CA2235688A1 - Method for handoff type selection by a target base station in a wireless telecommunications system - Google Patents

Abstract

In a wireless telecommunications system including a mobile switching center, a source base station and a target base station, a method for allowing the target base station to choose a handoff type is disclosed.
More particularly, the source base station specifies handoff type options which are sent to the target base station. The target base station chooses a particular handoff type and the call handoff is processed in accordance with that selection. Advantageously, multiple messages between the source base station and the target base station for agreement of handoff type are eliminated.

Description

~ CA 0223S688 1998-04-23 METHOD FOR HANDOFF TYPE SELECTION BY A TARGET BASE
STATION IN A VVIRELESS TELECOMMUNICATIONS SYSTEM

5 Cross Reference to Related Application:
This application is related to the applications of:
Michael F. Dolan, Thomas L. McRoberts, Eshwar Pill~llpalli and Thomas T.
Towle entitled "Wireless Telecommunications System For Improving Performance And Compatibility";
Deborah L. Barclay, Michael F. Dolan, Thomas L. McRoberts, Larry E.
Pelletier, Albert J. Sawyer and Joseph E. Seitz entitled "Method For Source Transfer In A Wireless Teleconllllul~ications System"; and Michael F. Dolan and Thomas T. Towle entitled "Method For Addressing Call Glare In Wireless Telecoll,.nulfications Systems" which applications are ~qsignP(l to 15 the ~sign~e of the present application, and are being filed con~ elllly herewith.
Technical Field:
This invention relates to wireless telecolllll.ullications n~lw~llk~, and more particularly, to enhancing flexibility available in such wireless telecollllllul~ications systems during call handoff procedures.
20 Background of the Invention:
The world-wide proliferation of wireless telecommunications presents an oppol~ ity for service providers positioned to benefit from an ever-growing ~lçm~n~l for convenient, reliable wireless service. As these service providers are well aware, controlling expenses while providing such service, via the procurement and 25 maintenance of state-of-the-art wireless telecommunications equipment, poses a significant challenge. Fxi~ting wireless service providers meet this challenge by implem~nting wireless telecommunications n~lwolk~ compri~ed of mobile switching centers (MSCs) interconnected to base stations. The MSC completes calls between mobile stations (that is, any mobile tçrmin~l using radio tr~n~mi.~sion) and other 30 parties. These "other parties" may be mobile stations or parties served by the public-switched telephone n~;lw~ Each base station is associated with a specific geographic region and is an interface between mobile stations within its region and the MSC.
Existing wireless telecommunications equipment places severe limitations on 3 5 the ability of service providers to devise varied network configurations. One area in which this rigidity is especially noticeable is during call handoff procedures. Call handoff is a process in which radio connections to the mobile station are added to and CA 0223~688 1998-04-23 deleted from various base stations as a mobile user travels from one geographic region to another. In exi~ting wireless telecommunications n~lwulh~, a source base station (that is, the base station ~ ell~ly serving the call) negotiates with a target base station (that is, the base station anticipated to receive control of the call) to determine which type of call handoffprocedure will take place. These negotiations are often accomplished via a mobile switching center and must occur during a predeterminedtime period so that the mobile user participating in the call does not experience interruptions in voice qualitv. In some circllm~t~nces, these call handofftype negotiations are not accomplished within the predetermined time and voice qualitv is 10 degraded or the call is dropped. Inconveniences associated with degraded voice quality and dropped calls result in wireless subscriber fl.~ lion and a loss of revenue for service providers.
Therefore, there is a need in the art for ensuring sllccecsful call handoff procedures in a wireless teleco~ fications network.
15 Summary of the Invention:
This need is addressed and a technological advance is achieved by allowing a source base station to provide multiple handoff type choices to a target base station.
More particularly, a source base station provides ~ set of handofftype choices to a target base station which chooses a type of call handoff (e.g., hard handoff, soft 20 handoff) and conveys its choice to the source base station. Advantageously, multiple messages between base stations for confirming a handofftype are avoided resulting in a more efficient transfer of an active mobile call.
Brief Description of the Drawings:
FIGs.1-4 are illustrative embo~liment~ of wireless telecommunications 25 systems in which the present invention may be practiced;
FIG.Sis a message flow diagram of mobile station call origination as p~lr~ ed in the wireless telecommunications system of FIG.l;
FIG.6is a message flow diagram of soft handoff source transfer as performed in the wilelcss telecommunications system of FIG.3;
FIG.7is a message flow diagram of inter-base station collllllu,~ications for soft handoff support as performed in the wireless telecommunications system of FIG.3;FIG.8is a message flow diagram of handoff type selection by a target base station as p~lrolllled in the wireless telecommunications system of FIG.3;
FIG.9is a message flow diagram addressing call glare as performed in the 35 wireless telecolll~llunications system of FIG.3;
FIG.lOis a message flow diagram of si~n~lin~ connection clearing by a target base station as performed in the wireless telecommunications system of FIG.3.

