CA2265400A1

CA2265400A1 - Improved mobile switching centre and method for handling a dropped connection between a mobile station and a basestation transceiver

Info

Publication number: CA2265400A1
Application number: CA002265400A
Authority: CA
Inventors: Henry Lee; Len Lewis
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1998-05-06
Filing date: 1999-03-17
Publication date: 1999-11-06
Also published as: US6246872B1

Abstract

A system and method for recovering from a lost connection between a mobile station and a mobile communications system after a traffic channel has been successfully assigned are provided. After the connection is lost, rather than dropping the call altogether, the party on the other end of the call is connected to an announcement. While the announcement is playing, the system pages the mobile station in an effort to re-locate it and re-establish a traffic channel. When a traffic channel is successfully re-established, the call is connected. The connection may be lost during handoff, conversation (normal use), mobile termination or mobile origination for example.

Description

?10l5202530CA 02265400 l999-03- l7_ 1 _IMPROVED MOBILE SWITCHING CENTRE AND METHOD FOR HANDLING ADROPPED CONNECTION BETWEEN A MOBILE STATION AND A BASESTATIONTRANSCEIVERField of the InventionThe invention relates to a method and mobileswitching centre for handling a dropped connection between amobile station and a mobile switching centre in a cellularcommunications system.Background of the InventionMobile communications systems typically consist of amobile switching centre connected to a number of basestationcontrollers, each basestation controller controlling a numberof basestation transceivers each located in a respective cell.A first connection can be established between a mobile stationlocated in a particular cell and the mobile switching centre byestablishing a traffic channel (RF link) between thebasestation transceiver in the particular cell, and acorresponding channel from the basestation transceiver throughthe relevant basestation controller to the mobile switchingcentre. A call between a mobile terminal and a second terminal(which may be a wireless terminal, a wireline terminal, or aninter-mobile switching centre agent for example) is connectedwhen a second connection from the mobile switching centre tothe second terminal is made, and the first and secondconnections are bridged.In the normal course of events, a traffic channel isassigned through a traffic channel assignment procedure andthen communications over the assigned traffic channel commence.This ânormal course of eventsâ includes handoff, conversationmode, mobile origination and mobile termination scenarios. Ina handoff, a traffic channel assignment is sent and the mobilestarts communicating with a new basestation over a new traffic?1015202530CA 02265400 l999-03- l7_ 2 _channel. In conversation mode the mobile simply continues tocommunicate over the assigned channel. In a mobileorigination, a traffic channel is assigned as a result of auser of the mobile originating a call and then communicationsover the traffic channel commence. In a mobile termination, atraffic channel is assigned as a result of another party makinga call to the mobile terminal, and then communications over theassigned traffic channel commence.In any of the above scenarios, the traffic channelmay be lost after the successful completion of the trafficchannel assignment. This may be due to failure to connect tothe newly assigned traffic channel as in the case of a handoff,mobile origination or mobile termination, or simply due to thefailure to maintain communications over the traffic channelafter it has been connected. In existing systems, when atraffic channel is dropped in any of the above-identifiedcircumstances, the entire call is torn down meaning that thesecond connection to the second terminal is also disconnected,and either the caller or the callee must redial the otherparty's number to be reconnected.One problem with this situation is that dropped callsand connect failures are annoying to customers. Moreimportantly, any time that a call is dropped or fails to beconnected, there exists the possibility that the partiesinvolved will not bother to attempt to reconnect, and thisrepresents lost revenue to the operating companies.Another drawback with dropped calls is that some ofthem may be emergency calls such as 911 calls and a personmaking such a call may not have the opportunity to attempt itagain should the call be dropped.?1015202530CA 02265400 l999-03- 17Summary of the InventionIt is an object of the invention to obviate ormitigate one or more of the above identified disadvantages.According to a first broad aspect, the inventionprovides in a communications system, wherein a call connectionbetween a first mobile terminal and a connection entitycomprises a first connection over an assigned traffic channelbetween the mobile terminal and a mobile switching centre and asecond connection between the mobile switching centre and theconnection entity, wherein said first connection may be lostafter successfully assigning said traffic channel, a method ofrecovering from such a lost first connection comprising thesteps of: a) maintaining said second connection; b) pagingthe first mobile terminal to determine its location; c)establishing a new first connection between the mobile terminaland the mobile switching centre; and d) bridging the secondconnection with said new first connection thereby reâconnectingthe