Reconnection of dropped call in mobile communication system

Abstract: There is provided a method of reconnecting a communication link terminated by a service impediment during a service between a mobile station (MS) subscriber and the other party communicating with the MS subscriber through a mobile communication system having a plurality of mobile switching centers (MSCs) connected to one another, each MSC being connected to a plurality of base stations (BSs). When the service impediment lasts for at least a predetermined first time period, the MS subscriber sends a reconnection request signal. Then, the service is reinitiated between the MS subscriber and the other party through one of the plurality of BSs and one of MSCs connected to the BS in response to the reconnection request signal.

Description: The present invention relates generally to the field of wireless communication, and more particularly, to a method of reconnecting a dropped call in a mobile communication system. In a public land mobile network (PLMN), a connection is made between a base station (BS) and a mobile station (MS) in a radio environment. In some cases, call service cannot be provided in shadow areas due to physical characteristics of the radio environment which adversely affect signal propagation. In the case where a terminal subscriber roams on foot or is moving in a vehicle, a call can be temporarily dropped. In accordance with conventional technology, if a call drop lasts for a predetermined time period, it is determined that a call service cannot be provided and the call is released. In IS-95, the predetermined time is approximately 5 seconds (i.e., equal to one frame duration 20ms x 270). Upon this unintended call release, a calling party should resume the attempt to call to a called party. To overcome the aforestated problem of call drops, several approaches have been suggested in the prior art, including: (1) U.S. Patent No. 5,546,382: reconnection of a circuit to continuously provide a data communication service upon generation of a transmission failure; (2) U.S. Patent No. 5,239,571: reconnection of an abnormally terminated communication line by adding a separate device to a terminal or modifying the terminal. That is, with the aid of a RAM in the terminal for storing information on call origination and call termination, a call is automatically set up in the case of an abnormal disconnection of a call; and (3) U.S. Patent No. 5,566,236: reconnection of a disconnected telephone communication by use of a nearby telecommunication system (e.g., PBX: Private Branch Exchange and Centrex: Centralized PBX Service) with the concept of an intelligent network introduced. EP-A-0 872 982 (cited under Articles 51(3) and 51(4) EPC) describes a method and apparatus for re-establishing an interrupted data packet call on a channel between two transceiving devices in a telecommunications system. When packet data has not been sent or received in the call for a predetermined time period, the call is released and call configuration information is saved in at least one of the transceiving devices. If it is then necessary to transmit further packet data for the call, either one of the transceiving devices may initiate, within a predetermined time period, the sending of a call re-establishment message to re-establish the call. Information in the re-establishment message is used to retrieve the call configuration information that was saved upon call release. The call is then re-established using the information in the re-establishment message and the retrieved information. EP-A-0 871 316 (cited under Articles 51(3) and 51(4) EPC) describes a method and apparatus for processing dropped calls. If, during an established call between two communication devices, a telecommunication network node determines that a communication link to one of the devices has been dropped, the node determines the cause of the dropped communication link and sends a status message to the non-dropped communication device. If a reconnection attempt is appropriate, the node attends to reconnect to the dropped device. If the reconnection attempt is successful the call between the two communication devices is re-established. JP97177576 relates to preventing the communication from being interrupted when an intensity of an electric field from a mobile terminal equipment is lower than a communication enable electric field strength in a network for a short time. It is suggested, that when the intensity of electric field from a mobile terminal equipment A is lower than a communication enable electric field intensity in a network while the mobile terminal equipment A and a terminal equipment B are busy with each other, the network sets the communication with the terminal equipment B in the holding state. A guidance representing the holding communication is given to the terminal equipment B because the terminal equipment A moves to a zone to which no radio wave arrives. When a command of communication restart reached the network from the mobile terminal equipment A within a specified time, the network releases the holding and restarts the communication. EP-A-0 708 571 relates to a distributed control in wireless cellular and personal communication systems. It is the object of the present invention to provide a method of reconnecting a call which is dropped while a call service is being provided in a mobile communication system. The object is solved by the subject matter of the independent daims. Preferred embodiments of the present invention are defined by the dependent claims. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a method of automatically reconnecting a dropped call which is while a call service is being provided in a mobile communication system. Another object of the present invention is to provide a method of automatically reconnecting a dropped call while a call service is being provided in a mobile communication system. A further aspect of the present invention to provide a method of manually reconnecting a call which is dropped while a call service is being provided in a mobile communication system. It is a further aspect of the present invention to provide a method of reconnecting a dropped call upon detection of the call drop by a mobile station. Another aspect of the present invention is to provide a method of reconnecting a dropped call upon detection of the call drop by a system (i.e., base station (BS) or mobile switching center (MSC)). Another aspect of the present invention is to provide a method of reconnecting a call dropped due to an abnormality of bidirectional traffic channels for communication between a mobile station (MS) and a system, upon detection of the call drop by both the MS and the system. A still further aspect of the present invention is to provide a method of reconnecting a dropped call when a mobile station (MS) detects the call drop and requests a location registration. Briefly, these and other aspects are accomplished by a method of reconnecting a communication link terminated by a service impediment during a service between an MS subscriber and its counterpart subscriber communicating with the MS subscriber through a mobile communication system having a plurality of MSCs connected to one another, each MSC being connected to a plurality of base stations Bss. When the service impediment lasts for at least a predetermined first time period, the MS subscriber sends a reconnection request