CA 0223~688 1998-04-23 Detailed Description:
FIGs. 1-4 are illustrative embo-limente of wireless telecommunications systems in which the present invention may be practiced. Although four embodiments are shown for clarity, those skilled in the art will recognize that the first and second 5 packet interconnection protocols may enable numerous other arrangements of wireless telecommunications systems.
FIG. 1 is a simplified block diagram of wireless telecommunications system 100 including mobile switching center (MSC) 102, first base station 110 and second base station 120. MSC 102 includes control processor 104 for executing tasks 10 associated with call control and mobile station mobility management. Control processor 104 is interconnected to call control processors of the first and second base stations via sign~ling links 131 and 133, respectively. In alternative embodiments, ~ign~ling links 131 and 133 resources are conserved by multiplexing into a common channel for acces~ing control processor 104 of MSC 102. Switch fabric 106 is interconnected to SDU 108 via user traffic (voice/data) link 135. In this embo-liment SDU 108 provides frame selection and voice coding for all base stations in wireless network 100 (in this example, base stations 110 and 120).
First base station 110 comprises call control processor 112 for ~lmini~tering functions associated with call origination and tç~rnin~tion, and controlling SDU 108 on a call-by-call basis; interconnection processor 114 for mapping proprietary connections 137 into the standard user traffic interface 139 to the SDU; and channel element 116 for establishing con~nu"ications between the base station and subscriber equipment, such as mobile station 160. Call control processor 112 and interconnection processor 114 comnlunicate with channel element 116 over proprietary int~ ces 137, as known in the art. SDU 108is interconnected to interconnection processor 114 via a first packet interconnection protocol over packet transport-based link 139. SDU 108is also interconnected to call control processor 112 via a second packet interconnection protocol over packet transport-based link 143 for allowing first base station 110 to control SDU 108, as necessary.
Similarly, second base station 120 comprises call control processor 122, interconnection processor 124 and channel element 126. Call control processor 122 and interconnection processor 124 co~"~"u-~icate with channel element 126 over proprietary int.?rf~es 129. Interconnection processor 124 is interconnected to SDU
108 via a first packet interconnection protocol over packet transport-based link 141 and call control processor 122 is interconnected to SDU 108 via a second packet interconnection protocol over packet transport-based link 145. Call processor 112 and call control processor 122 communicate directly via sign~ling link 105.

