BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to the field of wireless communications, and in particular, to a method of reconnecting a dropped call in a mobile communication system.

2. Description of the Related Art 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 affects 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, the calling party must attempt to call the called party again.

To overcome the aforestated problem of call drops, several approaches have been suggested in the prior art including: (1) U. S. Patent No. 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) whereby the concept of an intelligent network introduced.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method of automatically reconnecting a dropped call during a call service by location registration in a mobile communication system.

Another object of the present invention is to provide a method of automatically reconnecting a call dropped during the call by location registration in a mobile communication system.

Briefly, these and other objects are accomplished by an automatic dropped call reconnection method in a mobile communicating system. In the method of reconnecting a communication link terminated by a service impediment during service between a mobile terminal of a first subscriber and a terminal of a second subscriber with the first subscriber communicating through one of a plurality of base stations (BSs) connected to the mobile terminal and at least one of a plurality of mobile switching centers (MSCs) connected to the BS in a mobile communication system having the plurality of BSs and the plurality of MSCs, an MSC detects a location register which registers or stores the previous location of the mobile terminal when one of the plurality of BSs and the MSC connected to the BS receives a location registration request message from the mobile terminal upon the service impediment. Then, the MSC determines whether information about the mobile terminal registered or stored in the location register indicates service in progress, and attempts reconnection paging to the mobile terminal to thereby reinitiate service between the mobile terminal of the first subscriber and the terminal of the second subscriber, if the mobile terminal information indicates service in progress.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: FIG. 1 is a block diagram of an example of a mobile communication system to which an embodiment of the present invention is applied; FIG. 2 is a flowchart illustrating a method for processing a dropped call according to an embodiment of the present invention;

FIG. 3 is a flowchart depicting an MS control operation according to an embodiment of the present invention; FIG. 4 is a flowchart depicting a BS control operation according to an embodiment of the present invention; FIG. 5 is a flowchart depicting an MSC (Mobile Switching Center) control operation according to an embodiment of the present invention; FIG. 6 is a flowchart depicting call processing in a conventional paging method based on the IS-634 standard; FIG. 7 is a flowchart depicting a VLR (Visitor Location Register) control operation according to an embodiment of the present invention; FIGs. 8A, 8B, and 8C are flowcharts depicting automatic reconnection of a call which was dropped in an old MSC service area after an MS subscriber moves to a new MSC service area; FIG. 9 is a state transition diagram for conventional call processing; FIG. 10 is a state transition diagram for call processing according to an embodiment of the present invention; FIG. 11 illustrates the format of a signal <Registration Notification Return Result> including a reconnection flag according to an embodiment of the present invention; FIG. 12 illustrates the format of a location registration cancellation command message constructed by adding <MSD ID> and <Location Area ID> to <Registration Cancellation>, as defined in IS-41; FIG. 13 illustrates the format of a registration reconnection message according to an embodiment of the present invention; FIG. 14 is a block diagram of a BS method for detecting a call drop according to an embodiment of the present invention; FIG. 15 illustrates paging areas each including a corresponding cell and its adjacent cells; and FIG. 16 illustrates an exemplary use of an intersystem page according to the embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described hereinbelow with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

Terms and Definitions "A call service in progress" : the state where voice communication and/or data communication of still images, moving pictures, and the like are being performed.

"Dropped call" : 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 unavailable service situations.

"Call disconnection" : the state where a call remains disconnected for a predetermined time.

An embodiment of the present invention will be described within 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 an embodiment of the present invention is applied includes an HLR (Home Location Register) 80, MSCs (Mobile Switching Centers) MSCO 70a and MSC1 70b, BSCs (Base Station Controllers) BSC00 64a, BSC01 64b, and BSC10 64c, BTSs (Base Station Transceiver Subsystems) BTS000 62a to BTS101 62f, and an MS 50. A plurality of HLRs and MSCs are interconnected in the PLMN to perform subscriber management and call switching. As shown in FIG. 1, a single HLR 80 is connected to a plurality of MSCs MSCO 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 single BSC and multiple BTSs.

An MSC controls a connection to the PSTN (Public Switch Telephone Network) and the PLMN. A BSC controls a radio link and performs handoffs, 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 region of tall buildings.

A call drop as defined in an embodiment 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.

FIG. 2 is a flowchart of a call process in which a dropped call is released and then reconnected according to an embodiment of the present invention. FIGs. and 7 are flowcharts depicting control operations in an MS, a BS, an MSC, and an VLR respectively according to an embodiment of the present invention.

