Abstract:
Disclosed is a push-to-talk-over-cellular (PoC) service that transmits information on a group talk in a wireless connection section for operation in a mobile environment with a low traffic burden and higher efficiency. The PoC service transmits a PoC call request message of an originating PoC client to a terminating PoC server, receives a response message to the PoC call request message from a terminating PoC client, and receives a message including information of the terminating PoC client transmitting the response message, from an originating PoC server.

Description:
PRIORITY  
       [0001]     This application claims the benefit under 35 U.S.C. § 119(a) from an application entitled “METHOD AND SYSTEM FOR TRANSMITTING INFORMATION OF RESPONDENT PARTICIPATING IN PUSH-TO-TALK OVER CELLULAR NETWORK SESSION” filed in the Korean Intellectual Property Office on Apr. 1, 2005 and assigned Serial No. 2005-27431, the entire contents of which are hereby incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to push-to-talk-over-cellular (PoC) communication technology capable of transmitting information on a group talk in a wireless connection section, for operation in a mobile environment with a low traffic burden and higher efficiency, and more particularly, to a method and system for transmitting session participation information of PoC users participating in a PoC group session.  
         [0004]     2. Description of the Related Art  
         [0005]     Significant developments in mobile communications technology and the extension of mobile communication networks have resulted in the development of a vast array of services and applications for use with a cellular phone. Concurrently, there is an increasing demand from cellular phone users for additional services, such as a location, multimedia, and push-to-talk (PTT) service. Among these extra services, the PTT service supports various supplementary functions such as instant messenger and a status display, as well as a group call and a voice call, which are also provided by an existing radio or a trunk radio system (TRS).  
         [0006]     Standardization of a push-to-talk-over-cellular (PoC) service that utilizes the PTT function in a mobile communication network is currently taking place. One unique feature of the PoC service is that a user can participate in a plurality of PoC sessions and thus can use a call service while moving among the PoC sessions as desired. This feature is a requirement that is specified in the open mobile alliance (OMA), which is a forum for specifying mobile communications services.  
         [0007]      FIG. 1  is a schematic diagram of a conventional PoC service system. Referring to  FIG. 1 , a PoC client  10 , as a service requester installed in a mobile station, is generally connected to a Session Initiation Protocol/Internet Protocol (SIP/IP) core  30  that supports SIP and IP multimedia via an access network  20 .  
         [0008]     The PoC client  10  resides in a PoC user terminal to provide access to the PoC service. The PoC client  10  mainly serves to establish, participate in, and terminate a PoC session. In addition, the PoC client  10  makes and transfers a talk burst, supports an instant personal alert, and authenticates when accessing the PoC service. Hereinafter, unless otherwise stated, the PoC client  10  is assumed to be the same as a PoC service subscriber.  
         [0009]     The SIP/IP core  30  is connected to a PoC server  60 , a Group List and Management System (GLMS)  50 , and a presence server  70 , in order to support the PoC service. At this time, the PoC server  60  has a Controlling PoC Function for maintaining and managing a PoC session, or a Participating PoC Function for participating in a PoC session for a one-to-one PoC call or a one-to-two or more PoC call.  
         [0010]     In general, SIP or Extended SIP, an application-layer protocol for controlling Internet multimedia communication (IP telephony), are mainly used to transmit session participation information of PoC group talks.  
         [0011]     Generally, SIP is a standard defined in the Internet Engineering Task Force (IETF) Request for Comments (RFC) 2543. 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 communication. SIP exists over a User Datagram Protocol (UDP)/TCP/IP layer, which supports both unicast and multicast sessions for initiating a session by inviting participants to a multimedia conference with a client/server protocol capable of exchanging SIP Request and Response messages in a request/response fashion.  
         [0012]     The SIP Request message provides six functions in RFC 2543 as follows:  
         [0013]     INVITE (Invitation to participate in a session);  
         [0014]     ACK (Acceptance of an INVITE request);  
         [0015]     BYE (Termination of a call), REGISTER (Registration with the database of a redirect server by a user agent);  
         [0016]     CANCEL (Cancellation of a pending request); and  
         [0017]     OPTIONS.  
