a result of the successful reconnect by the mobile switching center. 
     Advantageously, all preferred embodiments of the present invention require holding some portion of network resources during call reconnection attempts. Holding network resources allows calls to be reestablished more quickly because only portions of the call connection need to be set up and thereby eliminates the need for duplicated network effort. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified block diagram of a preferred embodiment of a wireless telecommunications system; 
     FIG. 2 is a flow diagram illustrating the steps performed by a base station shown in the wireless telecommunications system of FIG. 1 in accordance with the preferred embodiment of the present invention; 
     FIG. 3 is a flow diagram illustrating the steps performed by a base station shown in the wireless telecommunications system of FIG. 1 in another preferred embodiment of the present invention; 
     FIG. 4 is a flow diagram illustrating the steps performed by a mobile terminal in the wireless telecommunications system of FIG. 1 in accordance with the preferred embodiment of the present invention; and 
     FIG. 5 is a flow diagram illustrating the steps performed by a mobile switching center in the wireless telecommunications system of FIG. 1 in accordance with the preferred embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a simplified block diagram of wireless telecommunications system  100  including mobile switching center  120  and mobile switching center  140  interconnected to the rest of the public switched telephone network  101  via established trunks  121  and  141 , respectively. Mobile switching center  120  includes processor  128  interconnected to database  130  via data link  129 . Processor  128  includes a timing means and is responsible for performing call processing functions (such as set up and tear down of calls) and retrieving information such as routing information used to set up a call from database  130 . In this embodiment, mobile switching center  120  serves base stations  122 ,  124  and  126  via established base station links  123 ,  125  and  127  respectively. In the preferred embodiment, each base station includes a processor (with embedded memory and timing means) for administering call reconnection protocols as described below. Base stations  122 ,  124  and  126  include processors  132 ,  134  and  136 , respectively. In this example, mobile terminal  135  is served by base station  122 . Established air interface connection  131  indicates that mobile terminal  135  is active in a call. 
     Mobile switching center  140  includes processor  148  interconnected to database  150  via data link  149 . Processor  148  includes a timing means and is responsible for performing call processing functions (such as set up and tear down of calls) and retrieving information such as routing information used to set up a call from database  150 . In this example, mobile switching center  140  serves base stations  142 ,  144  and  146  via base station links  143 ,  145  and  147 , respectively. Similar to mobile switching center  120 , each base station includes a processor with embedded memory and timing means for administering reconnection protocols, as described below. In this example, base stations  142 ,  144  and  146  include processors  152 ,  154  and  156 , respectively. Mobile terminal  155 , served by base station  142 , is interconnected to the base station via air interface connection  151 . In the preferred embodiment, mobile terminal  155  includes antenna  157  and reconnection processor  159  for responding to a base station using a reconnection protocol. The reconnection protocol is administered by the base station over an established reconnection channel. In this embodiment, base station  142  has dedicated reconnection channel  153  for operating the reconnection protocol. 
     Calls involving mobile terminals  135  and  155  are established using known procedures which allow voice and data transmissions to be relayed from a base station to a mobile terminal and vice versa. Occasionally, the air interface between the base station and the mobile terminal is obstructed resulting in a loss of synchronization and call release if resynchronization cannot be achieved. In accordance with the preferred embodiment, an active attempt to reconnect the portion of the connection affected by loss of synchronization calls is made before network call release procedures are invoked. 
     FIG. 2 illustrates the steps performed by a base station to reconnect a call in accordance with one preferred embodiment of the present invention. More particularly, FIG. 2 illustrates the steps performed by base station  122  (that is, a base station without a dedicated reconnection channel) in reestablishing a call. 
     For purposes of example, assume that a subscriber using mobile terminal  135  is involved in a call with a party served by public switched telephone network  101 . Accordingly, air traffic channel  131  is established to transmit voice between mobile terminal  135  and the customer premises equipment of the other party (not shown) via base station  122 , mobile switching center  120  and trunk  121 . Also assume that the data flow between mobile terminal  135  and base station  122  is interrupted due to loss of synchronization. The reconnection process begins in step  200  in which base station  122  detects disconnect of the call (that is, the loss of synchronization is detected and the resynchronization time-out sequence has expired without successfully resynchronizing to the mobile terminal). The base station recognizes the disconnect to be involuntary (as opposed to a user simply ending a call by hanging up) because no formal call release message is received. 