Reconnection of a dropped call in accordance with an embodiment of the present invention will be described referring to FIGs. 2 to 5, and FIG. 7.

When a call is established and then the traffic channel in current use for the call service (call) is disconnected, the MS and the BS determines whether the call has been dropped. In steps 300 of FIG. 3 and 400 of FIG. 4, the MS and BS determine that the call service is in progress. Then, upon disconnection of the traffic channel, the MS and the BS determine whether the call has been dropped in steps 302 of FIG. 3 and 402 of FIG. 4. As disclosed in U. S. Application No.

09/294,046, if no frames have been received for a predetermined time (one to ten

seconds) on the current traffic channel or if successively received frames have errors, the MS and the BS consider the call as having been dropped. The predetermined time is preferably 1.2 seconds. The time of 1.2 seconds is about 5 seconds shorter than the time set for releasing a call in the prior art.

The ways an MS and a BS detect a call drop will be described in more detail.

FIG. 14 is a block diagram of the BS way of detecting a call drop.

Referring to FIG. 14, a CDMA signal received through an antenna 100 is converted to an IF (Intermediate Frequency) signal by an RF & XCVB (Radio Frequency & Transceiver Block) 102. A CMDB (CDMA Modulation and Demodulation Block) 104 converts the IF signal to a QCELP (Qualcomm Code Excited Linear Predictive coding) packet. During this operation, the CMDB 104 determines whether a packet frame is normal by checking its CRC (Cyclic Redundancy Code) and how much the frame is defective, and adds the abnormality indicating information (Quality Matrix: H'00-H'ff) to the converted packet. If the abnormality indicating information is zero, a TSB 106 subjects the packet frame received from the CMDB 1040 to an abnormality operation. If it is one, the TSB 106 converts the received packet frame to a PCM (Pulse Code Modulation) signal and sends the PCM signal to an MSC.

Upon determination that a packet frame is bad, the TSB 106 counts continuous errors and unreceived frames using a timer interrupt generated every 20ms to make a final decision whether 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 104, the TSB 106 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 106 determines that the call is dropped. Assuming that the first predetermined time is 2 seconds, the TSB 106 declares a call drop if it receives 80 consecutive bad frames. However, if two consecutive normal frames are received, a bad frame counter is initialised and the TSB 106 returns to a normal operation. On the other hand, if any of 20 consecutive frames is not received, the TSB 106 considers it to be predictive of a call drop. Then, if no consecutive frames are received for a second predetermined time, the TSB 106 determines that the call in progress is dropped. The second predetermined time is preferably shorter than the first

predetermined time.

Now, an MS detection of a call drop will be described. An MS monitors a forward traffic channel in an MS control on the traffic channel state. When the MS receives L (L is a natural numeral) consecutive bad frames on the forward traffic channel, it disables its transmitter. Then, if M (M is a natural numeral) consecutive frames are received, the MS enables the transmitter. The MS activates a fade timer for the forward traffic channel when the transmitter starts to act, in a traffic channel initialisation substate of the MS control on the traffic channel state. The fade timer is reset to N (N is a natural numeral) seconds when M consecutive good frames are received. When no consecutive good frames are received until the fade timer has expired, the MS disables the transmitter and declares a call drop.

Upon detection of a call drop in steps 302 of FIG. 3 and 402 of FIG. 4, the MS notifies the MS user that it is waiting for reconnection in step 304 of FIG.

3, and the BS performs a call release procedure in step 404 of FIG. 4. The MS can notify the MS user by illuminating an LED (Light Emitting Diode), displaying a message on a terminal display, or sounding specific tones. The MS user can also be informed of the automatic reconnection of a dropped call through the above methods.

Meanwhile, upon detection of the call drop, the BS notifies the MSC of the call drop by using an existing message or a new message. In the former case, element values in the existing message are combined without any modification to the existing message or a newly defined element is added to the existing message, in order to notify the MSC of a call drop.

Upon detection of the call drop, the BS notifies the MSC of the call drop with a message used in a call release procedure. The call release procedure in the BS (step 404 of FIG. 4) will be described in connection with FIG. 2.

Upon detection of the call drop, the BS transmits a conventional signal to the MSC, notifying the MSC that a call drop has occurred. The release signal is <Clear Request (DROP) > or <Release (DROP) >, as shown in FIG. 2. If the call drop notification information included in the release signal is set to, for example, 1 (=DROP), the release signal can be defined as a dropped call-caused call release signal. If it is set to 0 (=NORMAL), it can be defined as a normal call release

signal.