         [0018]     The SIP Response message provides status codes as follows:  
         [0019]     1xx (Information response);  
         [0020]     2xx (Successful response);  
         [0021]     3xx (Redirection response);  
         [0022]     4xx (Client Error, Request Failure);  
         [0023]     5xx (Server Error); and  
         [0024]     6xx (Global Failure).  
         [0025]      FIG. 2  illustrates a schematic configuration of a conventional PoC server. The PoC server performs both a Controlling PoC Function (hereinafter referred to as CF) for controlling overall maintenance and management of a PoC session and the Participating PoC Function (hereinafter referred to as PF) for controlling maintenance and management between each PoC session, which will be explained below with reference to Tables 1 and 2.  
                         TABLE 1                       Controlling PoC Function (CF)                                    Provides centralized PoC session handling           Provides centralized Media distribution           Provides centralized Talk Burst Arbitration functionality           including talker identification           Provides SIP session handling, such as SIP session origination,           termination, etc.           Provides policy enforcement for participation in group sessions           Provides participant information           Collects and provides centralized media quality information           Provides centralized charging reports           May provide transcoding between different codecs           Supports Talk Burst Control Protocol Negotiation                        
         [0026]     As shown in Table 1, the PoC server performing the CF (or the Controlling PoC server) manages a PoC session. In particular, the Controlling PoC server receives requests for the 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 Controlling 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.  
         [0027]     As shown in Table 2 below, the PoC server performing the PF (or the Participating PoC server) manages a PoC session between the Controlling PoC server and each PoC client. In particular, the Participating PoC server relays the floor between the PoC client and the Controlling PoC server when the PoC client makes a request for the floor or when the Controlling PoC server gives the floor to the PoC client. In addition, the Participating PoC server relays media between the Controlling PoC server and the PoC client, performs transcoding between different codecs, and filters one of two concurrent PoC sessions according to the choice of a PoC user when there is simultaneous talking in the two active PoC sessions.  
                     TABLE 2                       Participating PoC Function (PF)                                Provides PoC session handling       May provide the Media relay function between PoC client and       Controlling PoC server       May provide user media adaptation procedures       May provide the Talk Burst control message relay function between       PoC client and Controlling PoC server       Provides SIP session handling, such as SIP session origination,       termination, etc, on behalf of the represented PoC client       Provides policy enforcement for incoming PoC session (e.g. access       control, incoming PoC session barring, availability status, etc.)       May collect and provide media quality information       Provides participant charging reports       May provide filtering of media streams in the case of       simultaneous sessions       May provide transcoding between different codecs       May support Talk Burst Control Protocol Negotiation       Stores the current Answer Mode and Incoming PoC Session Barring       preferences of the PoC client                  
 
         [0028]     In the PoC service system described above, the PoC user can input information on groups and their members into the GLMS  50  through the PoC user&#39;s terminal, and can receive information about other PoC users with whom the PoC user can talk through an individual or group list transmitted from the GLMS  50 .  
         [0029]     Alternatively, in order to create, modify and manage groups and their members, the information on the groups and their members may be input into the GLMS  50  via a communication network, such as an Internet or Intranet.  
         [0030]     In order to use the PoC call service, the PoC user registers his/her PoC address with the SIP/IP core  30 . The SIP/IP core  30  stores information on the PoC user by request of the PoC user. Thus, when another PoC user tries to request the group PoC call, the PoC user registers his/her information with the SIP/IP core  30  in advance, and requests the group PoC call to his/her SIP/IP core  30  by using group identification information transmitted from the GLMS  50 . The SIP/IP core  30  performs addressing and domain locating using information of the requesting PoC user and then transfers a PoC call request to a home PoC server  60  with which the requesting PoC user is registered.  
         [0031]     In regard to the PoC call request, the PoC server  60  prepares to establish a PoC session, obtains each user&#39;s information from the GLMS  50 , and then transfers a PoC call request signal to the SIP/IP core  30 . When the PoC call request is made to users within an Intradomain, the PoC server  60  performs both the CF and the PF. The PoC server  60  managing the call-requested PoC user requests the PoC call to the PoC user after locating the SIP/IP core  30  by use of the PoC user&#39;s information transmitted to the PoC server  60 .  