     In decision step  202 , base station  122  determines whether reconnection procedures should be attempted. More particularly, decision step  202  allows base station  122  to selectively deploy the reconnection process. In some circumstances, such as an unusually heavy call volume, base station  122  may be programmed to immediately release call resources upon detection of a disconnect. If the outcome of decision step  202  is a “NO” determination, the process continues to step  203  in which base station  122  initiates call release procedures and sends a call release message indicating disconnect to mobile switching center  120 . If the outcome of decision step is a “YES” determination, the process continues to step  204  in which base station  122  sets a reconnection retry count to “zero”. In step  206 , the base station begins a reconnection time-out sequence during which the base station attempts to establish new air traffic channels to be used between base station  122  and mobile terminal  135 . In step  207 , base station  122  waits for an event to occur while holding call resources associated with the interrupted call. Each of the possible events is discussed in detail below. 
     The process continues through connector A when the reconnection time-out sequence has expired. In step  208 , base station  122  makes an attempt to reestablish the portion of the connection affected by the loss of synchronization. In decision step  210 , base station  122  determines whether the reconnection attempt was successful. If the outcome of decision step  210  is a “YES” determination, the process continues to step  212  in which base station  122  uses new air interface resources (i.e., traffic channels) in the call with the mobile terminal. If the outcome of decision step  210  is a “NO” determination, the process continues to step  214  in which the reconnection retry count is incremented. The process continues to step  216  in which base station  122  determines whether the reconnection retry count has been incremented beyond a predetermined threshold. If the outcome of decision step  216  is a “NO” determination, the process returns to step  206  in which the reconnection time-out sequence is initiated. If the outcome of decision step  216  is a “YES” determination, the process continues to step  218  in which base station  122  sends a call release message indicating disconnection to the mobile switching center. In step  220 , the base station releases all call resources associated with the disconnected call. 
     The process continues through connector B when another incoming call directed to mobile terminal  135  has been received during the time-out sequence. The process continues to step  222  in which the incoming call request is rejected and the time-out sequence is continued. In alternative embodiments, the incoming call request may be honored. In these embodiments, the time-out sequence is halted, call resources associated with the disconnected call are released, and the incoming call is processed as a new call request. 
     The process continues through connector C when the subscriber using mobile terminal  135  attempts to originate a call during the time-out sequence. The process continues to step  224  in which the time-out sequence is terminated, call resources for the existing call are released and the new call request is processed. 
     FIG. 3 is a flow diagram illustrating the steps performed during another preferred embodiment of the present invention. More particularly, FIG. 3 describes the steps performed by base station  142  which includes reconnection channel  153  for relaying messages to a mobile terminal. For the reconnection channel to be functional, the mobile terminal involved in the call must be equipped with a reconnection processor. For purposes of explanation, assume that a subscriber is involved in a call using mobile terminal  155  (that is, a mobile terminal including a reconnection processor) when a loss of synchronization occurs and a resynchronization timer expires. The process begins in step  300  in which base station  142  detects call disconnect 
     The process continues to decision step  302  in which base station  142  determines whether reconnection procedures should be attempted for this particular call. If the outcome of decision step  302  is a “NO” determination, the air traffic channel interconnecting base station  142  to mobile terminal  155  is released and a call release message indicating the disconnect is sent to the mobile switching center in step  303 . If the outcome of decision step  302  is a “YES” determination, the process continues to step  304  in which base station  142  sets a reconnection retry count to “zero”. In step  305 , the base station initiates a reconnection time-out sequence. In step  306 , base station  142  broadcasts a reconnection message to mobile terminal  155  over reconnection channel  153  while holding all resources associated with the call. Although reconnection channel  153  is shown to exclusively serve terminal  155 , other embodiments may use a global channel to serve all mobile terminals. In the preferred embodiment, the reconnection message instructs the mobile terminal to tune to new air traffic channels so that the call may be reconnected. In other embodiments, base station  142  sends a message over the reconnection channel informing mobile terminals to request new resources. 
     During the broadcast, a variety of events may occur. First, the subscriber associated with mobile terminal  155  may elect to originate a call. If this event occurs, the process continues through connector A to step  310  in which the time-out sequence is halted, a call a release message is sent to the mobile switching center and the new call request is processed. If the subscriber calls the disconnected party, resources may be bridged without tearing down the existing call. Secondly, the mobile subscriber associated with mobile terminal  155  may also explicitly elect to reconnect the call. This election is signaled by the wireless subscriber sending a message to the serving base station (e.g., the subscriber may depress keypad buttons *56 to indicate that reconnection is desired) over the reconnection channel. If the subscriber requests reconnection, the process continues through connector B to step  314  in which base station  142  accesses its database to retrieve subscriber information relating to mobile terminal  155 . The process continues to decision step  316  in which base station  142  determines whether there are any air traffic channels available to reconnect the previously disconnected call. If the outcome of decision step  316  is a “YES” determination, the process continues to step  318  in which the base station stops the reconnection time-out sequence and bridges the held call resources with those connecting the base station to the mobile terminal. If the outcome of decision step  316  is a “NO” determination, the process continues to step  322  and all call resources associated with the call are released. Also, a call release message indicating disconnect is sent to the mobile switching center. 
