Note: Descriptions are shown in the official language in which they were submitted.
CA 0223~669 1998-04-23
METHOD FOR SOURCE TRANSFER IN A
WIRELESS TELECOl\/IMUNICATIONS SYSTEM
Cross Reference to Related Application:
This application is related to the applications of:
Michael F. Dolan, Thomas L. McRoberts, Eshwar Pilk~npalli and Thomas T.
Towle entitled "Wireless Telecommunications System For Improving Performance
And Compatibility";
Deborah L. Barclay, Michael F. Dolan, Thomas L. McRoberts and Thomas T.
10 Towle entitled "Method For Handoff Type Selection By A Target Base Station In A
Wireless Telecommunications System"; and
Michael F. Dolan and Thomas T. Towle entitled "Method For Addressing Call
Glare In Wireless Telecommunications Systems" which applications are assigned tothe assignee of the present application, and are being filed con~ lly herewith.
15 Technical Field:
This invention relates to wireless teleco~ ications networks, and more
particularly, to source transfer among base stations in such wireless
telecommunications networks.
Background of the Invention:
The world-wide proliferation of wireless telecommunications presents an
opportLmity for service providers positioned to benefit from an ever-growing demand
for convenient, reliable wireless service. As these service providers are well aware,
controlling expenses while providing such service, via the procurement and
mai~llenallce of state-of-the-art wireless telecomlnullications eq-lipm~nt poses a
25 significant challenge. Fxi.cting wireless service providers meet this challenge by
implementing wireless telecommunications networks comprised of mobile switching
centers (MSCs) interconnected to base stations. The MSC completes calls between
mobile stations (that is, any mobile terminal using radio tr~n~mi~.sion) and other
parties. These "other parties" may be mobile stations or parties served by the public-
30 switched telephone network. Each base station is associated with a specific
geographic region and is an interface between mobile stations within its region and the
MSC.
A common occurrence in existing wireless telecollllllunications systems is the
need to transfer call control (hereinafter, referred to as source transfer) from a first
35 base station to a second base station as a mobile user travels from one geographic
region to another. Source transfer requires precise and rapid communication among
the mobile switching center and the affected base stations. Fxi~tin~ wireless
CA 0223~669 1998-04-23
telecommunications equipment places severe limitations on the ability of serviceproviders to devise various network configurations which might enhance the
efficiency of source transfer. This is because telecommunications equipment vendors
use rigid mobile switching center/base station interconnection protocols and routinely
5 dispose integral functions in a number of systems which inhibits the ability of
networks to pass call control among base stations. As a result, it is impossible to
choose and allocate, on a call-by-call basis, individual network components for
supporting source transfers. Indeed, service providers cannot create, either call-by-call
or network-wide, a customized wireless telecommunications configuration for
10 exploiting a synergy or minimi~inp problems associated with source transfer in a
particular geographic region. Re~L,dilling the use of wireless telecommunications
equipment for source transfer raises the cost of doing business for all wireless service
providers. These costs and inconveniences associated therewith are ultimately borne
by wireless service subscribers. Therefore, there is a need in the art for enhancing the
15 efficiency with which source transfers are accomplished in wireless
telecommunications networks.
Summary of the Invention:
This need is addressed and a technological advance is achieved by
interconnection protocols for supporting packet switched messages between the MSC
20 and base stations in wireless telecommunications systems. More particularly, a first
packet interconnection protocol establishes an interface b~lw~ll a selection
distribution unit (SDU) for performing frame selection and voice transcoding, and a
base station interconnection processor for ll~lslllillhlg sign~ling and user data to
mobile stations. A second packet interconnection protocol establishes an interface
25 between the SDU and a base station controller for transmitting control information.
In a prc;r~ d embodiment of the present invention, a wireless
telecG ll~ications system includes a mobile switching center which serves at least
two base stations. To transfer call control from a first (source) base station to a second
(target) base station, the source base station extends a handoff request message to the
30 mobile ~wilching center for forwarding to the target base station. A packet-based
connection is established between the selection/distribution unit (SDU) associated
with the call and the target base station. Control information is then exchanged to
accomplish source transfer from the source base station to the target base station.
