as) United States
`a2) Patent Application Publication (10) Pub. No.: US 2004/0246990 Al
`
` Krishnamurthiet al. (43) Pub. Date: Dec. 9, 2004
`
`
`US 20040246990A1
`
`(52) US. CM.
`
`cecccssssssssssststnsneeetstee 370/466; 370/331
`
`(54) SYSTEM AND METHOD FOR HANDING
`OVER A CALL FROM A
`PACKET-SWITCHED NETWORK TOA
`CIRCUIT-SWITCHED NETWORK
`
`(75)
`
`ABSTRACT
`(67)
`A communications system and method are provided for
`Inventors: Govind Krishnamurthi, Arlington, MA
`handing overa call from a packet-switched network(e.g., IP
`(US); Hemant M. Chaskar, Woburn,
`network) to a circuit-switched network(e.g., PLMN, PSTN,
`MA(US)
`etc.). The system includesafirst terminal capable of com-
`municating via a packet-switched network and a circuit-
`switched network, and a second terminal capable of com-
`municating via a circuit-switched network. The system also
`includes a galeway capable of mapping Communications
`etween a packet-switched network and
`at
`least one circuit-
`switched network. The gateway can support communication
`between the first terminal and the second terminal such that
`the first terminal has a packet-switched connection with the
`gateway and the second terminal has a circuit-switched
`connection with the gateway. The first terminal can then
`establish a circuit-switch connection with the gateway.
`Thereafter,
`the gateway can connect the circuit-switched
`connection established between the first terminal and the
`gateway with the circuit-switched connection between the
`gateway and the second terminal.
`
`Correspondence Address:
`ALSTON & BIRD LLP
`BANK OF AMERICA PLAZA
`
`101 SOUTH TRYON STREET, SUITE 4000
`CHARLOTTE, NC 28280-4000 (US)
`:
`.
`.
`.
`(73) Assignee: Nokia Corporation, Espoo (FI)
`(21) Appl. No.:
`10/454,685
`
`(22)
`
`Filed:
`
`Jun.4, 2003
`
`Publication Classification
`
`(SL)
`
`TInt, C0?ccc ccceseeeeesccssssssneeeecceccsesnneeeees H04J 3/16
`
`70
`
`SETUP VOICE CALL
`FROM PS NETWORK TO
`CS NETWORK
`
`
`START
`
`
`
`
`
` 72
`
`GENERATE CALL
`SESSION ID AND
`
`
`SHARED SECRET
`
`
`
` RECEIVE CALL SESSION ID
`AND SHARED SECRET AT
`GATEWAY AND MOBILE
`
`STATION
`
`
`
`SIGNAL STRENGTH
`BELOW THRESHOLD
`
`
`FOR MORE THAN
`
`
`PREDEFINED TIME?
`
`YES
`
`74
`
`
`
`
`
`
`76
`
`
`
`RECEIVE HANDOFF
`IDENTIFIER AT MOBILE
`
`
`STATION
`
`78
`
`IPR2021-00297
`
`Bose Exhibit 1102
`Bose Exhibit 1102
`Bose v. Koss
`Bose v. Koss
`IPR2021-00297
`
`

`

`Patent Application Publication Dec. 9, 2004 Sheet 1 of 4
`
`US 2004/0246990 Al
`
`34
`
`
`FIXED PS
`TERMINAL
`
`s7 28
`
`s 30
`
`TERMINAL
`
`a 16
`
`SERVER
`
` FIXED CS
`
`
`cs
`MOBILE FF
`STATION
`PLMN
`
`22
`
`VLR
`
`HLR
`
`24
`
`26
`
`
`
`

`

`Patent Application Publication Dec. 9, 2004 Sheet 2 of 4
`
`US 2004/0246990 Al
`
`
`
`TRANSMITTER
`
`
`
`53
`
`58
`
` DISPLAY
`
`56
`
`KEYPAD
`
`
`RECEIVER
`CONTROLLER
`
`eae]
`SPEAKER |
`
`52
`
`
`
`MICROPHONE
`
`VOLATILE
`MEMORY
`
`64
`
`NON-
`VOLATILE
`MEMORY
`
`60
`
`SIM
`
`FIG. 2.
`
`