call.According to another broad aspect, the inventionprovides a mobile switching centre for use in a communicationssystem, wherein a call connection between a first mobileterminal and a connection entity comprises a first connectionover an assigned traffic channel between the mobile terminaland the mobile switching centre and a second connection betweenthe mobile switching centre and the connection entity, whereinsaid first connection may be lost after said traffic channelhas been successfully assigned, the mobile switching centrecomprising: a) a connection holding mechanism for maintainingsaid second connection after said first connection is lost; b)a paging mechanism for paging the first mobile terminal todetermine its location; c) connection establishment circuitryfor coordinating the establishment of a new first connectionbetween the mobile terminal and the mobile switching centre;and d) connection bridging circuitry for bridging the second?1015202530CA 02265400 l999-03- l7-4-connection with said new first connection thereby reâconnectingthe call.Brief Description of the DrawingsPreferred embodiments of the invention will now bedescribed with reference to the attached drawings in which:Figure 1 is a block diagram of a mobilecommunications system according to an embodiment of theinvention;Figure 2 is a message flow diagram for recoveringfrom a connection dropped during handoff according to anembodiment of the invention;Figure 3 is a functional block diagram of a mobileswitching centre according to an embodiment of the invention;Figure 4 is a message flow diagram for recoveringfrom a traffic channel dropped during mobile originationaccording to an embodiment of the invention;Figure 5 is a message flow diagram for recoveringfrom a traffic channel dropped during mobile terminationaccording to an embodiment of the invention; andFigure 6 is a message flow diagram for recoveringfrom a traffic channel dropped during normal conversationaccording to an embodiment of the invention.Detailed Description of the Preferred EmbodimentsReferring firstly to Figure 1, a conventional mobilecommunications system includes an MSC (mobile switching centre)10, a plurality of Bss (basestations, only one shown) 12 eachconnected to the MSC through a respective link 13, and aplurality of BTSS (basestation transceivers, three shown)14,l6,18 for each BS connected to the respective BS throughrespective links l5,l7,l9. Each basestation transceiverprovides service to mobile stations located inside theirrespective cells 20,22,24. A single mobile station 26 is shown?1015202530CA 02265400 l999-03- l7_ 5 _located inside the cell 20 of one of the basestationtransceivers 14. In order to establish a call connectionbetween the mobile station 26 and a second terminal 28, a firstconnection shown schematically by a dotted line 30 from themobile station is made which includes a traffic channel over anRF link 32 between the mobile station and the BTS 14, and achannel from the BTS to the MSC 10 on links 15,13. A secondconnection shown schematically by a dotted line 33 is made fromIn the MSC, the firstthe MSC 10 to the second terminal 28.connection 30 is identified by a first PID (process identifier)34 and the second connection 33 is identified by a second PID36. A connection between the mobile terminal 26 and the secondterminal 28 is made when the first and second connections 30,33are bridged.When a connected mobile station 26 moves from onecell to another, for example from cell 20 to cell 22 it mustbreak its communications with the BTS 14 involved in theand establish communicationsexisting connection (source BTS)with the BTS (target BTS) 16 of the new cell 22 in a processknown as handoff. A handoff can be classified as either a softor a hard handoff.station 26 breaks communication with the source BTS 14 beforeA hard handoff occurs when the mobileit establishes communications with the target BTS 16. A softhandoff occurs when the mobile station 26 establishescommunications with the target basestation 16 before breakingcommunications with the source basestation 14.During hard handoff, when the mobile station 26 isdirected to tune to an assigned target traffic channel, itceases communications over its current traffic channel. Afterthe mobile station ceases communications over its currentchannel, if the mobile is unable to tune to the target trafficchannel, the mobile station is no longer communicating with anybasestation, and thus, it is eventually dropped from the call.The second terminal 28 is also dropped and the resources?1015202530CA 02265400 l999-03- l7-5-allocated to both the first and second PIDs 34,36 are de-allocated.During conversational or normal use in a non-handoffsituation, the mobile station 26 may cease to be properly tunedto its currently assigned traffic channel. In such situations,in conventional systems after a certain time period, the callis again dropped.Finally, during mobile origination and mobiletermination there is a signalling phase during which thelocation of the mobile station 26 is determined, and theparticular traffic channel to be used is assigned. Undernormal circumstances, the mobile station then tunes to theparticular traffic channel and a connection is made. However,in some cases, the mobile station 26 fails to be tuned to theparticular traffic channel. In conventional systems, the callis again dropped.According to an embodiment of the invention, a systemand method of recovering from a hard handoff failure areprovided. During a hard handoff, when the mobile