Advantageously, a single, centrally accessible SDU accomplishes all processing
35 necessary for the source transfer. Multiple SDUs are not required. In other words, the
functionality of the SDU need not be duplicated in each base station as is required in
existing wireless telecommunications systems.
- CA 0223~669 1998-04-23
B~ef Description of the D~ a~
FIGs. 1-4 are illustrative embo-1iment.s of wireless telecommunications
systems in which the present invention may be practiced;
FIG. 5 is a message flow diagram of mobile station call origination as
5 performed in the wireless telecommunications system of FIG. 1;
FIG. 6 is a message flow diagram of soft handoffsource transfer as p~lrolllled
in the wireless telecolllmul~ications system of FIG. 3;
FIG. 7 is a message flow diagram of inter-base station communications for soft
handoff support as p~ lrolllled in the wireless telecommunications system of FIG. 3;
FIG. 8 is a message flow diagram of handoff type selection by a target base
station as p~,.rolllled in the wireless telecoll.~ ~ications system of FIG. 3;
FIG. 9 is a message flow diagram addressing call glare as performed in the
wireless telecommunications system of FIG. 3;
FIG. 10 is a message flow diagram of ~ign~ling connection clearing by a target
base station as ~t;lrolllled in the wireless telecon~llul~ications system of FIG. 3.
Detailed De~ ."tion:
FIGs. 1-4 are illustrative embo-liment~ of wireless telecommunications
systems in which the present invention may be p ~cticed. Although four embo(liment~
are shown for clarity, those skilled in the art will recognize that the first and second
packet inlel~;o~ ection protocols may enable numerous other arrangements of wireless
telecon~ ications systems.
FIG. 1 is a simplified block diagram of wireless telecoll"nul~ications 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
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 ~ipn~lin~ links 131 and 133, respectively. In alternative embodiments,
~i~n~lin~ links 131 and 133 resources are conserved by multiplexing into a common
channel for accessing control processor 104 of MSC 102. Switch fabric 106 is
intercol"~e~;led to SDU 108 via user traffic (voice/data) link 135. In this embodiment,
SDU 108 provides frame selection and voice coding for all base stations in wireless
network 100 (in this example, base stations I l O and 120).
First base station l lO comprises call control processor 112 for ~mini~tering
functions associated with call origination and termin~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 communications between the base station and subscriber
CA 0223~669 1998-04-23
equipment, such as mobile station 160. Call control processor 112 and
interconnection processor 114 communicate wvith channel element 116 over
proprietary int~ ces 137, as known in the art. SDU 108 is interconnected to
intelco~ e~;lion processor 114 via a first packet interconnection protocol over packet
transport-based link 139. SDU 108 is 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 compri~es call control processor 122,
interconnection processor 124 and channel element 126. Call control processor 122
10 and interconnection processor 124 conmlul~icate with channel element 126 overproprietary int~ ces 129. Interconnection processor 124 is interconnected to SDU108 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 ll~l~olL-based link 145. Call processor 112 and
15 call control processor 122 communicate directly via ~ign~ling link 105.
In this illu~l~d~ive embodiment, SDU 108 is positioned within MSC 102. The
separation of the SDU function from a base station and its centralization enh~n~es the
ability of service providers to enhance the efficiency of exi~ting call processin~
functions as described below.
FIG. 2 is a simplified block diagram of wireless teleco~ ications 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
switching 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
25 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 initi~te~l or handed off by means of hard handoff to base
station 220.
First base station 220 comprises call control processor 222, SDU 224 and
channel element 226. Channel element 226 is interconnected to the rest of the
components within the base station via proprietary intçrf~ es 227. Second base station
240 comprises call control processor 242 and hl~rcol~l ection processor 244 which are
interconnected to channel element 246 via proprietary interfaces 247. In this
35 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 ~i~n~ling and user traffic link 233, and
CA 0223~669 1998-04-23
packet transport-based ~i~n~ling link 231, respectively. Sign~ling link 231 allows
SDU 224 to be controlled by other base stations while ~ign~lin~ and user traffic link
233 enables co~ nullication of coded voice between a base station (in this case,second base station 240) and an SDU located in a dirrelclll base station (in this
example, first base station 220) on a call by call basis. Although SDU 224 is shown
positioned within first base station 220, second base station 240 may include the SDU
in alternative embol1imenti. Further, in alternative embo~1iment~, a common sign~ling
channel is created by multiplexing multiple instances of ii~n~ling link 205 into a
single int~rf~ce between base station 220 and call control processor 204 of MSC 202,
lO or multiple in~t~n~es of ~iFn~ling link 207 into a single interface between base station
240 and call control processor 204.