`

`Patent Application Publication Dec. 9, 2004 Sheet 3 of 4
`
`US 2004/0246990 Al
`
`
`
`
`
`
`
`
`
`RECEIVE CALL SESSION ID
`AND SHARED SECRET AT
`GATEWAY AND MOBILE
`
`STATION
`
`
`
`RECEIVE HANDOFF
`
`
`IDENTIFIER AT MOBILE
`STATION
`
`70
`
`f2
`
`74
`
`76
`
`78
`
`
`
`START
`
`
`
`SETUP VOICE CALL
`
`
`FROM PS NETWORK TO
`
`CS NETWORK
`
`GENERATE CALL
`SESSION ID AND
`SHARED SECRET
`
`
`
`
`SIGNAL STRENGTH
`
`
`BELOW THRESHOLD
`FOR MORE THAN
`
`
`PREDEFINED TIME?
`
`YES
`
`FIG. 3A.
`
`

`

`Patent Application Publication Dec. 9, 2004 Sheet 4 of 4
`
`US 2004/0246990 Al
`
`80
`
`82
`
`88
`
`
`
`INITIATE COMMUNICATION
`WITH GATEWAY OVER CS
`NETWORK BASED UPON
`
`
`HANDOFF IDENTIFIER
`
`
`
`ACCEPT COMMUNICATION
`AND RECEIVE CALL
`SESSION ID AND SHARED
`
`SECRET
`
`
`SHARED
`
`SECRET
`VERIFIED?
`
`
`DROP COMMUNICAITON
`YES
`
`AND CONTINUE CALL
`
`
`
`
`BETWEEN PS NETWORK
`ESTABLISH CIRCUIT OVER
`
`AND CS NETWORK
`
`CS NETWORK BETWEEN
`
`MOBILE STATION AND
`
`GATEWAY
`
`90
`
`
`
`CONNECT CIRCUIT BETWEEN
`
`
`MOBILE STATION AND GATEWAY
`
`
`WITH CIRCUIT BETWEEN
`GATEWAY AND TERMINAL
`
`92
`CONTINUE VOICE CALL
`
`OVER CS NETWORK(S)
`
`
`
`
`FIG. 3B.
`
`