station 26 isinstructed to tune to a target traffic channel of the targetbasestation 16, it first ceases communications with the sourcebasestation 14 and then attempts to establish communicationwith the target basestation 16. In the event that the mobilestation 26 is unable to tune to the target traffic channel, theMSC 10 is notified by the target basestation 16 that the mobilestation has not tuned to the target traffic channel. At thispoint, the MSC 10 deallocates the source basestation radioresource.A hard handoff recovery mechanism provided by thisembodiment of the invention which is then executed in an effortto reconnect the mobile station 26 without dropping the callwill now be described by way of overview. Firstly, the secondterminal 28 is connected to a connection holding mechanismwhich may for example be an announcement 40 (the announcement?1015202530CA 02265400 l999-03- l7-7-played on some type of announcement playing mechanism). Whilethe announcement 40 is playing, the MSC 10 pages the mobilestation 26. If the page fails, the MSC 10 may initiate a re-page. The cells specified in the page are based on one or moreof the following:target cells used in the hard handoff;cell information in the VLR (visitor location registry);andexisting paging algorithms for mobile termination.Existing paging algorithms include zone paging andbroadcast paging among others.When the mobile station 26 is paged, a specialringing tone is preferably applied to the mobile station and/ora message text is displayed on the mobile station to indicatethat the mobile station is being reconnected to an existingcall. When the mobile station answers, it is connected to thecall, the other party is disconnected from the announcement 40,and is reconnected to the call.A detailed implementation of the above summarizedhard-handoff recovery mechanism will now be described withreference to Figure 2 (which includes Figures 2A and 2B) inwhich it is assumed that the interface between the mobilestation 26 and the basestation 14 is ISâ95A, and the interfacebetween the basestation and the mobile switching centre 10 isTSB-80. The bold text highlights the modifications to theexisting handoff and paging message flows. Figure 2 contains amessage flow diagram in which the various components in thesystem are represented by vertical time lines, one each for theMSC,MS when in communication with the source BS, source BS,target BS, and the MS when in communication with the target B8.A message from one component to another is represented by anumbered and labelled arrow from one of the time lines toanother. The messages will now be described in sequence:Message 1. The basestation sends a Handoff Required?1015202530CA 02265400 l999-03- l7-8-message with a target cell list to the MSC to request a hardhandoff.Message 2. The MSC sends to the target basestation aHandoff Request message with a target cell list based on thereceived target cell list in the Handoff Required message.Message 3. The target basestation allocates the radioresource and sends to the MSC a Handoff Request Acknowledgemessage with the target cell(s) for which the radio resourceshave been successfully allocated.Message 4. The MSC sends a Handoff Command message to thesource basestation to convey the information received in theHandoff Requested Acknowledge message.Message 5. The source basestation sends an ExtendedHandoff Direction message to the mobile to instruct it to tuneto the target basestation.Message 6. The mobile sends a Mobile StationAcknowledgment Order message to the source basestationconfirming the receipt of the Extended Handoff Directionmessage.Message 7. The source basestation sends to the MSC aHandoff Commenced message indicating that the mobile has beeninstructed to tune to the target basestation and that it is nolonger communicating with the source basestation.Message 8. The mobile station is unable to tune to thetarget basestation. When the target basestation timer expiresit sends a Handoff Failure message to the MSC. The MSCconnects the other party (not shown) to an announcement.Message 9. The MSC sends a Clear Command message to thesource basestation to clear the source basestation radioresource.Message 10. The BS sends a Clear Complete message to theMSC when the radio resources have been cleared.Message 11. The MSC sends a Clear Command message to thetarget basestation to clear the target radio resources.?1015202530CA 02265400 l999-03- l7_ 9 _Message 12. The target basestation sends a Clear Completemessage to the MSC when the radio resources have been cleared.Message 13. The MSC builds a cell list based on the targetcell list used in the hard handoff, based on the VLR, or usingone of the existing paging algorithms for terminations. TheMSC sends a Paging Request message to one or more basestationswith the cell list.Message 14. The basestations send a Page messagecontaining the mobile station address over the paging channel.Message 15. The mobile station receives the Page messagefrom one of the basestations and acknowledges the Page messageby sending a Page Response message to the basestation.Message 16. The basestation sends a Paging Responsemessage to the MSC.Message 17. The basestation sends a BasestationAcknowledgement Order message to the mobile station toacknowledge receipt of the Page Response.Message 18. The MSC sends a Setup message (contained in aSCCP Connection Confirmed message) to the basestationcontaining all the required information necessary to initiateterminating call establishment.Message 19. Upon