signal. Then, the service is reinitiated between the MS subscriber and its counterpart subscriber through one of the plurality of BSs and one of MSCs connected to the BS in response to the reconnection request signal. BRIEF DESCRIPTION OF THE DRAWINGS The above objects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: FIG. 1 is a block diagram of a mobile communication system according to the present invention; FIG. 2 illustrates a field of a call release signal, indicating whether the call release signal is generated in a normal state or due to a dropped call; FIG. 3 illustrates a parameter of a reconnection request signal to be used for a connection to an established call on a temporary hold state; FIG. 4 illustrates a parameter indicating whether a dropped call reconnection function is individually registered as an additional service for each MS; FIG. 5 illustrates a parameter discriminating a normal call release signal from a dropped call-caused call release signal; FIG. 6 is a flowchart illustrating a method for releasing an MS originated call based on IS-634 standards; FIG. 7 illustrates an MS originated call process; FIG. 8 is a comprehensive flowchart illustrating a method for processing a dropped call in an MS, BS, or MSC according to embodiments of the present invention; FIG. 9 is a flowchart illustrating a method of processing a dropped call upon detection of the call drop by an MS; FIG. 10 is a flowchart illustrating a method for processing a dropped call upon detection of the call drop by a system; FIG. 11 is a flowchart illustrating a method for processing a dropped call upon detection of the call drop caused by bidirectional traffic channels by both the MS and the system; FIG. 12 is a flowchart illustrating a method for processing a dropped call in the case where the MS detects the call drop and issues a location registration request; FIG. 13 is a diagram of conventional call processing states; FIG. 14 is a diagram of call processing states according to the embodiments of the present invention; and FIG. 15 is a block diagram of a BS for detecting a dropped call. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Like reference numerals denote the same components in the drawings. It is to be noted that a detailed description of the known structure and function of the present invention will be omitted if it is deemed to obscure the subject matter of the present invention. Terms and Definitions "a call service in progress" : a state where voice communication and data communication of still images, moving pictures, and the like are being performed. "dropped call": is defined as a call provided in a call service which can no longer be maintained either temporarily or permanently. A dropped call may be caused by either a call disconnection, noise channel on a specific channel, or non-serviceability. "call disconnection" : is the state where a call remains disconnected for a predetermined time. The embodiments of the present invention will be described with in the context of the North American digital mobile communication system standard (IS-95, IS-634, and IS-41 series) by way of example. The present invention is also applicable to 3G IS-95 which provides the additional services of high quality voice, high speed data, moving pictures, and Internet browsing. Referring to FIG. 1, the mobile communication system to which the present invention is applied includes an HLR (Home Location Register) 80, MSCs (Mobile Switching Centers) MSC0 70a and MSC1 70b, BSCs (Base Station Controllers) BSC00 64a, BSC01 64b, and BSC10 64c, BTSs (Base Station Transceiver Subsystems) BST000 62a to BTS101 62f, and an MS 50. A plurality of HLRs and MSCs are interconnnected in the PLMN to perform subscriber management and call switching. As shown in FIG. 1, the single HLR 80 is connected to a plurality of MSCs, MSC0 70a and MSC1 70b. Each MSC is in turn connected to a plurality of BSCs, and each BSC to a plurality of BTSs. A BS is typically comprised of a BSC and BTSs. An MSC controls a connection to the PSTN (Public Switch Telephone Network) and the PLMN, (public land mobile network). A BSC controls a radio link and performs a handoff, a BTS forms a radio link with an MS and manages radio resources, and an HLR registers subscriber locations and serves as a database for storing subscriber information. Each MSC has a VLR (Visitor Location Register) for temporarily storing the information of an MS entering the service area of the MSC. If the MS moves into another service area, the stored information is discarded. For a call set-up, the mobile communication system assigns radio channels between an MS and a BTS, forms communication links between the BTS and a BSC, between the BSC and an MSC, and between the MSC and the PLMN or an external network such as PSTN. If the established call cannot be maintained for a predetermined time because the MS is in a shadow area or due to characteristics of the radio environment, the mobile communication system typically disconnects the call. Shadow area problems may arise from a number of situations including, for example, an elevator, a relay-free tunnel, a long tunnel located between adjacent cells, a shadow area among dense tall buildings. A call drop as defined in the embodiments of the present invention can occur in such locations as indicated by reference numerals 10, 12, 14, and 16 in FIG. 1. Reference numerals 10, 12, and 14 indicate locations of a dropped call within the same MSC area, and reference numeral 16 indicates a location of a dropped call between BTSs covered by different MSCs. Specifically, reference numeral 10 denotes a call dropped location of the MS 50 communicating with the BTS000 62a within the coverage area of the BTS000 62a, reference numeral 12 denotes a call dropped location of the MS 50 communicating with the BTS000 62a or the BTS001 62b in the service area of the BSC00 64a, and reference numeral 14 denotes a call dropped location of the MS 50 communicating with the BTS001 62b or the BTS010 62c on the border of their coverage areas. A BS and an MSC of FIG. 1 controls the reconnection of a dropped call, possibly using the same algorithm. A dropped call reconnecting function can be provided in two ways: (a) a system applies the function indiscriminately to all MSs by modifying their operations; and (b) the function is provided on an individual basis to each MS as an additional service. The former method is viable if all the subscriber terminals are similarly configured to implement the dropped call reconnecting function. The latter method allows a flexible application of the reconnection function, and only subscribers requiring the service need modify their terminals. In the embodiments of the present invention, either existing signals are modified or new signals are used. Existing call release signals including a elease signal □communicated between a BS and an MSC and a "Release Order signal" which is communicated between an MS and a BS, can be used or added with a new function. If a call drop can be detected from the call release signals, they can be used. Otherwise, a parameter as shown in FIG. 2 is added to them to separately indicate a normal call release signal or a dropped call-caused call release signal. In FIG. 2, a parameter RELEASE_STAT, indicating release status is added to a call release signal for discriminating normal call release signal from a call release signal due to a dropped call. The parameter