`

`US 2004/0246990 Al
`
`Dec. 9, 2004
`
`In another dimension,footprints of wireless access
`[0005]
`networks that are enabled with VoIP (e.g., WLAN, UMTS,
`cdma2000) will overlap with footprints of those using
`traditional circuit switched technology for voice (e.g., GSM,
`GPRS, IS-136, IS-95). In this regard, consider a mobile node
`that has two interfaces, such as WLAN and GSM. Assume
`that the mobile nodeis currently in the coverage of an indoor
`WLAN network and engaged in a voice call over a WLAN
`interface using VoIP with a correspondentnode whois using
`a traditional (non-IP) telephone connected to the PSTN.In
`this regard, provisions exist within SIP to enable establish-
`mentof such a call, a part of which spans IP network and a
`part the PSTN. When the mobile node moves out of the
`coverage of WLAN network, however, the call is typically
`dropped. Dropping suchcalls, as will be appreciated,is often
`an annoyanceto endusers since they have to initiate a new
`call to resume the voice call. However, there are currently no
`techniques, to best knowledge of the inventors, that allow
`the user in an IP access network to undergo handoff to a
`circuit-switched access network without
`interrupting the
`voice call.
`
`SUMMARYOF THE INVENTION
`
`In light of the foregoing background, embodiments
`[0006]
`of the present invention provide a system and method of
`handing off a call from a packet-switched network, such as
`an IP network,
`to a circuit-switched network, such as a
`Public Land Mobile Network (PLMN)or a Public Switched
`Telephone Network (PSTN). Advantageously,
`the system
`and method of embodiments of the present invention are
`capable of handing off such a call in a seamless manner such
`that the call can be handedoff such that the end points of the
`communication are not aware of the handover and/or do not
`have to take any specific action to effectuate, or as a result
`of, the handover. Additionally, such calls can be handed off
`to the circuit-switched network without being dropped. Calls
`can therefore be initiated and conducted between a terminal
`in a packet-switched network and a terminal in a circuit-
`switched network, while increasing the likelihood that the
`call can be completed uninterrupted. Embodiments of the
`present invention are therefore particularly advantageousin
`instances such as when a mobile station, engaged in such a
`call from a packet-switched network, movesoutside an area
`serviced by the packet-switched network and within an area
`serviced by a circuit-switched network.
`
`dance with this aspect of the present invention, the gateway
`is capable of supporting communication between the first
`terminal and the second terminal such thatthe first terminal
`
`has a packet-switched connection with the gateway and the
`second terminal has a circuit-switched connection with the
`gateway.
`
`[0008] After one of the first and second terminals has
`setup communication with the other terminal, the first ter-
`
`[0003] At the same time as voice communicationis evolv-
`ing, wireless networks are evolving from circuit-switched
`voice networks (e.g., GSM,
`IS-136,
`IS-95)
`to packet-
`switched networks
`(e.g, WLAN, UMTS, cdma2000)
`capable of supporting multimedia applications to mobile
`end-users over IP. General Packet Radio Service (GPRS),
`whichis an evolution of GSM,can support packet data(e.g.,
`web browsing, email) in a cellular environment. Further