receipt of the Setup message, thebasestation sends a Call Confirmed message to the MSC.Message 20. The MSC sends an Assignment Request message tothe target basestation with a special ringing tone and/ormessage text. The target basestation sets up the radioresources.Message 21. The target basestation sends a ChannelAssignment message to the mobile station with a special ringingtone and/or message text.Message 22. The mobile station begins sending the trafficchannel preamble over the designated reverse channel.Message 23. Once the basestation acquires the reversetraffic channel, it sends a Basestation Acknowledgment Order?l0l5202530CA 02265400 l999-03- 17message to the mobile station.Message 24. The mobile station acknowledges the receipt ofthe Basestation Acknowledgment Order message by sending aMobile Station Acknowledgment Order to the basestation.Message 25. The basestation sends a Service OptionResponse Order message to the mobile station specifying theservice configuration for the call by way of a service optionnumber.accordance with the specified service option number.The mobile station begins processing traffic inMessage 26. After the radio traffic channel andterrestrial circuit have both been established and fullyinterconnected, the basestation sends the Assignment Completemessage to the MSC.Message 27. The basestation sends the Alert WithInformation message to the mobile station with a specialringing tone and/or message text.Message 28. The mobile station acknowledges the receptionof the Alert With Information message by sending a MobileStation Acknowledgment Order message to the basestation.Message 29. The call is answered at the mobile station, aConnect Order message is sent to the basestation.Message 30. The basestation acknowledges the Connect Ordermessage with the Basestation Acknowledgement Order over theforward traffic channel.Message 31. The basestation sends a Connect message to theMSC to indicate that the mobile is answered. The MSCdisconnects the other party from the announcement andreconnects the other party and the mobile to the call.Message 32. The basestation acknowledges receipt of theConnect message by sending a Connect Acknowledge message to theMSC.Referring now to Figure 3, a mobile switching centreaccording to an embodiment of the invention which is adapted toparticipate in the above described method comprises a?l015202530CA 02265400 l999-03- 17.11.connection holding mechanism 50, a paging mechanism 52,connection establishment circuitry 54, and connection bridgingcircuitry 56.The connection holding mechanism is for maintainingthe second connection after the first connection is lost. Thepaging mechanism is for paging the first mobile terminal todetermine its location. In the above described example, thesecond connection between the mobile switching centre and thesecond terminal is maintained by connecting it to anannouncement. any connection holding mechanismMore generally,may be used to achieve this. Such a connection holdingmechanism may be realized in hardware, software, or acombination of hardware or software.The connection establishment circuitry is forcoordinating the establishment of a new first connectionbetween the mobile terminal and the mobile switching centre.The connection bridging circuitry is for bridging thesecond connection with said new first connection thereby re-connecting the call.Both the connection establishment circuitry and theconnection bridging circuitry may be realized in software,hardware, or a combination of software and hardware.Interface ModificationsThis section describes the interface changes requiredfor a CDMA system using the TSB-80 BS to MSC andISâ95A BS to MS interfaces.The paging process is the same as for mobiletermination with the exception that a special ringing toneand/or message text is applied at the mobile. Changes arerequired in the messaging between the MSC and BS and betweenthe BS and MS.?1015202530CA 02265400 l999-03- 17.12..Basestation to Mobile Switching Centre Messages (TSB-80)In Table I below, the unmodified existing informationelements of the Assignment Request message are shown in plaintext. Existing elements which have been modified are shown initalics. In addition, an information element which is beingadded to the definition of the message is shown in bold.TABLE IInformation Element for Assignment Request Message0 Channel Type- Signalling Type- Priority0 Circuit Identity0 Cell Identifier0 Channel Number0 Encryption Information- Service Option0 Transcoder Mode- Signal0 Calling Party ASCII Number- DisplayIn the Assignment Request message, the âSignalâinformation element is modified. A new ringing tone value isdefined to identify the fact that a reconnect is beingperformed. The âDisplayâ information element is added to themessage definition. The Display information element containsthe message text to be applied to the mobile display.Mobile Station To Basestation Messages (ISâ95A)In Table II below the existing information elementsof the Alert With Information message are shown. The modifiedinformation elements are shown in italics.?l01520253035CA 02265400 l999-03- l7- 13 _TABLE IIInformation Elements for Alert With Information Message0 Encryption0 Information Record0 Display0 Called Party Number0 Calling Party ASCII Number0 SignalSignalIn the Alert With Information message, the âSignalâinformation element is modified. A new ringing tone value isdefined to cause a special ringing tone to sound identifyingthe fact that a reconnect is being performed.The above described