FIG. 3 illustrates a network configuration embodiment in which the SDU
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
15 in wireless network 300 and can be ~cces~ed by multiple base stations. More
particularly, wireless telecommunications system 300 comprises 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 interconnPcted to first base station 320 and
20 second base station 340 via si~n~ling links 301 and 303, respectively. SDU 310 is
interconnected to switch fabric 306 of MSC 302 via user traffic link 307. SDU 310
also ,,.~i"~ c packet transport-based user traffic and ~ign~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
25 the second interconnection protocol and allow first base station 320 and second base
station 340, respectively, to control the SDU when necessary as described in detail
below.
First base station 320 comprises call control processor 322, interconnection
processor 324 and channel element 326. Channel element 326 collllllullicates with
30 other 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 connected to channel element 346 via proprietary interfaces 347.
Packet transport-based user traffic and ci~n~ling links 313, 315 are subject to
35 the first interconnection protocol and enable the communication of coded voice and
associated ~i~n~lin~ between the base stations and SDU 310 on a call by call basis. In
alternative embodiments, multiple instances of si~n~ling links 301 and 303,
CA 0223~669 1998-04-23
respectively, may be multiplexed into common ~ign~ling channels to reduce the
overall number of .~i~n~lin~ links which may be transmitted by the system. Call
control processor 322 and call control processor 342 can con~ icate directly viasign~lin~ link 305.
FIG. 4 is a simplified diagram illustrating yet another embodiment of the
present invention including an "hltel~volking processor" for performing functions
associated with transforming data from a format used within the public switched
telephone network to one used across an air interface.
Wireless telecollJl,.u.lications system 400 comprises MSC 402, interworking
10 processor 410, SDU 420, first base station 430 and second base station 440. MSC 402
includes control processor 404 which co.."llu,licates with the call control processors
432 and 442 of first base station 430 and second base station 440, respectively, via
~ign~ling links 403 and 405, respectively. Also shown is switch fabric 406 which is
interconn~cted to illlel~JlLng processor 410 via user data traffic link 407. In turn,
interworking processor 410 is connected to SDU 420 via user data traffic link 411.
SDU 420 provides frame selection and tçrmin~tion of the radio link protocol used for
data tr~n~mi~.~ion for all base stations in wireless network 400. SDU 420 m~int~in~
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 collllllullicate 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 hll~lcol~llected to SDU 420 via
packet transport-based l;nk 413 which is subject to the second interconnection
protocol. Interconnection processor 434 is hll.,lcolmected to SDU 420 via packettransport-based link 415 subject to the first inlelcolmection protocol. Second base
station 440 comprises call control processor 442 and interconnection processor 444
which are connçcted to channel element 446 via proprietary interface 447. Call
control processor 442 is interconnected to SDU 420 via packet transport-based
~i~n~lin~ link 417 while ~ com1ection processor 444 is connected to the SDU via
packet transport-based link 419. Signslling links 413 and 417 allow each base station
to control SDU 420 as n~cess~. ~ for various processes including call handoff.
Sign~ling links 415 and 419 enable the co.lllllunication of coded data and associated
~ign~lin~ between each base station and the SDU on a call by call basis. Call control
processor 432 and call control processor 442 can cO~ icate directly via ~ign~ling
link 405.
The central, and independent location of interworking processor 410 and SDU
420 allows wireless service providers great flexibility in network configuration since
CA 0223~669 1998-04-23
the functionality associated with these two processes can be accessed by a number of
base stations. In other words, allocation of the interworking process and the SDU
function on a per base station basis is not required. Although interworking processor
410 is shown in a central location, alternative embodiments may deploy the
interworking processor in many other locations, such as a base station, MSC or within
the SDU.