Upon receipt of the release signal <Clear Request> or <Release> in step 502 of FIG. 5 during a call (call service) in step 500 of FIG. 5, the MSC determines based on the call drop notification information whether the dropped call is to be reconnected in step 504 of FIG. 5. For example, if the call drop notification information is set to 1 (=DROP), the MSC determines that the dropped call should be reconnected.

The call drop notification information can be included in or added to the existing release signal <Clear Request> or <Release> in two ways. In one way, a call drop notification parameter can be set by use of available values defined in a "Cause"information element among message types, that is, Cause, Circuit Identity, and Code Extension, in the format of the release signal <Clear Request> or <Release> based on IS-64. Values defined in the"Cause"information element available as the call drop notification parameter include Uplink Quality (Cause: 0x02), Uplink Strength (Cause: 0x03), MS not equip (0x20), and BS not equip (0x25). In the second way, a call drop notification element is added to the signal format of <Clear Request> or <Release> based on IS-634.

Besides the existing release signal, the BS can use a newly defined message for transmitting the call drop notification information to the MSC. The new message is configured in the same manner as the signal <Clear Request> or <Release>.

If, in step 504 of FIG. 5, the dropped call is not to be reconnected, the MSC releases the call in step 520 of FIG. 5. On the contrary, if the dropped call is to be reconnected, the MSC transmits an information message notifying the other subscriber of the waiting for a reconnection in step 506 of FIG. 5. The information message takes the form of a voice message, music, tone, or mute for the other subscriber in communication, whereas it takes the form of null data for a data communication subscriber and a data service provider.

In step 508 of FIG. 5, the MSC activates a timer with a timer value T- Vall. The timer value T-Vall indicates the time period for which the MSC should receive a location registration request message <Location Update Request> from the BS to which the MS registers its location after the MSC receives the dropped call-caused call release signal. The timer value T-Vall

ranges from several seconds to several hundreds of seconds, is registered as initialisation data, and is varied by an operator according to system operation status and subscriber characteristics. For T-Vall, typical call release procedures (Clear Command, Complete, SCCP RLSD, and SCCP RLC) based on IS-634 are performed between the BS and the MSC in steps 404 of FIG. 4 and 510 of FIG. 5.

Returning to FIG. 3, when it detects a call drop, the MS notifies the user that it is waiting for reconnection in step 304 and activates a timer set to timer value T-Val2 in step 306. T-Val2 is several tens of seconds, preferably 30 seconds, and is the predetermined time in which a page message should be received from the BS after detection of a call drop. In step 308, the MS is initialised. System reacquisition is performed during the initialisation, and an MS idle state is maintained.

In this state, a radio channel on which the call has been dropped, for example, a service channel between the MS and the BS, is released, while service channels between the BS and the MSC, between MSC and another MSC, and/or the MSC and the other party communicating with the MS.

If the MS moves out of the area of service impediment, the MS transmits a location registration request message <Registration Message> to the BS in step 310 of FIG. 3. The BS determines whether a <Registration Message> has been received from the MS in step 406 of FIG. 4. Upon receipt of a <Registration Message>, the BS transmits the signal <Location Update Request> to the MSC, thus requesting a location update in step 408 of FIG. 4.

The MSC determines whether the location update request signal <Location Update Request> is received from the BS before the T-Vall timer expires in step 512 of FIG. 5. Upon receipt of a <Location Update Request>, the MSC determines whether the MSC requesting the location registration is within the coverage area of the MSC and if it is, the MSC sends a location registration request to its VLR in step 514 of FIG. 5. That is, the MSC transmits a signal <Registration Notification> to its VLR.

The case that the MS sending a location registration message is a subscriber to the MSC which receives the subsequent location registration request, will be described referring to FIG. 2. In FIG. 2, if the MS experiences a call drop in areas indicated by reference numerals 10,12, and 14, the MS is a subscriber to

the VLR 72a of the MSCO 70a.

However, when a call is dropped in the area indicated by reference numeral 16 as the MS moves out of the service area of the MSC1 70b and enters the service area of the MSCO 70a, the MS is not a subscriber to the MSCO 70a, from the perspective of the MSCO 70a.

A call reconnection for an MS which is a subscriber to the registering MSC will be described referring to FIG. 2, and then a call reconnection for an MS which is not a subscriber to the registering MSC will be described referring to FIGs. 8A and 8B.