message flow is specific to thecase where a recovery from a dropped connection during handoffis to be performed. The example includes messages which areparticular to the IS-95A and TSB-80 protocols. However, it isto be understood that this and other embodiments or theinvention are not limited to application in the context ofthese protocols.In the above described example, embodiments of theinvention provide a system and method for dealing with a firstconnection which is dropped during handoff. In such a case, afirst connection between the mobile station and the MSC throughthe target BS fails to be connected and is thus lost, and theMS is eventually connected to a BS (which may or may not be theMoreoriginal target BS) over a new first connection.generally, embodiments of the invention can be applied torecovering from a first connection which is lost duringconversation or during mobile origination or mobilethe first connection may betermination. During normal use,dropped due to interference for example. A new connection tothe MSC is established which may be through the same BS or?1015202530CA 02265400 l999-03- l7- 14 _through a different BS, this being dependent upon whichbasestation the Page Response message is received from.Similarly during mobile origination or mobile termination, thefirst connection may fail to be connected and thus become lost.A new first connection to the MSC is established which may bethrough the same BS or through a different BS again dependingupon which basestation the Page Response message is receivedfrom.According to another embodiment of the invention, asystem and method for the recovery from a dropped trafficchannel during mobile origination are provided, and will bedescribed with reference to Figure 4 which is a message flowdiagram.Message 1. The mobile stations transmits anOrigination message with layer 2 acknowledge over the accesschannel of the air interface to the basestation to requestservice.Message 2. The basestation acknowledges the receiptof the Origination message with a Basestation AcknowledgmentOrder to the mobile station.Message 3. The basestation constructs a CM ServiceRequest message, places it in a Complete Layer 3 Informationmessage, and sends the message of the MSC.Message 4. The MSC uses the SCCP Connection Confirmedprimitive without (layer 3) user data to complete establishmentof the signalling channel connection. The MSC sends the SCCPConnection Confirmed message to the basestation.Message 5. Once the signalling connection isconfirmed, the basestation sends a Setup message to the MSC,providing the necessary information for call setup to thecalled party.Message 6. The MSC returns a Call Proceeding messageto the basestation to indicate that the call establishment hasbeen initiated in the network and that additional call?1015202530CA 02265400 l999-03- 17.15..establishment information will no longer be accepted.Message 7. The MSC sends an Assignment Requestmessage to the basestation to request assignment of radioresources and traffic channel.Message 8. The basestation sends a Channel Assignmentmessage over the paging channel of the radio resource toinitiate the establishment of a radio traffic channel.Message 9. The mobile station begins sending thetraffic preamble over the designated reverse traffic channel.Message 10. Once the basestation acquires the reversetraffic channel, it sends a Basestation Acknowledgment Ordermessage to the mobile station over the forward traffic channel.Message 11. The mobile station acknowledges thereception of the Basestation Acknowledgment Order message bytransmitting the Mobile Station Acknowledgement Order messageand by sending null traffic channel data over the reversetraffic channel.Message 12. The basestation then sends the ServiceOption Response Order message to the mobile station specifyingthe service configuration for the call. The mobile stationbegins precessing traffic.Message 13. After the radio traffic channel andterrestrial circuit have been both established and fullyinterconnected, the basestation send the Assignment Completemessage to the MSC.Message 14. The mobile station loses communicationwith the basestation. The basestation sends a Clear Requestmessage with an abnormal cause value to release the call.Message 15. Upon receipt of the Clear Requestmessage, the MSC sends a Clear Command message to thebasestation to instruct the basestation to release theassociated dedicated resource.Message 16. In response to the Clear Command message?l015202530CA 02265400 l999-03- 17.16.the basestation releases the associated dedicated resource andreturns a Clear Complete message to the MSC.Message 17. The MSC sends the SCCP RLSD message tothe basestation to release the SCCP connection.Message 18. In response to the SCCP RLSD message, thebasestation releases the SCCP connection and returns a SCCP RLCmessage to the MSC.Message 19. Description for messages 19 through 38 isthe same as in the hard handoff message flow (message 13through 32) described previously with reference to Figure 2.According to another embodiment of the invention, amessage flow for the recovery from a dropped channel duringmobile termination is provided, and will be described withreference to Figure 5.Message 1. The MSC determines that an incoming callterminated to a mobile within its serving region and sends aPaging Request message to the basestation to initiate a mobileterminated call setup scenario.Message 2. The basestation issues a Page messagecontaining the mobile station address over the paging