         [0032]      FIG. 3  is a schematic diagram illustrating CF and PF blocks of a PoC server. Referring to  FIG. 3 , PoC clients  111 ,  121 ,  131 , and  141  provide access to a CF  100  through PFs  110 ,  120 ,  130 , and  140 , respectively, thereby establishing a PoC session. When the floor is granted to a requester qualified as a talker by the CF  100 , speech media of the corresponding PoC client is transmitted to each PoC client. At this time, the PoC user who is granted the floor cannot appropriately speak until he/she confirms information of the participants participating in the PoC group session.  
         [0033]     Meanwhile, the PoC system required by the OMA has the following features.  
         [0034]     First, the terminating side can set its own answering modes according to the request of a PoC user. The answering modes can be either auto or manual.  
         [0035]     If the terminating side is registered in an auto answer mode user list, the terminating side can immediately send an answer to the originating side in a corresponding network in place of the manual answer of a recipient. The automatic answer is sent instead of operating the terminal in the network because the PoC server has a function of storing the answering mode and the corresponding user list according to a request of the terminal to set the answering mode.  
         [0036]     The manual answer mode corresponds to when the user is not included in an automatic answer user list, the answer is ambiguous, or the recipient sets all users to make the manual answer. In the manual mode, a PoC call request is transmitted to the user&#39;s terminal through a terminating network and then a call is connected by acceptance of the PoC user.  
         [0037]     Second, the PoC system is divided into an on-demand session mode and a pre-established (or early) session mode, according to the type of connection with a PoC server within a user&#39;s home network. The pre-established session mode is designed so that the PoC user sets a session between a PoC client and a PoC server belonging to a PoC user&#39;s home network in advance by PoC user&#39;s request. The pre-established session enables the PoC user to negotiate media parameters to be used with the PoC server in advance, and thus advance rapid session establishment without renegotiating the media parameters to be used in the future between the PoC server and client. In order to set the pre-established session, the PoC client provides supported media parameters to a Session Description Protocol Multipurpose Internet Mail Extensions (SDP MIME) body through a SIP INVITE method, and responds to the media parameters provided from the PoC server. The PoC client sends, to the PoC user, identification information of the pre-established session for a response message received from the PoC server, together with a conference Uniform Resource Identifier (URI). When using the pre-established session, it is possible to pre-negotiate parameters such as an IP address, a port number, a codec to be used, and a Talk Burst Control Protocol (TBCP) for controlling a talk burst. The on-demand session mode refers to a state in which the PoC user does not set the pre-established session, and indicates that the PoC user performs a PoC call connecting procedure after receiving an invitation message of another PoC user.  
         [0038]     Hereinafter, a process of establishing a PoC session of the PoC system will be described with distinction between the originating side and the terminating side.  
         [0039]     First, an originating PoC client A sends an SIP INVITE request message, which includes the SIP address of a recipient to whom the PoC client A desires to talk, to a corresponding SIP/IP core A. The INVITE request message includes information such as a PoC address of the call-requesting client, requested media parameters (because the requested session is based on multimedia having various media attribute values such as an audio and video encoding method, a rate and a payload type), and an attribute value informing PoC service, and is forwarded to a PF via corresponding IP Multimedia Subsystem (IMS) servers (Proxy Call Session Control Function (P-CSCF) and Serving Call Session Control Function (S-CSCF)) in an IMS network through a route query at a Dynamic Host Configuration Protocol (DHCP) server or Domain Name System (DNS) server. Because the PF, to which a PoC user is connected at a general call request, can be implemented as an entity different from a CF that manages the talk burst of an established session, the INVITE request message forwarded previously is transmitted to the CF via an SIP/IP core of the corresponding network.  