CA 0223~688 1998-04-23 In this illustrative embodiment, SDU 108 is positioned within MSC 102. The separation of the SDU function from a base station and its centralization enhances the abilit,v of service providers to enhance the efficiency of existing call proces~ing functions as described below.
FIG. 2 is a simplified block diagram of wireless telecommunications system 200 including mobile switching center 202, first base station 220 and second base station 240. Also shown is mobile station 260 served by the first base station. Mobile ~wilchillg center 202 includes call control processor 204 and switch fabric 206. Call control processor 204 is interconnected to call control processors of the first and 10 second base stations via ~i~n~ling links 201 and 203, respectively. Switch fabric 206 is interconnected to SDU 224 (positioned within the first base station) via user traffic (voice/data) link 209. In this embodiment, SDU 224 provides frame selection and voice coding for all calls initiated or handed offby means of hard handoffto base station 220.
First base station 220 comprises call control processor 222, SDU 224 and channel element 226. Channel element 226 is intercollllected to the rest of the components within the base station via propl;ctary interf~ces 227. Second base station 240 compri~es call control processor 242 and interconnection processor 244 which are illtelcolmected to channel element 246 via proprietary interfaces 247. In this 20 embodiment, SDU 224 not only serves the first base station but is shown interconnected to call control processor 242 and interconnection processor 244 of the second base station via packet transport -based sign~ling and user traffic link 233, and packet transport-based ~ign~ling link 231, ~s~e~;lively. Sign~ling link 231 allows SDU 224 to be controlled by other base stations while ~iFn~ling and user traffic link 25 233 enables communication of coded voice between a base station (in this case, second base station 240) and an SDU located in a di~e." base station (in this ~llple7 first base station 220) on a call by call basis. Although SDU 224 is shown positiQn~l within first base station 220, second base station 240 may include the SDU
in ~lt~n~tive embo liment~ Further, in al~ ive embol1iment~ a common sign~ling 30 ~.h~nr ~l is created by multiplexing multiple instances of sign~lin~ link 205 into a single int~ ce between base station 220 and call control processor 204 of MSC 202, or multiple instances of sign~ling link 207 into a single int~rf~ce between base station 240 and call control processor 204.
FIG. 3 illustrates a network configuration embodiment in which the SDU
35 function is located independently of both the MSC and base stations. In this embodiment SDU 310 provides frame selection and voice coding for all base stations in wireless network 300 and can be accessed by multiple base stations. More CA 0223~688 1998-04-23 s particularly, wireless telecommunications system 300 compri~es mobile switching center 302, SDU 310, first base station 320, and second base station 340.
MSC 302 comprises control processor 304 and switch fabric 306. In this embodiment, control processor 304 is interconnected to first base station 320 and second base station 340 via ~ign~lin~ links 301 and 303, respectively. SDU 310 is interconnected to switch fabric 306 of MSC 302 via user traffic link 307. SDU 310 also IllA~ n~ packet L~ o~l-based user traffic and si~n~ling links 313 and 315 to interconnection processors associated with first base station 320 and second base station, respectively. Packet transport-based .~ign~ling links 321 and 323 are subject to the second interconnection protocol and allow first base station 320 and second base station 340, respectively, to control the SDU when necess~. ~ as described in detail below.
First base station 320 comprises call control processor 322, interconnection processor 324 and channel element 326. Channel element 326 collllllunicates withother components within the base station over proprietary links 327. In this embodiment, first base station 320 serves mobile station 360. Similarly, second base station 340 comprises control processor 342 and interconnection processor 344 which are connPcted to charmel element 346 via proprietary interfaces 347.
Packet transport-based user traffic and sign~ling links 313, 315 are subject to the first intercormection protocol and enable the collllllullication of coded voice and associated ~ign~ling between the base stations and SDU 310 on a call by call basis. In alternative embo~liment~ multiple instances of ~ign~ling links 301 and 303, respectively, may be multiplexed into common c i~n~ling channels to reduce the overall number of c i~nAling links which may be transmitted by the system. Call control processor 322 and call control processor 342 can colll~llunicate directly via .~ign~lin~ link 305.
FIG. 4 is a simplified diagram illustrating yet another embodiment of the present invention including an "interworking processor" for p~.rOllllillg functions associated with lldllsrollllhlg data from a format used within the public switched telephone nclwolk to one used across an air interface.
Wireless telecommunications system 400 comprises MSC 402, interworking processor 410, SDU 420, first base station 430 and second base station 440. MSC 402 includes control processor 404 which communicates with the call control processors 432 and 442 of first base station 430 and second base station 440, respectively, via .sign~ling links 403 and 405, respectively. Also shown is switch fabric 406 which is interconn~ctecl to illlcl ~olkillg processor 410 via user data traffic link 407. In turn, interworking processor 410 is connected to SDU 420 via user data traffic link 411.