If the MS requesting a location registration is a subscriber to the MSC, the MSC sends a location registration request to its VLR in step 514 of FIG. 5.

That is, the MSC transmits a <Registration Notification> to its own VLR.

The VLR receives <Registration Notification> from the MSC in step 700 of FIG. 7 and determines in step 702 of FIG. 7 whether the internal registered information about the corresponding MS indicates that call service (call) is in progress for the MS. If the call service (call) is in progress for the MS, it implies that the location registration request of the MSC is for the reconnection of an existing call. Hence, the VLR transmits a location registration request response message <Registration Notification Ack> with the reconnection flag set (reconnection flag = 1) to the MSC in step 704 of FIG. 700. On the contrary, if the call service (call) is not in progress for the MS, the VLR transmits the location registration request response message <Registration Notification Ack> with the reconnection flag unset (reconnection flag = 0) to the MSC in step 706 of FIG.

700.

FIG. 11 illustrates the structure of the location registration request response message <Registration Noti. Ack> based on IS-41, with an additional reconnection flag according to an embodiment of the present invention. Referring to FIG. 11, the reconnection flag occupies 1 bit in a spare area. To indicate that a dropped call can be reconnected due to successful location registration, the reconnection flag is set to 1 (=DCR: Drop Call Reconnection).

The MSC determines whether a location registration request response message <Registration Notification Ack> with the reconnection flag set to 1 has

been received from the VLR in step 516 of FIG. 5, and, if it has been received, transmits a location registration acknowledgement response <Location Update Accept> to the BS in step 518 of FIG. 5.

The BS determines whether the location registration acknowledgement response <Location Update Accept> has been received in step 410 of FIG. 4, and, if it has been received, transmits a location registration acknowledgement response <Registration Accepted Order> to the MS in step 412 of FIG. 4. Then, the MS receives the location registration acknowledgement response <Registration Accepted Order> in step 312 of FIG. 300.

After transmission of the location registration acknowledgement response <Location Update Accept>, the MSC transmits a signal <SCCP RLSD> to the BS and receives a signal <SCCP RLC> from the BS.

Then, the MSC transmits a paging request for dropped call reconnection <Paging Request> to the BS in step 520 of FIG. 5. The MSC transmits the signal <Paging Request> to the corresponding BS alone or the corresponding BS and its adjacent BSs together. The latter case is preferable.

A paging area for reconnection of a dropped call according to an embodiment of the present invention will be described in detail. The paging area for reconnection can be preset as internal data in the MSC. The single cell where a called subscriber is located is paged for reconnection or a paging area including adjacent cells is paged. Or the MSC can page a broad area including all the cells covered by the MSC. In an embodiment of the present invention, paging the corresponding cell only, the paging area, and the broad area are termed cell paging, PAI (Paging Area Identification) paging, and broad paging, respectively.

Primarily, cell paging is implemented and PAI paging is used to obtain a higher paging success rate than cell paging. It is preferable to use PAI paging as secondary paging after the primary paging or as the primary paging. Broad paging is used when the location of an MS is not detected or to increase the paging success rate by using a wider paging area after a failure of primary paging.

Meanwhile, the BS determines whether a <Paging Request> has been received in step 414 of FIG. 4. Upon receipt of a <Paging Request>, the BS transmits a page message to the MS as shown in FIG. 2 and in step 416 of FIG. 4.

While FIG. 2 illustrates the case that the page message occurs only once, it is preferable that the BS transmit the page message a plurality of times when it attempts a reconnection paging. In accordance with an embodiment of the present invention, it is preferable that the BS attempts cell paging as primary paging, PAI paging as secondary paging, and then another PAI paging across a wider area than the previous PAI paging. Broad paging is preferably performed when the location of an MS is not detected. The paging success rate can be increased by setting the paging area according to the environment and the number of pagings.

FIG. 15 illustrates an example of a paging area including a corresponding cell and its adjacent cells for call reconnecting paging. In FIG. 15, Cl to C-18 denote cells. If an MS which had a call drop is located in cell Cl, the MSC issues a primary paging request to all the cells Cl to C6 within the area PAI1.

The cells may be within the service area of the same MSC or different MSCs. If there is no response to the primary paging, the MSC can issue a paging request to an area wider than the area PAI-1. That is, the MSC can issue a secondary paging request to the area PAI 2 or to a broad paging area consisting of all the cells within the MSC.

In the case that a cell to which a reconnection paging is requested is within an MSC different from an MSC of its adjacent cells, the former MSC should issue a paging request to the latter MSC by an intersystem page.