occupies one bit. If the bit is zero, it indicates a normal call release signal. If the bit is one, it indicates a call release signal due to a dropped call. A typical protocol for using the existing signals "Release" and "Release Order" is shown, by way of example, in FIG. 6 which illustrates the signal flow for releasing an MS originated call based on the IS-634 standard. Reference numeral 20 denotes the signals "Release" and "Release Order". Alternatively, the parameter RELEASE_STAT can be added to a signal "Clear" to achieve the same objective. As an additional alternative solution, a new signal can be used. To obtain information to be used for a connection to an established call on a temporary hold, a conventional reconnection request signal "Reconnection" directed from an MS to a BS, from a BS to an MSC, from an MSC to a VLR, and from an MSC to another MSC can be used or modified. When directed from an MS to a BS or from a BS to an MSC, messages used for an orgination call use an additional parameter, OLD_BS_ID, shown in FIG. 3 as information to be used to make a connection to an established call on a temporary hold. This parameter includes 16 bits and can be generated by modifying an existing origination message and a set-up signal. FIG. 7 illustrates the procedure for an MS originated call proceeding. In FIG. 7, reference numeral 30 denotes signals "Origination Order" and "Setup" added with the parameter OLD_BS_ID. To indicate whether a dropped call reconnection function is provided as an additional service to each MS. As previously described, a parameter DCAR_FLAG, as shown in FIG. 4 is added to data in the HLR 80, and VLR0 72a & VLR1 72b of FIG. 1. This parameter is one bit. If the bit is zero, it indicates a deactivation state (0=DACT) and if the bit is one, it indicates an activation state (1=ACT). A parameter WATING_FLAG, shown in FIG. 5 for discriminating a normal call release signal from a dropped call-caused call release signal is added to the subscriber data in VLR0 72a and VLR1 72b of FIG. 1. This parameter is one bit. If the bit is zero, it indicates a normal call release state (0=NORMAL), and if the bit is one, it indicates a call drop release state (1=DROP). The initial value of the parameter is zero. Drop Call Detection A dropped call is recognized in the following ways according to the embodiments of the present invention: First Embodiment: Detection by an MS Detection of dropped call information by MS: an MS detects the drop of a call in progress due to an abnormality of a traffic channel directed from the system to the MS. For example, the MS would store information on a call drop generating location, and if the MS transits to a normal state, it then notifies the system of the call drop. That is, the MS notifies the BS and MSC. Second Embodiment: Detection by a System Detection of dropped call by system: in this case, the system can determine whether a call in progress drops or not. The call drop is caused by an abnormality of a traffic channel directed from an MS to the system. The system sends a reconnection request for the MS to perform a reconnection function, or forces the reconnection function to be implemented. Third Embodiment: [PLEASE PROVIDE DESCRIPTION] Abnormality of bidirectional traffic channels between MS and system. Fourth Embodiment: Location registration request from MS: after a call is dropped, an MS transits to a normal state without information on a call drop-caused location, reserves information different from existing parameters, and requests a location registration to the system due to the call drop. Upon generation of a call drop in this situation, the MS is normally initialized and registers its location. If the updated value of the parameter WAITING_FLAG in a VLR is one, the previously established call is reconnected. The location registration is the process by which an MS notifies a BTS of its location, status, identification, slot cycle, and other characteristics. Drop Call Processing Processing a dropped call varies depending upon how the dropped call is detected. Prior to describing dropped call processing according to the previously described four detection methods, dropped call processing performed by an MS, a BS, and an MSC alone or in combination will be described comprehensively referring to FIGs. 1 and 8. In FIG. 8, a call is set up in step 100 and proceeds in step 101. An MS or a BS determines in step 102 whether there is a manual reconnection request from an MS subscriber during the call. The MS subscriber can manually request a call reconnection by pressing a manual reconnection request keywhen serious noise is generated in voice or images, or the voice is mute during the call. The manual reconnection request key can be added to a key pad of the terminal, or may be provided using an existing function key, or constructed by a combination of existing digit keys. In the absence of the manual reconnection request from an MS subscriber, it is determined whether frames are being continuously received on a current traffic channel, in step 104. Upon receipt of a frame, it is determined whether the received frame has an error, in step 105. If no errors exist in the frame, a counter for counting unreceived or bad frames (hereinafter, referred to as a non-reception counter) is cleared in step 106. If the frame has an error in step 105, a non-reception count is incremented in step 108. Then, it is determined whether the non-reception count is equal to or greater than a predetermined time Timer_Val1, in step 110. The predetermined time Timer_Val1 varies depending upon a system operation state and subscriber characteristics, ranging from zero to ten seconds, preferably 1.2 seconds, to determine whether a call is dropped or not. The time of 1.2 seconds is about 5 seconds shorter than the release time for a call in the prior art. The value Timer_Val1 is registered as system initialization data and may be changed by an operator. When the dropped call reconnection function is uniformly registered for all MSs, the value Timer_Val1 is set as a predetermined value commonly applied each MS. On the other hand, if the function is provided individually for each MS, the value Timer_Val1 is unique to each MS. The ways an MS and a BS detect a call drop will be described in more detail. BS Detection of a Call Drop FIG. 15 is a block diagram of the BS for detecting a call drop. Referring to FIG. 15, a CDMA signal received through an antenna 1000 is converted to an IF (Intermediate Frequency) signal by an RF & XCVB (Radio Frequency & Transceiver Block) 1020. A CMDB (CDMA Modulation and Demodulation Block) 1040 converts the IF signal to a QCELP (Qualcomm Code Excited Linear Predictive coding) packet. During this operation, the CMDB 1040 determines whether a packet frame is normal or abnormal by checking its CRC (Cyclic Redundancy Code), and if abnormal it adds the abnormality indicating information (Quality Matrix: H'00-H'ff) to the converted packet. If the abnormality indicating information is zero, a TSB 1060 subjects the packet frame received from the CMDB 1040 to an abnormality operation. If it is one, the TSB 1060 converts the received packet frame to a PCM (Pulse Code Modulation) signal and sends the PCM signal to an MSC. Upon a determination that the packet frame as bad, the TSB 1060 counts continuous errors and unreceived frames using a timer interrupt generated every 20ms to make a final decision wether a call is dropped