CA 0223~688 1998-04-23 SDU 420 provides frame selection and termination of the radio link protocol used for data tr~nsmission for all base stations in wireless network 400. SDU 420 m~int~ins packet transport-based links to the first and second base stations, as described below.
First base station 430 comprises call control processor 432 and interconnection processor 434 which communicate with channel element 436 over proprietary interfaces 437. Also shown is mobile station 460 served by the first base station 430.
In this embodiment, call control processor 432 is interconnected to SDU 420 via packet transport-based link 413 which is subject to the second interconnection protocol. Interconnection processor 434 is interconnected to SDU 420 via packet 10 transport-based link 415 subject to the first interconnection protocol. Second base station 440 comprises call control processor 442 and interconnection processor 444 which are connected to channel element 446 via proprietary int~ ce 447. Call control processor 442 is interconnected to SDU 420 via packet transport-based sign~ling link 417 while interconnection processor 444 is connected to the SDU via 15 packet transport-based link 419. SiFn~ling links 413 and 417 allow each base station to control SDU 420 as necess~- ~ for various processes including call handoff.
Sign~ling links 415 and 419 enable the co"ll"~ulication of coded data and associated sign~ling between each base station and the SDU on a call by call basis. Call control processor 432 and call control processor 442 can co~ licate directly via sign~ling 20 link 405.
The central, and independent location of i~lelwo~king processor 410 and SDU
420 allows wireless service providers great flexibility in n~lwulk configuration since the functionality associated with these two processes can be ~cce~se~l by a number of base stations. In other words, allocation of the il~ king process and the SDU
25 function on a per base station basis is not required. Although illl~wulking processor 410 is shown in a central location, alternative embo~liments may deploy the interworking pl~cessor in many other locations, such as a base station, MSC or within the SDU.
The above-described illustrative embo-limPnts are presented to exemplify the 30 network configuration flexibility enabled by the first and second packet interconnection protocols for cull~ lication between the MSC and base stations vi an SDU. Although the most common implen ent~tions of the present invention have been shown, those skilled in the art may devise numerous other arrangements using these packet transport protocols.
Predictably, the first and second packet ll~lsl)oll illlel~;û~ection protocols which enable the location flexibility also affect call processin~ To exemplify the impact on existing call processes, a series of message flow diagrams is presented in CA 0223~688 1998-04-23 FIGs. 5-10. For purposes of clarity, each message flow diagram is associated with a wireless telecom~ ~ications system depicted in FIGs. 1-4. Although the association with a wireless teleconllllul~ications system is made for clarity, those skilled in the art will recognize that these messages may be deployed in any number of wireless network configurations.
FIG. 5 is a message flow diagram depicting the exchange of messages required for origination of a call from a mobile station to another party. For purposes of example, assume that the messages described below are exchanged within wireless telecommunications system 100 as shown in FIG. 1. In this example, a user associated with mobile station 160 wishes to place a call to another party (not shown).
Accordingly, mobile station 160 transmits an origination message to its serving base station (that is, base station 110). Base station 110 receives the origination message and extends a service request message to MSC 102 over ~ign~ling link 131. In response to this service request message, base station 110 receives a connectionconfirm~tion message from MSC 102 over ~ign~ling link 131. Subsequently, MSC
102 sends an ~ nment request message to base station 110 over ~i~n~ling link 131.