         [0040]     A PoC session controlling network including the CF transmits the INVITE request message to the terminating network, and then receives a response message. The SIP response message with which the terminating network responds may a provisional response message of 1xx, a successful response message of 2xx, and an error response message of 4xx, 5xx, or 6xx. If an AUTO-ANSWER mode is set, the CF can receive a SIP 183 Session Progress signal, and thus perform connection between the PoC server and the PoC client in the IMS network of the call requester. The call acceptance signal of the recipient responds with the SIP 183 Session Process or SIP 200 OK response and is forwarded to the PoC client A via the CF and PF, the PoC servers. After receiving the 200 OK or 183 Session Progress response from the terminating PoC server, the CF determines that a PoC call is connected and then sends a Floor Granted signal, which gives the talk burst floor to the PoC client A. Granting the talk burst authority according to the response (200 OK or 183 Session Progress) can be divided into confirmed and unconfirmed types. When receiving the unconfirmed response, the CF requires a buffering function.  
         [0041]     After receiving the response signal to the INVITE request signal, the originating PoC client A receives the Floor Granted signal forwarding a talk burst transmission enable signal (e.g., a ring back tone) using Real-time Transport Protocol (RTP) Control Protocol (RTCP). At this time, the Floor Granted signal is generated from the CF having the authority to arbitrate the talk burst, and transmitted to the corresponding PoC client via the PF, which manages the corresponding PoC client. The Floor Granted signal can be transmitted without passing through the SIP/IP core because it uses a bearer&#39;s route rather than the SIP. Finally, the PoC user who confirms the ring back tone transmits a media stream (e.g. voice) using an RTP.  
         [0042]     Conventional art associated with respondent information transmission will be described below.  
         [0043]      FIG. 4  illustrates a flow of signals for an SIP call processing procedure of a PoC client for receiving continuous information of participants when a conventional group session is connected. A PoC client A requests an on-demand ad-hoc group session from PoC clients B 1 , B 2 , and C (since the procedure is performed on a pre-established group session by the same principle, the ad-hoc group session is shown by way of example), and a different response message is received from each client.  
         [0044]     Referring to  FIG. 4 , an originating PoC client A  111  sends an SIP INVITE request message including conference session information (a URI list including address information of terminating PoC clients, a provisioned conference identity) (S 1  and S 2 ), and thus the SIP INVITE request message is sent to a corresponding PoC client (S 3   a  and S 4   a ). When a first 200 OK response message is received from a terminating PoC client C  131 , the originating PoC client A  111  initiates a PoC media session (S 5   a , S 6   a , S 7  and S 8 ). The originating PoC client A  111  requests a SIP SUBSCRIBE message from a conference server in order to obtain information on group session participation (S 11  and S 12 ). The originating PoC client A  111  extracts a PoC session identity from the previously received 200 OK response (see steps S 7  and S 8 ) in order to identify conference information that the originating PoC client A  111  intends to obtain, sets a request URI of the SIP SUBSCRIBE message as the extracted PoC session identity, and includes the “conference” in an event header. Accordingly, the originating PoC client A  111  can request the session information from the corresponding conference server.  
         [0045]     Thereafter, a CF operating as an SIP user agent by request of the SIP SUBSCRIBE message draws up a conference state event package whenever an event occurs.  
         [0046]     Finally, whenever receiving a session participation message in the future according to the session information obtaining request, the CF X  100  sends a result of the conference state event package including the event by way of an SIP NOTIFY message (S 15  and S 16 ). Further, a terminating PoC client B 1   121  participating in the session in the future receives the conference state information by the same procedure as the foregoing.  
         [0047]     However, the conventional method of obtaining session participant information has the following drawbacks.  
         [0048]     First, a session initiator needs to request the SIP message (SIP SUBSCRIBE message) in order to obtain the conference state information. As a result, additional SIP signaling is required.  
         [0049]     Second, when the established group session is an ad-hoc group session, the SIP SUBSCRIBE message is requested after the first 200 OK response message to the initial SIP INVITE message is received. (As described above, after the PoC session identity is received through the response of steps S 7  and S 8 , the SIP SUBSCRIBE message can be requested.)  