channel.Message 3. The mobile station acknowledges the pageby transmitting a Page Response message over the accesschannel.Message 4. The basestation constructs a PagingResponse message and sends the message to the MSC.Message 5. The basestation acknowledges the receiptof the Page Response message from the mobile station with aBasestation Acknowledgment Order message to the mobile station.Message 6. The MSC sends the Setup message to thebasestation to initiate terminating call establishment.Message 7. Upon receipt of the Setup message(contained in the SCCP Connection Confirmed message), thebasestation sends the Call Confirmed message to the MSC.?1015202530CA 02265400 l999-03- l7_ 17 _Message 8. The MSC sends an Assignment Requestmessage to the basestation to request assignment of radioresources and a traffic channel.Message 9. The basestation sends a Channel Assignmentmessage over the control channel of the radio interface toinitiate the establishment of the radio traffic channel.Message 10. The mobile station begins sending thetraffic channel preamble over the designated reverse trafficchannel.Message 11. Once the basestation acquires the reversetraffic channel, it sends the Basestation Acknowledgment Orderto the mobile station over the forward traffic channel.Message 12. The mobile station acknowledges thereception of the Basestation Acknowledgment Order message bytransmitting the Mobile Station Acknowledgment Order.Message 13. The basestation sends the Service OptionResponse Order message to the mobile station specifying theservice configuration for the call.Message 14. After the radio traffic channelterrestrial circuit have both been established, the basestationsends the Assignment Complete message to the MSC.Message 15. The mobile station loses communicationwith the basestation. The basestation sends a Clear Requestmessage with an abnormal cause value to release the call.Message 16. Upon receipt of the Clear Requestmessage, the MSC sends a Clear Command message to thebasestation to instruct the basestation to release theassociated dedicated resource.Message 17. In response to the Clear Command message,the basestation releases the associated dedicated resource andreturns a Clear Complete message to the MSC.Message 18. The MSC sends the SCCP RLSD message tothe basestation to release the SCCP connection.theMessage 19. In response to the SCCP RLSD message,?1015202530CA 02265400 l999-03- 17.18.basestation releases the SCCP connection and returns a SCCP RLCmessage to the MSC.Message 20. Description for messages 20 through 39 isthe same as in the hard handoff message flow (messages 13through 32) described previously with reference to Figure 2.According to another embodiment of the invention, amessage flow for the recovery from a dropped channel duringnormal conversation is provided, and will be described withreference to Figure 6.Message 1. The mobile station and basestation losecommunication. The basestation sends a Clear Request messagewith an abnormal cause value to release the call.Message 2. Upon receipt of the Clear Request message,the MSC sends a Clear Command message to the basestation toinstruct the basestation to release the associated dedicatedresource.Message 3. In response to the Clear Command message,the basestation releases the associated dedicated resource andreturns a Clear Complete message to the MSC.Message 4. The MSC sends the SCCP RLSD message to thebasestation to release the SCCP connection.Message 5. In response to the SCCP RLSD message, thebasestation releases the SCCP connection and returns a SCCP RLCmessage to the MSC.Message 6. Description for messages 6 through 25 isthe same as in the hard handoff message flow (messages 13through 32) described previously with reference to Figure 2.Preferably, the re-paging methods and systemsprovided by embodiments of the invention are invoked as afunction of considerations such as the importance of the call,capacity considerations, and handoff failures. For example, adropped 911 call should be given a high priority for re-if a hardware outage resultsestablishment. On the other hand,in thousands of dropped calls, it may not be desirable to?101520CA 02265400 l999-03- 17.19..repage all of the mobiles involved in the dropped calls.In the illustrated examples, a second terminal isconnected through a second connection directly to the MSCcommunicating with the mobile station. It is to be understoodthat the second terminal may be any connection entity includingan inter-for example a wireless terminal, a wireline terminal,MSC agent, or another switch. Furthermore, it is beunderstood that the âsecond connectionâ may be any type ofconnection between a connection entity and the MSC including awireless connection, a wireline connection to another MSC, awireline connection to a PSTN switch, or combinations of all ofthe above and/or other connection types.It is to be noted that this invention is used whenthe traffic channel has been successfully assigned and themobile station loses communication with the basestation. Thisis to be differentiated from Page Request timeâout for callterminations and Assignment Request failures for callorigination and terminations. Error handling for these twoscenarios is already covered in the TSBâ8O specification.Numerous modifications and variations of the presentinvention are possible in light of the above teachings. It istherefore to be understood that within the scope of theappended claims, the invention may be practised otherwise thanas specifically described herein.

