Method and system for guaranteeing seamless session when replacing PoC terminal in PoC system

CLAIM OF PRIORITY

0001This application claims all benefits accruing under 35 U.S.C. §119 from an application for METHOD AND SYSTEM FOR GUARANTEEING SEAMLESS SESSION WHEN REPLACING PoC TERMINAL IN PoC SYSTEM filed in the Korean Intellectual Property Office on Jan. 26, 2005 and assigned Serial No. 2005-7287, contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

00021. Field of the Invention

0003The present invention relates to a method and system for guaranteeing a seamless session when replacing a push-to-talk-over-cellular (PoC) terminal in a PoC system, capable of replacing the PoC terminal without loss of a media stream for a running PoC group session when the PoC terminal is replaced for a specified purpose.

00042. Description of the Related Art

0005Due to significant development of mobile communications technology and extension of mobile communications networks, various extra services and applications which make use of a cellular phone are being provided. At the same time, demand among cellular phone users for various extra services, such as a location service, a multimedia service, and a push-to-talk (PTT) service, is increasing. Among these extra services, the PTT service supports various supplementary functions such as an instant messenger function and a status display function, as well as a group call and a voice call which are also provided by an existing radio or a trunk radio system (TRS).

0006Currently, standardization of a push-to-talk-over-cellular (PoC) service which employs the PTT function in a mobile communication network is actively proceeding. One unique feature of the PoC service is that a user can participate in a plurality of PoC sessions, and can move among the PoC sessions to use a call service. Requirements that the user should move among the plurality of PoC sessions to use the call service are specified in the Open Mobile Alliance (OMA) which is a forum for specifying mobile communications services. The structure of an ordinary PoC service system will be explained below with reference to <figref idref="DRAWINGS">FIG. 1</figref>, which is a schematic diagram illustrating a conventional PoC service system. Referring to <figref idref="DRAWINGS">FIG. 1</figref>, a PoC client <b>10</b>, as a service requester installed in a mobile station, is connected to a Session Initiation Protocol/Internet Protocol (SIP/IP) core network <b>30</b> which supports SIP and IP multimedia functions via an access network <b>20</b>.

0007The PoC client <b>10</b> resides in a PoC user terminal to provide access to the PoC service. The PoC client <b>10</b> serves to establish a PoC session, participate in a PoC session that is currently proceeding, and terminate a PoC session. In addition, the PoC client <b>10</b> acts to make and transfer a talk burst, support an instant personal alert, and perform authentication when accessing the PoC service. Hereinafter, unless otherwise stated, both the PoC user and the PoC client <b>10</b> are assumed to be the same as a PoC service subscriber.

0008The SIP/IP core network <b>30</b> is connected to a PoC server <b>60</b>, a GLMS (Group List and Management System) <b>50</b>, and a presence server <b>70</b> in order to support the PoC service.

0009Generally, SIP is a standard defined in IETF (Internet Engineering Task Force) RFC (Request for Comments) <b>2543</b>. SIP is an application-layer control protocol that is used to set up, modify and terminate a session or call for multimedia communication such as video and voice. SIP is a protocol that exists over an UDP (User Datagram Protocol)/TCP/IP layer, which supports both unicast and multicast sessions so as to be able to initiate the session by inviting participants to a multimedia conference with a client/server protocol capable of transceiving SIP Request and Response messages in a request/response fashion.

0010A SIP Request message provides six functions in RFC 2543 as follows: INVITE (Invitation to participate in a session), ACK (Permission to an INVITE request), BYE (Termination of a call), REGISTER (Registration with a redirect server by a user agent), CANCEL (Cancellation of a pending request), and OPTIONS. The SIP Response message provides status codes as follows: 1xx (Informational response), 2xx (Success response), 3xx (Redirection response), 4xx (Client Error, Requesf Failure), 5xx (Server Error), and 6xx (Global Failure).

0011The PoC server <b>60</b> serves as a Controlling PoC Function (CF) for maintaining and managing a PoC session, or a Participating PoC Function (PF) for participating in a PoC session for a one-to-one PoC call or a one-to-many PoC call (or group PoC call).

0012Functional blocks of the PoC server will be explained below with reference to the schematic diagram of <figref idref="DRAWINGS">FIG. 2</figref>.

0013The PoC server is classified into a Controlling PoC Function of taking charge of overall maintenance and management of a PoC session and a Participating PoC Function (PF) of taking charge of maintenance and management between each PoC session, which will be explained below with reference to relevant tables. <tables id="TABLE-US-00001" num="1"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="1"><colspec colname="1" colwidth="217PT" align="center" /><thead><row><entry namest="1" nameend="1" align="center">TABLE 1</entry></row><row><entry /></row><row><entry namest="1" nameend="1" align="center" rowsep="1" /></row><row><entry>Controlling PoC Function (CF)</entry></row><row><entry namest="1" nameend="1" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry /></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="2"><colspec colname="OFFSET" colwidth="14PT" align="left" /><colspec colname="1" colwidth="203PT" align="left" /><tbody valign="top"><row><entry /><entry>Provides centralized PoC session handling</entry></row><row><entry /><entry>Provides centralized Media distribution</entry></row><row><entry /><entry>Provides centralized Talk Burst Arbitration functionality including</entry></row><row><entry /><entry>talker identification</entry></row><row><entry /><entry>Provides SIP session handling, such as SIP session origination,</entry></row><row><entry /><entry>termination, etc.</entry></row><row><entry /><entry>Provides policy enforcement for participation in group sessions</entry></row><row><entry /><entry>Provides participant information</entry></row><row><entry /><entry>Collects and provides centralized media quality information</entry></row><row><entry /><entry>Provides centralized charging reports</entry></row><row><entry /><entry>May provide transcoding between different codecs</entry></row><row><entry /><entry>Supports Talk Burst Control Protocol Negotiation</entry></row><row><entry /><entry namest="OFFSET" nameend="1" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

