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METHOD OF AUTOMATICALLY RECONNECTING A DROPPED
CALL IN A MOBILE COMMUNICATION SYSTEM
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. 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) whereby the concept of
an intelligent network introduced.
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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;
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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.
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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, BSCO1 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
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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
S 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 SO 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 SO communicating with the
BTS001 62b or the BTSO10 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.
3, 4, 5, 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
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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'ffj 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
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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
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signal.
Upon receipt of the release signal <Clear Request> or <Release> in step
502 of FIG. 5 during a call (call service) in step S00 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. S. 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
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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
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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 onset (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
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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.
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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, C_1 to
C_18
denote cells. If an MS which had a call drop is located in cell C_1, the MSC
issues a primary paging request to all the cells C_1 to C 6 within the area
PAI_1.
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, MSC
1
makes a call reconnection paging request to the cells within the area PAI_1,
that is,
C 2 to C 5. The MSC 1 can issue the paging request directly to the cells C_2
and
C 3 but must transmit a message including the IDs of the cells C_4 and C_5 to
MSC2 by Intersystem Page 2 so that MSC2 can issue a paging request to cells
C 4 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 MSC 1 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
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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 4I8 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 a 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 a 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.
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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
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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.
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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
S 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
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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.