After base station 110 receives the ~Signment request message from the mobile switching center over ~ign~ling link 131, b~e st~ on 110 ~signs radio resources to the call and initiates a packet transport b~ed channel establi~hment procedure for sign~lin~ between call control processor 112 and SDU 108 over si~n~lin~ link 143 to allow b~e station 110 to control SDU 108. B~e station 110 also establishes a packet transport b~ed colll~llul~ication link 139 between interconnection processor 114 and SDU 108. Subsequently, b~e station 110 establishes a traffic channel with mobilestation 160 and a call connection is made. B~e station 110 extends an ~c~ignmentcompletemessagetoMSC 102Over~i~n~linglink 131 toindicatethatitconsidersthe call to be in a "conversation state." In the p~fe.l~;d embodiment, the ac~ignment colllplete message includes a time parameter which indicates a more nearly exact time at which the mobile began to use the traffic channel. Advantageously, this time of origination allows the service provider to more accurately bill for the call.
FIG. 6 is a message flow diagram depicting messages exch~nged during soft handoff source transfer occurring when a user of a mobile station travels outside of the geographic area of a first base station. For purposes of this example, assume that the mobile station is mobile station 360 served by wireless telecoll~llul~ications system 300 shown in FIG. 3. Also ~sume that the mobile station is traveling out of the geographic region served by the first b~e station 320 (also known ~ the "source"b~e station) to the geographic area served by second b~e station 340 (also known ~
the "target" b~e station). Initiation of call control transfer from the source b~e station to the target base station is commenced when source base station 320 realizes that source transfer is necessary and extends a soft handoff source transfer message to MSC 302 over sign~ling link 301. MSC 302 receives the soft handoffsource transfer message and fo~ds it to target base station 340 over sign~ling link 303. The soft 5 handoff source transfer message includes information identifying the call currently served by source base station 320. In this example, assume that target base station 340 determinPs that it will accept the source transfer (in ~ltçrn~tive embodiments, the target base station may decline to accept the source transfer call). Accordingly, target base station 340 extends a packet connection request message to SDU 310 to create 10 si~n~lin~ link 323 in response to receiving the soft handoffsource transfer message from MSC 302. The packet connection request message çxtPn~led to the SDU
includes information which uniquely identifies the call ~ el-lly served by the source base station. SDU 310 then sends an acknowleclgment message to target base station 340. Target base station extends a soft handoffsource transfer acknowleclgment message to MSC 302 via .sign~ling link 303. Subsequently, MSC 302 forwards the soft handoff source transfer acknowlel1gment message to source base station 320 over si~n~ling link 301. In alternati,ve embo-liments, the soft handoffsource transfer message could have been sent directly from the source base station 320 to the target base station 340 across .si~n~lin~ link 305. The soft handoffsource transfer 20 acknowledgment message could also have been sent across .sign~lin~ link 305. Upon receipt of the soft handoff ~ re, acknowleclgm~nt message, source base station 320 extends a transfer prepare message to SDU 310 over link 313. SDU 310 responds with a transfer prepare acknowle~gment message to source base station 320 indicating its re~.1iness for source transfer. Upon receipt of the transfer prepare acknowledgment 25 message, base station 320 sends a source transfer commit message across sign~ling link 321 to SDU 310 to cause the transfer of call control. SDU 310 forwards the source transfer commit message to target base station 340 over si n~ling link 323.
Target base station 340 then responds to the SDU with a source llal~rel commit acknowle~1gm~nt message indicating that it now has control of SDU 310. SDU 310 30 fonvards the source transfer commit acknowledgment message to base station 320 across si~n~ling link 321. Next, target base station 340 sends a soft handoff source transfer complete message to MSC 302 via sign~ling link 303. This message notifies the MSC that base station 340 now has control of the call which was previously served by base station 320. Base station 320 then disconnects its connection 321 with SDU
35 310.
FIG. 7 is a message flow diagram outlining the messages exchanged among base stations during soft handoff add target procedures. "Soft handoff add target"