0014As shown in Table 1, the CF serves to maintain and manage a PoC session on the whole. The PoC server receives requests for a floor from PoC clients, arranges an order in which to give the clients the floor, and gives the clients the floor in that order. The PoC server also distributes a talk burst, for which an arbitrary PoC client makes a request, to all other PoC clients participating in a group PoC call, and provides information of the PoC clients participating in the group PoC call.

0015As shown in Table 2 below, the PF manages a PoC session between the CF and each PoC client. In particular, the PF acts to relay the floor between the PoC client and the CF when the PoC client makes a request for the floor or when the CF gives the floor to the PoC client. In addition, the PF serves to relay media between the CF and the PoC client, perform transcoding between different codecs, and filter one of two concurrent PoC sessions according to the choice of a PoC user when there is simultaneous talking in the two concurrent PoC sessions. <tables id="TABLE-US-00002" num="2"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="1"><colspec colname="1" colwidth="217PT" align="center" /><thead><row><entry namest="1" nameend="1" align="center">TABLE 2</entry></row><row><entry /></row><row><entry namest="1" nameend="1" align="center" rowsep="1" /></row><row><entry>Participating PoC Function (PF)</entry></row><row><entry namest="1" nameend="1" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry /></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="1"><colspec colname="1" colwidth="217PT" align="left" /><tbody valign="top"><row><entry>Provides PoC session handling</entry></row><row><entry>May provide the Media relay function between PoC client and Controlling</entry></row><row><entry>PoC server</entry></row><row><entry>May provide user media adaptation procedures</entry></row><row><entry>May provide the Talk Burst control message relay function between PoC</entry></row><row><entry>client and Controlling PoC server</entry></row><row><entry>Provides SIP session handling, such as SIP session origination,</entry></row><row><entry>termination, etc, on behalf of the represented PoC client</entry></row><row><entry>Provides policy enforcement for incoming PoC session (e.g. access</entry></row><row><entry>control, incoming PoC session barring, availability status, etc.)</entry></row><row><entry>May collect and provide media quality information</entry></row><row><entry>Provides the participant charging reports</entry></row><row><entry>May provide filtering of the media streams in the case of simultaneous</entry></row><row><entry>sessions</entry></row><row><entry>May provide transcoding between different codecs</entry></row><row><entry>May support Talk Burst Control Protocol Negotiation</entry></row><row><entry>Stores the current Answer Mode and Incoming PoC Session Barring</entry></row><row><entry>preferences of the PoC client</entry></row><row><entry namest="1" nameend="1" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

0016In the PoC service system as described above, the PoC user can input information on a group and its members to the GLMS <b>50</b> through his/her PoC terminal, and can be aware of information about PoC users who he or she can call through an individual or group list transmitted from the GLMS <b>50</b>. Alternatively, the information on the group and its members may be input, corrected and managed in the GLMS <b>50</b> via a reliable communication network such as the Internet or Intranet which a PoC service provider can trust.

0017In order to make use of the PoC service, the PoC user registers his/her PoC address with the SIP/IP core network <b>30</b>. The SIP/IP core network <b>30</b> stores PoC user information at the request of the PoC user. Thus, when another PoC user tries to request a group PoC call, the PoC user registers his/her information in the SIP/IP core network <b>30</b> in advance as described above, and requests the group PoC call to his/her SIP/IP core network <b>30</b> by using group identification information transmitted from the GLMS <b>50</b>. At this time, the SIP/IP core network <b>30</b> performs address determination and domain location determination by using information of the call requesting PoC user and then transfers a PoC call request to a home PoC server <b>60</b> with which the call requesting PoC user is registered. In regard to the PoC call request, the PoC <b>60</b> server prepares for establishment of a PoC session, obtains each user's information from the GLMS <b>50</b>, and then transfers a PoC call request signal to a corresponding SIP/IP core network <b>30</b>. Here, in the case of a PoC call request to users within an Intradomain, the PoC server <b>60</b> performs both the CF and PF. The PoC server <b>60</b>, which manages a call-requested PoC user, requests a PoC call to the PoC user after the SIP/IP core network <b>30</b> performs the location determination procedure, by using information of the PoC user that is transmitted to the PoC server <b>60</b>.

0018<figref idref="DRAWINGS">FIG. 3</figref> is a schematic diagram illustrating CF and PF blocks of a PoC server.