or not. That is, it is determined whether a frame is received at each 20ms interrupt. If 20 consecutive bad frames are received from the CMDB 1040, the TSB 1060 considers it to be predictive of a call drop. Then, if a predetermined number of consecutive bad frames are received for a first predetermined time, the TSB 1060 determines that the call is dropped. Assuming that the first predetermined time is 2 seconds, for example, the TSB 1060 declares a call drop if it receives 80 consecutive ball frames. However, if two consecutive normal frames are received, the non-reception counter is initialized and the TSB 1060 returns to a normal operation. On the other hand, if 20 consecutive frames are not received, the TSB 1060 considers it to be predictive of a call drop. Then, if no consecutive frames are received for a second predetermined time, the TSB 1060 determines that the call in progress is dropped. The second predetermined time is preferably shorter than the first predetermined time. MS Detection of Call Drop Now, an MS detection of a call drop will be described. An MS monitors a forward traffic channel on the traffic channel state. When the MS receives L consecutive bad frames on the forward traffic channel, it disables its transmitter. Then, if M consecutive good frames are received, the MS enables the transmitter. The MS activates a fade timer for the forward traffic channel when the transmitter is enabled, in a traffic channel initialization substate of the MS control on the traffic channel state. The fade timer is reset to N seconds when M consecutive good frames are received. When no consecutive good frames are received during the fade timer sequence, the MS disables the transmitter and declares a call drop. Returning to FIG. 8, if the MS and/or the BS detects a call drop in steps 104 and 106, it holds the current call in step 112 while the radio links between the MS and the BS, between the BS and an MSC, and between the MSC and the PSTN (or another MSC) are still maintained. When necessary, the radio links between the MS and the BS and between the BS and the MSC can be released. In step 114, an information message notifying a wait for a reconnection is sent to the other subscriber or a service provider. The information message takes the form of voice message, music, tone, or mute for the other telephone subscriber. The message takes the form of null data for a data communication subscriber and a data service provider. In step 116, a timer is set for a predetermined time Timer_Val2. When a reconnection waiting state is entered in step 118, the timer is activated in step 120. It is determined in step 122 whether there is a reconnection request or a location registration request is issued from the MS or the system (BS or MSC) before the timer expires. The timer value Timer_Val2 varies depending upon a system operating state and subscriber characteristics, ranging from several tens of seconds to several hundreds of seconds (e.g., 10-180 seconds) to determine the action time of call releasing. Timer_Val2 is preferably 30 to 60 seconds. When the dropped call reconnection function is uniformly registered for all MSes, the value Timer_Val2 is set as a predetermined value commonly applied to the MSes. On the other hand, if the function is provided individually for each MS, the value Timer_Val2 varies for each MS. Upon request of a call reconnection or location registration within the value Timer_Val2 in step 122, the timer is deactivated in step 124. In step 126, the established call is released from the hold stated and reconnected on a traffic channel suitable for a new location of the MS. The information message is withdrawn in step 128. Thus, the MS can continue communication with the other subscriber or the service provider in the existing call service. However, if no reconnection request is issued within the value Timer_Val2, an information message notifying a call drop is sent to the other subscriber or the service provider in step 130, and the call is released in step 132. Now, a description of a dropped callprocessing according to the four embodiments for drop call detection, previously described, will be given hereinbelow referring to FIGs. 1 and FIGs. 8 to 12. Prior to the description, it should be noted that in the same MSC/VLR area and in a different MSC/VLR area as shown in FIGs. 9 to 12 respectively, indicate that the MS is located in the same MSC/VLR area at the time when a call is dropped and a reconnection request and a location registration request are issued, and that the MS is located in a different MSC/VLR areas at the time when a call is dropped and a reconnection request and a location registration request are issued. First Embodiment: Call Drop Processing for a Drop Call Detected by an MS FIG. 9 is a flowchart of processing a dropped call when the MS detects the call drop. This situation occurs due to an abnormal traffic channel directed from the system to the MS. Referring to FIGs. 1 and 9, when an MS subscriber presses a reconnection request key during a call in step 200, the MS perceives the reconnection request in step 201 and is initialized in step 204. If a call is dropped due to non-reception of frames during the call in step 200, it is determined whether the call drop state lasts as long as or longer than the predetermined time Timer_Val1 in step 202. If it does, the MS is initialized in step 204. Here, step 204 can be omitted. Then, if the call drop is overcome, for example, due to a change in the location of the MS, the MS sends a reconnection request signal "Reconnect having the parameter OLD_BS_ID shown in FIG. 3, to its serving BS in step 206. The serving , BS sends the signal "Reconnect" to a corresponding MSC in step 208. In step 210, the MSC sends a signal "Reconnect_Ack" back to the serving BS and in step 212, the BS sends the signal "Reconnect_Ack" back to the MS. In step 213, the MSC determines whether the call reconnection function is uniformly registered for all MSs or as an additional service for an individual MS. If it is individually provided as an additional service, the MSC further determines whether the reconnection function is registered to be implemented for the MS requiring the call reconnection by analyzing the parameter DCAR_FLAG stored in its VLR. If the call reconnection function is not registered, a call release operation is performed in step 234. If the call reconnection function is registered, the MSC determines whether the parameter OLD_BS_ID of the received signal "Reconnect" indicates whether the MS is within the service area of the MSC, in step 214. If it does indicate the serving BS, the MSC sends an information message notifying a reconnection waiting to the other subscriber or the service provider, in step 216. For example, a call in progress is dropped when the MS 50 is within the coverage area of BTS000 62a, and the MS 50 moves into the area of BTS001 62b and requests a call reconnection. Then, MSC0 70a recognizes that the MS 50 is within its service area and sends the information message notifying "a reconnection waiting" to the other subscriber or the service provider. If the MS is not within the service area of the MSC in step 214, the MSC sends the received signal "Reconnect" to a different MSC in step 218. The new MSC sends the signal "Reconnect_Ack" to the previous MSC in step 220, and sends the information message notifying " reconnection waiting" to the other subscriber or the service