CA 0223~688 1998-04-23 refers to the process in which additional base stations become involved in the connection to the mobile station without disruption to the voice link. A traditional soft handoff scenario requires base stations participating in the handoffto exchange required control data. These control messages are passed between the base stations via the MSC. The latency introduced due to this procedure often does not meet the stringent timing requirements for succes~ful soft handoff in a wireless telecollm~ ications system. FIG. 7 illustrates direct base station to base station colll,nul~ications design~d to improve the timing for the e~ch~nge of data and thus, allow for con~iit~ntly successful soft handoffs. For purposes of example, assume that 10 the messages described below are exchanged within wireless telecommunication system 300 as shown in FIG. 3. For clarity, first base station 320 will be referred to as the "source" base station indicating that it is the base station which ~;ullcnlly has control of a call to which second base station 340 (also referred to as the "target" base station) is to be added. In accordance with the plefellcd embodiment, source base 15 station 320 cletennin~s that a handoff is required and issues a handoff request message to target base station 340 via ~i n~ling link 305. Target base station 340 dele~ es that it will accept the handoff. Accordingly, interconnection processor 344 in the target base station extends a packet-based connection request to SDU 310 to create ~ign~ling and user traffic link 315. SDU 310 completes connection 315 and returns a 20 connection acknowletlgment message to target base station 340 indicating that the connection has been established.
Target base station 340 then extends a handoffrequest acknowledgment message to source base station 320 over ~ign~ling link 305. SDU 310 begins to send packetized user trafffic messages to target base station 340 across link 315 immediately 25 after the connection acknowledgment message is sent. In turn, the target base station channel element 346 extends ~l~v~d traffic channel data frames to the mobile station particip~ g in the call which is being handed off. Upon receiving the first forward traffic charmel data frame, target base station channel element 346 begins to send reverse idle frames to SDU 310 via interconnection processor 344 over link 315.
30 Upon dete~ ;on by SDU 310 that link 315 to base station 340 is appro~liately established, the SDU extends a packet-based connected message to source base station 320 via sign~ling link 321. Subsequently, source base station 320 extends a handoff direction message to the mobile station participating in the call. More particularly, source base station 320 sends a ~ign~ling message to SDU 310 co~ i l-i l-g a handoff 35 direction message. SDU 310 sends the handoff direction message to the mobile station via link 313 which is int~rn~lly connected to base station channel element 326.
The mobile station extends a mobile station acknowleclgment order to acknowledge CA 0223~688 1998-04-23 the handoff direction message received. The mobile station acknowledgment order is delivered to SDU 310 via ~ign~ling link 313. SDU 310 then informs source base station 320 of successful delivery of the handoff direction message via a data forward sign~lin~ delivered message which is sent on si~n~ling link 321.
The mobile station extends a handoff completion message to SDU 310 via links 327/313 and 347 /315 after completion of the soft handoff to the target base station. Subsequently, the SDU forwards the handoff completion message to sourcebase station 320 via sign~ling link 321 and source base station 320 extends a handoff performed message to MSC 302 via sign~lin~ link 301 to inform it that the mobile10 station's active location has been changed.
The introduction of the first and second interconnection protocols enables several types of call handoffs in a wireless teleconll~ lications system as the mobile station moves from one base station to another. More particularly, the various types of handoff which occur include hard handoff, semi-soft handoff, soft handoff and soft 15 handoffwith consolidation. In the pler~ d embodiment, when a target base station receives a request from a source base station indicating that a handoff of a call is requested, the target base station determines which resources are available for the call.
For example, the source base station may request a soft handoffbut the target base station may only have resources for a hard handoff. This resource data is conveyed to 20 the source base station so that agreement of the handofftype is reached before the handoff procedure is commenced.