     Another event which may occur during the time-out sequence is that a new incoming call may be directed to mobile terminal  155 . In this case, the process continues through connector C to step  320  in which the incoming call request is rejected and the time-out sequence is continued until the time-out expires. If the time-out sequence expires, the processor continues to step  322  in which base station  142  sends a call disconnect message to the mobile switching center so that all call resources may be released. 
     FIG. 4 is a flow diagram illustrating the steps performed by a mobile terminal during a reconnection attempt. For purposes of clarity, continue with the example of mobile terminal  155  involved in a call when a loss of synchronization and expiration of a resynchronization timer occurs. In this example, base station  142  serving mobile terminal  155  has established reconnection channel  153  for administering a reconnection protocol. Reconnection processor  159  contained within mobile terminal  155  allows the terminal to interact with base station  142  over reconnection channel  153 . The process begins in step  400  in which mobile terminal detects call disconnect. 
     The process continues to step  402  in which reconnection processor  159  tunes to reconnection channel  153  using antenna  157 . In step  404 , mobile terminal  155  waits for an event. If the process continues through connector A to step  406 , the user of mobile terminal  155  releases its resources associated with the call. If a reconnection message is received, the process continues through connector B to step  408 . In step  408 , mobile terminal  155  interacts with the base station using the reconnection protocol. In the preferred embodiment, the reconnection protocol involves transmitting information (either initiated by the mobile terminal or by the base station) for reestablishing the air interface portion of the call. The process continues to step  410  in which the call is in progress. 
     FIG. 5 illustrates the steps performed by a mobile switching center equipped with call reconnection capabilities. The process begins in step  500  in which the mobile switching center receives a call release message indicating disconnect from a base station. In step  502 , the mobile switching center sets a reconnection retry count to “zero”. In step  504 , a reconnection time-out sequence is initiated during which all call resources are held. In step  506 , the mobile switching center waits for an event to occur. If a subscriber using mobile terminal  155  attempts to place a call during the time-out sequence, the process continues through connector A to step  508  in which the time-out sequence is terminated, call resources for the existing call are released, and the new call request is processed. If an incoming call request is directed to mobile terminal  155 , the process continues through connector B to step  512  in which the incoming call request is rejected and the tine-out sequence is continued. In alternative embodiments, however, the incoming call request may be honored. If the time-out sequence expires, the process continues through connector C to step  514  in which the mobile switching center attempts to set up an incoming call to the mobile subscriber associated with mobile terminal  155  via base station  140 . In decision step  516 , the mobile switching center determines whether the incoming call setup request was successful. If the outcome of decision step  516  is a “YES” determination, the process continues to step  518  in which held call resources are bridged to the new call setup to the mobile terminal. If the outcome of decision step  516  is a “NO” determination, the process continues to step  520  in which the reconnection retry count is incremented. In decision step  522 , the mobile switching center determines whether the reconnection retry count has been incremented beyond a predetermined threshold. If the outcome of decision step  522  is a “YES” determination, the call is released in step  524 . If the outcome of decision step  522  is a “NO” determination, the process return to step  504  in which the reconnection time-out sequence is reinitiated. 
     During all of the above-described preferred embodiments, it is assumed that the other party involved in the call (that is, the party other than the user of wireless terminals  135  or  155 ) maintains the connection while reconnection attempts occur. Preferably, an announcement message informing the party to hold the connection while reconnect is attempted is issued. 
     Advantageously, the preferred embodiments of the present invention allows a mobile terminal, base station and a mobile switching center in a wireless telecommunications system to hold call resources while reconnection attempts are made resulting in a more efficient use of network for reconnection. Further, the preferred embodiments of the present invention allow the mobile switching center and the base station to make a determination as to whether reconnection attempts will occur. For example, if it is determined that there is heavy call volume, either the base station or the mobile switching center may elect to forego reconnection attempts. In this manner, the wireless telecommunications service providers are afforded with flexibility regarding reconnection attempts. 
     Although this invention has been described with respect to preferred embodiments, those skilled in the art may devise numerous other arrangements without departing from the scope of the invention defined in the following claims.