provider. For example, a call in progress is dropped when the MS 50 is within the coverage area of BTS011 62d, and the MS 50 moves into the area of BTS100 62e and requests a call reconnection. Then, MSC0 70a sends MSC1 70b the signal "Reconnect" because the MS 50 is beyond its service area and the MSC 1 70b sends the information message notifying a reconnection waiting to the other subscriber or the service provider. After step 216 or 222, the MSC releases the existing links and connects an initiated call according to the current location of the MS in step 224. If the MS is within the service area of the same MSC, an origination call is set up, and the existing links are released and the initiated call is connected in the same manner as a hard handoff between BSes. For example, the MS 50 moves out of the coverage area of BTS000 62a into that of BTS001 62b and requests a reconnection, the existing links between the MS 50 and BTS000 62a and between BTS000 62a and BSC00 64a are released, and links are established between the MS 50 and BTS001 62b and between BTS001 62b and BSC00 64a. Then, those new links are connected to the other subscriber or the service provider. On the other hand, If the MS is within the service area of a different MSC, an origination call is set up, and the existing links are released and the initiated call is connected in the same manner as a hard handoff between MSCs. For example, the MS 50 moves out of the coverage area of BTS100 62e into that of BTS011 62d and requests a reconnection, the existing links between the MS 50 and BTS100 62e, between BTS100 62e and BSC10 64c, and between BSC10 64c and MSC1 70b are released, and links are established between the MS 50 and BTS011 62d, between BTS011 62d and BSC01 64b, between BSC01 64b and MSC0 70a, and between MSC0 70a and MSC1 70b. Then, those new links are connected to the other subscriber or the service provider. In step 226, the MSC withdraws the information message notifying the waiting for reconnection, and in step 228, determines whether the initiated call is normally connected to the other subscriber or the service provider. If it is normally connected, the call proceeds in step 230. Otherwise, the MSC sends an information message notifying a call release to the other subscriber or the service provider in step 232 and performs a call release operation in step 234. It is noted from FIG. 9 that the timer set for Time_Val2 in steps 116 and 122 of FIG. 8 does not operate. This is because the MS issues a reconnection request shortly after its initialization in steps 204 and 206 and thus the BS need not determine whether a predetermined time passes after the reconnection request. Second Embodiment: Call Drop Processing According to Drop Call Detection by the System (i.e., BS or MSC) FIG. 10 is a flowchart of processing a dropped call when the system detects the call drop. This case happens due to an abnormal traffic channel directed from the MS to the system. Referring to FIGs. 1 and 10, if a call is dropped due to non-reception of frames during the call in step 300, the BS determines whether the call drop state lasts as long as or longer than the predetermined time Timer_Val1 in step 302. If it does, the BS sends a signal "Release Order" including the parameter RELEASE_STAT set to one (DROP=1) to the MS in step 304 and a signal "Release" including the parameter RELEASE_STAT set to one (DROP=1) to the MSC in step 306. Then, the MSC sends a signal "Release Complete" to the BS in step 308. In step 309, the MSC determines whether the call reconnection function is uniformly registered for all MSs or as an additional service for an individual MS and whether the reconnection function, if it is individually provided as an additional service, is registered to be implemented for the MS by analyzing the parameter DCAR_FLAG stored in its VLR. If the call reconnection function is not registered regardless of indiscriminate provision or individual provision, a call release operation is performed in step 338. If the call reconnection function is registered, the MSC sends an information message notifying a reconnection waiting to the other subscriber or the service provider and activates a timer by setting the timer for Timer_Val2, in step 310. In step 311, the MS, which received the signal "Release Order" having the parameter RELEASE_STAT set to one (DROP=1) from its serving MSC or a different MSC, is initialized. This step can be omitted because the MS can change the used traffic channel to a new traffic channel without its initialzation. After step 311, if the call drop is overcome due to a variation in the location of the MS, the MS sends a signal "Reconnect" having the parameter OLD_BS_ID to the BS, in step 312. The BS sends the signal "Reconnect" to the MSC in step 314. The MSC determines in step 316 whether the parameter OLD_BS_ID represents the ID of a BS within its service area. If it does, the MSC sends a signal "Reconnect_Ack" to the BS in step 322, and the BS sends the signal "Reconnect_Ack" to the MS in step 324. If the parameter does not represent a BS covered by the MSC in step 316, the MSC sends the received signal "Reconnect" to a corresponding MSC in step 318. The new MSC sends the signal "Reconnect_Ack" to the old MSC in step 320. Then, the old MSC sends the BS the signal "Reconnect_Ack" in step 322, and the BS sends the MS the signal "Reconnect_Ack" in step 324. In step 326, the MSC releases the existing links and connects an initiated call. If the MS is within the service area of the same MSC, an origination call is set up, and the existing links are released and the initiated call is connected in the same manner as a hard handoff between BSs. On the other hand, If the MS is within the service area of a different MSC, an origination call is set up, and the existing links are released and the initiated call is connected in the same manner as a hard handoff between MSCs. In step 328, the MSC withdraws the information message notifying a wait for reconnection, and in step 330, determines whether the initiated call is normally connected to the other subscriber or the service provider. If it is normally connected, the call proceeds in step 332. Otherwise, the MSC sends an information message notifying a call release to the other subscriber or the service provider in step 336 and performs a call release operation in step 338. Meanwhile, if the timer set for Timer_Val2 (e.g., 30 to 60 seconds) has expired without receiving the reconnection request signal "Reconnect" from the MS in step 334, the MSC sends an information message notifying a call release to the other subscriber or the service provider in step 336 and performs a call release operation in step 338. While the BS sends the signal "Reconnect Order" of DROP=1 to the MS in FIG. 10, the signal "Reconnect Order" can be replaced by a signal "Reconnect". In this case, steps 310, 312, and 324 can be omitted. Third Embodiment: Call Drop Processing According to Drop Call Detection By MS and System Due FIG. 11 is a flowchart of processing a dropped call caused by bidirectional abnormal traffic channels between the MS and the system. Referring to FIGs. 1 and 11, if a call is dropped due to non-reception of frames during the call in step 400, the BS and the MS determine whether the call drop state lasts as long as or longer than the predetermined time Timer_Val1 in steps 401 and 402, respectively. If it does, the BS sends a signal"Release