         [0050]     Third, in the case of the PoC client invited to the session, an ACK signal to the received 200 OK response is returned, and then the group information can be requested. In this case, the participant information cannot be obtained in real time when the group is initially established. As a result, there is a possibility of the PoC user not properly transmitting media.  
       SUMMARY OF THE INVENTION  
       [0051]     It is an object of the present invention to provide a PoC network for a mobile environment with a low traffic burden, by transmitting information of a PoC client responding to session participation in an initial session establishment procedure with the information included in a TBCP signal.  
         [0052]     Another object of the present invention is to provide a session initiator with all information created according to a session participation response type.  
         [0053]     According to the present invention, there is provided a method for transmitting information of a respondent participating in a push-to-talk-over-cellular (PoC) network session, including transmitting, by an originating PoC server, a PoC call request message of an originating PoC client to a terminating PoC server; receiving, by the originating PoC server, a response message to the PoC call request message from a terminating PoC client; and receiving, by the originating PoC client, a message including information of the terminating PoC client transmitting the response message, from the originating PoC server.  
         [0054]     According to the present invention, there is provided a method for transmitting information of a respondent participating in a push-to-talk-over-cellular (PoC) network session, including transmitting, by a PoC server managing the session, a PoC call request message of an originating PoC client to a terminating PoC server, receiving, by the PoC server, a response message to the PoC call request message from a terminating PoC client, and receiving, by the originating PoC client, a message including information of the terminating PoC client transmitting the response message, from the PoC server.  
         [0055]     According to the present invention, there is provided a system for transmitting information of a respondent participating in a push-to-talk-over-cellular (PoC) network session, including an originating PoC client for transmitting a session call request message to a terminating PoC system, an originating PoC server for receiving a response message to the call request message, and the originating PoC client for receiving a message including information of a terminating PoC client transmitting the response message from the originating PoC server.  
         [0056]     According to the present invention, there is provided a push-to-talk-over-cellular (PoC) system for transmitting information of a respondent participating in a PoC network session, including an originating PoC client for transmitting a session call request message to a terminating PoC system, a PoC server for overall management of the session and receiving a response message to the call request message, and the originating PoC client for receiving a message including information of a terminating PoC client transmitting the response message from the originating PoC server.  
         [0057]     According to the present invention, there is provided a push-to-talk-over-cellular (PoC) server for transmitting information of a respondent participating in a PoC network session, wherein, when a response message to a session call request message of an originating PoC client is transmitted to a terminating PoC client, and when the session is connected, the PoC server provides the originating PoC client with a message including information of the terminating PoC client transmitting the response message.  
         [0058]     According to the present invention, there is provided a push-to-talk-over-cellular (PoC) client for transmitting information of a respondent participating in a PoC network session, in which the PoC client transmits a call request message to a terminating PoC system, and when a response message to the session call request message is transmitted from a terminating PoC client to an originating PoC server, the PoC client receives a message including session connection information of the terminating PoC client transmitting the response message from the originating PoC server.  
         [0059]     According to the present invention, there is provided a push-to-talk-over-cellular (PoC) client for receiving information of a respondent participating in a PoC network session. The PoC client transmits a call request message to a terminating PoC system, and when a response message to the session call request message is transmitted from a terminating PoC client to a PoC server managing the session, the PoC client receives a message including information on session connection of the terminating PoC client transmitting the response message from the PoC server. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0060]     The above and other objects, features, and advantages of the present invention will be more clearly understood from the following detailed description, taken in conjunction with the accompanying drawings, in which:  
         [0061]      FIG. 1  is a schematic diagram of a conventional PoC service system;  
         [0062]      FIG. 2  shows a schematic configuration of a conventional PoC server;  
         [0063]      FIG. 3  is a schematic diagram illustrating CF and PF blocks of a conventional PoC server;  
         [0064]      FIG. 4  illustrates flow of signals for a SIP call processing procedure of a PoC client for receiving continuous information of participants when a conventional group session is connected;  
         [0065]      FIG. 5  is a schematic diagram illustrating flow of signals related to the concept of transmitting session participation information through an RTCP message in accordance with the present invention;  
         [0066]      FIG. 6  illustrates flow of signals for a process of transmitting session participation information with the session participation information included in a TBCP message in accordance with the present invention;  
         [0067]      FIG. 7  illustrates a format of a TBCP message including an RTCP payload in accordance with the present invention; and  
         [0068]      FIG. 8  illustrates a table of TBCP response types according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0069]     Hereinafter, preferred embodiments of the present invention will be described in full detail with reference to the accompanying drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for the sake of clarity and conciseness.  