0019Referring to <figref idref="DRAWINGS">FIG. 3</figref>, PoC clients <b>111</b>, <b>121</b>, <b>131</b> and <b>141</b> provide access to a CF <b>100</b> through PFs <b>110</b>, <b>120</b>, <b>130</b> and <b>140</b> respectively, thereby establishing a PoC session. Here, when a floor is granted to a requester qualified as a talker from the CF <b>100</b>, media based on speaking of the corresponding PoC client is transmitted to each PoC client.

0020First, in order to describe in full detail a session connecting procedure, etc. of a PoC client in a terminal, the following features of the PoC system defined in the OMA will be discussed. The PoC system according to setup of the originating and terminating sides of the PoC session in the OMA has the following features.

0021The PoC system is divided into two types, an on-demand session mode and a pre-established (or early) session mode, according to how the connection with a PoC server <b>60</b> in the home network of a user is set up.

0022The pre-established session mode means that the PoC user sets up a specified session between the PoC client and the PoC server <b>60</b> within his/her home network in advance by his/her request. The pre-established session is necessary to enable the PoC user to negotiate media parameters to be used with the PoC server <b>60</b> in advance, and thus perform rapid call setup without negotiating again the media parameters to be used in the future between the server and the client. In order to establish the early session, the PoC client provides media parameters supported to a SDP (Session Description Protocol) body through an SIP INVITE method, and responds to media parameters provided from the server. The PoC client returns identification information of the early session which is newly set up for a response message to the PoC user including a conference URI (Uniform Resource Identifier). In the case of using the early session, it is possible to set up an IP address, a port number, a codec to be used, a talk burst control protocol, etc., in advance.

0023The on-demand session mode refers to a state where the PoC user does not set up the early session, and means that the PoC user performs a PoC call connecting procedure after receiving an INVITE message of another PoC user.

0024Meanwhile, the PoC system makes a half-duplex group PoC call possible, including the above features. This multilateral conferencing function is an exemplary feature of the PoC system, and may be divided into an ad hoc PoC group, a pre-arranged PoC group, and a chat PoC group according to the feature of a group that is set up.

0025In the PoC system having the foregoing features, the respective elements such as the PoC client, PoC server, SIP/IP core network, group list server, presence server, and so on, as well as procedures of initiating and connecting an initial PoC session through signaling between these elements can be found from the OMA standard draft as the conventional SIP-based technology, and so their description will be omitted.

0026Meanwhile, in the conventional PoC system having the foregoing features, there may occur situations where a PoC client participating in a currently proceeding PoC session should be replaced.

0027As concrete examples of this situation, in a first situation, a currently participating PoC session has a feature of supporting not only a voice, but there is a need to replace media, such as a video, including the voice as a call is made. In this case, some PoC users using a voice support terminal have to replace their own PoC client with video support terminal.

0028In a second situation, power of a currently used terminal is almost consumed, but there is a need to continuously participate in a current PoC session without termination.

0029In a third situation, a user is participating in a PoC session using a stationary PoC terminal (e.g. a VoIP terminal) at his/her office, but feels a need to move. In this case, there is a need to connect a currently participating session using his/her mobile wireless terminal (mobile PoC terminal).

0030In addition to these examples, a need to replace the PoC terminal by a special request of the user may frequently occur.

0031With respect to this requirement, a conventional process of replacing a PoC session on the basis of OMA PoC <b>1</b> standard technology will be described with reference to the flow diagram of <figref idref="DRAWINGS">FIG. 4</figref>.

0032<figref idref="DRAWINGS">FIG. 4</figref> shows a process where a PoC client A<b>1</b><b>111</b>, as a terminal that has already participated in a running session, attempts to participate in a running session using a PoC client A<b>2</b><b>112</b> by request of a user.

0033First, a PoC client A<b>1</b><b>111</b> transmits an SIP BYE message to a PF A <b>110</b> of his/her home network in order to terminate a participating PoC session (S<b>101</b>), and thus the PF A <b>110</b> transmits the BYE message to a CF <b>100</b> through information on an SIP address contained in the BYE message (S<b>102</b>).

0034Next, after receiving the session BYE message, the CF <b>100</b> transmits a 200 OK response signal for terminating a PoC conference (S<b>111</b> and S<b>112</b>), thereby releasing the PoC client A<b>1</b><b>111</b> and stopping transmitting media at the same time.

0035Meanwhile, a PoC user who terminates the session transmits an INVITE message to the PF A <b>110</b> via the PoC client A<b>2</b><b>112</b> in order to participate again in the previous session using a new PoC client (S<b>121</b>). The INVITE message is transmitted through the PF A <b>110</b> to the CF <b>100</b> having control over the running conference session (S<b>122</b>). At this time, the PoC client A<b>2</b><b>112</b> must have unique identity information of the PoC session (session ID) in order to participate the proceeding PoC session again.

0036Next, the CF <b>100</b> transmits a 200 OK response (S<b>131</b> and S<b>132</b>), thereby allowing the new PoC client A<b>2</b><b>112</b> to participate in the session, receives an ACK signal (S<b>141</b> and S<b>142</b>), and confirms the participation of the session. Simultaneously, the CF <b>100</b> transmits a corresponding floor message to the PoC client A<b>2</b><b>112</b> according to a floor in the current session. When another PoC client transfers media, the CF <b>100</b> transmits a Floor Taken message to the PoC client A<b>2</b><b>112</b> using RTCP (Real-time Transport Control Protocol) (S<b>151</b> and S<b>152</b>).