`evolution of GPRS, often referred to as the Universal Mobile
`Telecommunication System (UMTS), is expected to support
`real-time multimedia over IP (e.g., VoIP, video over IP,
`streaming media) in a cellular environment. In addition, the
`Third Generation Partnership Project (GPP) has specified
`the IP Multimedia Subsystem (IMS) in UMTSto accomplish
`the control and service functions of wireless IP multimedia.
`[0007] According to one aspect of the present invention, a
`In this regard, the 3GPP has adopted SIP as the signaling
`
`communications system is provided that includesafirst
`terminal, such as a mobile station or a fixed terminal,
`protocol in IMS. At the sametime, in the cdma2000 world,
`the 3GPP2 has been developing the IP Multimedia Sub-
`capable of communicating via a packet-switched network
`system (IMS), formerly referred to as the IP Multimedia
`(e.g.,
`IP network) and a circuit-switched network (e.g.,
`Domain (MMD),
`to implement
`the control and service
`PLMN, PSTN,etc.). The system also includes a second
`functions of wireless IP multimedia. The 3GPP2 has also
`terminal, such as another mobile station or fixed terminal,
`capable of communicating via a circuit-switched network.In
`addition, the system includes a media gateway capable of
`mapping communications between a packet-switched net-
`work and at least one circuit-switched network. In accor-
`
`SYSTEM AND METHOD FOR HANDING OVER A
`CALL FROM A PACKET-SWITCHED NETWORK
`TO A CIRCUIT-SWITCHED NETWORK
`
`FIELD OF THE INVENTION
`
`[0001] The present invention relates generally to voice
`communication systems and methods of providing voice
`communications crossing between packet-switched net-
`worksandcircuit-switched networks and, more particularly,
`relates to systems and methods for handing off such calls
`from the packet-switched network to a circuit-switched
`network.
`
`BACKGROUND OF THE INVENTION
`
`[0002] Voice communication is evolving from circuit-
`switched technology, such as provided by the Public
`Switched Telephone Network (PSTN) or Public Land
`Mobile Network (PLMN), to packet-switched technology,
`such as provided by Voice over IP (VoIP) techniques across
`IP Networks. Indeed, the Internet Engineering Task Force
`(IETF) has developed IP-based protocols to perform various
`functions
`in VoIP communications.
`In particular,
`for
`example, Session Initiation Protocol (SIP), specified in IETF
`request for comment document RFC 2543, has been devel-
`oped for establishing voice calls between two parties. In
`contrast, Real-time Transport Protocol (RTP), specified in
`IETF request for comment document RFC 1889, has been
`developed to format packetized voice to be carried over the
`Internet once the call has been established.
`
`adopted SIP into the IMSspecification.
`
`[0004] The mass deployment of IP networks supporting
`VoIP is expected to happen in the future. At first, VoIP is
`expected to penetrate into fixed (desktop) phone and wire-
`less LAN (WLAN) segments of the communications indus-
`try, followed by penetration into the cellular (3G) segment.
`Thus, two kinds of heterogeneity are conceivable in the near
`future. In one dimension, there will be a large number of
`VoIP phones, as well as a large number of PSTN phones. To
`address this heterogeneity, provisions are made in SIP to
`allow a VoIP phoneto call a telephone number in PSTN and
`vice versa.
`
`