         [0070]      FIG. 5  is a schematic diagram illustrating flow of signals related to the concept of transmitting session participation information through an RTCP message in accordance with the present invention. More specifically,  FIG. 5  illustrates a concept in which a PoC client A requesting a group session receives a first 200 OK response to establish the group session, receives an SIP response from another PoC client in the same group, and transmits the resulting RTCP message to the established session.  
         [0071]     Referring to  FIG. 5 , a conference server CF has a basic function of transmitting no RTCP message because a provisional response received from PoC clients B 1  and B 2  as shown in steps S 101  and S 102  is not a final response for session participation. (However, the RTCP message may be selectively transmitted depending on the function of the CF.)  
         [0072]     When a successful response is received from another PoC client (S 103 ), the CF transmits automatically an identity of the PoC client participating in the session, a type of the response, and a content of the response through a payload of the RTCP message (or a TBCP message according to an OMS PoC policy) (S 104 ). In certain cases, the CF may be able to transmit them intermittently or late. Further, when receiving SIP final responses continuously transmitted from other PoC clients, the CF transmits contents of successful and rejected responses with the contents of successful and rejected responses included in the RTCP payload (S 106  through S 111 ).  
         [0073]     Although not shown in  FIG. 5 , the PoC client transmitting the successful response to the session can receive information of participants connected to the session in the future by the same method as the present invention. That is, in  FIG. 5 , a PoC client C can receive future information on session participation of a PoC client B 1 . No entity other than a session initiator is necessary to send a provisional or reject response for a session participation request.  
         [0074]      FIG. 6  illustrates a process of a terminating PoC system transmitting a response message to a session connection request message of an originating PoC system, and transmitting a message including information on the terminating PoC system to an originating PoC server and client, in accordance with the present invention. Further,  FIG. 6  illustrates a flow of signals for a process of transmitting session participation information of a terminating PoC client transmitting 200 OK with the session participation information included in a TBCP. Here, the originating PoC system includes at least one originating PoC server and an originating PoC client, while the terminating PoC system includes at least one terminating PoC server and at least one terminating PoC client.  
         [0075]     An RTCP message defined may be expressed as a “TBCP Report” or “TBCP Inform” message.  
         [0076]     The session participation information can be transmitted by creating a “TBCP Report” or “TBCP Inform”, or by adding a field proposed in the present invention to an existing TBCP message such as a TBCP Connect message. However, the session participation information includes, but is not limited to, the two types of TBCP messages. Thus, various types of TBCP messages can be used within the present invention. For convenience of description, the session participation information is defined herein as the “TBCP Report.” 
         [0077]     Referring to  FIG. 6 , a PoC client A  1101  requesting a group session requests session connection including a URI list of recipients in an SIP INVITE (S 201  and S 202 ), and thus a CF X requests session participation from each PoC client (S 203   a , S 203   b , S 203   c , S 204   a , S 204   b , and S 204   c ). Next, the CF X receives a response from a PoC client  1301  (S 205   a , S 206   a , S 207 , and S 208 ), and then from each of PoC clients B 1   1201  and PoC clients B 2   1202  (S 205   b , S 206   b , S 205   c , and S 206   c ).  
         [0078]     The PoC client A  1101  establishes a first session with the PoC client C  1301 , and then receives the session participation information in response later through the defined TBCP Report message (S 209  and S 210 ).  
         [0079]     Accordingly, the CF X  1000  acting as a conference server creates and transmits a data payload, which is included in the TBCP message, from a status line of the received SIP response message and an SIP identity of a respondent.  