0037Thereafter, when the session is connected, the CF <b>100</b> transmits media to the PoC client A<b>2</b><b>112</b> using an RTP (Real-time Transport Protocol).

0038The foregoing conventional process has the following problems.

0039First, when the PoC user participates again in the same PoC conference session in which he/she participates as the PoC client A<b>1</b> using the PoC client A<b>2</b>, he/she must have the PoC session identity. However, the PoC session identity, that is generated arbitrarily as in an ad hoc session, should be transmitted to the PoC client A<b>2</b> by manual input of the user.

0040Further, the PoC client A<b>2</b> cannot make use of SIP session dialog information which the previous PoC client A<b>1</b> is putting to use. As such, the PoC client A<b>2</b> fails to receive a media stream using a session dialog that has been already used.

0041Therefore, there is a possibility that the QoE (Quality of Experience) of the user will deteriorate because the PoC client A<b>2</b> fails to receive the media stream of the PoC session while replacing the terminal of the PoC client.

SUMMARY OF THE INVENTION

0042It is an objective of the present invention to provide a method and system for guaranteeing a seamless session when replacing a PoC terminal in a PoC system, in which address information of a PoC terminal to be replaced and session identity information are transmitted to a server or a PoC client that makes a request to replace the PoC terminal before the session is terminated, thereby replacing only the PoC terminal without loss of a running media stream.

0043According to an aspect of the present invention, there is provided a method for guaranteeing a seamless session when replacing a push-to-talk-over-cellular (PoC) terminal in a PoC system. The method includes transmitting, by a PoC client requesting replacement of a PoC terminal, a terminal replacement request message for the replacement of the PoC terminal to a session management server while maintaining the seamless session; receiving, by the session management server, the replacement request message, and transmitting an INVITE message to a PoC client to be replaced; and receiving, by the terminal replacement target PoC client, the INVITE message, and media through an existing session.

0044According to another aspect of the present invention, there is provided a method for guaranteeing a seamless session when replacing a push-to-talk-over-cellular (PoC) terminal in a PoC system. The method includes transmitting, by a PoC client requesting replacement of a PoC terminal, a terminal replacement request message for the replacement of the PoC terminal to a PoC client to be replaced while maintaining the seamless session; receiving, by the terminal replacement target PoC client, the terminal replacement request message, and transmitting an INVITE message including information on an existing session to a session management server; and transmitting, by the session management server receiving the INVITE message, media to the terminal replacement target PoC client.

0045According to yet another aspect of the present invention, there is provided a push-to-talk-over-cellular (PoC) system for guaranteeing a seamless session when replacing a push-to-talk-over-cellular (PoC) terminal. The PoC system includes a terminal replacement requesting PoC client that makes a request to replace a PoC terminal; a terminal replacement target PoC client that becomes a replacement target of the PoC terminal; and a session management server that receives a terminal replacement request message from the terminal replacement requesting PoC client, transmits an INVITE message to the terminal replacement target PoC client with reference to information on the terminal replacement target PoC client included in the terminal replacement request message, and transmits media to the terminal replacement target PoC client while maintaining an existing session.

0046According to still yet another aspect of the present invention, there is provided a push-to-talk-over-cellular (PoC) system for guaranteeing a seamless session when replacing a PoC terminal. The PoC system includes a terminal replacement requesting PoC client that makes a request to replace a PoC terminal; a terminal replacement target PoC client that receives a terminal replacement request message from the terminal replacement requesting PoC client, refers to information on an existing session, and transmits an INVITE message including the existing session information; and a session management server that receives the INVITE message, and transmits media to the terminal replacement target PoC client while maintaining the existing session.

0047According to still yet another aspect of the present invention, there is provided a push-to-talk-over-cellular (PoC) terminal for maintaining a seamless session when the PoC terminal is replaced in a PoC system, which transmits to a session management server a terminal replacement request message including a session identity and information on a PoC terminal to be replaced, and receives a session termination message to terminate the session when a process of replacing the terminal is terminated by the session management server.

0048According to still yet another aspect of the present invention, there is provided a push-to-talk-over-cellular (PoC) terminal for maintaining a seamless session when the PoC terminal is replaced in a PoC system, which directly transmits a terminal replacement request message, that includes a session identity and information on a PoC terminal to be replaced, to the replacement target PoC terminal through a SIP/IP core network, and receives a session termination message from a session management server to terminate the session.

0049According to still yet another aspect of the present invention, there is provided a push-to-talk-over-cellular (PoC) terminal for maintaining a seamless session when the PoC terminal is replaced in a PoC system, which receives a terminal replacement request message from a replacement requesting PoC terminal, transmitting an INVITE message to a session management server according to information on an existing session received from the replacement requesting PoC terminal, and receives media from the session management server.