The above-described illustrative embo-liment~ are presented to exemplify the
network configuration flexibility enabled by the first and second packet
interconnection protocols for communication between the MSC and base stations vi an
10 SDU. Although the most common implementations 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 transport illhlcontlection protocols
which enable the location flexibility also affect call proces~ing. To exemplify the
15 impact on existing call processes, a series of message flow diagrams is presented in
FIGs. 5-10. For purposes of clarity, each message flow diagram is associated with a
wireless telecollllllul~ications system depicted in FIGs. 1-4. Although the association
with a wireless telecolll-llul,ications system is made for clarity, those skilled in the art
will recognize that these messages may be deployed in any number of wireless
20 network configurations.
FIG. 5 is a message flow diagram depicting the exch~n~e 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
25 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 (t_at is, base station 110). Base station 110 receives the origination message
and extends a service request message to MSC 102 over si~n~lin~ link 131. In
response to this service request message, base station 110 receives a connection30 Co~ ion message from MSC 102 over si~n~ling link 131. Subsequently, MSC
102 sends an ~signment request message to base station 110 over ~ign~ling link 131.
After base station 110 receives the ~csignment request message from the mobile
switching center over iign~ling link 131, base station 110 assigns radio resources to
the call and initi~tes a packet transport based channel establi~hment procedure for
35 sign~ling between call control processor 112 and SDU 108 over ~ign~ling link 143 to
allow base station 110 to control SDU 108. Base station 110 also establishes a packet
transport based communication link 139 between interconnection processor 114 and
CA 0223~669 1998-04-23
SDU 108. Subsequently, base station 110 establishes a traffic channel with mobile
station 160 and a call connection is made. Base station 110 extends an ac.~ignment
complete message to MSC 102 over ~i~n~ling link 131 to indicate that it considers the
call to be in a "con~ alion state." In the prere.l~d embodiment, the ~signment
S complete 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 exchanged during soft
handoff source transfer occurring when a user of a mobile station travels outside of the
10 geographic area of a first base station. For purposes of this example, assume that the
mobile station is mobile station 360 served by wireless telecollll,lullications system
300 shown in FIG. 3. Also assume that the mobile station is traveling out of thegeographic region served by the first base station 320 (also known as the "source"
base station) to the geographic area served by second base station 340 (also known as
15 the "target" base station). Initiation of call control Lld.~r~. from the source base
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 ~ign~ling link 301. MSC 302 rec~ves the soft handoffsource transfermessage and fo.~v~ds it to target base station 340 over sign~ling link 303. The soft
20 handoffsource transfer message includes information identifying the call cullellLly
served by source base station 320. In this example, assume that target base station 340
determines that it will accept the source transfer (in alternative embo~liment.~, 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
25 ~ign~ling link 323 in response to receiving the soft handoff source transfer message
from MSC 302. The packet connection request message çxten-le~l to the SDU
includes infollllalion which uniquely identifies the call currently served by the source
base station. SDU 310 then sends an acknowledgme~t message to target base station
340. Target base station extends a soft handoff source transfer acknowledgment
30 message to MSC 302 via ~i~n~lin~ link 303. Subsequently, MSC 302 forwards thesoft handoff source transfer acknowledgment message to source base station 320 over
.sign~ling link 301. In alternative embo~liment~, the soft handoff source transfer
message could have been sent directly from the source base station 320 to the target
base station 340 across sign~ling link 305. The soft handoffsource transfer
35 acknowledgment message could also have been sent across .sign~ling link 305. Upon
receipt of the soft handofftransfer acknowle.lgment message, source base station 320
extends a transfer prepare message to SDU 310 over link 313. SDU 310 responds
CA 0223~669 1998-04-23
with a transfer prepare acknowledgment message to source base station 320 indicating
its re~-liness for source transfer. Upon receipt of the transfer prepare acknowledgment
message, base station 320 sends a source transfer commit message across signaling
link 321 to SDU 310 to cause the ~ sr~ of call control. SDU 310 forwards the
source ll~lsfel commit message to target base station 340 over sign~ling link 323.
Target base station 340 then responds to the SDU with a source transfer commit
acknowle~lgmrnt message indicating that it now has control of SDU 310. SDU 310
forwards the source transfer commit acknowle~lgment message to base station 320
across si~n~lin~ link 321. Next, target base station 340 sends a soft handoffsource
10 transfer complete message to MSC 302 via ~i~n~lin~ 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
310.