FIG. 16 illustrates an example of an intersystem page, entitled Intersystem Page 2, according to an embodiment of the present invention.

Referring to FIG. 16, if a cell in which a call is terminated is cell C 3, MSC1 makes a call reconnection paging request to the cells within the area PAI-1, that is, C2 to C 5. The MSC1 can issue the paging request directly to the cells C2 and C 3 but must transmit a message including the IDs of the cells C4 and C5 to MSC2 by Intersystem Page 2 so that MSC2 can issue a paging request to cells C4 and C 5. Upon receipt of the message, the MSC2 attempts a paging request to cells C 4 and C 5. Upon receipt of a response to the paging request from a BS, the MSC2 transmits the received response to the MSC1 by Intersystem Page 2.

Intersystem Page 2 can increase th paging success rate for a subscriber at the boundary between MSCs.

Returning to FIG. 3, after receipt of a location registration acknowledgement response message <Registration Accepted Order> from the BS

in step 312, the MS determines whether a page message has been received from the BS within T-Val2 in step 316 of FIG. 3. If the MS fails to receive the page message within T-Val2, the MS releases the information message notifying the user that it is waiting for reconnection in step 318 of FIG. 3 and then enters an idle state in step 320 of FIG. 3. Upon receipt of the page message within T-Val2 in step 316 of FIG. 3, the MS deactivates the T-Val2 timer in step 322 of FIG. 3 and establishes a communication link by an IS-634 conventional paging method in steps 324 to 334 of FIG. 3.

Referring to FIG. 2, block 200 denotes the conventional paging procedure based on IS-634, and the signals in block 200 are shown for better understanding of an embodiment of the present invention. FIG. 6 is a flowchart depicting a call connection process by the conventional IS-634 paging method, as specified in block 200 of FIG. 2.

There will be hereinbelow given a description of a call reconnection in an MS, BS, and MSC after receipt of a page message by the MS with reference to FIGs. 2 through 6. Upon receipt of the page message, the MS transmits a page response message to the BS in steps 324 of FIG. 3 and c of FIG. 6. Upon receipt of the page response message in steps 418 of FIG. 4 and c of FIG. 6, the BS transmits a signal indicating successful paging, <Complete L3 Info: Paging Response> to the MSC in steps 420 of FIG. 4 and d of FIG. 6. The MSC determines whether the paging was successful or not by the reception or non- reception of the signal <Complete L3 Info: Paging Response> in step 522 of FIG.

5. Upon receipt of the signal <Complete L3 Info: Paging Response>, the MSC re- establishes a communication link in steps 524 of FIG. 5 and f to u of FIG. 6.

Then, the MSC releases the notification of waiting for reconnection from the other subscriber for communication in step 526 of FIG. 5. Meanwhile, if the signal <Complete L3 Info: Paging Response> has not been received in step 522 of FIG.

5, the MSC transmits an information message notifying the other subscriber of a reconnection failure in step 528 of FIG. 5 and then releases the call in step 530 of FIG. 5.

Upon receipt of a page response message in steps 418 of FIG. 4 and c of FIG. 6, the BS performs the subsequent paging steps (steps d to u of FIG. 6 and step 420 of FIG. 4) by the conventional paging method, like transmitting the signal <Complete L3 Info: Paging Response> to the MSC in step d of FIG. 6.

Thus, call service (a call) is in progress in step 424 of FIG. 4.

After transmitting the page response message to the BS in step 324 of FIG. 3, the MS performs the subsequent paging steps by the conventional paging method. More specifically, after the MS transmits the page response message to the BS, the MS determines whether a signal <Alert with Info> has been received in step 326 of FIG. 3. Upon receipt of the signal <Alert with Info>, the MS releases the waiting for reconnection notification in step 328, and then generates a ring sound in step 330 of FIG. 3. If the MS user responds in step 332 of FIG. 3, a call service state (a call state) is entered in step 334 of FIG. 3.

A call reconnection when the MS is not a subscriber to the registering MSC will be described referring to FIGs. 8A, 8B, and 8C. FIGs. 8A, 8B, and 8C are flowcharts depicting automatic call reconnection in a new MSC service area into which an MS subscriber moves, after moving out of the old MSC service area where the call was dropped.