BRIEF DESCRIPTION OF THE DRAWINGS

0050A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, in which like reference symbols indicate the same or similar components, wherein:

0051<figref idref="DRAWINGS">FIG. 1</figref> is a schematic diagram illustrating a conventional PoC service system;

0052<figref idref="DRAWINGS">FIG. 2</figref> is a schematic diagram illustrating the structure of a conventional PoC server;

0053<figref idref="DRAWINGS">FIG. 3</figref> is a schematic diagram illustrating CF and PF blocks of a PoC server;

0054<figref idref="DRAWINGS">FIG. 4</figref> is a flow diagram showing a conventional process of changing a PoC session in order to replace a PoC terminal;

0055<figref idref="DRAWINGS">FIG. 5</figref> is a flow diagram showing a process for busy change of a PoC compliant terminal in accordance with a first embodiment of the present invention;

0056<figref idref="DRAWINGS">FIG. 6</figref> shows the format of a REFER message used in the process of <figref idref="DRAWINGS">FIG. 5</figref>;

0057<figref idref="DRAWINGS">FIG. 7</figref> shows the format of an INVITE message used in the process of <figref idref="DRAWINGS">FIG. 5</figref>;

0058<figref idref="DRAWINGS">FIG. 8</figref> is a flow diagram showing a process for busy change of a PoC compliant terminal in accordance with a second embodiment of the present invention;

0059<figref idref="DRAWINGS">FIG. 9</figref> shows the format of a REFER message used in the process of <figref idref="DRAWINGS">FIG. 8</figref>; and

0060<figref idref="DRAWINGS">FIG. 10</figref> shows the format of an INVITE message used in the process of <figref idref="DRAWINGS">FIG. 8</figref>.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

0061Hereinafter, preferred embodiments of the present invention will be described more fully with reference to the accompanying drawings so as enable those skilled in the art to easily implement the present invention.

0062The following description will be made with respect to application services of a push-to-talk-over-cellular (PoC) system that enables an immediate call according to a call request using IMS (IP Multimedia System) Core Network (CN) that is being standardized or completed in the 3GPP (3rd Generation Partnership Project) or 3GPP2 ((3rd Generation Partnership Project 2), as well as a half duplex type call, a group of users and presence information.

0063The present invention is constructed on the basis of at least one PoC client and PoC server (PF and CF) defined in an OMA PoC release <b>1</b> system, and an SIP and its extension protocol. The basic construction is the same as a general PoC basic structure of <figref idref="DRAWINGS">FIG. 1</figref>, and so its description will be omitted.

0064<figref idref="DRAWINGS">FIG. 5</figref> is a flow diagram showing a process for busy change of a PoC compliant terminal in accordance with a first embodiment of the present invention, where, when a PoC user wants to replace only a terminal without terminating a session, and thus selects a terminal which he/she wants to replace, the resulting information is transmitted to a server, and the terminal is replaced by the server.

0065Referring to <figref idref="DRAWINGS">FIG. 5</figref>, a PoC client A<b>1</b><b>1110</b> is connected with a PF A <b>1100</b> that is the PoC server of a home network, thereby having a session pre-established with a CF <b>1000</b> having control over a group session. At this time, the PoC client A<b>1</b><b>1110</b> stores a PoC session identity received through participation in the session, as well as session dialog information (dialog identifier) which is obtained through an INVITE message and its 200 OK response message, and designated between the PoC client A<b>1</b> and the PoC server.

0066The dialog identifier is a global identifier, and is composed of a From tag, a To tag and a Call-ID of the SIP INVITE message.

0067The PoC client A<b>1</b><b>1110</b> storing such information creates an SIP REFER message, and transmits it to the session management PoC server, the CF <b>1000</b> (S<b>1001</b> and S<b>1002</b>). Here, when transmitting the SIP REFER message, the PoC client A<b>1</b><b>1110</b> sets up a unique identity of the PoC session (conference URI:conf_uri_cfx, which is generally managed by the CF) as a Request URI of the REFER message, and sets up address information of the PoC client, which a user wants to use, to Refer-To header information.

0068<figref idref="DRAWINGS">FIG. 6</figref> shows the format of a REFER message used in the process of <figref idref="DRAWINGS">FIG. 5</figref>. As shown in <figref idref="DRAWINGS">FIG. 6</figref>, a Refer-To header field P<b>3</b> includes a proper SIP message method which is to be taken as address information of a destination.

0069In the present invention, the INVITE message is transmitted when the session is established. The information on the INVITE message will be described after description of the information of the REFER message.

0070Address information of a target terminal (a terminal to be changed to) included in the Refer-To header field P<b>3</b> may be an SIP URI of which a current PoC client A<b>1</b> makes use, or another SIP URI.

0071If the same SIP URI is used, a new SIP message is adapted to be transmitted to the PoC client A<b>2</b><b>1120</b>, including preference information (characteristic value: mobile, video, or explicit) of an SIP user agent, which is consistent with capability of the SIP user agent of the PoC client A<b>2</b><b>1120</b> in order to discriminate between a target PoC client A<b>2</b><b>1120</b> to be changed to and the current PoC client A<b>1</b><b>1110</b>.

0072Meanwhile, the REFER message includes session connection information (session dialog identifier) (From-tag, To-tag and Call-ID in P<b>2</b> of <figref idref="DRAWINGS">FIG. 6</figref>) which makes it possible to seamlessly receive content of the currently running PoC session using the target PoC client A<b>2</b><b>1120</b>, and information on the target terminal (Refer-To in P<b>3</b> of <figref idref="DRAWINGS">FIG. 6</figref>).