Order" including the parameter RELEASE_STAT set to one (DROP=1) to the MS in step 404 and a signal "Release" including the parameter RELEASE_STAT set to one (DROP=1) to the MSC in step 406. Then, steps 406 and 438 are performed. A description of those steps will be omitted since they are the same as steps 306 to 338 of FIG. 10. Fourth Embodiment: Call Drop Processing According to a Location Registration Request FIG. 12 is a flowchart of processing a dropped call when the MS detects the call drop and request a location registration. This case happens an MS transits to a normal state without information on a call drop-caused location after a call drop, reserves information different from existing parameters, and requests a location registration to the system due to the call drop. Referring to FIGs. 1 and 12, if a call is dropped due to non-reception of frames during the call in step 500, the MS determines whether the call drop state lasts as long as or longer than the predetermined time in step 502. If the time duration exceeds Timer_Val1, the MS sends a signal "Release Order" including the parameter RELEASE_STAT set to one (DROP=1) to the BS in step 504 and the BS sends the signal Release including the parameter RELEASE_STAT set to one (DROP=1) to the MSC in step 506. In step 507, the MSC determines whether the call reconnection function is uniformly registered for all MSs or as an additional service for an individual MS and then whether the reconnection function, if it is individually provided as an additional service, is registered to be implemented for the MS by analysing the parameter DCAR_FLAG stored in its VLR. If the call reconnection function is not registered regardless of indiscriminate provision or individual provision, a call release operation is performed in step 546. If the call reconnection function is registered, the MSC sends a signal "DROP SET" to its VLR in step 508, and the VLR sets the parameter WAITING_FLAG indicating a normal call release or a dropped call-caused call release to DRP=1 in step 510. Then, the VLR sends the MSC a signal DROP CONF representative of drop confirmation in step 512. Meanwhile, the MSC sends an information message notifying a reconnection waiting to the other subscriber or the service provider and activates a timer by setting the timer for Timer_Val2, in step 509. In step 514, the MS, which sent the signal "Release Order" having the parameter RELEASE_STAT set to one (DROP=1) to the BS, is initialized. This step can be omitted. After step 511, if the call drop is overcome due to a change in the location of the MS, the MS sends a location registration request signal "Registration Message" to the BS, in step 516. In response, the BS sends a signal "Location Updating Request" to the MSC in step 518 and the MSC sends the received signal to the VLR in step 519. Then, the VLR determines in step 520 whether the MS is within the service area of the VLR. If it is not, the VLR requests an HLR or another VLR to determine whether the call proceeds for the MS, in step 522, and receives a response message for the request in step 524. On the contrary, if the MS is within the service area of the VLR, the VLR checks the parameter WAITING_FLAG for the MS, in step 526. If the parameter is set to zero, the VLR performs a general location registration procedure in step 530. If the parameter is set to one, the VLR sends a signal "Reconnect" to the MSC in step 528. In step 531, the MSC releases the existing links and connects an initiated call. If the MS is within the service area of the same MSC, an origination call is set up, and the existing links are released and the initiated call is connected in the same manner as a hard handoff between BSs. On the other hand, If the MS is within the service area of a different MSC, an origination call is set up, and the existing links are released and the initiated call is connected in the same manner as a hard handoff between MSCs. In step 532, the MSC sends a signal DROP UNSET to the VLR and in step 534, the VLR changes the parameter WAITING_FLAG to zero. Then, the VLR sends a signal DROP CONF to the MSC in step 536. In step 538, the MSC withdraws the information message notifying the waiting for reconnection, and in step 540, determines whether the initiated call is normally connected to the other subscriber or the service provider. If it is normally connected, the call proceeds in step 542. Otherwise, the MSC sends an information message notifying a call release to the other subscriber or the service provider in step 544 and performs a call release operation in step 546. Meanwhile, if the timer set for Timer_Val2 (e.g., 30 to 60 seconds) has expired without receiving the reconnection request signal "Reconnect" from the MS in step 543, the MSC sends an information message notifying a call release to the other subscriber or the service provider in step 544 and performs a call release operation in step 546. FIGs. 13 and 14 illustrate call processing state transitions according to both conventional technology and in the present invention. As shown in FIG. 13, upon a call release request or a call drop for a predetermined time, for example, 5 seconds, a call release state is entered in the prior art. In comparison to the prior art, as shown in FIG. 4, upon a call drop for a first time (e.g., 1.2 seconds) during a call service, an idle state is entered in the present invention. Then, if a reconnection request is issued, the idle state transitions to a conversation state. However, if the call drop occurs a second time (preferably, 30 to 60 seconds) in the idle state, a call release state is entered. If a call release is requested in the conversation or idle state, the call release state is immediately entered. As described above, the present invention is advantageous in that the inconvenience of resuming a call temporarily dropped in an elevator or a tunnel overcome by providing an automatic dropped call reconnecting function in a PLMN. Therefore, subscribers are relieved of the concerns associated with call drops and call service quality can be increased. While the present invention has been described in detail with reference to the specific embodiments, they are mere exemplary applications. Thus, it is to be clearly understood that many variations can be made by anyone skilled in the art within the scope of the present invention. The following is a list of further preferred embodiments of the present invention: Embodiment 1. A method of reconnecting a communication link terminated by a service impediment during a service between a mobile station (MS) subscriber and the other party communicating with the MS subscriber through a mobile communication system having a plurality of mobile switching centers (MSCs) connected to one another, each MSC being connected to a plurality of base stations (BSs), the method comprising the steps of: sending a reconnection request signal by the MS subscriber when the service impediment lasts for at least a predetermined first time period; and reinitiating the service between the MS subscriber and the other party through one of the plurality of BSs and one of MSCs connected to the BS in response to the reconnection request signal. Embodiment 2. The method of embodiment 1 further comprising the step of maintaining an existing service channel between a service providing MSC and the other party by the MEC' when the service impediment lasts for the at least predetermined first