FIG. 7 is a message flow diagram outlining the messages exch~nged among
15 base stations during soft handoffadd target procedures. "Soft handoffadd target"
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 handoff to exchange
required control data. These control messages are passed b~lw~en the base stations
20 via the MSC. The latency introduced due to this procedure often does not meet the
stringent timing requirements for successful soft handoff in a wireless
teleco~ nullications system. FIG. 7 illustrates direct base station to base station
comnlul~ications designed to improve the timing for the exrl~nge of data and thus,
allow for con~i~t~ntly s~cces~ful soft handoffs. For purposes of example, assume that
25 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 ~ ly 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 plef~lled embodiment, source base
30 station 320 ~letermin~s that a handoff is required and issues a handoff request message
to target base station 340 via sign~ling link 305. Target base station 340 determines
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
sign~ling and user traffic link 315. SDU 310 completes connection 315 and returns a
35 connection acknowledgment message to target base station 340 indicating that the
connection has been established.
CA 0223~669 1998-04-23
Target base station 340 then extends a handoffrequest acknowledgment
message to source base station 320 over si~n~ling link 305. SDU 310 begins to send
packetized user traffic messages to target base station 340 across link 315 immediately
after the connection acknowle~lgment message is sent. In turn, the target base station
5 channel element 346 extends forward traffic channel data frames to the mobile station
participating in the call which is being handed off. Upon receiving the first forward
traffic channel data frame, target base station channel element 346 begins to send
reverse idle frames to SDU 310 via interconnection processor 344 over link 315.
Upon determin~tion by SDU 310 that link 315 to base station 340 is al)pl.p,iately
10 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 sign~ling message to SDU 310 co~ llg a handoff
direction message. SDU 310 sends the handoff direction message to the mobile
station via link 313 which is internally connected to base station channel element 326.
The mobile station extends a mobile station acknowle-lgm~nt order to acknowledgethe handoff direction message received. The mobile station acknowledgment order is
delivered to SDU 310 via si n~ling link 313. SDU 310 then informs source base
station 320 of successful delivery of the handoff direction message via a data forward
.sign~ling delivered message which is sent on sign~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 handoffto the target base
station. Subsequently, the SDU fol~v~ds the handoffcompletion message to source
base station 320 via si~n~lin~ link 321 and source base station 320 extends a handoff
performed message to MSC 302 via sign~ling link 301 to inform it that the mobilestation's active location has been changed.
The introduction of the first and second hll~rconnection protocols enables
several types of call handoffs in a wireless teleconllllullications 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
handoffwith consolidation. In the plef~lled 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 d~t~ . ."il~es 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
the source base station so that agreement of the handoff type is reached before the
handoffprocedure is commenced.
CA 0223~669 1998-04-23
FIG. 8 is a message flow diagram depicting the messages exchanged in
wireless telecommunications system 300 for detçrmining handofftype selection by a
target base station. In the plere.led embodiment, the source base station may allow
one or more handofftype options which are conveyed to the target base station. This
S particular embodiment supports both a m~n-l~te-l handofftype (i.e., the source base
station allows only one handoff) or multiple handofftypes. Advantageously, there is a
reduction in the number of messages exchanged during a handoff scenario due to an
increased efficiency in the handoffexecution as a result of the handofftype selection
process. Further, all the handoffmessages including the list of allowed handofftypes
10 can be circulated through the MSC across ii~n~ling links 301 and 303, thereby also
allowing the MSC to exercise control ofthe handofftypes allowed.