FIG. 8 is a message flow diagram depicting the messages exchanged in wireless telecommunications system 300 for d~le, ., ~ E handofftype selection by a target base station. In the pl~rell~;d embodiment, the source base station may allow 25 one or more handofftype options which are conveyed to the target base station. This particular embodiment supports both a m~ntl~tecl handofftype (i.e., the source base station allows only one handoffl or multiple handoff types. Advantageously, there is a reduction in the number of messages exchanged during a handoff scenario due to an increased efficiency in the handoff execution as a result of the handofftype selection 30 process. Further, all the handoffmessages including the list of allowed handofftypes can be circulated through the MSC across .si~n~ling links 301 and 303, thereby also allowing the MSC to exercise control of the handofftypes allowed.
For purposes of example, assume that control of mobile station 360, ~ elllly served by source base station 320, requires a handoff. The handofftype selection35 process begins when call controller 322 of source base station 320 extends a handoff required message to MSC 302 via ~ign~ling link 301. MSC 302 receives the handoffrequired message and extends a handoffrequest message to call controller 342 of CA 0223~688 1998-04-23 target base station 340 over sign~ling link 303. The handoffrequest message includes a list of allowed handofftypes as formulated by call controller 322 of the source base station.
Target base station 340 dele ...in~s which, if any, of the handofftype options it S will select to process this call. If the target base station detelmines that it may accommodate the requested handoff, interconnection processor 344 extends a connect message to the SDU to establish user traffic and ~i~n~ling link 315. SDU 310 responds to the connect message by establishing sign~ling link 315 to interconnection processor 344 of the target base station. Next, the target base station remains idle 10 while waiting to receive forward traffic channel frames from the source base station.
As soon as the first forward traffic channel data frame is received in target base station channel element 346, channel element 346 begins to send reverse idle frames to SDU
310 via links 315 and 347. Upon receipt ofthe idle frames, SDU 310 determin~s ifthe connection between the mobile station and channel element 346 of the target base 15 station has been approlll;ately established and SDU 310 extends a packet connection established message to source base station 320 via sign~ling link 313. In addition, target base station call controller 342 extends a handoffrequest acknowledgment message to MSC 302 via ~ign~ling link 303. MSC 302 then extends a handoff comm~n~ message to source base station call controller 322 so that the handoff can be 20 completed.
Another common occurrence in wireless telecommunications systems is referred to as "glare". A glare situation occurs when a user a~elllp~ to make a call at the same time that another party is a~Le~ tillg to call the same user. Traditionally, wireless telecolllmul~ications systems have been unable to accommodate call glare. In 25 other words, the mobile origin~tecl call is serviced by default. With the establishment of the first and second interconnection protocols, a call glare situation is detectable by both the MSC and the mobile station. However, it is the responsibility of the MSC to resolve the situation by allowing only one call to be connected. More particularly, when the mobile station has initiated a call and the MSC has elected to reject the 30 initiated call and instead deliver the incoming call to the mobile station, the MSC must transmit this information to the serving base station so that activities in the network may be synchronized. The base station must signal its accep~lce of the delivery of the incoming call. In some embo-liment~, the base station may reject delivery of the incoming call in which case the MSC must proceed to service the call ori~in~tç~l by 35 the mobile user.
FIG. 9 describes how ~i~n~ling between the MSC and a base station can be used to synchronize the network to a new call direction in glare situations (that is, how CA 0223~688 1998-04-23 to allow a mobile initiated call to be interrupted for delivery of a call to the mobile).
For purposes of example, assume that the messages described in message flow diagram FIG. 9 are exch~nged within wireless telecon,l,l~ications system 300 as shown in FIG. 3. The process begins when mobile station 360 transmits an origination message over an air interf~ce to its serving base station (in this example, first base station 320). Base station 320 acknowledges receipt of the origination message with a base station acknowledgment order which is delivered to the mobile station. Subsequently, base station 320 extends a service request message to MSC 302 to create eign~ling link 301 and to forward the origination request. MSC 302 responds 10 to base station 320 with a connection confirm~tion message indicating establishment of link 301. In this embodiment, the service request message includes mobile identity information such as its electronic serial number. MSC 302 then extends an ~ceignment request message to base station 320 requesting that the base station allocate radio resources for the call. The ~ceignment request message includes a call 15 direction element indicating the MSC's desire to change the direction of the call from mobile ori~in~ted to mobile termin~te~l In this embodiment, functions performed by SDU 310 are separated from the base station so MSC 302 identifies SDU 310 in its~esignment request message. Upon receipt of the ~ssignment request message, basestation 320 initiates the packet-based channel establiehm~nt procedure as described in 20 FIG. 5. Next, base station 320 sends a ch~nnel aeeignment message over the control channel of the radio interface to initiate an establiehment of a radio traffic channel to the mobile station. The mobile station and network then exch~nge necessary messages to acquire the mobile station and plopelly connect it.