0073The CF <b>1000</b> as PoC server obtaining the REFER message responds with an Accepted message in order to return successful SIP processing (S<b>1011</b> and S<b>1102</b>).

0074Then, the CF transmits an INVITE message to an address of the SIP URI (conf_uri_cfx in P<b>1</b> of <figref idref="DRAWINGS">FIG. 6</figref>) received through the REFER message (S<b>1201</b> and S<b>1202</b>).

0075<figref idref="DRAWINGS">FIG. 7</figref> shows the format of an INVITE message used in the process of <figref idref="DRAWINGS">FIG. 5</figref>.

0076As shown in <figref idref="DRAWINGS">FIG. 7</figref>, a session request is adapted to be transmitted to the PoC client A<b>2</b><b>1120</b> with preference information (Q<b>3</b> of <figref idref="DRAWINGS">FIG. 7</figref>) of the user agent, which is set up by request of the user, included in the INVITE message.

0077Further, the CF <b>1000</b> recognizes an indicator replacing the PoC client A<b>1</b><b>1110</b> with the same dialog. To the end, the CF <b>1000</b> transmits information on a session dialog (to be replaced in order to receive media through the same session dialog with respect to the currently running session) through a Replaces header field (Q<b>4</b> of <figref idref="DRAWINGS">FIG. 7</figref>) of the INVITE message.

0078Next, the INVITE message routed to the PF A <b>1100</b> according to corresponding address information (SIP URI) (conf_uri_cfx) is transmitted to an SIP/IP core network. The SIP/IP core network routes the INVITE message to the corresponding PoC client A<b>2</b><b>1120</b> according to a client characteristic value (Fixed, mobile, video etc.) of the user agent which the PoC client A<b>2</b><b>1120</b> registers using an SIP REGISTER message, and information (Q<b>3</b> of <figref idref="DRAWINGS">FIG. 7</figref>) included in an Accept-Contact header field.

0079Then, the PoC client A<b>2</b><b>1120</b> returns a 200 OK response to the CF <b>1000</b> according to a response of the user (S<b>1301</b> and S<b>1302</b>). At this time, newly created session dialog information (dialog ID created by transaction in steps S<b>1201</b> to S<b>1302</b>) is transmitted to the CF <b>1000</b>. Thereby, the PoC server, the CF <b>1000</b>, replaces the previous dialog with the newly created dialog, and transmits media.

0080When the CF <b>1000</b> receives the 200 OK response, the CF <b>1000</b> managing a conference returns an ACK signal to the PoC client A<b>2</b><b>1120</b> (S<b>1401</b> and S<b>1402</b>). Simultaneously, the CF <b>1000</b> decides to transmit the media transmitted to the PoC session to a newly connected PoC client A<b>2</b><b>1120</b>, and transmits a talk burst control message that notifies the decision (S<b>1501</b> and S<b>1502</b>).

0081Further, a media stream in the session is transmitted to the new PoC client A<b>2</b><b>1120</b> after the 200 OK response is received (S<b>1600</b>). At this time, the media is transmitted up to the PF A <b>1100</b> through the same route, and is routed to the corresponding PoC client A<b>2</b><b>1120</b> via the SIP/IP core network.

0082Meanwhile, the PoC server, the CF <b>1000</b>, receiving the 200 OK message transmits a BYE message for terminating the session to the PoC client A<b>1</b><b>1110</b> in order to terminate unnecessary connection of the session (S<b>1701</b> and S<b>1702</b>), and confirms the termination of the session by receiving the 200 OK response (S<b>1801</b> and S<b>1802</b>).

0083Now, a second embodiment of replacing a terminal will be described, in which the terminal to be replaced continues a session, acting as a subject.

0084<figref idref="DRAWINGS">FIG. 8</figref> is a flow diagram showing a process for busy change of a PoC compliant terminal in accordance with a second embodiment of the present invention.

0085<figref idref="DRAWINGS">FIG. 8</figref> shows a process where, when a PoC user wants to replace only a terminal without terminating a session, information on a currently running session is transmitted to a terminal to be replaced, and replacement of the terminal is carried out by the terminal to be replaced.

0086Referring to <figref idref="DRAWINGS">FIG. 8</figref>, in the same situation as the first embodiment of <figref idref="DRAWINGS">FIG. 5</figref>, a PoC client A<b>1</b><b>1110</b> transmits information (conference URI, dialog ID, etc.) of his/her session and information indicating replacement of a PoC session to a corresponding client A<b>2</b><b>1120</b> through a REFER message. At this time, information included in the REFER message is similar to that of <figref idref="DRAWINGS">FIG. 6</figref>, but has the following difference.

0087The PoC client A<b>1</b><b>1110</b> creates an SIP REFER message, and then the REFER message is transmitted to the PoC client A<b>2</b><b>1120</b> that wants to replace the session through routing of SIP/IP core network (S<b>2001</b> and S<b>2002</b>). Here, since the REFER message is forwarded through a general SIP proxy server, it has no alternative to pass through a PF A <b>1100</b>.