`

`US 2004/0246990 Al
`
`Dec. 9, 2004
`
`minal can establish a circuit-switched connection with the
`gateway. For example,
`the first
`terminal can receive a
`handoff identifier hosted by the gateway, and thereafter
`establish a circuit-switched connection with the gateway
`based upon the handoff identifier. In this regard, the first
`terminal is capable of establishing the circuit-switched con-
`nection with the gateway via a second circuit-switched
`station moves outside a geographic support area for such a
`network, where the gateway is capable of supporting com-
`munication betweenthe first terminal via a packet-switched
`call. Also, for example, the system and method of embodi-
`network and the second terminal viaafirst circuit-switched
`ments of the present invention are capable of handing off a
`voice call of a fixed terminal in an IP network when the IP
`network. Advantageously, according to various embodi-
`ments of the present invention, the first and second circuit-
`switched networks maybedifferent or the same. Irrespective
`of whetherthe first and second circuit-switched networksare
`the same or different, however, after the circuit-switched
`connection is established betweenthefirst terminal and the
`
`conducted between a terminal in a packet-switched network
`and a terminal in a circuit-switched network, while increas-
`ing the likelihood that the call can be completed uninter-
`rupted. For example, in accordance with embodimentsof the
`present invention, the system and method of embodiments
`of the present invention are capable of handing off the voice
`call of a mobile station in an IP network when the mobile
`
`network experiences heavytraffic, transmission difficulties
`or the like, and thus becomes unable to adequately support
`the call. Therefore, the system and method of embodiments
`of the present invention solve the problems identified by
`prior techniques and provide additional advantages.
`
`gateway, the gateway is capable of connecting the circuit-
`switched connection established between the first terminal
`
`and the gateway with the circuit-switched connection
`between the gateway and the second terminal. In this man-
`ner, the gateway is capable of handing off communication
`from the packet-switched network to the circuit-switched
`network.
`
`[0009] The first terminal can be adapted to establish the
`circuit-switched connection with the gateway when the
`signal strength of the packet-switched connection between
`the first terminal and the gateway, or more particularly an
`access point in communication between the first terminal
`and the gateway, is below a predetermined threshold for
`more than a predefined period of time. In such an embodi-
`ment,
`the gateway can then connect the circuit-switched
`connection established between the first terminal and the
`
`gateway with the circuit-switched connection between the
`gateway and the second terminal in instances in which the
`first
`terminal establishes the circuit-switched connection
`with the gateway.
`
`the circuit-
`terminal establishes
`first
`the
`[0010] As
`switched connection with the gateway, the first terminal can
`be capable of transmitting a call session ID and/or a shared
`secret
`to the gateway. In this regard,
`the first
`terminal,
`gateway and/or a server (e.g., SIP server) in communication
`with the first terminal and the gateway can generate the call
`session ID and shared secret in conjunction with one of the
`first and second terminals setting up communication with the
`other terminal. By transmitting the call session ID,
`the
`gateway can be capable of connecting the circuit-switched
`connection established between the first terminal and the
`gateway with the circuit-switched connection between the
`second terminal and the gateway based uponthecall session
`ID. And by transmitting the shared secret, the gateway can
`be capable of verifying the shared secret before the circuit-
`switched connection is established with the first terminal.
`
`{0011] A method and computer program productare also
`provided for handing off communication from a packet-
`switched network to a circuit-switched network. Embodi-
`ments of the present invention therefore provide a system
`and method for handing off communication from a packet-
`switch network, such as an IP network,to a circuit-switched
`network, such as a PLMNor PSTN network. The system and
`method of embodiments of the present invention are capable
`of handing off such a call in a seamless mannersuch that the
`call can be handed off to the circuit-switched network
`
`without being dropped. Calls can therefore be initiated and
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0012] Having thus described the invention in general
`terms, reference will now be made to the accompanying
`drawings, which are not necessarily drawn to scale, and
`wherein:
`
`[0013] FIG.1 is a schematic block diagram of a commu-
`nications system according to one embodiment of the
`present invention including a packet-switched network and
`two circuit-switched networks, in accordance with embodi-
`ments of the present invention;
`
`[0014] FIG.2 is a schematic block diagram of a mobile
`station according to one embodimentof the present inven-
`tion; and
`
`[0015] FIGS. 3A and 3B are flow charts illustrating
`various steps in a method of handing off a call from a
`packet-switched network to a circuit-switched network in
`accordance with one embodiment of the present invention.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`[0016] The present invention now will be described more
`fully hereinafter with reference to the accompanying draw-
`ings, in which preferred embodiments of the invention are
`shown. This invention may, however, be embodied in many
`different forms and should not be construedas limited to the
`embodimentsset forth herein; rather, these embodiments are
`provided so that this disclosure will be thorough and com-
`plete, and will fully convey the scope of the invention to
`those skilled in the art. Like numbersrefer to like elements
`
`throughout.
`
`{0017] Referring to FIGS. 1 and2, an illustration of one
`type of communications system including terminals, such as
`mobile stations and fixed terminals, that would benefit from
`the present invention is provided. It should be understood,
`however, that the mobile stations and fixed terminals illus-
`trated and hereinafter described is merely illustrative of two
`types of terminals that would benefit from the present
`invention and, therefore, should not be taken to limit the
`scope of the present invention. While several embodiments
`of the mobile station are illustrated and will be hereinafter
`
`described as a mobile telephone for purposes of example,
`other types of mobile stations, such as portable digital
`assistants (PDAs), pagers, and other types of voice and text
`communications systems, can readily employ the present
`
`