For purposes of example, assume that control of mobile station 360, ~;u~ lly
served by source base station 320, requires a handoff. The handofftype selectionprocess begins when call controller 322 of source base station 320 extends a handoff
15 required message to MSC 302 via ~i~n~ling link 301. MSC 302 receives the handoff
required message and extends a handoffrequest message to call controller 342 of
target base station 340 over ~i~n~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 determines which, if any, of the handofftype options it
will select to process this call. If the target base station det~nnines that it may
accommodate the requested handoff, interconnection processor 344 extends a connect
message to the SDU to establish user traffic and sign~lin~ link 315. SDU 310
responds to the col~le-;l message by establishing ~ign~ling link 315 to interconnection
25 processor 344 of the target base station. Next, the target base station remains idle
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 frarnes to SDU
310 via links 315 and 347. Upon receipt of the idle frames, SDU 310 detçnnines if
30 the connection bclw~ell the mobile station and channel element 346 of the target base
station has been applopliately established and SDU 310 extends a packet connection
established message to source base station 320 via ~i~n~ling link 313. In addition,
target base station call controller 342 extends a handoffrequest acknowledgment
message to MSC 302 via si~n~ling link 303. MSC 302 then extends a handoff
35 comm~ncl message to source base station call controller 322 so that the handoffcan be
completed.
CA 0223~669 1998-04-23
12
Another common occurrence in wireless telecommunications systems is
referred to as "glare". A glare situation occurs when a user ~lle~ to make a call at
the same time that another party is ~lle.l,l,ling to call the same user. Traditionally,
wireless telecon~ ications systems have been unable to accommodate call glare. In
other words, the mobile origin~te~l 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
10 initiated call and instead deliver the incoming call to the mobile station, the MSC must
transmit this inforrnation to the serving base station so that activities in the network
may be synchronized. The base station must signal its acceptance 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~ted by
15 the mobile user.
FIG. 9 describes how sign~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
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
20 diagram FIG. 9 are exchanged within wireless telecoll,l"ul,ications system 300 as
shown in FIG. 3. The process begins when mobile station 360 transmits an
origination message over an air interface 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 acknowle~lgment order which is delivered to the mobile
25 station. Subsequently, base station 320 extends a service request message to MSC 302
to create si~n~ling link 301 and to folv~d the origination request. MSC 302 responds
to base station 320 with a connection confirm~tion message indicating establi~hment
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
30 ~si nm.ont request message to base station 320 requesting that the base station
allocate radio resources for the call. The ~signment request message includes a call
direction element indicating the MSC's desire to change the direction of the call from
mobile origin~te~l to mobile t.?rmin~te-l In this embodiment, functions performed by
SDU 310 are separated from the base station so MSC 302 identifies SDU 310 in its35 ~c~ignment request message. Upon receipt of the ~signment request message, base
station 320 initiates the packet-based channel establishment procedure as described in
FIG. 5. Next, base station 320 sends a channel ~.cignment message over the control
CA 0223~669 1998-04-23
channel of the radio interface to initiate an establi~hment of a radio traffic channel to
the mobile station. The mobile station and network then exchange necessary
messages to acquire the mobile station and properly connect it.
After the radio traffic channel and packet mode channel have been established,
base station 320 extends an ~c~ignment complete message to MSC 302 and indicatesits 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
answered, a connect order is transmitted to base station 320. Base station 320 then
10 extends a connect message to MSC 302 indicating that the call has been answered at
the mobile station and is in a convels~ion state.
FIG. 10 is a message flow diagram depicting messages exch~nged within
wireless telecollll.lul~ications system 300 to remove unnece~s~ connections. More
particularly, during operation of a call that makes use of a soft handoff, a target base
15 station may be supplying a set of resources to support the call. A ~ign~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
must be removed from the call under direction of the MSC. In the pre~ d
embodiment of the present invention, the target base station directly interacts with the
20 MSC to remove such a ~i~n~ling connection.
For purposes of example, assume that soft handoffhas occurred and resources
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 forwarded to target base
station 340. Target base station 340 removes its packet connection 315 to SDU 310
25 and sends a soft handoffdrop target acknowle~lgment message to MSC 302 via link
303 to be rOl ~ded to source base station 320. Target base station 340, re~ ing that
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
30 instruct it to release the associated dedicated resource (that is, ~ign~ling link 303). In
response to the clear command message, the target base station sends a clear complete
message and releases ~ign~ling link 303. Note that in alternative embodiments, the soft
handoff drop target and soft handoff drop target acknowle~lgment 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-describednetwork configuration and call processing and control. Although the present invention
CA 02235669 1998-04-23
14
has been illustrated using plcrc~l~d embodiments, those skilled in the art may devise
other arrangements without departing from the scope of the invention.