0088To this end, when the SIP REFER message is created, the PoC client A<b>1</b><b>1110</b> sets up a unique identity of the PoC session (conference URI:conf_uri_cfx, which is generally stored in a CF) as the value of a Refer-To header field, and sets up address information of the PoC client, which a user wants to use, as the value of a Request URI.

0089Description will be made about content of the REFER message with reference to <figref idref="DRAWINGS">FIG. 9</figref>.

0090The Refer-To header field R<b>5</b> includes a proper SIP message method which is to be taken as address information of a destination. In the present invention, the SIP message method is adapted to establish the session by designating as an INVITE message.

0091In the present invention, the SIP message method is designated as the INVITE message when the session is established, and the information on the INVITE message will be described after description of the information of the REFER message.

0092Address information included in the Request URI may make use of an SIP URI used by the current PoC client A<b>1</b> or another SIP URI.

0093Like the description of <figref idref="DRAWINGS">FIG. 5</figref>, if the same SIP URI is used, a new SIP INVITE message is adapted to be transmitted to the PoC client A<b>2</b><b>1120</b>, including preference information (characteristic value: mobile, video, or explicit) of an SIP user agent, which is consistent with the capability of the SIP user agent of the PoC client A<b>2</b><b>1120</b> in order to discriminate between a target PoC client A<b>2</b><b>1120</b> and the current PoC client A<b>1</b><b>1110</b>. The new SIP INVITE message is transmitted including the preference information in an Accepted-Contact header field (R<b>4</b> of <figref idref="DRAWINGS">FIG. 9</figref>) of the REFER message, thereby being routed to the PoC client A<b>2</b><b>1120</b> in the SIP/IP core network.

0094Meanwhile, the REFER message includes connection information of the session (the session dialog identifier) (From-tag, To-tag and Call-ID in R<b>2</b> of <figref idref="DRAWINGS">FIG. 9</figref>) which enables the currently running PoC session to be seamlessly connected using the PoC client A<b>2</b><b>1120</b>, as well as information on the target terminal to be replaced (Refer-To in R<b>5</b> of <figref idref="DRAWINGS">FIG. 9</figref>).

0095Further, the REFER message includes a Referred-By header R<b>6</b> and a Multipurpose Internet Mail Extension type (MIME) body portion containing the corresponding contents in order to transmit dialog information that the PoC client A<b>1</b><b>1110</b> is putting to use.

0096The PoC client A<b>2</b><b>1120</b> receiving the REFER message responds with an Accepted message in order to return successful SIP process (S<b>2101</b> and S<b>2102</b>).

0097Then, the PoC client A<b>2</b><b>1120</b> transmits an INVITE message to an address of the SIP URI (conference URI of the PoC session) received through the Refer-To header of the REFER message (S<b>2201</b> and S<b>2202</b>).

0098Information on the INVITE message transmitted at this time will be described with reference to <figref idref="DRAWINGS">FIG. 10</figref>.

0099As shown in <figref idref="DRAWINGS">FIG. 10</figref>, the PoC client A<b>2</b><b>1120</b> transmits corresponding session dialog information through a Replaces header field (T<b>4</b> of <figref idref="DRAWINGS">FIG. 10</figref>) of the INVITE message in order to replace a session dialog (to be replaced in order to receive media through the same session dialog with respect to the currently running session) which the PoC client A<b>1</b><b>1110</b> puts to use.

0100The PoC server, the CF <b>1000</b>, receives a request to connect the session from the PoC client A<b>2</b><b>1120</b>, and then returns a 200 OK response (S<b>2301</b> and S<b>2302</b>). At this time, newly created session dialog information (dialog ID created by transaction in steps S<b>2201</b> to S<b>2302</b>) is stored in the CF <b>1000</b>, and it is confirmed that a RTP media stream transmitted through the previous dialog can be transmitted as a new dialog. The PoC client A<b>2</b><b>1120</b> returns an ACK signal with respect to the 200 OK response (S<b>2401</b> and S<b>2402</b>).

0101Meanwhile, the CF <b>1000</b> transmits the 200 OK response, and simultaneously transmits a talk burst control message notifying transmission of the media to the PoC client A<b>2</b><b>1120</b> (S<b>2501</b> and S<b>2502</b>). Further, the resulting media stream is transmitted to new PoC client A<b>2</b><b>1120</b> after the 200 OK response is transmitted (S<b>2600</b> and S<b>2700</b>). At this time, the media is transmitted to the PF A <b>1100</b> through the same route, and is routed to the corresponding PoC client A<b>2</b><b>1120</b> after the route of the SIP/IP core network is decided.

0102Next, the PoC server, the CF <b>1000</b>, after transmitting the 200 OK message transmits a BYE message for terminating the session to the PoC client A<b>1</b><b>1110</b> in order to terminate unnecessary connection of the session (S<b>2801</b> and S<b>2802</b>), and confirms the termination of the session by receiving the 200 OK response S<b>2901</b> and S<b>2902</b>).

0103Now, the message shown in <figref idref="DRAWINGS">FIGS. 6, 7</figref>, <b>9</b> and <b>10</b> will be described in full detail.