time period. Embodiment 3. The method of embodiment 2 further comprising the step of reconnecting the maintained service channel by sending the reconnection request signal to the service providing MSC by at least one BS which receives the reconnection request signal. Embodiment 4. The method of embodiment 1 further comprising the step of maintaining an existing service channel between the service providing MSC and the other party by the MSC and sending an information message to the other party by the MSC on the maintained service channel indicating that the MSC is waiting for a reconnection when the service impediment lasts for the at least predetermined first time period. Embodiment 5. The method of embodiment 2 wherein the reinitiating step further comprises: searching by the service providing MSC for a new service channel between the MSC and the MS subscriber requiring a reconnection in response to the reconnection request; and reinitiating the service between the MS subscriber and the other party by connecting the new service channel to the service channel between the MSC and the other party. Embodiment 6. The method of embodiment 5 further comprising the step of: sending the other party an information message by the MSC indicating a call release on the maintained service channel, and releasing the maintained service channel by the MSC, if the search for the new service channel is not completed within a predetermined second time period. Embodiment 7. The method of embodiment 4 wherein the information message is one of an announcement and null data. Embodiment 8. The method of embodiment 1 wherein the predetermined first time period is between zero and several seconds. Embodiment 9. The method of embodiment 6 wherein the predetermined second time period is between several tens of seconds and several hundreds of seconds. Embodiment 10. The method of embodiment 8 wherein the predetermined first time period is registered as system initialization data and may be changed by an operator. Embodiment 11. The method of embodiment 9 wherein the predetermined second time period is registered as system initialization data and may be changed by the operator. Embodiment 12. A method of reconnecting a communication link terminated by a service impediment during a service between a calling terminal and a called terminal, at least one of the calling and called terminals being wirelessly connected to at least one BS, through a mobile communication system having a plurality of MSCs connected to one another, each MSC being connected to a plurality of BSs, the method comprising the steps of: storing information related with the BS by the terminal wirelessly connected to the BS; and sending the BS-related information by the calling or called terminal which stored the BS-related information during the service impediment. Embodiment 13. The method of embodiment 12 further comprising the step of detecting the service impediment by the terminal wirelessly connected to the BS prior to the sending step. Embodiment 14. The method of embodiment 13 wherein if the service impediment lasts for at least a predetermined first time period, the service impediment is considered to be detected. Embodiment 15. The method of embodiment 13 wherein if the terminal wirelessly connected to the BS receives a manual reconnection request, the service impediment is considered to be detected. Embodiment 16. The method of embodiment 13 wherein the service impediment is considered to be detected when the service impediment lasts for a predetermined first time period, or the terminal wirelessly connected to the BS receives a manual reconnection request, the service impediment is considered to be detected. Embodiment 17. The method of embodiment 14 wherein the predetermined first time period is between zero and several seconds. Embodiment 18. The method of embodiment 18 wherein the predetermined first time period is between zero and several seconds. Embodiment 19. The method of embodiment 13 wherein it is considered that the service impediment has occurred if the BS sends call-drop detection information. Embodiment 20. The method of embodiment 12 further comprising the step of maintaining an existing service channel between a service providing MSC and the other terminal by the MSC when the service impediment lasts for the at least predetermined first time period. Embodiment 21. The method of embodiment 20 further comprising the step of reconnecting the maintained service channel by sending the reconnection request signal to the service providing MSC by at least one BS which receives the reconnection request signal. Embodiment 22. A method of reconnecting a communication link terminated by a service impediment during a service between an mobile terminal and the other terminal communication with the mobile terminal through one of BSs connected to the mobile terminal and at least one of MSCs connected to the BS in a mobile communication system having the plurality of BSs and the plurality of MSCs, the method comprising the steps of: determining by one of the plurality of MSCs connected to one of the plurality of BSs, upon reception of a reconnection request signal including BS-related information from the mobile terminal, whether the BS-related information is related with the MSC; searching for a new service channel between the MSC and the mobile terminal by the MSC, if the BS-related information is related with the MSC; and connecting the new service channel to an existing service channel between the MSC and the other terminal by the MSC. Embodiment 23. The method of embodiment 22 further comprising the step of sending the other terminal an information message by the MSC indicating that the MSC is waiting for a reconnection on the service channel between the MSC and the other terminal, prior to the searching step. Embodiment 24. The method of embodiment 22 further comprising the step of: sending the other terminal an information message by the MSC indicating a call release on the service channel, and releasing the service channel, if the search for the new service channel is not completed within a predetermined second time period. Embodiment 25. The method of embodiment 24 wherein the predetermined second time period is between several tens of seconds and several hundreds of seconds. Embodiment 26. The method of embodiment 22 further comprising the step of sending a corresponding MSC which is related to the BS-related information the reconnection request signal including the BS-related information by the MSC, if the BS-related information is not related to MSC. Embodiment 27. A method of reconnecting a communication link terminated by a service impediment during a service between an mobile terminal and the other terminal communicating with the mobile terminal through one of BSs connected to the mobile terminal and at least one of MSCs connected to the BS in a mobile communication system having the plurality of BSs and the plurality of MSCs, the method comprising the steps of: sending service impediment information by the mobile terminal during the service impediment; and sending a location registration request message by the mobile terminal when a serviceable state is entered. Embodiment 28. The method of embodiment 27 further comprising the step of detecting the service impediment by the mobile terminal before the sending step. Embodiment 29. The method of embodiment 28 wherein the service impediment is