`

`US 2004/0246990 Al
`
`Dec. 9, 2004
`
`invention. In addition, the system and method of the present
`invention will be primarily described in conjunction with
`mobile communications applications. It should be under-
`stood, however, that the system and methodof the present
`invention can be utilized in conjunction with a variety of
`other applications, both in the mobile communications
`industries and outside of the mobile communications indus-
`tries.
`
`[0018] As shown,a circuit-switched (CS) mobile station
`10 is capable of transmitting signals to and receiving signals
`from a base site or base station (BS) 14. The basestation is
`a part of a cellular network that includes a mobile switching
`center (MSC) 16, voice coder/decoders (vocoders) (VC) 18,
`data modems (DM)20, and other units required to operate
`the PLMN. The MSC is capable of routing calls and
`messages to and from the CS mobile station when the CS
`mobile station is making and receiving calls. The MSC
`controls the forwarding of messages to and from the mobile
`station when the station is registered with the cellular
`network, and also controls the forwarding of messages for
`the mobile station to and from a message center (not shown).
`As will be appreciated by those skilled in the art, the cellular
`network may also be referred to as a Public Land Mobile
`Network (PLMN)22.
`
`[0019] Subscriber data of a CS mobile station 10 is stored
`permanently in a Home Location Register (HLR) 24 of the
`PLMN22 and temporarily in the Visitor Location Register
`(VLR) 26 in the area of which the CS mobile station is
`located at a given moment.In this regard, the VLR contains
`selected administrative information necessary for call con-
`trol and provision of the subscribed services for each CS
`mobile station currently located in the geographical area
`controlled by the VLR. Although each functional entity can
`be implemented as an independent unit, manufacturers of
`switching equipmentgenerally implement the VLR together
`with the MSC 16 so that the geographical area controlled by
`the MSC corresponds to that controlled by the VLR, thus
`simplifying the signaling required.
`
`[0020] The PLMN22is capable of providing voice com-
`munications in accordance with a number of different cir-
`
`the PLMN is
`In this regard,
`cuit-switched techniques.
`capable of providing voice communications in accordance
`withfirst, second, third-generation wireless communication
`protocols and the like. For example, the PLMN may provide
`voice communications in accordance with second-genera-
`tion
`(2G) wireless
`communication protocols
`IS-136
`(TDMA), GSM, and/or IS-95 (CDMA). Within the PLMN,
`signaling communications may be provided in accordance
`with any of a numberof different techniques, but signaling
`communications are typically provided in accordance with
`the Signaling System 7 (SS7) standard.
`
`[0021] The MSC 16, and thus the PLMN 22, can be
`coupled to a Public Switched Telephone Network (PSTN) 28
`that, in turn, is coupled to one, or more typically, a plurality
`of fixed CS terminals 30. Like the PLMN, the PSTN is
`capable of providing signaling communications in accor-
`dance with any of a numberof different techniques, includ-
`ing SS7. The PSTN is also capable of providing voice
`communications in accordance with any of a number of
`different circuit-switched techniques. For example,
`the
`PSTN may provide voice communications in accordance
`with Time Division Multiplexing (TDM)techniques, such as
`
`64 Kbps (CCIT), and/or Pulse Code Modulation (PCM)
`techniques, such as 56 Kbps (ANSI).
`
`[0022] The PSTN 28 and the PLMN 22 (via the MSC 16)
`can also be coupled to, electrically connected to, or other-
`wise in electrical communication with a packet-switched
`network, such as an Internet Protocol (IP) network 32. The
`IP network may be coupled to one or more fixed packet-
`switched (PS) terminals 34. Additionally, the IP network
`may be coupled to one or more wireless access points 36,
`which may be coupled to one or more PS mobilestations 38.
`As shown, the IP network comprises a Wireless Local Area
`Network (WLAN), thereby providing communications in
`accordance with IEEE 802.11, for example. The packet-
`switched network can equally comprise any of a numberof
`other types of networks. For example, the packet-switched
`network can comprise Universal Mobile Telecommunica-
`tions Service (UMTS) or cdma2000 networks without
`departing from the spirit and scope of the present invention.
`As such, the packet-switched network can equally be con-
`figured to provide voice communications in accordance with
`the Internet Protocol (IP), for example, without departing
`from the spirit and scope of the present invention.
`
`[0023] The IP network 32 may be capable of providing
`signaling communications in accordance with any of a
`numberof different techniques. For example, the IP network
`may be capable of providing signaling in accordance with
`the Session Initiation Protocol (SIP). In this regard, SIP
`typically provides signaling for initiating a session between
`two or more endpoints in the IP network by making those
`endpoints aware of the session semantics. Accordingly,
`devices (or users that run certain applications on these
`devices) are registered with the SIP backbone so that an
`invitation to a particular session can be correctly delivered
`to these endpoints. To achieve this, SIP provides a registra-
`tion mechanism for devices and users, and it applies mecha-
`nisms such as an SIP server 40 to route the session invita-
`tions appropriately.
`
`[0024] To facilitate voice communications between the IP
`network 32, and the PLMN 22 and PSTN 28, the commu-
`nications system typically also includes a gateway 42. The
`gateway is capable of mapping signaling and voice com-
`munications between the IP network, and the PLMN and/or
`PSTN to thereby permit communications between the IP
`network, and the PLMN and/or PSTN.In accordance with
`SIP, for example, the gateway may include a Media Gateway
`42a that converts transmissions between a packet stream
`from the IP-network and the voice-encoded communication
`(e.g., TDM or PCM) from the PLMNand/or PSTN. Thus,
`voice is carried in packets over a packet-switched connec-
`tion between the PS mobilestation 38 or PS-terminal 34 and
`the MGW,while voice is carried over voice-encoded com-
`munication circuits
`in
`a
`circuit-switched connection
`between the MGW and the PSTN CSterminal 30.
`
`[0025] Also in accordance with SIP, the gateway 42 may
`include a Media Gateway Control Function (MGCF) 42b
`that operates to terminate SIP signaling and control the
`functions of the MGW 42a. In this regard,
`the MGCF
`controls the functions of MGW in accordance with the
`Media Gateway Control protocol (MEGACO),such as that
`described in the IETF request for comment document RFC
`3015, entitled: Megaco Protocol Version 1.0, the contents of
`whichare hereby incorporated by reference in its entirety. In
`
`