0104<figref idref="DRAWINGS">FIG. 6</figref> shows the format of the REFER message used in the flow diagram of <figref idref="DRAWINGS">FIG. 5</figref> when the PoC clients A<b>1</b> and A<b>2</b> have the same SIP URI.

0105A Request URI P<b>1</b> of the REFER message of <figref idref="DRAWINGS">FIG. 6</figref> is set up as a conference URI that is managed by the CF, and the value of a Refer-To header field P<b>3</b> is designated as the value of an SIP URI of the PoC client A<b>2</b> to which the CF makes a request for an INVITE message. At this time, the SIP URI of the Refer-To header field P<b>3</b> is transmitted including preference (characteristic value) of the PoC user in order to discriminate from the PoC client using the same SIP URI. To this end, the present invention is defined so as to add preference (fixed, mobile, video, explicit etc.) of a PoC session requester, which serves as an URI parameter of the Refer-To header as in <figref idref="DRAWINGS">FIG. 6</figref>.

0106The PoC client A<b>1</b> makes use of a Referred-By header field P<b>4</b> in order to transmit dialog information of a session to which he/she is connected. The Referred-By header field P<b>4</b> includes SIP URI information of the PoC client A<b>1</b> and an identity CID of an attached content. The PoC client A<b>1</b> is adapted to transmit his/her session dialog information through a corresponding MIME body portion P<b>5</b>. When the PoC server, the CF, transmits a new INVITE message using these content identity and dialog information, the CF can use the same session dialog.

0107<figref idref="DRAWINGS">FIG. 7</figref> shows the format of the message when the INVITE message is created after the REFER message is received. As discussed above, the preference information of the PoC client which is included in the Refer-To header is used in an Accept-Contact header portion Q<b>3</b> of the INVITE message, discriminating among the PoC clients having the same SIP URI to act to route the INVITE message. Meanwhile, a Replaces header Q<b>4</b> includes session dialog information transmitted through the REFER message, and serves to receive media for the same session dialog when the PoC client is replaced.

0108<figref idref="DRAWINGS">FIG. 9</figref> shows the format of a REFER message when PoC clients A<b>1</b> and A<b>2</b> have the same SIP URI. A Request URI R<b>1</b> of the REFER message is set up as the SIP URI of a new PoC client, and the value of a Refer-To header field R<b>5</b> is set up as the value of an SIP URI of the CF in which the PoC clients participate.

0109Further, for the purpose of routing to the PoC client A<b>2</b> using the same SIP URI, a preferred user agent characteristic is set up for an Accepted-Contact header R<b>4</b>, and a Refer-To header R<b>5</b> sets up a type of the message, which the PoC client A<b>2</b> has to transmit in addition to information on a PoC session identity, as an INVITE message.

0110Meanwhile, the PoC client A<b>1</b> makes use of a Referred-By header field R<b>6</b> in order to transmit dialog information of a session to which he/she is connected. The PoC client A<b>1</b> is adapted to transmit his/her session dialog information with SIP URI information of the PoC client A<b>1</b> as well as an identity CID of an attached content included in the Referred-By header field R<b>6</b>. When transmitting a new INVITE message using these content identity CID (denoted by 20398823.2UWQFN309shb3@domain.com) and dialog information (denoted by 88upf11a@client_apc.domain.com; to-tag=7743; from-tag=6472), the PoC client A<b>2</b> can use the same session dialog.

0111<figref idref="DRAWINGS">FIG. 10</figref> shows the format of the message when the INVITE message is created after the REFER message is received. As discussed above, the capability information of the PoC client which is included in the Accepted-Contact header is given to a Contact header T<b>3</b> of the INVITE message, discriminating from the PoC client A<b>1</b> using the same SIP URI. Meanwhile, a Replaces header T<b>4</b> includes session dialog information transmitted through the Referred-By header and MIME body portion of the REFER message, and allows the PoC client A<b>2</b> to receive media for the same session dialog instead of the PoC client A<b>1</b>.

0112The present invention is not limited to a PoC system, but it may be applied to all systems where a call is made by request of the call using IMS (IP Multimedia System) Core Network (CN) that is being standardized or completed in the 3GPP (3rd Generation Partnership Project) or 3GPP2 (3rd Generation Partnership Project 2), as well as a half duplex type call.

0113The present invention as mentioned above enables the target PoC client to be connected to the PoC session instead of the current PoC client by the instruction of the PoC client, instead of the procedure of connecting the session again after the session is terminated.

0114Therefore, the media is prevented from being transmitted discontinuously, so that the PoC user can be provided with improved service.

0115Further, the currently busy PoC client can be replaced in the existing busy PoC session using the PoC compliant client capable of connecting the PoC session, so that the transmitted media stream of the running PoC session can be prevented from being lost when the PoC client is replaced.

0116For this reason, in the situation where it is necessary to replace the PoC terminal according to change of the media for the PoC call, request of mobility, etc., by the PoC user, the user is allowed to make a continuous call. Thus, it is expected to improve QoE of the user and expand the markets for the PoC terminal and its services.

0117Although exemplary embodiments of the present invention have been described with reference to the attached drawings, the present invention is not limited to these embodiments, and it should be appreciated to those skilled in the art that a variety of modifications and changes can be made without departing from the spirit and scope of the present invention.