Referring to FIGs. 8A and 8B, when the MS requesting location registration from a current MSC is not a subscriber to the current MSC, the current MSC transmits a location registration request signal <Registration Notification> to its VLR (current VLR). The current VLR determines whether it stores information about the corresponding MS in response to <Registration Notification> but it has no information about the MS because the MS is not a subscriber to the current MSC. Thus, the current VLR transmits the location registration request signal <Registration Notification> to the old HLR. The old HLR issues a location registration cancellation command, <Registration Cancellation>, to the old VLR.

FIG. 12 illustrates the structure of a location registration cancellation command message as defined in IS-41, with the addition of an MSD ID and a Location Area ID to the <Registration Cancellation> message according to an embodiment of the present invention. The MSC ID and Location Area ID are used for the Intersystem Page 2.

Upon receipt of the <Registration Cancellation>, the old MSC determines whether the information about the corresponding MS indicates that the MS is in call service. When normal call release is performed (when a call is terminated by the other party or an MS subscriber), the old MSC cancels the MS information registered in its VLR in response to the registration cancellation command of the

VLR and then notifies an HLR of successful implementation of the registration cancellation command. However, when the call has been dropped, the MS information registered in the VLR is maintained as call service in progress.

Therefore, the old VLR deletes the MS information and performs a normal operation if the MS information does not indicate call service in progress.

FIG. 8C is a flowchart depicting normal operation in this case. Referring to FIG.

8C, the old VLR deletes the MS information and transmits a response message to the HLR, notifying that the registration cancellation command result is success.

That is, the old VLR transmits a <Registration Cancellation Return Message (success) > to the HLR. The HLR transmits the signal <Registration Cancellation Return Message (success) > to the current VLR, which in turn transmits the signal to the current MSC. The current MSC transmits the location registration acknowledgement response message <Location Update Accept> to the current BS.

Then, the current BS transmits the location registration response message <Registration Accepted Order> to the MS.

Returning to FIG. 8B, if the corresponding MS information indicates call service in progress, the old VLR transmits <Registration Notification Return (failure) > to the HLR without deleting the MS information. The old VLR transmits a registration reconnection message to the old MSC.

FIG. 13 illustrates the structure of the registration reconnection message according to an embodiment of the present invention. The registration reconnection message includes the parameters of an electronic serial number (ESN), a mobile identification number (MIN), MSC ID, and a location area ID, which are used for Intersystem Page 2. The ESN is the product number of a terminal, the MIN is a subscriber number, the MSC ID is the ID of an MSC, and the location area ID indicates a paging area.

Returning to FIG. 8B, upon receipt of a <Registration Notification Return Result (failure) > from the old VLR, the HLR in turn transmits a <Registration Notification Return Result (failure) > to the current VLR, which transmits a <Registration Notification Return Result (failure) > to the current MSC. The current MSC transmits a location registration rejection response message <Location Update Reject> to the BS. The BS in turn transmits a <Location Update Reject> to the MS.

Meanwhile, the old MSC which received the registration reconnection message from the old VLR attempts intersystem paging to the area indicated by the location area ID of the MSC ID included in the registration reconnection message. That is, the old MSC transmits a message <Intersystem Page2> to the current MSC and the current MSC attempts paging the MS through the BS. Then the current MSC transmits a paging request signal to the BS and the BS transmits a page message to the MS in response to the paging request signal.

Then, as shown in FIG. 8A, the MS transmits a page response message to the BS and then the BS transmits the signal <Complete L3 Info: Paging Response> to the current MSC. In response to <Complete L3 Info: Paging Response>, the current MSC transmits a signal <InterSystem Page2 Return Result> to the old MSC.

The subsequent call reconnection steps are the same as those of FIG. 2 and thus their detailed description is omitted.

FIGs. 9 and 10 are state transition diagrams for call processing in the prior art and according to an embodiment of the present invention, respectively.

In the prior art, as shown in FIG. 9, upon a call release request or a predetermined time period after a call drop, for example, 5 seconds during a call, the idle state is directly entered. In comparison to the prior art, in an embodiment of the present invention, as shown in FIG. 10, upon a call drop after a first predetermined time (preferably, 1.2 seconds) during call service, a standby state is entered to await a call reconnection request, and the standby state transits to the conversation state by paging through a call reconnection request by location registration. However, if the call drop lasts for a second predetermined time (preferably, 30 to 60 seconds) in the standby state, the idle state is entered. If a call release is requested in the conversation or standby state, the idle 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 is alleviated by automatically reconnecting a dropped call through location registration in a PLMN. Therefore, subscribers are relieved of the concerns associated with call drops and call service quality can be increased.

While the invention has been shown and described with reference to a certain preferred embodiment thereof, it should be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.