Claims

We Claim:

1. In a communications system, wherein a call connection between a first mobile terminal and a connection entity comprises a first connection over an assigned traffic channel between the mobile terminal and a mobile switching centre and a second connection between the mobile switching centre and the connection entity, wherein said first connection may be lost after successfully assigning said traffic channel, a method of recovering from such a lost first connection comprising the steps of:
a) maintaining said second connection;
b) paging the first mobile terminal to determine its location;
c) establishing a new first connection between the mobile terminal and the mobile switching centre; and d) bridging the second connection with said new first connection thereby re-connecting the call.

2. A method according to claim 1 wherein said first connection was lost during a handoff.

3. A method according to claim 1 wherein the first connection was lost during normal use.

4. A method according to claim 1 wherein the first connection was lost during mobile origination or mobile termination.

5. A method according to claim 1 wherein said step of maintaining said second connection comprises the step of connecting the second connection to an announcement.

6. A method according to claim 2 wherein said step of maintaining said second connection comprises the step of connecting the second connection to an announcement.

7. A method according to claim 3 wherein said step of maintaining said second connection comprises the step of connecting the second connection to an announcement.

8. A method according to claim 4 wherein said step of maintaining said second connection comprises the step of connecting the second connection to an announcement.

9. A method according to claim 2 wherein in said step of paging the mobile station, the mobile station is paged in a plurality of cells defined by a target cell list generated during handoff.

10. A method according to claim 1 wherein in said step of paging the mobile station, the mobile station is paged in one or more cells defined by cell information stored in a visitor location registry.

11. A method according to claim 1 wherein in said step of paging the mobile station, the mobile station is paged with a broadcast page.

12. A method according to claim 1 wherein in said step of paging the mobile station, the mobile is paged using zone paging.

13. A method according to claim 1 further comprising the step of:
c1) after said step of establishing a new first connection between the mobile terminal and the mobile switching centre, alerting the mobile station with a special ringing tone and/or message text;

wherein said step of bridging the second connection with said new first connection is executed after the call is answered at the mobile station.

14. A mobile switching centre for use in a communications system, wherein a call connection between a first mobile terminal and a connection entity comprises a first connection over an assigned traffic channel between the mobile terminal and the mobile switching centre and a second connection between the mobile switching centre and the connection entity, wherein said first connection may be lost after said traffic channel has been successfully assigned, the mobile switching centre comprising:
a) a connection holding mechanism for maintaining said second connection after said first connection is lost;
b) a paging mechanism for paging the first mobile terminal to determine its location;
c) connection establishment circuitry for coordinating the establishment of a new first connection between the mobile terminal and the mobile switching centre; and d) connection bridging circuitry for bridging the second connection with said new first connection thereby re-connecting the call.

15. A mobile switching centre according to claim 14 wherein connection holding mechanism comprises an announcement playing mechanism.

16. A mobile switching centre according to claim 14 further comprising means for generating a target cell list which identifies cells to be paged by said paging mechanism.

17. A mobile switching centre according to claim 14 further comprising:
means for alerting the mobile station with a special ringing tone and/or message text when a new second connection is ready to be established.