considered to be detected when the service impediment lasts for a predetermined first time period. Embodiment 30. The method of embodiment 29 wherein the predetermined first time period is between zero and several seconds. Embodiment 31. The method of embodiment 27 further comprising the steps of: storing information indicating that the service for the mobile terminal is on hold by an MSC connected to the serving BS in a location register of the MSC, when the MSC receives the service impediment information through the BS; determining by one of the plurality of MSCs connected to one of the plurality of BSs whether the service impediment information is stored in a location register of the MSC when the MSC receives the location registration request message; searching by the MSC for a new service channel between the MSC and the mobile terminal if the service impediment information is stored in the location register of the MSC; and reinitiating the service between the two terminals by causing the MSC connecting the new service channel to a service channel between the MSC and the other terminal communicating with the mobile terminal. Embodiment 32. The method of embodiment 31 further comprising the step of sending the other terminal an information message by the MSC indicating that the MSC is waiting for a reconnection on the service channel between the MSC and the other terminal, prior to the storing step. Embodiment 33. The method of embodiment 31 further comprising the step of sending the other terminal an information message by the MSC indicating a call release on the service channel between the MSC and the other terminal, and releasing the service channel by the MSC, if the search for the new service channel is not completed within a predetermined second time period. Embodiment 34. The method of embodiment 31 wherein the predetermined second time period is between several tens of seconds and several hundreds of seconds. Embodiment 35. The method of embodiment 31 further comprising the step of requesting a home location register by the MSC to determine the service status for the mobile terminal, and receiving a response for the request by the MSC from the home location register, if the information is not registered. Embodiment 36. A method of reconnecting a communication link terminated by a service impediment during a service between an MS subscriber and the other subscriber communicating with the MS subscriber through a mobile communication system having a plurality of BSs and a plurality of MSCs, the method comprising the steps of: sending service impediment detection information by a service providing BS among the plurality of BSs to the MS subscriber and an MSC connected to the BS, when the service impediment lasts for at least a predetermined first time period; and reinitiating the service between the MS subscriber and the other subscriber through one of the plurality of BSs and one of MSCs connected to the BS in response of the reconnection request by the mobile terminal. Embodiment 37. The method of embodiment 36 further comprising the step of maintaining an existing service channel by one of the MSCs between the MSC and the other subscriber, when the service impediment lasts for the at least predetermined first time period. Embodiment 38. The method of embodiment 36 further comprising the step of maintaining the service channel by one of the MSCs between the MSC and the other subscriber and sending the other subscriber an information message by the MSC indicating that the MSC is waiting for a reconnection on the service channel, when the service impediment lasts for the at least predetermined first time period. Embodiment 39. The method of embodiment 36 wherein the reinitiating step further comprises the steps of: searching for a new service channel by a service providing MSC between the MSC and the MS subscriber requiring a reconnection upon request service of the reconnection; and connecting the new service channel to the service channel between the service providing MSC and the other subscriber. Embodiment 40. The method of embodiment 33 further comprising the step of sending the other subscriber an information message by the MSC indicating a call release on the maintained service channel and releasing the maintained service channel, if the search for the new service channel is not completed within a predetermined second time period.

Claims: A method of reconnecting a communication link terminated by a service impediment during a service between a mobile terminal (MS) and another terminal communicating with the MS through one base station (BS) and at least one mobile switching center (MSC) connected to the BS in a mobile communication system comprising a plurality of MSCs connected to one another, each MSC connected to a plurality of BSs and comprising a visitor location register (VLR), the method comprising the steps of : detecting (502) the service impediment in the MS; sending (504, 506) a release order signal by the MS via its serving BS to the connected MSC when the service impediment lasts for at least a predetermined first time period; setting (510) a drop call parameter for the MS by the VLR of the connected MSC in response to the release order signal; sending (516, 518, 519) a location registration request signal by the MS via its serving BS to the VLR of the connected MSC checking (526) the drop call parameter for the MS by the VLR after the VLR receives the location registration request signal; and releasing the communication link and reconnecting an initiated call (531) for the MS according the drop call parameter. The method of claim 1, further comprising the step of sending (509), upon reception of the release order signal by the MSC, an information message to the other terminal by the MSC indicating that the MSC is waiting for a reconnection. The method of claim 2 further comprising the steps of: determining (520) by the VLR whether the MS is within a service area of the VLR; sending (528) a reconnect signal by the VLR to the MSC if the MS is within the service area of the VLR; searching for a new service channel between the MSC and the MS by the MSC; and reconnecting the MS with the other terminal via the new service channel between the MSC and the MS and the existing service channel between the MSC and the other terminal by the MSC. The method of claim 3, further comprising the step of sending (514) the other terminal an information message by the MSC indicating a call release on the service channel between the MSC and the other terminal, and releasing the service channel by the MSC, if the search for the new service channel is not completed within a predetermined second time period. The method of claim 4, wherein the predetermined second time period is between several tens of seconds and several hundreds of seconds. The method of any of claims 3 to 5, further comprising the steps of: requesting (522) by the VLR a home location register or another VLR to determine the service status for the mobile terminal; and receiving (524) by the VLR a response for the request from the home location register, if the MS is not within the service area of the VLR. The method of any of claims 1 to 6, wherein the predetermined first time period is between zero and several seconds.

Inventors: CHUN KYUNG JOON, HAM SEOK JIN, CHUN, KYUNG JOON, HAM, SEOK JIN
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1999
Main classification: H04Q7
IPC codes: H04W76