`

`US 2004/0246990 Al
`
`Dec. 9, 2004
`
`addition to terminating SIP signaling and controlling the
`MGW,
`the MGCF also typically performs conversion
`between SIP signaling in the IP network 32 and SS7sig-
`naling in the PLMN 22 and/or PSTN 28, such as for call
`control.
`
`Thefixed CS terminal 30 and fixed PS terminal 34
`[0026]
`can comprise any of a number of known devices capable of
`providing voice communications in accordance with circuit-
`switched and packet-switched techniques, respectively. For
`example, the CS terminal can comprise any of a numberof
`known, conventional fixed wireline or wireless telephones.
`The PSterminal, for example, can comprise any of a number
`of known, conventional IP-enabled wireline or wireless
`telephones. Also, for example, the PS terminal can comprise
`a processor, such as a personal computer, laptop computer or
`the like, capable of operating a software application for
`providing voice communications in accordance with packet-
`switched techniques. As will be appreciated by those skilled
`in the art, and in accordance with embodiments of the
`present invention, the PS terminal may be capable of oper-
`ating in accordance with the CS terminal, and vice versa. As
`such, as described herein, the term “fixed terminal” will be
`used to describe a single fixed terminal capable of perform-
`ing the functions of both the PS terminal and the CS
`terminal.
`
`[0027] Reference is now drawn to FIG. 2, which illus-
`trates a block diagram of a mobilestation that may comprise
`the CS mobile station 10 or the PS mobile station 38. Like
`with the Fixed CS terminal 30 and Fixed PS terminal 34, in
`accordance with embodiments of the present invention, the
`PS mobile station may be capable of operating in accordance
`with the CS mobile station, and vice versa. Thus, as
`described herein, the term “mobile station” will be used to
`describe a single mobile station capable of performing the
`functions of both the PS mobile station and the CS mobile
`station. The mobile station includes a transmitter 46, a
`receiver 48, and a controller 50 that provides signals to and
`receives signals from the transmitter and receiver, respec-
`tively. These signals include signaling information in accor-
`dance with the air interface standard of the applicable
`cellular system, and also user speech and/or user generated
`data. In this regard, the mobile station can be capable of
`operating with one or moreair interface standards, commu-
`nication protocols, modulation types, and access types.
`Moreparticularly, the mobile station can be capable of
`operating in accordance with any of a numberoffirst,
`second and/or third-generation communication protocols or
`the like. For example, the mobile station may be capable of
`operating in accordance with second-generation (2G) wire-
`less communication protocols IS-136 (TDMA), GSM, and
`IS-95 (CDMA).
`
`It is understood that the controller 50 includes the
`[0028]
`circuitry required for implementing the audio and logic
`functions of the mobile station. For example, the controller
`may be comprised of a digital signal processor device, a
`microprocessor device, and various analog to digital con-
`verters, digital
`to analog converters, and other support
`circuits. The control and signal processing functions of the
`mobilestation are allocated between these devices according
`to their respective capabilities. The controller thus also
`includes the functionality to convolutionally encode and
`interleave message and data prior to modulation and trans-
`mission. The controller can additionally include an internal
`
`voice coder (VC) 50A, and may include an internal data
`modem (DM) 50B. Further, the controller 50 may include
`the functionally to operate one or more software programs,
`which may be stored in memory (described below). For
`example,
`the controller may be capable of operating a
`connectivity program that allows the mobile station to
`transmit and receive content and/or voice communications
`from the IP network 32, such as according to the Wireless
`Application Protocol (WAP), for example.
`
`[0029] The mobile station also comprises a user interface
`including a conventional earphone or speaker 52, a ringer
`53, a microphone 54, a display 56, and a user input interface,
`all of which are coupled to the controller 50. The user input
`interface, which allows the mobile station to receive data,
`can comprise any of a number of devices allowing the
`mobile station to receive data, such as a keypad 58, a touch
`display (not shown)or other input device. In embodiments
`including a keypad, the keypad includes the conventional
`numeric (0-9) andrelated keys (#, *), and other keys used for
`operating the mobile station.
`
`[0030] The mobile station 10 can also include memory,
`such as a subscriber identity module (SIM) 60, a removable
`user identity module (R-UIM)or the like, which typically
`stores information elements related to a mobile subscriber.
`In addition to the SIM, the mobile station can include other
`memory.
`In this regard,
`the mobile station can include
`volatile memory 62, such as volatile Random Access
`Memory (RAM) including a cache area for the temporary
`storage of data. The mobile station can also include other
`non-volatile memory 64, which can be embedded and/or
`may be removable. The non-volatile memory can addition-
`ally or alternatively comprise an EEPROM,flash memoryor
`the like. The memories can store any of a numberof pieces
`of information, and data, used by the mobile station to
`implementthe functions of the mobilestation. For example,
`the memories can include an identifier, such as an interna-
`tional mobile equipment identification (IMEI) code, capable
`of uniquely identifying the mobile station, such as to the
`MSC 16.
`
`[0031] Referring now to FIGS. 3A and3B,in accordance
`with embodiments of the present invention, the communi-
`cations system is capable of handing off the voice call of a
`mobile station or fixed terminal from a packet-switched
`network (e.g., IP network 32) to a circuit-switched network
`(e.g., PLMN 22 or PSTN 28). In this regard, a mobile station
`or fixed terminal engaged in a voice call via a packet-
`switched network can be handed off to a circuit-switched
`
`network in instances in which the packet-switched network
`cannot continue to adequately support the call. For example,
`in accordance with embodiments of the present invention,
`the system is capable of handing off the voice call of a
`mobile station from the IP network when the mobilestation
`
`moves outside a geographic support area for a VoIP call,
`such as outside the geographic region supported by the AP
`36. Also, for example, in accordance with embodiments of
`the present invention, the system is capable of handing off
`a voice call of a fixed terminal from the IP network when the
`IP network experiences heavytraffic, transmission difficul-
`ties or the like, and thus becomes unable to adequately
`support the call.
`
`[0032] As described below, the system and methodwill be
`explained in the context of handing off a call of a mobile
`
`