After the radio traffic channel and packet mode channel have been established, 25 base station 320 extends an ~eeignment complete message to MSC 302 and indicates its acceptance of the call direction change by including a call direction acknowledgment element. Base station 320 then extends an alerting message to themobile station to cause ringing at the station via established links. When the call is ~ ,.e~, a connect order is transmitted to base station 320. Base station 320 then 30 extends a col"le~il message to MSC 302 indicating that the call has been answered at the mobile station and is in a conversation state.
FIG. 10 is a message flow diagram depicting messages exchanged within wireless telecoll~ ~ications system 300 to remove unnecessary connections. More particularly, during operation of a call that makes use of a soft handoff, a target base 35 station may be supplying a set of resources to support the call. A eign~ling connection specific to the call is also created between the MSC and the target base station.
Traditionally, when resources at the target base station are no longer required, they ' CA 0223~688 1998-04-23 must be removed from the call under direction of the MSC. In the pl~felled embodiment of the present invention, the target base station directly interacts with the MSC to remove such a ~ign~ling connection.
For purposes of example, assume that soft handoff has occurred and resources 5 at the target base station are no longer required. Accordingly, source base station 320 extends a soft handoffdrop target message to MSC 302 to be fol~ ded to target base station 340. Target base station 340 removes its packet connection 315 to SDU 310 and sends a soft handoffdrop target acknowledgment message to MSC 302 via link 303 to be fol~v~ded to source base station 320. Target base station 340, re~li7in~ that 10 it has no more radio resources allocated to the call, sends a clear request message to MSC 302 to request clearing of ~ign~ling link 303.
MSC 302 extends a clear command message to the target base station to instruct it to release the associated dedicated resource (that is, ~i n~ling link 303). In response to the clear command message, the target base station sends a clear complete 15 message and releases ~ign~ling link 303. Note that in ~lt~rn~tive embodiments, the soft handoff drop target and soft handoff drop target acknowle~lgm~nt messages is exchanged via ~ign~ling link 305.
Advantageously, the first and second interconnection protocols allow flexibility in MSC/base station communications which enables the above-described20 network configuration and call processing and control. Although the present invention has been illustrated using pr~r~lled embollim~nt~, those skilled in the art may devise other arrangements without departing from the scope of the invention.

Claims (7)

Claims:

1. In a wireless telecommunications system comprising a mobile switching center in a plurality of base stations, a method for accomplishing call handoff comprises the steps of:
a source base station extending a message including handoff type options;
a target base station selecting a handoff type option as received from the source base station; and performing call handoff between the source base station and the target base station in accordance with the handoff type selection.

2. In a wireless telecommunications system comprising a mobile switching center serving at least one source base station and at least one target base station, a method for selecting handoff type comprises the steps of:
the source base station extending a message to the mobile switching center wherein the message includes handoff type options;
the mobile switching center receiving the message and extending a handoff request message to the target base station; and the target base station selecting a handoff type.

3. The method of claim 2 further comprising the target base station extending a request acknowledgment message to the mobile switching center.

4. The method of claim 2 further comprising the mobile switching center extending a handoff command message to the source base station.

5. The method of claim 2 further comprising the mobile switching center determining the handoff type options.

6. The method of claim 2 further comprising the source base station extending a handoff request message including handoff type options to the target base station.

7. The method of claim 6 further comprising the target base station indicating the selected handoff type to the source base station.