         [0080]     The information transmitted through the TBCP message can include an identity of the PoC client (or a nickname or SIP address information that can be selectively set according to privacy settings) and a successful response (OK) as in steps S 207   b  and S 208   b.    
         [0081]     Further, the information transmitted through the TBCP message can include an identity of the PoC client (or a nickname or SIP address information that can be selectively set according to privacy settings) and an error response (such as 480 Temporarily Unavailable, 486 Busy Here, 488 Not Acceptable Here, 403 Forbidden, etc.) as in steps S 209   c  and S 210   c.    
         [0082]     As described above, in addition, the PoC client invited to the group session can send a 200 OK response or an ACK response to participate in the session, and receives the session participation information (i.e., the PoC client identifier and 200 OK response information) of another PoC client participating in the session later. Thus, the PoC client participating in the session can receive the information of the PoC client participating in the session later without an SIP subscription request of the group information, so that media transmission can be efficiently performed.  
         [0083]     The case in which the PoC client A establishes the general PoC group session with three terminating PoC clients is described with reference to  FIG. 6 . However, the present invention can also be applied to an establishment procedure modified according to a group type of PoC service (such as ad-hoc group, pre-arranged group and chat group), a session type (pre-established session or on-demand session), an answering mode (auto or manual), and response content. For example, with respect to the ad-hoc group session, a PoC server taking the concurrent roles of PF and CF can directly transmit the TBCP message to a PoC client. Further, in  FIG. 6 , the CF X  1000  acts as an independent server herein, but this configuration is not limited to network topology. More specifically, the CF X can act as the originating PoC server in the originating PoC system by transmitting the TBCP Report to the originating PoC client in step S 207   b , or as the terminating PoC server in the terminating PoC system by transmitting the TBCP Report including the information of the PoC client transmitting the 200 OK to the terminating PoC client.  
         [0084]      FIG. 7  illustrates a format of a TBCP message including an RTCP payload in accordance with the present invention. Referring to  FIG. 7 , the RTCP APP packet includes the identity of a respondent, the identity of an established PoC group or session, and the response type and content of a terminating PoC client, together with general RTCP transmission relevant information such as a version, a subtype, a packet type and a packet length.  
         [0085]     The subtype information is set to “10000” to indicate an inherent TBCP Report message, or any one selected from the other reserved values.  
         [0086]     The conference server includes its own address information for managing a corresponding group session and PoC Release information in a Synchronization Source (SSRC) and name fields, respectively. As information on a final response is received from the PoC client, the conference server transmits an identity and response content of the corresponding PoC client through additional field definition. For example, in  FIG. 7 , a PoC client “John Doe” sends a successful response (Reason Code=1), which can be expressed and transmitted into a response reason “session joined.” 
         [0087]     As another example, when the Reason Code is 2, whether an error such as an SIP 4xx response occurs can be indicated, and the present response reason can be expressed in text.  
         [0088]      FIG. 8  illustrates a table of TBCP response types of the present invention. Referring to  FIG. 8 , when the TBCP “Reason Code” is 1 in the TBCP Report message, this indicates successful session connection. Text (“session joined”) indicating a content of the present successful session connection is included. When the TBCP “Reason Code” is 2, a session establishment error is indicated, and error content (such as forbidden, unavailable or busy) is transmitted to a session initiator so as to be suitable for each error response situation.  
         [0089]     The TBCP message can be modified into such a message as a Multimedia Burst Control Protocol (MBCP) message, in order to express multimedia in the future. According to the present invention, when the SIP session connection response is received after the first response in the PoC group talk, the session connection information of the responding, terminating PoC client is transmitted in the connected session using the TBCP, and traffic burden can be reduced in the wireless environment.  
         [0090]     Further, the initiator of the PoC group talk and the PoC user responding to the group talk request first provide the information on the members participating in the group in the future without an additional request of the SIP message. Particularly, the session initiator provides the reject information of a PoC user who does not gain acceptance to participate in the session. Thus, reliability of information provision is improved.  
         [0091]     While the present invention has been described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in from and detail may be made therein without departing from the scope of the present invention as defined by the following claims.