`

`US 2004/0246990 Al
`
`Dec. 9, 2004
`
`station from the IP network 32 to the PLMN 22, where the
`mobile station is engagedin a call with a fixed terminal via
`the PSTN 28. It should be understood, however, that the
`system and method are equally applicable to any of a
`number of other contexts. For example,
`the system and
`method can be applied to the context of handing off a call of
`a fixed terminal from the IP network to the PSTN, where the
`fixed terminal is engaged in a call with a mobile station via
`the PLMNoranother fixed terminal via the PSTN.
`
`[0033] To effectuate a call handoff from a packet-switched
`network to a circuit-switched network, the gateway 42 is
`capable of hosting a handoff identifier, such as a handoff
`telephone number,that the gateway can utilize to provide the
`handoff.
`In operation,
`the mobile station and the fixed
`terminal can establish or setup a voicecall via the IP network
`32 and the PSTN 28, respectively, such as in accordance
`with conventional techniques, as shown in block 70.In this
`regard, once the voice call is setup, the mobile station and
`fixed terminal can transmit and receive voice communica-
`tions in the following manner:
`the mobile station and
`gateway maintain a packet-switched connection, while the
`gateway and fixed terminal maintain a circuit-switched
`connection. Either during or after the call setup, a call
`session ID and a shared secret (e.g., password) can be
`generated in conjunction with the voice communication
`sctup between the mobile station and the fixed terminal, and
`thereafter received by the gateway and the mobilest